Probably something to do with engine type/power and propellor type... Just guessing as I know sweet ....all, you know ;)

Dive acceleration is not solely a function of gravity.

One must also account for frictional coefficients and powerplant thrust as they relate to the specific "dive" profile being discussed, as well as each individual aircraft's operational guidelines and parameters. It's not a simple answer by any means.

If anyone has a direct link to that TAIC study report, please post it, I'd be very interested to read it.

http://img338.imageshack.us/img338/3665/p47fw190.jpg

http://img571.imageshack.us/img571/2213/p47fw1902.jpg

http://img155.imageshack.us/img155/806/p47fw1903.jpg

http://img805.imageshack.us/img805/6062/p47fw1904.jpg

Excellent point.

I know. So once more: If someone asks why dive accelerations are the _same_, he doesn't want to know about thrust, drag and whatnotelse. Because these are reasons for _different_ dive accelerations.

Who says that there is no difference in dive acceleration. Whole premises in initial question is wrong. There is a difference in dive acceleration.

You are missing the point here, people want dive acceleration "fixed" as there is some magical switch that is turned on when plane start to dive but there is no such thing. All you have are Thrust, Lift,Drag and Weight acting on a plane no matter what's the plane attitude.

If you change something you will change plane behavior in all flight regimes not only in dive. If you have plane behavior modeled reasonably well in level flight and climb than there is no reason to believe that dive behavior is wrong.

Where game has its problems are extreme parts of flight envelope but that's not what the thread starter asked.


BTW Long ago I made tests and posted it on CWOS but it's lost now. But anybody can repeat it.It's simple.

1. Start the plane at alt above the initial testing point. Use no cockpit view to get TAS and stabilize the plane at desired TAS.

2. Keep the TAS constant and measure the time required to pass from the start altitude to end altitude.

3. Repeat for all planes you want to test, try it with no power and full power.

4. Compare the results.


As the test measure the time required to get from StartAlt to EndAlt it also measure the distance traveled. Test requirement is that TAS is kept constant so difference in time from let's say 4000m to 2000m will mean that planes passed different distances which in turn means that their diving angle was different.

Plane that needed longest to get to EndAlt is the best diving among the tested planes because it needed smallest help from gravity to keep its speed. Consequently it will dive fastest in a dive that is performed at same diving angle.

I tried：4.11m Tempest mkv vs p51c engine overheat off, radiator closed

From 3000m altitude to 2000m, keeping speed=700TAS

2minutes and 2 second for tempest mkv
2minute and 30 second for p51c

So, p51c outdives tempest? No. Tempest mkv definitely outdives p51c!

BTW, this kind of "dive" is very shallow, smaller than 10 degree.

When P51 dives to 4500 m=15000ft altitude, and reaches 640km/h=400 mph IAS, that is 1.25*640=800 km/h=222m/s TAS, the mach number is equal to 222/322=0.69. That's fuselage speed.

However, the speed of tip of airscrew is far more 0.69 mach.

4-blade hamilton airscrew,10.5 feet diametre, the reduction ratio of airscrew rotating to engine is 0.477. 3000rpm engine, 1431rpm=23.85r/s airscrew, the rotating speed of tip is：3.14*10.5*0.303*23.85=238m/s

So the combination speed is: (238^2+222^2)^0.5=325 m/s.

Unfotunatly, sonic speed at 4500m altitude is 322m/s, that is to say, the tip of airscew is 1 mach. There is no mach number "concept" in il2's model at all, how can I believe that il2 simulates 1 Mach aerodynamics very well?
Tempest could dive to 450 mph IAS below 20000ft, aha, tempest's big rotol 14-ft airscrew, 3800rpm, 450mph IAS, the tip of rotol must be supersonic, so is P51's hamilton, thus the efficiency curves of their airscrews play an important role when they dive to high speed.

Why tempest outdives p51? For more efficiency airscew @ supersonic? Maybe. For much more heavier fuselage? Probably.

All in all, il2's model is lack of supersonic simulation, that's why we couldn't experience what tempest/p51 should be. That's why tempest couldn't outdive dora easily in game.

Why 400mph IAS ? Why not any other number?

Prop planes can't break sonic barrier so it's not that important for Il2 to have highly detailed Mach model.

Yes it does.

It does? Maybe, depends which models you take for comparison .

It should out dive Dora easily, why ?

It's very common to dive to 400-450MPH IAS for WWII late aircrafts such as P51, P47,Tempest, fw190, etc.


Prop planes can't break sonic barrier, but Prop planes' propeller CAN often break sonic barrier in a dive, it's an unfortunate fact for il2.

As early as 1904 when Wright brothers made the first a/c, they knew both airscrew and wing are "same thing".


Their original propeller blades were only about 5% less efficient than the modern equivalent, some 100 years later.........


That conclusion is based on low Mach data, for supersonic airscrew, the story is totally diefferent.


In my opinion, the airscrew theory/simulation is the weakness of il2's FM.

Tempest MKV 9lbs boost outdives P51B(Mustang III) 18lbs boost.

Quote:

Speed and acceleration in the dive is an essential quality to a successful fighter, but a decisive conclusion on the order of superiority is largely dependant on throttle settings, and the maximum speed in straight and level flight of the individual aircraft. Here again, however, by carrying out a number of tests under different conditions, it is reasonable to assume that the Meteor is well ahead of its rivals, followed by the Tempest, Thunderbolt, Mustang and Spitfire in that order. http://www.wwiiaircraftperformance.org/sl-wade.html

if you simply want to test 'drag' without digging into the code, I think it better to do it with the engine off (i.e., no thrust test).

- go into fmb and set your spawn kph to zero and start a track in cockpit view.

- spawn your plane say 5000 meters and leave the engine off.

- close your rads, neutral your trims, set you prop pitch to 100%.

-push nose into 90 degree vertical.

-end the track when your plane hits the beach at zero alt.

-go back and look at track. look at speedometer at say 20 second mark per track time. (e.g., speed says 400 kph at 20 seconds for this plane)

-repeat with another plane and compare results.

- For the thrust piece, you can use devicelink to get an idea. There is an acceleration parameter that can be graphed/logged. You can see the effect of adjusting throttle and prop pitch. Prop pitch changes and its effect on acceleration is modeled. The csp may be slower to change blade angle than the vdms. At least, that's how it feels like to me. The fw vdm has a torque limiter. I think p factor is also modeled. You can produce de-celeration by adjusting blade angle, according to devicelink.

Crumpp, your very good information.

Especially this one, P47d4 vs fw190a5? a6?

I bet that il2 4.11m can't simulate this.

Attachment 9504
There are some interesting records:

1) bf109g6as initially outdives spitfire IX LF, but spitfire overtakes 109 as speed building up.

2)fw190a5 initially outdives p47d, but p47d overtakes fw190a5 as speed building up.

3)Tempest and 109G's initial dive acceleration are roughly same, but Tempest outdives 109G easily as speed building up.


When speed building up, what happens to P47/P51/Tempest? There airscrew tips reach/break sonic barrier??? howabout 109/190's airscrew?

If Daidalos Team solve this "supersonic" issue, we''ll appreciate that.

Doesn't changing the blade angle keep the prop from going sonic in a dive?

That doesn't make sense because airscrew tip's speed is irrelevant to blade angle.

Airscrew aerodynamics is quite complex, one need to read a whole book to master that.

I'm not an engineer or a pilot. I understand that tip speed means velocity at the tip of the prop and it's faster at the tip then near the center. Theres some formula that describes rotational velocity.

my point is when they built these planes, I imagine the didn't want the tip to be breaking the sound barrier all the time, so they put governors on the engines and design the props to keep it from doing that. if your in a dive and ram air is pushing your prop to rpm limits, I'm pretty sure the operators manual is going to tell you that isn't so good and you need to change the pitch angle and slow the rpms/reduce the tip speeds or something might break or do damage when you get near your never exceed speeds.

Going back to your tempest example, will the pilot not try to do something to mitigate the effects of sonic tip speeds? Or simply, the prop design specs try to engineer that out of the equation as much as possible? It seems in your analysis, you assume not, that it is simply a function of prop length, max rpms of the engine and forward velocity. I just don't know if that is realistic. To me, it seems that tips speed breaking the sound barrier would be a rare event. So, not sure why it needs detailed modeling.

But then again, I only learn aviation stuff from playing this game.;)


Edit:

Relates to what I was thinking about. http://en.wikipedia.org/wiki/Scimitar_propeller

BlackBerry, I think what you're saying about Tempest vs. P-51C is about right, even if your sources are vague and not always the right ones (you're quoting a comparison between Tempest and Typhoon, for instance). A direct test between the P-51C and the Tempest V revealed that the "Tempest tends to pull away" - which is a marginal advantage for the Tempest. You've tested a marginal advantage for the P-51.
Given the average accuracy of the flight models, which was aiming at a 5%, this is something that simply may happen between individual planes, it is no indication that the general algorithm is wrong.
Mach effects are modelled, not extensive enough for accurate high speed performance imho, but they are there.
The differences in initial acceleration between individual planes is there, if you fly them properly. Some of the acceleration differences you see as simple statements "this one is better" has a lot to do with engine management. If say an Fw 190 and a P-47 cruise side by side and then go into a full power dive, the guy in the 190 slams the throttle forward and off he goes, while the guy in the P-47 adjusts mixture, then rpm and then the throttle and then starts to accelerate. Gives the 190 a two second head start. But even without considering this, if you compare a 190A-4FR with a P-47D-22 at medium altitude in 4.11, you'll be getting something similar to the test you quoted.

JTD, I'll make clear that I am not saying the il2 FM is wrong, on the contrary, I believe il2 is the best simulation of WII a/c, especially 4.11m has achieved "structute failure" at high G manoeuver. Well done Daidalos Team! And many of us just hope il2 go further to become PERFECT.


In il2 FM, the wing drag coeffiecent is a constant, since most a/c fly below 0.8 Mach, the result is very accurate. However, there is small flaw in high speed diving. See picture below:

Attachment 9519
Even the best prop diver---Tempest MKV can hardly reach 0.8 Mach in diving----maximum permissible airspeeds 540m.p.h. IAS below 10000 ft, but the airsrew==a twisted and rotating "wing" could exceed 0.8 Mach with its "tip" in a high speed dive.

As long as we could simulate "the balance of propeller power", il2 will be nearly perfect. Don't forget those exhaust tubes just behind airscrew! Exhaust boost!

Attachment 9515

In WWII, US had NACA-16 series airfoil(eg. 4-blade Hamilton), UK had ARA-D airfoil, German had Gottingen airfoil.

I just suspect that those engineers of Hamilton or Rotol intently designed very big 4-blade airscrews in order to optimise high mach performance at high speed.


P51D----Hamilton Standard, four-blade, hydraulic, constant speed, 11 feet 2 inches, non-feathering

Bf109----The propeller is a V.D.M.9 - 12087. Three bladed metal constant-speed with electric pitch change, hand controlled or automatic. Diam. 9' 10" Max. blade width 11 5/8".

Tempest MKV----All versions of the Sabre drove four-bladed, 14 ft (4.267 m) diameter de Havilland Hydromatic or Rotol propellers.

Fw190A9----Three types of propeller were authorised for use on the A-9: the VDM 9-112176A wooden propeller, 3.5 m (11 ft 6 in) in diameter, was the preferred option, however, many A-9s were fitted with the standard VDM 9-12067A metal propeller and some had a VDM 9-12153A metal propeller with external, bolt on balance weights.

P47D----The P-47D-16, D-20, D-22 and D-23 were similar to the P-47D-15 with minor improvements in the fuel system, engine subsystems, a jettisonable canopy, and a bulletproof windshield. Beginning with the block 22 aircraft, the original narrow-chorded Curtiss propeller was replaced by propellers with larger blades, the Evansville plant switching to a new Curtiss propeller with a diameter of 13 ft (3.96 m) and the Long Island plant using a Hamilton Standard propeller with a diameter of 13 ft 2 in (4.01 m). With the bigger propellers having barely 6 in (152 mm) of ground clearance, Thunderbolt pilots had to learn to be careful on takeoffs to keep the tail down until they obtained adequate ground clearance, and on landings to flare the aircraft properly.

Last but not least, I believe the high-speed dive and zoom advantage of P51P47Tempest is their most important tactic in combat, and is the most amazing aspect of their flight characters.

If they can outzoom from high-speed against opponent for 300 metres higher; if they can outdive rival for more kinetic energy(sth. equals to 300 metres Potential Energy ). What will happen?

As we all known, Bf109s are very good at climbing(low speed,max climb), usually, in low-medium altitude, Bf109 has 1000ft/minute climbing advantage to their opponents.

Bf109's Energy fight: When finding enemy at rear, same energy, 700-800m away , 109 will probably climb, after 2 minutes(Be patient! Be careful about the 3rd one!), 109 will establish 600 meters higher advantage over the opponent who follows the 109. And then, 109 will fight back by using this 600 meters "extra" energy. This kind of story takes place again and again and again in most il2 servers.

P51P47Tempest could also "E-fight" in different style: diving and zooming. If a P51 find a 109 at rear, same energy, 700-800m away, P51 can dive to 650km/h IAS ( by split S), if 109 follows, he will find P51 is gaining on him, that is, P51 is quite faster than him, and the distant between them has been enlarged to 1000 m, and then, P51 will zoom at 60 degree (Be patient! Be careful about the 3rd one!), of course 109 will cut the coner, but P51 has zoom advantage so that 109 could not get close to shooting range during zooming period. Roughly P51 will find himself 600 m higher than 109, and this is the time to fight back.


1v1 is quite funny, teamwork of E-fighting will be more attractive, believe it or not. If you have some advantage, be good at using it, don't waste it, don't spoil it, be patient.

Which is already modelled including Mach effects. Like I said, some aspects are there. This is one.

IIRC, the FW-190 tested is a G series by WerkNummer.

The maximum helical tip velocity is extremely important to any propeller design. At about mach .85 most propellers will begin to dramatically decrease efficiency as the normal shock formation disrupts flow. Yes, they break the sound barrier.

You can see the effect in any fast aircraft equipped with a CSP and the ability to over speed the propeller. Climb to about 12,500 feet, preferably on a hot summer day and set the aircraft for 75% cruise. If you increase to maximum rpm and manifold pressure, you will see a drop in your airspeed.

Maybe one could simulate the effect just by decreasing propeller efficiency sharply at Mach .85.

As you stated, propellers are extremely complicated and there are lots of trade-offs in design. For example, adding blades does increase the co-efficient of power but adding blades decreases propeller efficiency. The materials one chooses also has a large effect on propeller design. Metal blades have good power absorption but are fatigue limited. The primary reason for a metal propeller is cheap production and erosion resistance. Metal propellers have excellent erosion resistance so they be flown in the rain. Wooden blades have even better power absorption and unlimited fatigue life. Wooden blades can delaminate in the rain and require some sort of protection in order not to erode. The German wooden propellers were wrapped in metal mesh, fabric covered, and covered with a thick resin.

In the event of a prop strike, metal transferred more force to the engine resulting in more damage. Wooden propellers tend to act like a circuit breaker and disintegrate transferring less force to the engine. It is cheaper to replace a propeller than an engine.

Let's discuss this record.

Jtd's explanation:
It sound reasonable, in a real world combat, the engine management can't be neglected, but this is a "test" to determine dive accelaration so P47 should prepare for diving: at first setting rich-mixture, rpm/pitch to 3000rpm, throttle to 40% so that actual engine rpm is 2000 or so(although pitch is fine), criusing side by side with fw190 @250mph IAS. Next step for P47 is just slam the throttle forward and dive.

BTW, I am doubt fw190A4 could PULL AWAY RAPIDLY from P47d at initial dive in 4.11m.

My GUESS about that historical test:

When diving from 250mph to 380 mph or so, the speed of tip of P47's airscrew firstly reached 0.8 Mach than fw190's due to its greater rotating speed. When P47's reached 0.8 Mach and suffered from obvious airscrew efficience drop while 190's remained at BELOW 0.8 Mach, 190 had more thrust and outdove P47 rapidly.

But 380mph IAS(just my estimate) is the turning point, where P47's tip breaks sound barrier, and the drag coefficient of airscrew tip DECREASES, P47's efficiency INCREASES as we known that the majority of airscrew thrust is from tip section of propeller. From 380mph to 450mph, it was 190's turn to suffer from efficiency drop due to it's 0.8-1.0 Mach tip speed, P47 began to catch up with 190, and the P47's hugh weight advantage boosted its taking over because the higher speed, the more important role of weight. Therefore P47's diving accelarartion became astonishing-------quickly catched up fw190 200yard ahead, and passed 190 with MUCH GREATER SPEED like "thunderbolt".

It's good thinking. I'll take a shot.

I think if the props on both planes are nearing 450 TAS and running inefficiently, you must fall back to the drags of the planes themselves. We know that the wing loading on p47-22 is greater than fw 190A. We also know that fw190A out turns a p47-22 based on the fan plots. So, I think we can conclude that the p47 is simple more aerodynamically streamlined for diving (less draggy) and this is why it eventually catches up and surpasses the 190A in a dive. I don't think the p47 prop all of a sudden gets more efficient when it breaks the sound barrier, but I could be wrong about that. Anyway, it is not the weight of the p47, but more so that the 190 wing simply generates more lift and that creates a drag. Yes, there is a weight difference. But if both planes were shaped as same sized spheres and one is twice as heavy as the other, I think you won't get that much separation.

Also ot, don't ever dive after a p51 in 109. Climb, pursue and hope he turns. If he is diving away from his home base, you have him. Simply cut off the angle. ;)

The "no-lift" drag coefficeint for P47D-37 is "0.0256",it's a constant below 0.8 Mach. The test 0f 1943 December between fw190G and P47D was definitely below 0.8 Mach. You can see that P47 has a big wing of 25.87 square meters:

P-47D-27 = 0.0256 * 25.87 = 0.662272

0.662272 is also a constant, if you want to get drag force of the wind, multiple speed^2:

0.662272X(250mph)^2

I have no fw190G's data, fw190G-1 based on A4; G-2 based on A5, they are both 1.42ata( 42").

let's put aside propeller's thrust at first, only gravity and wind drag(there are some induced force but I just calculate roughly).


gXweightXcos(60)-dragcoefficentX(speed)^2= (dive-accelaration)Xweight

that is

dive-accelaration=gXcos(60)-dragcoefficentX(speed)^2/weight

P47's weight is almost twice of fw190A4, so at 250mph speed it's almost impossible for fw190 to outdive P47 in il2 4.11m. But in real world, fw190 pulled away rapidly!

The only factor we didn't include is the detailed airscrew efficeiency curve espicielly when tip reachs 0.8-1.0 Mach and above.
Spitfire.LF.IXC
[Mass]
Empty 2650.0
TakeOff 3300.0

[Squares]
Wing 19.0
Aileron 1.32
Flap 2.125
Stabilizer 1.90
Elevator 1.20
Keel 0.85
Rudder 1.10

[Polares]
lineCyCoeff 0.092
AOAMinCx_Shift 0.0
Cy0_0 0.1
AOACritH_0 16.0
AOACritL_0 -17.0
CyCritH_0 1.4
CyCritL_0 -0.7
CxMin_0 0.0232
parabCxCoeff_0 5.4E-4




P-47D-27
[Mass]
Empty 4630.0
TakeOff 6583.0

[Squares]
Wing 25.87
Aileron 1.45
Flap 2.76
Stabilizer 3.50
Elevator 2.05
Keel 1.30
Rudder 1.10

[Polares]
lineCyCoeff 0.092
AOAMinCx_Shift 0.9
Cy0_0 0.17
AOACritH_0 16.0
AOACritL_0 -15.0
CyCritH_0 1.25
CyCritL_0 -0.8
CxMin_0 0.0256
parabCxCoeff_0 4.8E-4


Bf-109G-2 = 0.027 * 16.16 = 0.43632
Spitfire.LF.IXC = 0.0232 * 19.0 = 0.4408
P-47D-27 = 0.0256 * 25.87 = 0.662272



Someone says


R-2800 engine, 2700rpm, 50% reduction for airscrew=1350rpm, 4m diametre. On the ground when engine at full rpm, the propeller's tip's rotating speed is:

3.14X4X1350/60=282m/s=282/340=0.83 Mach

Wow, it's seems that P47's designer just want to make the tip speed approach sonic as soon as posssible. Why? The supersonic state for airscrew's tip? We all know P47 was intently designed for high altitude escort where the sonic speed is samller than 340m/s on the ground, and P47 often dives at hight speed at high aititude, therefore P47's airscrew tip must often beyond 1 Mach.

airscrew=the twisted and rotating "wing" above 1 Mach, what does this mean in il2?

Again we analysis 1943's test.

Quote:

(C) (1) 10000 feet to 3000 feet, starting at 250 m.p.h., diving at angle of 65 degree with constant throttle setting. The FW-190 pulled away rapidly at the beginning but the P-47 passed it at 3000 ft with a much greater speed and had a decidedly better angle of pull out.

When p47 flew on 10000 feet@250 mph IAS,what's the speed of propeller's tip?

At 5,000' TAS = IAS + 9%
At 10,000' TAS = IAS + 16%
At 15,000' TAS = IAS + 25%
At 20,000' TAS = IAS + 36%
At 25,000' TAS = IAS + 49%
At 30,000' TAS = IAS + 64%

250 mph IAS=290mph TAS=130m/s, rotating speed is 282m/s, combination speed is 310m/s, Mach number=310/328=0.945Mach

When slam throttle full forwards and dives 60 degree, P47's airscrew will probably be the first one to suffer from sonic barrier.0.95-1.0 Mach. This is probably the reason why P47 was outdived by fw190G from 250 mph(initial diving stage). As speed building up to 650km/h or so (3000ft altitude), mach number=1.

(Probably)Fw190's airscrew tip entered 0.9-1.0 Mach later than P47, that's why 190 outdove P47 at the begining, but when both of them were all suffering from low airscrew efficiency at high speed, P47 will gain on 190, the formula I'v posted above demonstrates this clearly.


When P47 dives to 7500 altitude @800 km/h TAS, and tip mach number is 1.16. Hamilton standard airscrew is NACA-16 series which is laminar flow airfoil.

<<Static characteristics of Hamilton Standard propellers having Clark Y and NACA 16 series blade sections>>
http://digital.library.unt.edu/ark:/...etadc62146/m1/

hmm, before I read further, I think we need zero lift drag coefficient for 190 to say that weight is the deciding factor. wiki says 27.87 m² for p47 wing area and 18.30 m² for 190A. Agree with you that denominator (i.e., weight) is almost twice as large for p47 verses 190, but numerator? To keep simple math, assume zero drag coeff=1 for both planes, 190 weighs "1" and p47 weighs "2" (weight on relative basis to each other). Then drag coefficient portion of numerator

-dragcoefficentX(speed)^2/weight

where you did this -> P-47D-27 = 0.0256 * 25.87 = 0.662272 (i assume your using 25.87 for wing area)

(i.e., use 1 instead of .0256 and 1 for speed since same for both planes and 2 for weight p47 and 1 for weight of 190)

27.87/2 (p47) or 18.3/1 (190) is bigger??? The latter is bigger, and since it is a subtraction from this gXcos(60), wing area and/or differences in zero drag coeff may be the deciding factor in the calculation of dive acceleration, not the weight. And if this is the case, dive acceleration is less for 190 than p47. Sorry,if this is confusing. It's late here.

27.87/2=13.93 much smaller than 18.3, so P47 has smaller subtraction from this g*cos(60).


BTW, this link says VDM 3m diametre has a 0.54 prop reduction gearing. Fw190A8 :VDM 9-12176A10 ft, 11 ¾ in. diameter 390 lbs

BMW-801D 2700rpm



So in 1943's test, @10000ft ,250mph IAS, fw190A4's propeller tip speed=(254^2+130^2)^0.5=286m/s=0.87Mach, less than 0.95 Mach of P47's.

Some P47's has 16:9 reduction which provides higher Mach number (1.05Mach ).

In conclusion, My opinion is that when p47 and fw190a4 at full engine 2700rpm dive from @10000ft ,250mph IAS, fw190's tip speed is about 0.87mach while p47's is around 0.945-1.05 Mach. Probably at that time P47's propeller's efficiency is quite lower than fw190, so p47 was outdove rapidly at initial diving stage.
Attachment 9525

Your conclusion seems reasonable to me. I was digging into that hamilton clark link, it says the machs for best efficiencies were in the .7-.9 range. I would assume that plays out at cruising speeds.

4 meter diameter compared to a 3 meter diameter....

4 meters is a big prop and they have to push the tip speeds. Keep in mind, diameter is the most important factor in propeller design.

Good design can compensate, though.

They were not dumb at Republic!
:grin:

I did not check over your math but your final conclusion is absolutely right.

Crumpp, thank your comments，I think I've found the answer！ the NACA report of Hamilton standard tells us everything：1350rpm propeller 13ft diameter CSP just like P47's with the exception of 3-blade vs 4-blade.

http://digital.library.unt.edu/ark:/...etadc62146/m1/

There were two airfoil being tested， Clark Y airfoil was before WWII, not laminar. NACA16 was during WWII, NACA16 airfoil is laminar flow profile, and the test shows that there is no advantage of NACA16 airfoil when propeller's tip speed is 1Mach and when "advance ratio" is above 2.0, but there is no more than 3% efficiency benefit from NACA16 when "advanc ratio" is between 1.2 and 2.0.

advance ratio= TAS/(rpm*diameter)

fw190:3 meter propeller, 1400 rpm
p47d: 4 meter propeller, 1400 rpm

You can see Figure 24, when tip speed is 1 Mach, the more advance ratio, the lower efficiency. So we now come to know why Repulic engineer wanted as big propeller as possible because they wanted smaller advance ratio! When fw190 and P47 dive to same high TAS, the P47 has smaller advance ratio and higher efficiency AS LONG AS BOTH PROPELLER'S TIP SPEED IS AROUND 1 MACH. Republic engineers were right: since high TAS diving(efficiency loss)is inevitable for P47, why not prefer low "advance ratio" while accepting the high mach number of 4 meters big propeller's tip?

The complete formular of diving 65 degree is below:

acceleration=g*cos(65)-dragcoefficent*(TAS)^2/weight+Propellerthrust/weight

A simple math question: if you are P4D7's pilot fighting against a fw190G(both arr 250mph TAS @10000ft ), how can you get higher dive acceleration? On the right of formular there are three parts:

1) g*cos(65)
You have nothing to do with it, every a/c shares same value.

2) dragcoefficent*(TAS)^2/weight

Your huge weight is your advantage, and the bigger TAS, the more important role this part plays. So you should build up speed ASAP.

3) Propellerthrust/weight

Unfortunately the third part is your enemy's advantage. Although P47's efficiency is almost same as fw190's, your huge weight is your shortcoming.The NACA report says when tip speed is above 0.9 mach, the drag coefficient of tip increase rapidly, and when tip speed is 1.0 mach and advance ratio is above 2.0, the efficiency of propeller drops sharply. @250 IAS ,p47's advance ratio is about 1.32, not high, but as speed slightly building up, efficiency drops quicker than 0.5-0.8 Mach curve.

http://digital.library.unt.edu/ark:/...dc62146/m1/43/
http://digital.library.unt.edu/ark:/...dc62146/m1/13/
http://digital.library.unt.edu/ark:/...dc62146/m1/11/

You are now 250mph TAS @10000ft, your tip speed is 1 mach, and you will suffer from compressibility loss while your enemy dose NOT. The fw190 has more thrust than you, his weight is less than you, therefore his thrust/weight is much greater than you so that he can overcome your advantage----the second part of formular.

What should you do ? The answer is very simple and as same as the conclusion we've got from 2nd part of formular:

BUILD UP SPEED ASAP.

Drag him down!!! Yes, your tip speed will be always above 1 mach but now fw190's is also around 1 mach, he is now suffering from compressibility just like you, furthermore, his advance ratio(J) will much bigger than you, so his propeller efficiency drops more sharply than you. Now, the third part of formular is NOT enemy's advantage any more. You've succeded in eliminating his advantage and retaining and enlarging yours.

Congratulations from Republic engineers! You now have energy advantage by diving to high TAS, you are extending your distant now, do what you want to do. :)

The last thing is that if il2 4.11 models the compressibility loss of propeller efficiency . If not, there are 2 probem with its FM.

1) every piston a/c dives faster than it shoud be @ high TAS.

2)For those a/c like P47, the advantage of high TAS diving acceleration has been ignored, so is it's Low TAS diving accelaration shortcoming.

Now we can perfectly explain the fact of 1943 Dec test between fw190G and P47D, and many other comparation such as spitfire vs bf109 initial and final diving difference.

BTW, it's stupid for P47 to dive in a shallow angle with zeke which demonstrates the 6 ton thunderbolt has only a littile advantage(100yards)to the "kite"----Zeke52. I can image those angry faces of the Republic's engineers.:cool:


As for P51d, He has similar high TAS diving acceleration with P47, but the reason is not much depending on huge weight, it is very low coefficient of laminar wing. pls look at 2nd part of formular.

about this part of the equation, Propellerthrust/weight ->

Simply lower the rpms to bring the p47 tips speeds down to optimal range .7-.9? Won't that give you better acceleration? It seems to work that way in game. Starting at 250 mph, full throttle, and 100% pp, nose trim 2 notches down, rads closed. if I nose it down into dive and crank down the prop pitch to 0% quickly and then bring it back up to ~77% and gradually lower throttle to about 77% in a dive from 250 mph, the planes gets to ~ 400 mph ias very quickly.

Yes, that works. But even you can get same propeller efficiency as 190, your weight is too great to overcome, Propellerthrust/weight is still inferior to 190's.

We need to know if 190's airscrew tip compressibility loss is modelled or not at high mach number where 190's huge "advance ratio" making efficiency even worse than P47's.

agree, but better to do it than not if your p47 guy. for 190 guy, you have revealed something that I think goes ignored. sometimes it is good to over ride the vdm auto prop pitch control and go to manual mode for same reason, to keep tip speeds at optimal ranges.

Thanks to BlackBerry and everyone,

all your threads have done greatly to help me to understand more.

and may be there is a chance to involve the Prop calculation in IL2

to knock down Cliff of Dover and be the most perfect all the way?

Sorry for offensive language to someone l

I am tricky and want this topic to enjoy a long life and drag more men
to research in their full strenth,

So that I deliberately use somewhat offensive language.

sorry again JTD or so

That dosn't help much, I'll show you how terribly drop of propeller efficiency for SMALLER propeller.

Again, NACA Figure24, Both fw190's 3 meter prop. and p47's 4-meter prop @19500ft , 1350rpm for propeller CSP,

1)A point=126m/s TAS=453km/h TAS=323km/h IAS

P47's advance ratio=126/(4*22.5)=1.4

fw190's advance ratio=126/(3*22.5)=1.86

2)B point=180m/s TAS=648km/h TAS=462km/h IAS

P47's advance ratio=2.0
fw190's advance ratio=2.66

3)C point=216m/s TAS=777km/h TAS=555km/h IAS=345m.p.h. IAS

P47's advance ratio=2.4

fw190's advance ratio=3.2

Attachment 9534

B piont--->When both p47 and fw190 dive to 20000ft/462km/h IAS=287m.p.h. IAS, p47's efficiency is 70%, almost twice of fw190's 38%!

C point---->When dive to 20000ft/555km/h IAS=345 mph IAS, fw190 almost lost its propellerthrust while p47's remaining 55%.

So when speed building up @high altitude, the third part of acceleration formular will also be p47's advantage. Thus we could image how "Thunderbolt" really is!

P47's exhaust turbine boost leads to the lost of exhaust boost for propeller, but that can't help fw190 much.

Finally, we can completely understand why Republic's engineers want the biggest propeller as possible.

interesting. I'm suggesting that at point A with a canister of mw50 on board that the 190 guy trade high rpms and prop efficiency while TAS is low, for thrust 'instantaneously' (limited by rate of change of the blade angle) by over-riding auto prop pitch (slower response). The prop would not be spinning at 1350 rpm. It has a load on now, so is spinning at say the lower end of the power curve for the bmw engine, guesstimate, 1050 rpm. Point C for 190 is irrelevant because dive speed limitations are upon him. The race is for point B. How long does it take for p47 turbo to spool up??? How long does it take to get power from mw50???

1) German tried MW50 on BMW801 but finally gave up. It's said that MW50 is harmful to BMW801's piston. 1945 version Dora's Jumo213A(liquid cooled) was equipped with MW50. However, 20000ft(6000m) is probably above 190's FTH, so there is little benifit from WEP.

At "A" point, if fw190 uses 1050rpm propeller, tip rotating speed is 165m/s, TAS is 126m/s, Mach number=207/310=0.67. Quite below 0.9mach. But I'd to say the propeller's has a certain optimised rpm, if you use full 100% throttle while reducing propeller's rpm incorrectly, you will lose efficiency. Your propeller probably couldn't absorb engine's output.

2) C point is above fw190's max allowable diving limit?

3)I don't know how long does it take for p47 turbo to spool up. I guess it's several seconds?

4)Last but not least, Do you have the information of Dora's 41276.16 V propeller? The size of it, the reduction ratio etc. As we all known before 4.11m, Dora flys so fast.

.

I know you are trying to get best propeller efficiency by adjusting rpm manually, thinking that fw190 is quite an automatic plane but the pilot is oblidged to be so busy....

I'll carefully calculate Fw190 A8's prop. efficiency.


Prop diameter: 11.98feet=3.33 metre

Prop rpm: 2700*0.54=1458rpm =24.2 rps(100% pitch)

Notice that "B" point is just Fw190A8 max. level speed @19500 ft, that is 648km/h=180m/s

advance ratio=180/(3.33*24.2)=2.23

P47's advance ratio=2.0

So fw190's ratio is NOT so bad as 2.66, just 2.23. My 3-meter vs 4-meter comparation is just a demonstration of how important the diameter of prop. is.
http://www.wwiiaircraftperformance.o...90a8-level.jpg


You can see Fw190A8 gets its max. level by using 2700rpm-engine, this is the best rpm for A8, if you fly A8 at "B" point, don't decrease rpm because this will increase your advance ratio, which leads to the moving to rightside on the curve.

http://www.wwiiaircraftperformance.o...ed-13nov43.jpg

At "B" point, A8's tip speed=(180^2+253^2)^0.5=310m/s.

Mach number is 310/316=0.981 Mach, almost 1 Mach.

That means @19500ft, fw190A8 have to make it's propeller's tip speed just equal to 1 Mach, this is the best result, if you decrease rpm, you lose efficiency because "advance ratio" will be greater.

Actually, fw190A8's prop. working piont is "D" with efficiency 0f 60%+, less than P47's 70%, if P47 using 16:9 reduction, thundebolt's working point is "E", 75% efficiency.
Attachment 9555
I am not saying that manual operating rpm is useless, it may help you a bit, but not much.

P47's efficiency advantage may or may NOT overcome its huge weight at high speed, but at least P47 could shrink the third part of the formular which is propitious to fw190. Even fw190 could maintain 100% efficiency all along diving, the importance of 3rd part of firmualar will also be less and less. Don't forget another foumular:

output(HP)=speed(m/s)*thrust(KN)

When you double your speed, the 3rd part of formular will become 50% (and even smaller due to efficiency lose) important as before, the 2nd part will become 400% important as before.
When P47 building up speed quickly, fw190A is doomed to be outdived.

This thread is about diving acceleration, not level flying.

"B" point is just fw190a8 's max. level ,only 648km/h TAS, while P51B could reach 715km/h (444mph)@20000ft, and 670km/h for P47D.

It's very very easy for P51P47 dive to "C" point(777km/h TAS) which is only 100-60km/h higher than there max. level flght. At "C" point fw190a8 prop. efficiency only 36%!

The max. permitted dive speed @20000ft for Tempest is 450 mph IAS=1014 km/h TAS, P47P51 are probably the same. Sir, when diving at 850km/h TAS@20000ft, can you imagine how frustrated fw190A8 is?


I think I've expressed enough my opinion. Let's read a paragragh from wiki to understand why the author wrote "energy-saving" dive. "energy saving" means P47's opponents bleeding their energy heavily when fighting against P47 at high TAS. I wish Daidalos team could make il2 perfect by giving us REAL thunderbolt , Mustang, and Tempest.:)

yes. recall, at point A when you enter the dive at ~400 kph, your rpms are at ~2100. So going manual mode allows you to spool up the engine to 2700 rpm at cruise speeds and transfer that power in short order to the prop for better acceleration than auto mode would give you. most of the time you fly in auto mode. just certain situations, you apply manual mode adjustments to the power band, imo. entering a dive, nearing stall speed, ground taxi, air braking, etc.

But to maintain top speed at level flight, agree. don't use manual mode for that.

Maybe they called it 'thunderbolt' because of the dive limit?

Okay, I'm out. Thanks for the discussion.:cool:

It's dive acceleration not dive limit made P47 succesful in history.

It's totally useless for having 1 Mach dive limit when your enemy could outdive you with better acceleration within 0.7 Mach and pull to level run away. Even if you fly the same a/c 500m behind your wingman, you couldn't catch up him when following his dive, could you?


Although SpitfireIX/XIV has the same dive limit(if not better than) as fw190A/bf109G, he couldn't over take Germans in a dive.

mach 1, sound barrier and 'loud noise'. what is 'thunder'??? get it now??? What other WW2 prop planes had mach 1 dive limit??? I can't recall any other that did. Even the great Ta 152 falls short. So, if Republic makes the only plane that can get to sound barrier with out breaking up, calling it 'thunderbolt', seems a good fit to me. So, I simply was speculating on the origination of name. :rolleyes:

"it's totally useless for having 1 Mach dive limit when your enemy could outdive you with better acceleration within 0.7 Mach and pull to level run away. Even if you fly the same a/c 500m behind your wingman, you couldn't catch up him when following his dive, could you?"


Huh? We already know that the fw190a-4 was quicker off the line in the dive! Go back and read your own posts and theory as to why. I already stated your conclusion seemed reasonable. I don't know why you seem to be contradicting your own conclusion now? Most of the other prop planes couldn't touch that dive limit, so p47 acceleration is moot beyond their dive limits. But it did not have the best acceleration in the dive as per the test record.

Also, even if fw 190 does get some lucky shots off in the first part of the dive, p47 is built like a tank. Good chance it gets home.;)

It's totally useless for having 0.85 Mach dive limit when your enemy could outdive you with better acceleration within 0.7 Mach and pull to level run away.

It's totally useless for having 0.75 Mach dive limit when your enemy could outdive you with better acceleration within 0.65 Mach and pull to level run away.

It's totally useless for having 0.65 Mach dive limit when your enemy could outdive you with better acceleration within 0.55 Mach and pull to level run away.

Now, do you understand me?




It seems that you misundertand me. P47 can outdive fw190 WITHIN fw190's dive limit.
In your opinion, fw190 from 3000m @400km/h dive to deck will lost its wings?
No, no, no.
When hit ground, the speed of fw190 is BELOW its diving limit. But fw190 was outdived by P47, wasn't it?

I'll use estimated numbers to show you, if you don't aunderstand, I 'll give up.

acceleration=g*cos(65)-dragcoefficent*(TAS)^2/weight+Propellerthrust/weight


1)from 400km/h to 570km/h,Fw190G outdives P47D
acceleration=g*cos(65)-dragcoefficent*(TAS)^2/weight+Propellerthrust/weight

For fw190G:acceleration=4.1-1.5+3.5=6.1 m/s^2

For P47D:acceleration=4.1-1+2=5.1 m/s^2


6.1>5.1, So fw190G outdives P47D.



2)from 570km/h to 750km/h, P47D doutdives fw190G

For fw190G:acceleration==4.1-5+1=0.1m/s^2

For P47D:acceleration=4.1-3.3+1=1.8 m/s^2


1.8>0.1,So P47D outdives fw190G.

Okay, I guess it is just a words mix up. Look at the underline bold above. You posted it from the test record. It says 190 was quicker off the line. This is from the test record. You reasoned it out. I agreed. Then you said this:

which contradicts the test observation we have been talking about. So, imho, I don't think it's p47 acceleration abilities in a dive that gave it the name 'ThunderBolt'. http://en.wikipedia.org/wiki/Thunder It is the fact that it had a dive limit capability like no other prop plane at that time, near the sound barrier. It took time for p47 to catch up and pass the 190. It was not the quickest. It was the fastest. So, I don't think you can say dive acceleration was it's leading attribute in history. There's nothing to argue here. I think we just got a word mix up.:)


The wings obviously did not fall off the 190 in that test. So, I guess we can assume that at that dive angle, starting altitude, starting speed...etc, that the 190 stayed within the dive limits. Probably a vertical dive angle is a different story and the wings fall off.

What's name for P47 isn't important, you may call it "old woman", but p47's acceleration is still one of the best.

In 1943 July, p47 was equipped with old naca-16 propeller whose efficiency is low at low TAS, but when P47 had a paddle prop., story changed.

I guess fw190G can only slightly outdive P47 (paddle) at the beginning.

Hi Blackberry,

You have drawn some of the right conclusions but there is some work required still.

First of all, these are constant speed propellers. They change pitch as required. I am sure you got confused looking at that single F4U graph but it is a fact, you cannot compare CSP propellers at different advance ratios.

The advance ratio does not tell you a thing except in the context of that specific pitch angle. Now what you are doing is how that pitch stops are determined. A good propeller design will keep the polar at the flat area on the top as the pitch of the blade changes throughout the flight envelope.

This is what a complete CSP efficiency over advance ratio graph looks like:

http://img403.imageshack.us/img403/8...vanceratio.jpg

The best aircraft/engine for this propeller will achieve Vmax at ~2.2 advance ratio and the propeller will have the stops at 15 degrees and 45 degrees.

That is the advantage of a CSP, you maintain peak efficiency over a wide range of velocities.

The F4U graph looks like it comparing airfoil selection at a specific velocity.

I think he knows that. The example is a bit rigged. Same max rpms, similar reduction ratios. The main difference being the diameter. He started it out by calculating max tip speed as peak efficiency. So, have to look at his advance ratio calcs on a relative performance basis verses comparing one prop to the other.

Thanks for correcting me, you are right, the NACA16 vs Clark-Y diagram is at a specific velocity=640km/h TAS, and at a specific altitude= 6000m(19500ft)

advance ratio=J= V/(n*D)

V=177 m/s, D=4.1m(13.5ft), when J varies from 1.0 to 2.5, the propeller's rpm is from 2590 to 1036 respectively, and engine rpm is between 5180 and 2072rpm(reduction ratio=0.5:1). But 5158rpm is far more than engine's max. rpm, Thus the working range of 3-blade 4.1m diameter CSP is the "red curve", other part of curve is just the calculated result.


Larger prop. will always benifit from lower advance ratio when other things being equal.

Attachment 9568

3-blade CSP diagram
Attachment 9569
4-blade CSP diagram
Attachment 9570

It seems that 4-blade CSP with larger diameter prop, is FAR MORE efficient than 3-blade with smaller size, especially when advance ratio is very high(diving).

Assume 3-blade diagram is Fw190A8, 2700rpm engine , reduction 0.54:1, 1458rpm for 3.3m Propeller

Assume 4-blade diagram is P47D, 2700rpm engine , reduction 0.5:1 or 0.56:1, 1350rpm or 1518rpm for 4.0m Propeller

When both Fw190A8 and P47D dive to 6000m altitude @ 950km/h TAS=264m/s TAS=680km/h IAS=421m.p.h IAS. This speed is within Tempest MKV's permitted dive limit.

Propeller efficiency for P47D:82%-85% , advance ratio=3 or 2.6

Propeller efficiency for Fw190A8:0%. advance ratio=3.3

:shock:What a surprise! It's very wise for allied to drag fw190A to such a high speed, and make fw190a lost its power!

The most important is whether il2 4.11m models detailed prop. efficiency curve? Is there Mach number in il2's FM code? Crumpp Do you know?

To demostrate how important of 4-blade design compared to 3-blade, we can calculate Tempest MKV.

engine rpm 3700 or 3850, reduction ratio=0.274, prop. rpm=1013rpm or 1055 rpm ;propeller diameter=14ft=4.24m

When dive to 6000m altitude @ 950km/h TAS=264m/s TAS=680km/h IAS=421m.p.h IAS. This speed is within Tempest MKV's permitted dive limit.


advance ratio=J=V/(d*n)=264/(4.24*1012/60)=3.7 even bigger than Fw190A8!

If Tempest was equipped with 3-blade prop, it will be badly outdived by Fw190A8, but Tempest slightly outdive P47 and P51! The reason is 4-blade prop. which provides efficiency 78%@950km/h TAS.

During the war many types of fighter aircraft were produced out of the designers bag, some never even reached the prototype stage, others failed to reach Service requirements, but not a few made the grade and are now house hold words the world over. The best known in this country are, of course, the Hurricane and Spitfire, the Typhoon, Mustang and Thunderbolt, and latterly the Tempest and Meteor. Each came out in many guises and fulfilled many roles, some of which they were never designed for, but all did a grand job of work, and were at one time or another indispensable to the work of the R.A.F. Fighter Command.


The Aeroplane June 21st 1946.

http://www.wwiiaircraftperformance.org/wade-dive.jpg

1)The best diver, Meteor, of course, not" handicapped by airscrew drag"

2)Tempest MKV(9lbs boost), the best piston diver, 4-blade prop. 5 tons, streamlined

3)Thunderbolt(P47D), the best American piston diver, 4-blade prop, 7 tons.

4) Mustang(P51B/C 18lbs boost), one of the best diver, 4-blade prop, 3.5 ton , very low drag coefficiency of laminar flow wing.

5) Fw190A8A6? Bf109G6as Spitfire IX,XIV, these a/c are outdived by P47P51Tempy. Why?

Fw190A,as heavy as P51, but not laminar flow wing, lighter than P47/Tempy. only 3-blade prop. low prop efficiency at high speed.
Bf109G6as, very good at initial dive stage, but not very heavy, 3-blade prop.
Spitfire IX, XIV, 4-blade prop. not heavy, initially outdived by 109, but can catch up with 109 due to 4-blade efficienvy at high speed, over take 109? not sure.

3-blade and 4-blade diagrams are from university textbook of aerodynamics.

I don't know those props are modern or WWII era. So they are just for demonstrate, but apperently, xfu4-1 13.5ft naca16 or clark y propeller is much inferior to that 3-blade diagram. For example, 400mph @6000m xf4u's efficiency is 70%, advance ratio=2, while in that 3-blade diagrams, could reach 85%+. So this implies that fw190a8 will completely lose its power much below 950km/h@6000m. Sad news for germans, because bf109/fw190 including Ta152 are equipped with 3-blade prop. while allied had 4-blade after 1942. Even worse for soviet planes, la5/la7,yak are light, small size plane which means their dive acceleration are poor within 730km/h.

BWT, prop. efficiency decreases when altitude grows, and I don't know whether those diagrams are sea level or 9000 m.


I am afraid that the efficiency diagrams of WWII era are difficult to find. The last choice is "ansys" which is powerful software in simulating propeller.

That is a very generic graph with absolutely no conditions given. It is impossible to draw conclusions from about aircraft dive performance.

Good design can achieve the same efficiency and thrust with either 3 blades or 4 blades at the power levels of WWII aircraft.

As for propeller efficiency, there is a good reason why n=.85 is a good assumption to make for CSP propeller efficiency. Take the top of your single pitch effiiciency curve for the F4U and that is the efficiency a CSP will maintain throughout the envelope. It will adjust the blade angle to maintain that.

Examine n under various conditions and advance ratios in this article. This is a good primer for propeller performance btw.

You will see that n has a very small variance and even remains the same at different advance ratio because of the shape of the curve at that blade angle.

http://www.nar-associates.com/techni...ncy_screen.pdf

More blades = more drag but those airplanes have more thrust than the blades add drag because of their weight.

Align those aircraft by weight and you will see the important of it to achieving a high Vne.

That being said, mach limits and dynamic pressure limits have a much more practical impact on determining Vne.

Weight = additional available thrust. The propeller thrust is going to zero at the equilibrium point. It will vary but it not nearly the factor that weight becomes....

The excess propeller thrust is why the Bf-109 and FW-190 have such high initial dive acelerations under the conditions the article is talking about. If you dove all of those aircraft from Vmax, they would have no excess propeller thrust and would be using a component of weight as thrust.

Maybe, maybe not. Using the load plans, the P51D has the potential to add 550 more pounds of thrust at take off weight to increase its Vmax depending on the angle of dive. Granted that is not very much give the relationship of velocity and power. Power requirements are cubed in relation to velocity. Keep in mind your graphical representation from the 1940's flying magazine does not give us a scale but only shows relative advantage.

The radial of the FW190 will consume more gas and oil so its weight will change faster but the P51 has more gas and potential to change weight at a slower rate. The P51 also has a lower Drag picture so does not require as much thrust to achieve a higher speed. That is why it is faster than the FW-190A8 with a less powerful engine. Laminar flow has what is termed the "drag bucket" in the middle of the polar that occurs around cruise co-efficients of lift. It has no bearing on either low speed or Vmax performance except that laminar flow airfoils as a general characteristic exhibit lower CLmax. For your games purposes, that is irrelevant as you do not have to guess CLmax but can easily calculate it from stall speed with a given weight.

The Mustang achieves a higher Vmax in level flight so it was also achieve a higher Vmax in a dive provided it does not reach mach limitations or dynamic pressure limitations.

You can see from this sustained turn performance analysis the general effects of thrust and aerodynamic limitations of these designs.

http://img837.imageshack.us/img837/5...5vsfw190a8.png

Summary of propeller design:

Fewer blades = more efficiency

Fewer blades = lower power loading

More blades = better power loading

More blades = less efficiency

Larger disc size = better power loading

Larger disc size = faster tip speeds = lower efficiency = good for low speed work

Smaller disc = slower tip speeds = higher efficiency = good for high speed work

Propellers are undoubtedly the most complicated piece of engineering on an aircraft.

You can also bet that all the engineers during WWII did their homework. I know Mtt and Focke Wulf both tested 4 bladed designs on their aircraft. It was found that what one design made up in efficiency, it lost in power loading and vice versa. As such Focke Wulf concluded that was no appreciable difference other than weight savings on the 3 bladed propeller.

The German propeller designer took the approach of widening the blade chord to increase power loading and using a better material. The allies added more blades and accepted the weight increase. Both are perfectly acceptable approaches to increasing performance with very little to choose from.

The most efficient propeller would have one very long and wide blade. It would revolve rather slowly and acelerate rather poorly.

All the best,

Crumpp

Yes, but don't forget compressibility at 500mph=800km/h which P47P51Tempest could dive to. As for wing airfoil drag, there are extra 250HP is consumed by Compressibility.
http://history.nasa.gov/SP-4219/Chapter3.html
http://history.nasa.gov/SP-4219/4219-081.jpg
Graph and sketch hand-drawn by John Stack, 1933. The effect of compressibility on the power required for a hypothetical airplane.This sketch was subsequently sent to the October 1933 Committee Meeting of the NACA in Washington. From the John Stack papers at the NASA Langley Archives.


Yes, all the engineers during WWII did their homework. However, why allied engineers accepted the weight(drag) increased by the 4th blade, and why german engineers denied?

allied side:
German side:


Allied said laminar airfoil actually reduced drag in P51, but german believed it's an impossible goal when Reynold Numbers is high(real flight ).

Who made the mistakes?

This unclassified file<<where we stand>> at Page 45 says:

http://www.governmentattic.org/vonK/...VKarman_V2.pdf



Probablly German wind tunnel failed in testing laminar flow airfoil in WWII. German tested none-laminar 3-blade vs 4-blade prop. and drew the conclusion of "not appreciable" for 4-blade, but if they test laminar 3-blade vs laminar 4-blade prop, they will finally find the advantage of 4-blade low drag propeller.

Hamilton Standard :NACA-16 laminar flow airfoil,4-blade prop. widely used in P47P51 etc.
UK de Havilland Propellers was established in 1935, as a division of the de Havilland Aircraft company when that company acquired a license from the Hamilton Standard company of America for the manufacture of variable pitch propellers. The division was incorporated as a separate company on 27 April 1947. SpitfireIX,XIV, Tempest also have laminar flow airfoil,4-blade prop.

As XF4U-1 diagram indicated, 3-blade NACA16(laminar) and 3-blade Clark-Y propeller are roughly the same efficiency(within 3%), after 1942 alomost all allied laminar prop had 4 blade, later Spitfire even had 5-blade prop. There must be enough reason for allied engineers to prefer 4-blade. If allied found that was no appreciable difference other than weight savings on the 3 bladed propeller, they would drop 4-blade design just like Germans did. Those two diagrams from university textbook maybe demonstrate the difference between 3-blade laminar and 4-blade laminar propellers. We need exam it in future, Perhaps the diving difference mystery is just within propeller's efficiency diagrams.

If il2 FM couldn't model detailed compressibility of wing and propeller between 0.8-1.0 mach, it will never be perfect in simulating WWII late a/c.

http://history.nasa.gov/SP-4103/ch8.htm



So German's conclusion is not valid for allied laminar flow 3-blade vs 4-blade comparation. German never used laminar flow airfoil in wings, nor the propellers.

Xf4u-1 test speed is not high, merely 640km/h TAS, we don't know the difference between naca16 and Clark y at high speed, 750km/h,800km/h, etc.

Is that possible for 4-blade laminar type prop provides more power loading than 3-blade of traditional airfoil while keep the drag level remain same?

The Germans were well aware of the mustang and laminar flow. Their conclusions agreed with the NACA's, that laminar flow is very difficult to achieve under field conditions and the benefits would not be attainable in a frontline fighter.

http://img33.imageshack.us/img33/2650/p51wingdrag.jpg

http://img84.imageshack.us/img84/695...wreynolds3.jpg

I own and operate an aircraft with laminar flow wings.

You have to keep the wing and leading edge absolutely spotless and polished to see any benefit.

Dirt, bugs, and a rough surface will destroy the laminar flow drag bucket.

Lastly, the benefits of a laminar flow airfoil is not a factor at Vmax or Vs. It occurs in the vicinity of the cruise design point.

Look at the polar for a laminar flow airfoil.

You do know a Clark Y is not a laminar flow airfoil?

You use a propeller analysis for a Clark Y and then start talking about the benefits of laminar flow.

I am also not sure what I supposed to remember with compressibility effects. Transonic drag rise is included in the statements I made. It is one of the components of drag our thrust must overcome.

I am confused as to what you want to say now.

You are right in that the dive limits of WWII aircraft leave very little to chose. They all hit the wall about the same point. The diagram you form the 1940's enthusiast magazined has no scaling information at all.

I will attempt to answer your question as to why the Germans chose three blades and the allies four blades.

The Germans increased the chord to raise the coefficient of power. The Allies added a blade to increase the coefficient of power.

The Germans were resource and production limited so not having to produce another blade is attractive. Saving weight in any airplane is attractive. The German fighters had sychronized weapons firing through the propeller disc. Less blades means more bullets on any given target.

The Allies and especially the United States had much higher production capacity and nearly unlimited resources. Making more blades and the resources to make them was not an issue. The USAF main fighters used wing mounted weapons that did not fire through the propeller disc.

I don't understand why you guys keep saying weight = thrust. :confused:
Weight=mass*g
-> mass directly proportional to weight and g is constant
greater mass/weight in free fall gives you more inertia to overcome drag forces. Inertia is not thrust. p-47 was big plane with big torque radial engine (not the best drag profile to slip through the air). So it was a trade. A big engine to drive a big prop of a big plane with big drag profile. If p 47 want more acceleration off the line, simply take a steeper dive angle than fw 190 and fill up the tank with fuel and load up on bombs. So, inertia is not constant either. It depends on the loadout and dive angle.

Although there are some arguements about P51's laminar airfoil in a frontline role, Mustang is actually benifitted from this type of airfoil more or less. Isn't it? Same rules applies to NACA-16 laminar propeller airfoil. After WWII, NACA-16 was still widely used in various of propeller's with very low Cd(min) and high critical Mach number.

Attachment 9612

With regard to German tunnel test on P51 in 1943-1944, they even lost laminar effect when reynolds number reached 20 million due to the lack of low turbulence in wind tunnel which Prandtl had already mentioned. It's no need to remind you who is Prandtl.


Langley Two-Dimensional Low Turbulence Tunnel

http://crgis.ndc.nasa.gov/historic/L...ressure_Tunnel


Do you mean there were often Dirt, bugs, and a rough surface on the propellers of P47P51 in WWII?



Clark-Y was Before WWII, NACA-16 was during WWII. There was small peroid for allied using 3-blade laminar NACA16 airfoil beforce they moved to 4-blade. NACA16's section is very different from Clark-Y/RAF-6. Furthermore, although both Clark-Y and RAF-6 were very similar conventional pre-WWII design, there are even some difference between them:

1) Clark-Y has less drag than RAF-6, more suitable for cruising and high speed flying.
2) RAF-6 has more lift, more suitable for taking off.

Thus the difference between NACA16 and Clark-Y/RAF-6 is more profound. In fact RAF-6(UK), Clark-Y(USA) and Gottingen(German) airfoils were the best ones during WWI.

http://www.afwing.com/intro/p51/model.JPG
XP51 prototype model in wind tunnel , 3-blade prop.

http://www.afwing.com/intro/p51/na73x.jpg

NA-73X prototype , 3-blade ,looks like German's 3-balde sharp tip prop.

http://www.afwing.com/intro/p51/mustangI-2.JPG
RAF Mustang I, 3-blade

http://www.afwing.com/intro/p51/xp51-2.jpeg

Another picture of XP-51.

http://www.afwing.com/intro/p51/p51a.jpg
P-51A-10-NA

http://www.afwing.com/intro/p51/p51b5.jpg
P51B prototype , first time with 4-blade (Why 4-blade with 2-stage superchager Merlin engine? For high Mach number of propeller at high altitude?)

When crashed landing, wood propellers do less hatm to engine via shaft.

http://www.afwing.com/intro/p51/XP51g.JPG

Rotol wood 5-blade prop with XP-51G

To sum up, propeller is one of the most complicated components in WWII aircraft, thus deep invastigation should be paid in il2 FM about efficiency curve.

It was certainly advertised and pushed as such. However like many things advertised, buyer beware.

Page 124 tells the story...

http://www.scribd.com/doc/46042585/T...rams-1920-1950

No real benefit. Sounds cool though, laminar flow....

Believe it or not, the Davis wing on the B24 actually did see laminar flow benefits under certain conditions. It was total fluke of design but it did achieve laminar flow.

Yes.

Want some good dings in a propeller, taxi on new pavement. A propeller picks up dirt, rocks, bugs, and anything else in the aircrafts path. Operating from an unimproved strip will result in lots of nicks on the propeller to dress.

Even operating from a nice paved one, you will get nicks in the prop.

Find a Constant Speed Propeller that does not leak some grease too. Anything from the hub goes right up the blade.

Use the climb triangle:

When you dive that element of thrust is acting downward and an element of weight is added to thrust.

Our formula is rearranged to become T + W sin gamma = D

Our lift required increases in a dive as thrust acts against lift.

And this still applies at the equilibrium point:

http://www.theairlinepilots.com/foru...9895f5d7f6bd2f

How high speed? 400MPH TAS?

I coldn't open this link.

Athough 3-blade Clark-Y airfoil airscrew slightly outperformed 3-blade NACA-16 airfoil, you can't draw the conclusion that 3-blade NACA-16 outperforms or same as 4-blade NACA-16.

Why didn't German keep 2-blade Gottingen airfoil in WWII? 3-blade Gottingen airfoil is better than 2-blade Gottingen?

Don't forget 2-blade Gottingen prop. could save more resource and more friendly to sychronized weapons firing through the propeller disc.

Again, German's 3-blade Gottingen vs 4-blade Gotingen comparation is not valid for 3-blade NACA-16 vs 4-blade NACA-16.

Is a wonderful fighter aircraft. We are restoring one and I can't wait to fly it!

:)

With 4-blade prop. please.:)

lol. yes, vector quantities.:grin:

http://en.wikipedia.org/wiki/Propeller_(aircraft)

A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high-aspect blades can result in an excessive propeller diameter. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform, limiting the local Mach number - a significant performance limit on propellers.

A propeller's performance suffers as the blade speed nears the transonic. As the relative air speed at any section of a propeller is a vector sum of the aircraft speed and the tangential speed due to rotation, a propeller blade tip will reach transonic speed well before the aircraft does. When the airflow over the tip of the blade reaches its critical speed, drag and torque resistance increase rapidly and shock waves form creating a sharp increase in noise. Aircraft with conventional propellers, therefore, do not usually fly faster than Mach 0.6. There have been propeller aircraft which attained up to the Mach 0.8 range, but the low propeller efficiency at this speed makes such applications rare.

There have been efforts to develop propellers for aircraft at high subsonic speeds.[4] The 'fix' is similar to that of transonic wing design. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles; thin blade sections are used and the blades are swept back in a scimitar shape (Scimitar propeller); a large number of blades are used to reduce work per blade and so circulation strength; contra-rotation is used. The propellers designed are more efficient than turbo-fans and their cruising speed (Mach 0.7–0.85) is suitable for airliners, but the noise generated is tremendous (see the Antonov An-70 and Tupolev Tu-95 for examples of such a design).


//////

We will find the proof of 4-blade vs 3-blade, sooner or later.;) When a/c diving, it often creats a sharp increase in noise, which means tip of the blade reaches its critical speed.

http://digital.library.unt.edu/ark:/...dc62616/m1/19/
P47D tested with a Hamilton Standard 6507A-2 3-blade airfoil(NACA-16)

Cp=P/(r*n^3*D^5)

r=air density

We can see when dive to low altitude @0.7 Mach, prop efficiency will be as low as 63%. Fw190A8's 3.3m propeller advance ratio is quite bigger than P47's, so its efficiency should be quite less than 63% if the VDM prop. shares the same airfoil with 6507A-2.

But don't forget Hamilton Standard 6507A-2 has a NACA-16 airfoil. see here

http://digital.library.unt.edu/ark:/...adc63942/m1/3/


Allied also tested Hamilton Standard 6507A-2 with both 3-blade and 4-blade configuration at 0.4 Mach, also here

http://digital.library.unt.edu/ark:/...dc63942/m1/40/

http://digital.library.unt.edu/ark:/...dc63942/m1/43/


Fig 16 and Fig 18

Appearently, 4-blade NACA-16 airfoil shows around 5%-10%+ efficiency advantage over 3-blade cousin even at medium speed. That's one of the reasons p47D picked up 4-blade airscrew.

It's probably that when dive to high-speed fw190a8's 3-blade Gottingen prop. efficiency is much inferior than P47's 3-blade NACA-16 airfoil. But I know Crumpp will argue that Gottingen(WWI standard) outperforms NACA16 at high speed. I'll remind you that NACA-16 airfoil was widely used after WWII until 1970s when computer calculating method helped people designed better airfoils. To sum up, in high speed diving:

1)P47 has less advance ratio than fw190a8.
2)P47 has NACA-16 lower drag airfoil than fw190a8.
3)P47 has 4-blade prop rather than fw190a8's 3-blade.

In future, if someone finds the proof or calculates out that fw190A8 Gottingen 3-blade prop only has 30% efficiency in high speed diving while P47 has 75% with 4-blade NACA-16 airfoil, don't be surprised because that will perfectly explain why Fw190G was badly outdived by P47D at 65 degree in Italy 1943 summer.


How about 0.7 Mach comparation of 3-blade vs 4-blade efficiency? That's more interesting.:-P But I guess the truth will change il2's diving FM.

That is a poor assumption to make.

This is were your power loading effects the design. A two bladed propeller will not absorb 2000 hp very well.

A 3 Bladed propeller can absorb 2000 hp very well.

Yes, 3-blade is usually enough for 2000hp, but in some special envirenments, I doubt about it.

Crumpp, you have tons of information on a/c, try to find sth. interesting to make il2 diving FM perfect. I know you have all the data of VDM propellers. :)

Wow...interesting thread guys (but I did only read the actual page).

But I think you forgot something BckBr: the large bladed prop will fly easier in the airstream during the dive and will then have a tendency to raise the rpm much higher than a 4 bladed one.

More rpm -> pilot will have to reduce throttle during the dive in order to keep eng safe
More rpm -> more tip blade speed hence more drag

Transonic drag being far higher than low subsonic drag, low rpm is better either for your eng (max pow dive) and for your total drag coef.

But if you are comparing the Jug with the Fw, it 's far better to keep in mlind their difference in weight and the weight/power ratio.

With the latter, you'll understand easily that gravity did play a huge part during WWII in term of improvement of aircraft perf. Hence, a nose down Jug had far better "propulsive" power than a FW190 in the same configuration.

EDIT: oh... and let's not forget that the Jug had a metal prop when the 190 used ones made out of woods. The technology is quite different ( the latter being somewhat newer). Large blades might hve been something difficult to achieve with casted aluminium

TomcatViP, when fw190a8 flying at max. level speed, it uses 2700 rpm engine. Therefore if fw190A8 dive to a speed much more than its level speed, the engine rotation is definitly 2700rpm. Such as 2500 rpm in a high speed diving is stupid:loss efficiency a lot because of increase of advance ratio.

Let your CSP governer to maintain blade angle and rpm, don't bother thinking about it. And fw190a8's small prop's tip speed is inevitably around critical mach number in a dive, you can't avoid it.

Crummp, you are the expert on propeller aerodynamics. With your help, I've finally got the whole story.

In world war ONE, UK, Germany, USA developed RAF-6, Gottingen, and ClarkY airfoils for propellers respectively. These airfoils are "high drag high lift" conventianal airfoils. At the time, 2-blade fix pitch airscrew were used.

Before WWII, people found it's nessesary to add the 3rd blade to absorb growing horsepower of engine. eg. Bf109B/D->Bf109E. When you add more blade, there are two contrary effects:

1)good thing: better power loading ability
2)bad thing: more drag

At late 1930s, UK/USA/Germany engineers found it's almost no benifit from the 4th blade because the improvement on power loading is completely counteracted by drag increase added by the 4th blade. Allied tested RAF-6/ClarkY with 3-blade and 4-blade configration, drew that conclusion, German Mtt and Focke Wulf also tested , with same result.

http://aerade.cranfield.ac.uk/ara/19...report-640.pdf


During late period of WWII, every country faced same difficulty: how to improve prop efficiency when more powerful engine equipped with aircrafts?

German engineers found a clever method: use broad chord in 3-blade prop thus they could improve power loading while maintain lower drag than 4-blade. Result was quite good:

http://forum2.totalsims.com/viewtopic.php?t=2475

Allied finally accept 4-blade prop, eg P51A-P51B. in your opinion, both methods are perfect, and with little to choose.


Question: Since german 3-broad-blade obviuosly outperformed their old 3-blade design , so were allied new 4-blade prop. I've posted the proof of efficiency advantagde of P47's 4-blade hamilton over 3-blade. However, in late 1930s, allied reports on RAF-6/ClarkY already said there is little difference between 4 and 3 blade. What's the problem?


The answer is lamimar airfoil developed during WWII, NACA-16 series. I agree with you with the difficulty maintaining of laminar effect in actual combat envirenments. OK, let's regard NAVA-16 as conventianal airfoil, that is, NACA-16 is "fake" laminar flow airfoil. The next question is: Is there enough difference between two kinds of conventianal airfoils? Of course. In an aerodynamics textbook says:"RAF-6 is suitable for taking off while ClarkY is better in criusing and high speed flight." Notice that there is only slightly section shape difference between RAF-6 and ClarkY. Therefore, being a vast different shape, NACA-16 behavior should be "special". But in some allied test, 3-blade NAVA-16 is even slightly worse than 3-blade ClarkY especially during taking off. Notice that the test speed is probably within 400MPH.

http://www.scribd.com/doc/46042585/T...rams-1920-1950

at high speed......how high? 0.7 Mach TAS?

Is the NACA-16 the "new age ClarkY" just like Clark/RAF-6 comparation? that is to say, "new clarkY"--NACA16 is worse than old clark in taking off and better in REALLY high speed when propeller tip approching critical mach number? This is the key of mysterious diving performance difference.

After WWII, as piston engine's power increased to 2400-3000HP, people impelmented 5-6 blade low drag NACA-16 airfoil to absorb it, and this configaration worked perfectly at high mach subsonic flight. This fact reminds us that whether the 4-blade NACA16 propeller outperforms 3-blade high drag/high lift wide-chord airfoils at high diving speed(=0.7mach or so)? There is small clue as Crummp said in 2005:

Why decreased in max. level flght? Dose that suggest us the 3-broad blade prop was slightly outperformed by old 3-blade with narrow chord around 0.5 Mach? ? Because of the more drag of 3- broad blade especially when tip mach number approching 0.85 ?This is only level flight, just 0.5Mach/650km/h or so, how about 0.7Mach/850km/h? Could broad design shortcoming--more drag-- become more abvious?

while 4-blade NACA16 achived 90% in P47....

http://digital.library.unt.edu/ark:/...dc63942/m1/40/

In my opinon, there is the possibility of 4-blade NACA16 greatly outperformed 3-broad blade at high diving speed(0.7 Mach). To prove this ,we need more data while crummp tons of resource will play the key role. :)

"There is small clue as Crummp said in 2005"

LOL! Seven years ago. No where to run, no where to hide.

German 3-broad chord blade prop plane lost 4% top level speed compared with 3-narrow-blade, according to drag fomular, new broad design lost roughly 8% efficiency around 5.5Mach. Why? Because the bad thing--more drag--become more noticable when speed building up?How about 0.7 Mach? Broad chord design will lose 15% efficiency than narrow chord?

There is evidence that 3-blade NACA16 on P47 gets 63% efficiency at 0.7 Mach, so let's assume that ClarkY and Gottingen(narrow chord) share the same performance with everyelse being equal.

But fw190a8's diameter is quite smaller than P47's, then much bigger advance ratio, guess fw190a8 get 45% efficiency at 0.7 Mach?

Allied 4-blade NACA16 outperforms 3-blade NACA16 at 0.4 Mach, with extra 7-10%, probably at 0.7 Mach, 4-blade NACA16 will get 75% efficiency.

45% vs 75%? This is critical for P51P47Temepest's tactics. I strongly suggest il2 developing team simulate the efficiency of WWII late prop at high speed using software such as ANSYS, otherwise, this game could not perfectly simulate western line where the "battles are not forgotten".

what about windmilling and internal friction /cylinder compression of engine? bmw801 only had 14 cyl. p47 had 18 cyl. so, maybe some inefficiency of 3 blade wide was offset by less internal friction in the 801 and that is why they went with 3 blade wide.:-P

bf109k4 ：3-blade prop
spitfire xiv：4-blade prop even 5-blade

Both are liquid cooling engine.

btw，r2800 and bmw801 share almost same front area.

you miss the point.

What a wide 3 blade may lose in "thrust" vector efficiency from more drag , it may gain in 'weight" vector efficiency when "thrust" vector efficiency is at zero in a dive and high TAS. The wider blade means higher tip speed from ram air verses thinner 3 blade. This means greater torque to overcome internal friction from the engine. The pilot can reduce internal friction by lowering manifold pressure, but not completely. The engine will still tend to over-speed and this is mitigated by lowering rpms/coarsening blade pitch. So, internal friction of the engine is more easily overcome by the weight vector with the wider blade I speculate.

One way to measure the internal friction of the engine. What was the cranking force required to start the 801? What was the cranking force required to start the r-2800? If two engines are identical, it takes more cranking force to start an engine with a 4 blade wider diameter prop attached than a 3 blade smaller diameter. The engines were not identical. The r-2800 had more cylinders than the 801. If you take off the props, I suspect it takes more cranking force to start the r-2800. I suspect the p47 had more internal friction to overcome than the fw190. This would hamper dive acceleration.

Inncrease the area of our lifting surface = more drag

If you know how to do sections, then you know the common expression in propeller blade element theory is Cb for chord length and our local section Drag is expressed as:

~D = 1/2pVb^2CbCD

OK Gents,

I've read this thread with fascination, but the details are WAAAAY over my head. Do you have a conclusion, ie, IL2 is way off or it is close enough for a $40 flight sim that is a decade old?

Cloyd

lol, I have no idea.

maybe add some sound effects when the tip speeds go mach.

maybe add some sound effects to p47 turbine spinning up.

I did notice in up3, rc4, there seems to be turbo lag in the dial indicator verses the engine rpm indicator. so somebody was thinking good thoughts. i think it was a mod p47, maybe p47B. felt a little more agile than the others too.

Read this article and you will know high-speed propeller was a leading edge issue in WWII when people didn't know much about the transonic aerodynamiacs(0.8-1.0 Mach). They even could NOT precisely test the propeller efficiency at high speed in fullscale wind tunnel.

http://aerade.cranfield.ac.uk/ara/19...report-640.pdf

Il2 is a very closed simulation when speed is below 0.8 Mach, but when speed of anything(wing, propeller tip,etc) is beyond 0.8 Mach, not very accurate.

Try to find a WWII fighter with cowl guns and a 4 bladed propeller. I cannot think of a single one.

I have been flying this sim for over a decade, and I really only know how to fly two planes well - I-153 and F4F-3. (Yes, I realize that neither of these was available in the initial release.) Is it safe for me to assume that I don't have to worry about the performance of my preferred rides in the 0.8 to 1.0 Mach range? ;-) ;-) ;-)

Cloyd

If you ever go that fast in an I-153, you have a problem. And almost certainly, no wings... ;)

What I said most importantly :

Transonic drag being far higher than low subsonic drag, low rpm is better either for your eng (max pow dive) and for your total drag coef.


But I hve to agree that was not my best writing :oops:

I think that your answer lies in faisability (large series) and techniques.

Just remember that the very goal of the Clark's Y airfoils series is for an easy craftsmanship (russians abused of this with their wooden series of La, Yack etc... all were made out of Clark Y).

Propeller material:
German = wood (but some 109 had metal..... seems it was not so much a prob ?)
US = casted aluminium

It makes a huge difference in what kind of airfoil you can achieve.

But still it's only my own guess.

Thx for making that thread an interesting one. Pls go further on ;)

EDIT :OOOpss..just forgot to say that the Clark Y airfoils had a flat bottom to ease marksmanship.

Tomcatvip, if you exam the fw190a8 max level flight at 20000ft, you will find the tip of VDM propeller is just 1 Mach.

680km/h TAS, 2700rpm engine, 0.54:1 reduction ratio, 3.3m diameter

IMHO it's a by-design parameter. Don't we hev the same result with the larger Hamilton props such as fitted on the P47?

Yes, the engineers in the 1940's were at the top of their game for subsonic propeller design.

About the propeller on the Fw190V18?
 

Reading this very interesting topic a question has come to my mind: for what reasons did the enginneers at Focke-Wulf tried a four blades propeller on the Fw190V18 high altitude prototype (were the blades longer? was the propeller similar to those on the P51 and P47?). Anyone knows or have a guess?

They were looking for a counter to the B-29 being able to bomb at 30,000 feet and above.

The Germans had no ability to intercept anything at that altitude. The FW-190V18 was one of the designs examined and tested. The result was the Ta-152 series had better performance at altitude and the program was scrapped.

Thanks for the answer Crumpp,

In fact I was very surprised to see a German aircraft with a 4 blades propeller (which is very uncommon, the only other i know is He177 Greif bomber) , so i thought (in my noob mind) this solution would somehow be related to very high altitude flight (after all P47 used 4 blades prop and was designed as a high altitude aircraft, and when the P51 became one too, it switched its three blades for four). To sum up i thought the 4 blades were somehow related to very high altitude rather than to power dive performance. But it was just a noob question :) : anyway i recognize that i don't know anything about aircraft enginneering! Just trying to try to understand "in broad strokes"...

As for what you said about 4 blades propeller and cowling/wingroot weapons firing through the prop disc, although very rare, there were some aircraft with this configuration:

P39-Airacobra late 4 blades prop and P-63 Kingcobra, 2x50. cal (cowling) firing though the prop disc ;

Nakajima Hayate Ia, 4 blades prop and 2xHo103 MGs (cowling) firing through, and on later Ib version 2xHo5 20mm canon (cowling).

I've found any with as much guns (4) as the Fw190 though, perhaps four blades would have killed the high firing rate of these guns?

Hardly, the electrical priming on the Fw could deal with synchronised fire rather easily. Much better than other systems.

Yes it could compared to a mechanical sychronization but the electrical priming cannot change the limitations of the weapon itself.

Interesting and rare....

If the performance differences had been noteworthy, I think the Germans would have used a 4 bladed design. As it was, they had good propeller designs and increased chord width which accomplishes the exact same goal of being able to load more power onto the disc.

The Germans also used wood in many of their later designs as it is a much better material for power loading than metal.

I don't believe that. Broad chord 3-blade prop lost 8% efficiency when a/c speed is around 0.55 Mach, although this design outperforms old design a lot when speed is low.

German had no naca16 airfoil, what they used in WWII is just WWI standard-gottingen airfoils and the modified broad chord version.

For all of WWI airfoils(RAF6,ClarkY,Gottingen), 4-blade design is useless, but for the newly developed NACA16, story is different.

So?

Did you read the NACA's own findings on the Clark Y and NACA 16 series?

The 16 series has poor lift production and its only real application was in propellers. It was generally considered to be worse than the Clark Y even in that application. The NACA 16 series was supposed be low drag at high speed and designed for the very high transonic realm. It was a real disappointment to the NACA.

Go back a few pages and re-read it. It will confirm there was no difference at speed and the Clark Y was actually better overall.

This is my comprehension. Criticism welcome.

Attachment 9791

That is a nice chart, Blackberry. Couple of things to keep in mind.

NACA 16 is a whole series of airfoils each with their own characteristics. You can make some very general statements about them but for the most part, the only characteristic that really sets them apart is the method they were derived. A method with extremely mixed results and sometimes not so very good agreement between calculator and the wind.

Gottingen is also a series of airfoils each with its own characteristics. These were derived from practical work in the wind tunnel.

http://www.aerospaceweb.org/question...ls/q0197.shtml

Once more, just as the NACA was aware and used Gottingen airfoils, so did the German designers use NACA airfoils. The Focke Wulf FW-190A uses the NACA 23015.3 at the root and NACA 23009 at the tip.

Ah!.. the numbers ;)

So I suggest Daidalos Team make detailed prop efficiency model.

On what data???

There is a good reason why n = .85 in a CSP is a valid assumption in subsonic aerodynamics.

P47's 3-blade prop drops to 63% efficiency when diving to 0.7 Mach which is subsonic. When you have 20% efficiency advantage over your opponent, you have 400 extra Horse Power, that's a Huge difference.

To model detailed prop efficiency by softwares such as xfoil, ansys,etc.

You are asking TD to rewrite the entire flight physics modelling based on a single number? Yeah, that's going to happen...