Why still no dive acceleration difference?

[BlackBerry]

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.

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.

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.

a.JPG

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

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.

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

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.

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.


XP51 prototype model in wind tunnel , 3-blade prop.



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


RAF Mustang I, 3-blade



Another picture of XP-51.


P-51A-10-NA


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.



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.

[Crumpp]

Thus the difference between NACA16 and Clark-Y/RAF-6 is more profound.

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

To our dismay and disappointment, the 16-series propeller showed no advantage at high speeds; in fact the Clark Y appeared slightly
better.

Page 124 tells the story...

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

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.

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.

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

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.

[Crumpp]

I don't understand why you guys keep saying weight = thrust.

Use the climb triangle:

The balance of forces in a steady climb show thrust is acting upwards and an element of weight is adding to the drag

As the thrust assists the lift, the lift required is less than in level flight. Verify mathematically by the formula Lift = W.cos gamma

For a steady speed to be maintained the thrust and the two retarding effects of aerodynamic drag and the weight element must be equal.

If Thrust = T, Drag = D and Weight = W, then as a formula it can be written as:

T = D + W sin gamma

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:

For a steady speed to be maintained the thrust and the two retarding effects of aerodynamic drag and the weight element must be equal.

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

[Crumpp]

The P51

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

[BlackBerry]

Originally Posted by Crumpp

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

With 4-blade prop. please.

[MadBlaster]

lol. yes, vector quantities.

[BlackBerry]

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.

[BlackBerry]

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. But I guess the truth will change il2's diving FM.

[RegRag1977]

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?

[Crumpp]

for what reasons did the enginneers at Focke-Wulf tried a four blades propeller on the Fw190V18 high altitude prototype

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.