Why still no dive acceleration difference?

[BlackBerry]

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

The NACA's failure to discover and develop jet propulsion should not be allowed to mask its real and significant contributions to American aerial victory in World War II. Though air power was not the sole, [195] or even the most, important ingredient of American victory in the war, it was a key ingredient; without the NACA, American aerial superiority would have been less complete, less early. Every American airplane that fought in the war, every aircraft engine, had been tested and improved in NACA facilities. Most of this cleanup and testing was incremental and anonymous, hard to trace to the NACA, and difficult to evaluate. With military officers, NACA engineers, and aircraft designers and manufacturers all poring over the same test results in an effort to improve the flying qualities of an aircraft, the credit for improvements must be spread widely. Some examples of NACA contributions can be isolated, as when the Committee predicted that the B-32 would fail and recommended that its development be abandoned. In some cases, the prescribed NACA fix for a problem aircraft was rejected by the manufacturer, as when Kelly Johnson of Lockheed ignored the first solution proposed by the NACA for the problems his P-38 was experiencing.45

Two Committee achievements during the war were so obviously useful and noteworthy that the NACA took great pride in citing them. The first investigation undertaken at the new Ames laboratory - icing research - was so useful not only to military bombers operating at high altitudes and through all kinds of weather, but also to commercial operators, that it won for its principal investigator, Lewis A. Rodert, the Collier trophy of 1946. The low-drag wings of the P-51 Mustang, the result of years of NACA research on wing characteristics, became a hallmark of NACA achievement. Though some questioned that these laminar-flow wings (as they were often and incorrectly called) were responsible for the unparalleled performance of the Mustang, most agreed that they were a significant contribution to airfoil development and drag reduction. John Victory was pleased to report in later years that captured German documents revealed an inability by the Germans to account for the superior performance of the Mustang, even after they captured one intact and tested it, because their wind tunnels could not duplicate the low turbulence produced by the NACA.

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?

[Crumpp]

German never used laminar flow airfoil in wings, nor the propellers.

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.

[Crumpp]

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

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.

[Crumpp]

However, why allied engineers accepted the weight(drag) increased by the 4th blade, and why german engineers denied?

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.

[MadBlaster]

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

[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

[BlackBerry]

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

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?

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.

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.

[Crumpp]

The P51

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