Why still no dive acceleration difference?

[Crumpp]

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

[BlackBerry]

Power requirements are cubed in relation to velocity.

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

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.

More blades = more drag but those airplanes have more thrust than the blades add drag because of their weight.
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(drag) increase. Both are perfectly acceptable approaches to increasing performance with very little to choose from.

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:

The most valuable link of wind tunnel is here:
http://history.nasa.gov/SP-440/contents.htm

German side:

http://wp1113056.wp148.webpack.hoste...fluegel_en.htm

The measurements were repeated for different Reynold Numbers and different lift coefficients. For the lowest Reynold Number (4 millions) the point of transition was measured at 50% depth on the upper surface. It moved to the leading edge with increasing Reynold Number, arriving at 20% for Re=7,5 millions. Measurements with different laminar flow airfoils including the Mustang airfoil were later continued in the large high-speed wind tunnel of the DVL, Berlin up to Reynold Numbers of 20 millions. These measurements clearly revealed the fact that the laminar flow effect completely disappeared at real flight Reynold Numbers. This was an expected but sobering result.

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

According to the German aerodynamicist Schlichting, German work on laminarflow airfoils did not start until about the end of 1938. By 1940, Schlichting considered that the fundamentals were known. Drag coefficients as low as 0.0027 were reached at a Reynolds Number of 5 x 10^6, but the German scientists were unable to retain the low drag at higher Reynolds Numbers. They were handicapped by lack of suitable low-turbulence wind tunnels. On one occasion, Prandtl reported: "Suitable wind tunnels for the conduct of airfoil investigations at sufficiently high Reynolds Number and at low turbulence are lacking in Germany. On the other hand, it is known that in the U. S. A. particular installations created for this purpose are working exceptionally vigorously in this field."

Tests were made on a Japanese laminar flow airfoil, on·three airfoils derived from one member of an obsolete NACA Series 27215 (which was described in a captured French secret report), and on a few airfoils designed by Schlichting. The Germans also 45 had some information on a Russian laminar flow airfoil obtained from a captured report.
The Germans never used laminar flow airfoils on aircraft. They were astonished and mystified by the performance of the Mustang and made many wind-tunnel and flight "tests. They gave the following tabulation of wing profile drag coeffiicients
(obtained by momentum method) for a number of airplanes at lift coefficient of 0.2:
He-177 0.0109
Me-109B 0.0101
Mustang 0.0072
Ju-288 0.0102
FW-190 0.0089

The German comment is: "The drag of this only foreign original airfoil tested up till now is far below the drag of all German wings tested in which it should be remembered that it was tested without any smoothing layer." Another writer says: "A comparison of flight measurements shows quite unmistakably that the Mustang is far superior aerodynamically to all other airplanes and that it maintains this superiority in spite of its considerably greater wing area."
Allied Developments.
The NACA began investigations of laminar flow airfoils in a low-turbulence wind tunnel in the spring of 1938, and the encouraging nature of the results obtained (without details) were described in the Wilbur Wright Lecture of the Royal Aeronautical Society on 25 May 1939, and in the NACA Annual Report for 1939. In June, 1939, an advance confidential report by Jacobs was released. A summary was published in March, 1942 in confidential form. The most recent summary was relaesed in March, 1945, and this summary has been kept up to date by supplementary sheets.
As indicated in the summary of German developments, the Allies are far ahead in low-turbulence wind tunnel equipment and in knowledge of laminar flow airfoils and their application to aircaft. Drag coefficients as low as 0.003 at a Reynolds Number of 20 x 10^6 have been obtained. A summary of the present state of knowledge is given in the NACA restricted report L5C05, "Summary of Airfoil Data," by Abbott, von Doenhoff, and Stivers,
March, 1945.

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.

[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