Why still no dive acceleration difference?

[BlackBerry]

This condition is key. Of course, above Vmax we are outside of the aircrafts design envelope and our efficiency curve no longer approximates a slope of zero. Instead it takes on a negative slope.

Just though as it is a good assumption to have a slope of zero in the envelope, it is also a good assumption that all CSP designs will have a similar negative slope outside that envelope.

The most important factor of a propeller is its size/diameter. if you fly your P47 with a 3.3m diameter prop instead of a standard 4m, you'll definitely lose more efficiency, perhaps 10%, at 0.7Mach(above Vmax).

efficiency is Y axis, advance ratio is X axis, although 3.3m prop share same slope curve as 4m prop outside the envelope, the 3-meters prop working point is on the right side of 4-meter due to higher advance ratio, which means less efficiency.

I also think that individual and specific characteristic's are way more trouble than it is worth for dubious accuracy without the actual data. It would also open up a huge can of worms for your developers and people having to decide what data is applicable.

Look at all the arguments over such very well documented performance parameters as climb rates or Vmax. Now you want to add in propeller design?

The whining would never stop, not that it does now.

My suggestion would be to concentrate on accurately modeling the limits and behaviors found in the Operating Instructions.

Yes, this efficiency curve is a Pandora box. If il2 FM gets improved in propeller efficiency curve, the drag coefficient of many aircraft should be different from ealier, otherwise, how could aircrafts get accurate Vmax as historical data? For many years, blue side fans and red side fans argued a lot about engine output, boost level such as 1.8ata, 21lbs ,25lbs boost etc..... Now what ? Taking away 500HP from my engine when high speed dive? To degrade my bf109G6as into a bf109G6? OMG, just kill me.

I think it would be more realistic and easier to model the consequences of exceeding the dynamic pressure and mach limits of the aircraft than trying to find a generic braking point. You don't think, "my efficiency curve will drop off and drag rise due to compressibility will keep me safe" when you point an aircrafts nose down in a steep dive. You think, "don't exceed Vne...don't exceed Vne" as your butt cheeks suck up the seat.

For example, our FW190 tries to dive away from a P47. At ~466mphTAS, the FW encounters compressibility, and loses elevator control. The FW now happily sails to the dirt barrier or the pilot very very carefully uses the elevator trim to recover without overloading the airframe.

At 466 mph TAS, the P47 is in full control. He either:

1. Watches the FW hit the dirt barrier from the comfort of altitude.

2. Catches the FW on its straight path to the dirt barrier and shoots it down. His top speed is ~40 mph faster...

2. Follows the FW on its rather helpless recovery and shoots it down

When dive to 800km/h, the real world air compressibility costs more 200HP power than il2's none compressibility FM. It's not news and is already known by many people. Forthermore, as you said, in il2 4.11m, fw190 still has good elevator control above 466mph=750km/h.

However, my interest is not above 750km/h, just between Vmax(680km/h) and 750km/h. Could P47 get more efficiency than fw190 when steeply dives to 750km/h and then maintains 740km/h for 30 seconds in a shallow 10 degree dive?

Diving away is a very bad option for the FW190 if the limits are accurately modeled.

It is a good tactic for fw190 to dive away from spitfire. As <<Tempest in War>>said, many fw190 tried dive away when pursued by Tempestmkv, of course those fw190 usually got shot down. In history, those fw190 pilots succeeded in outdiving spitfire before they met tempest. That's why they tried dive away from tempest, they thought spitfire and tempest are similar in dive, but they were wrong.

Again, dive limit is NOT dive acceleration, IF my fw190 could dive faster than your P47 WITHIN 750km/h=466mph, I'll try dive away from P47 because that's a good idea. I'll keep dive speed within 466mph, so that your P47 has no chance to show higher dive limit. I'm sure I can get far away from your P47.

But the truth is that within 466mph, P47 still dives faster.

[Crumpp]

although 3.3m prop share same slope curve as 4m prop outside the envelope, the 3-meters prop working point is on the right side of 4-meter due to higher advance ratio, which means less efficiency.

Not sure what you are trying to say here...

You do understand you cannot compare propellers at different advance ratio's???
It is not in any way or form valid.

I think this where you are getting confused.

Advance ratio is analogous to angle of attack.

If you looked at angle of attack in isolation as a measure of turn performance for example, you would erroneously conclude that the aircraft at the higher angle of attack can outturn one at a lower angle of attack.

This is patently false and has no bearing on turn performance.

You can only make a valid comparison of propellers at the same advance ratio. The fact other propellers can achieve higher advance ratio's has no bearing on their performance when compared to lower advance ratio propellers anymore so that wing angle of attack can be used for turn performance prediction.

[Crumpp]

if you fly your P47 with a 3.3m diameter prop instead of a standard 4m, you'll definitely lose more efficiency, perhaps 10%, at 0.7Mach(above Vmax).

Not at top speed or dive performance.

In general....Smaller disc = higher top speed because a large disc means more tip lose.

Larger disc means better low speed performance, ie take off, climb, cruise, and turn.

Examine the diameter of supersonic propellers.....

[BlackBerry]

Not sure what you are trying to say here...

You do understand you cannot compare propellers at different advance ratio's???
It is not in any way or form valid.

I think this where you are getting confused.

I can't compare 2 propeller efficiency by using a single propeller curve.

But fw190 advance ratio from 2.54 to 3 when TAS 680km/h to 800km/h. 0.46 is difference.

P47 advance ratio from 2.1 to 2.47 when TAS 680km/h to 800km/h. 0.37 is difference.

if both share same efficiency drop slope above vmax, fw190 lose 5% more than p47. btw, German wide chord prop lose 8% efficiency compared to old narrow design because of 4% Vmax lose.

so if p47 equipped with 4 meter German old narrow prop, p47 should get at least 13% efficiency advantage over fw190a8 new wide chord propeller at 800km/h. furthermore, German wide chord dosen't share same efficiency slope as narrow prop above Vmax because within Vmax wide chord is better and at Vmax wide chord is worse. so wide chord has a steeper drop slope than old narrow chord design.

that is to Say, p47 with old narrow German airfoil 4meter prop could get almost 20% efficiency advantage at 800km/h TAS which is smaller than 466mph IAS.
IMG_1041.JPG
what allied propeller engineers did in WWII is just maintain German WWI standard airfoil performance? of course not.

story becomes complicated when allied developed naca16 and paddle wide chord propeller.

1944 early, both German and allied began to use wide chord airfoil in fighters:fw190a8 and p47d-25

with wide chord design,German get better climbe and turn performance, so was allied.

http://www.368thfightergroup.com/P-47-R2800.html

p47 got more than 10% climbe rate due to wide chord paddle propeller.

It is noticed that p47 propeller is so big that tip Mach too high, above 1 Mach, if you reduce rpm from 2700 to 2520(bigger advance ratio),you'll get 6% more efficiency at 800km/h. But that dosen't mean fw190 could get higher efficiency due to higher advance ratio than p47. fw190a8 prop tip Mach is usually less than 1 mach(1 Mach @800km/h). A bit complicated.
http://digital.library.unt.edu/ark:/...dc62616/m1/25/

Question:

1) Do allied wide chord paddle props suffer efficiency lose just like German wide chord cousin at 750-800km/h?

we don't know.

1) 3-blade naca16 is same as 3- blade clarkY at 750-800km/h. So is the 4-blade vs 4-blade compare?

we don't know.

But one fact is very clear: prop efficiency may lead to hundreds of horsepower difference above Vmax, so a simulation game must pay enough attention to detailed efficiency curve. otherwise, a big difference from history is inevitable.

[BlackBerry]

Originally Posted by Crumpp

Not at top speed or dive performance.

In general....Smaller disc = higher top speed because a large disc means more tip lose.

Larger disc means better low speed performance, ie take off, climb, cruise, and turn.

Examine the diameter of supersonic propellers.....

those supersonic propellers max tip Mach is more than 1.3 while WWII p47 max tip Mach is below 1.15. quite different story.

[Crumpp]

those supersonic propellers max tip Mach is more than 1.3 while WWII p47 max tip Mach is below 1.15. quite different story.

Come on man....

You understand the basic's of rotational mechanic's right?

On any radius of the circle, the point closest to the origin travels at a slower velocity than a point distal to the origin.

That is why as a generality, a smaller disc is better for Vmax performance.

[Crumpp]

I can't compare 2 propeller efficiency by using a single propeller curve.

I am not exactly sure what you are looking at.

CSP's are not compariable at different advance ratio.

[BlackBerry]

Originally Posted by Crumpp

Come on man....

You understand the basic's of rotational mechanic's right?

On any radius of the circle, the point closest to the origin travels at a slower velocity than a point distal to the origin.

That is why as a generality, a smaller disc is better for Vmax performance.

1.jpg
There are two shock wave areas in propeller.

One is near tip, the other is around root.

Ma----compound speed, =squareroot of (rotating speed^2+ TAS^2), Mach

Mak----critical shock wave stall speed for a certian airfoil, Mach

The propeller portion near root is usually thick and not very streamline, so Mak is quite low which means easily render shock wave. The tip portion, on the other hand, has a very high(near sonic)Ma, so shock wave inevitable although this portion is quite thin and streamline.

Afterall, there is a trade off upon propeller's diameter above Vmax, if you use a bigger one, the shock wave area near tip is quite big, bad thing. But you get a smaller advance ratio, that's a good thing.The art is to find a optimum point where whole propeller reaches maximum efficiency at a certain speed above Vmax.

Speed is an important concept in combat, just like altitude. Pilots know what's the best altitude for their aircraft, eg, P47D, are willing to fight fw190/bf109 above 6000m altitude. If fly a La7, the lower altitude, the better. Why altitude is so important? one reason is "engine output".So is speed.

If your opponent will lose 500HP at a certian speed between Vmax amd Vne due to lower propeller efficiency, you also wanna drag him to such high speed and beat him in an energy fight style.


The samller aircarft, the lower drag coefficent and smaller weight, thus easier reach high speed and better output/weight ratio. One couldn't have it both ways.Shouldn't those tiny soviet/German aircrafts pay the price during high speed dive?

[Crumpp]

There are two shock wave areas in propeller.

Right, as a function of angle of attack and not diameter....

You are getting into the weeds without keeping an eye on the big picture. Propeller designers are aware of of this and design accordingly. It is too easy to spot a bad propeller design very early on.

It is a general principle that smaller diameter is better for Vmax performance.

Keep the discussion to diameter effects.....

Take a lesson from Professor Von KlipTip.....

6. What is the importance
of propeller diameter?

Ideally, the propeller diameter should be
greater for efficient low airspeed operation
and smaller for high airspeeds.

http://www.google.com/url?sa=t&rct=j...KCOmcUBIgbeK8Q

[Crumpp]

Speed is an important concept in combat, just like altitude. Pilots know what's the best altitude for their aircraft, eg, P47D, are willing to fight fw190/bf109 above 6000m altitude. If fly a La7, the lower altitude, the better. Why altitude is so important? one reason is "engine output".So is speed.

All of this comes out in standard performance calculations. In this case, subsonic incompressible flow theory works very well at predicting subsonic propeller design behaviors.

For example, you don't have to add anything when crunching the numbers for a FW-190 regardless of the propeller.

If you plug in the data for a metal propeller, your drag is less which means less lift and your sustainable turn performance envelope is reduced.

If you plug the data for a wide chord wooden propeller, your drag increases resulting in more lift and your sustainable turn performance envelope increases!

It is all in the math!

But one fact is very clear: prop efficiency may lead to hundreds of horsepower difference above Vmax, so a simulation game must pay enough attention to detailed efficiency curve. otherwise, a big difference from history is inevitable.

What is the basis you will manipulate the curve? Propeller design is extremely complex and once again, it is too easy to spot a bad design very early on. Like first flight early, LOL.

This appears to be a license to manipulate aircraft behaviors based on intuiation and supposition.

Propeller design is just too complicated and easy to spot a bad design. There is a reason why a generic curve is acceptable!