WW2-fighter and critical Mach speed

[delcyros]

How about the critical Mach speed of fighter during world war 2? Speed remained of highest value for fighterplanes of ww2. In the early 40´s fast diving planes reached extreme speeds, bringing them close to the sound barrier. We may discuss on this board Mach speed figures of different planes. The Spitfire could be the example of the prop driven fighter with the highest Mach number of all times. What was the earliest plane to encounter it´s critical Mach Speed? P-38? Spitfire? Me-262? Anything else? (no unmanned designs like Goddarts or v. Brauns supersonic rockets, please)

[wmaxt]

Originally Posted by delcyros

How about the critical Mach speed of fighter during world war 2? Speed remained of highest value for fighterplanes of ww2. In the early 40´s fast diving planes reached extreme speeds, bringing them close to the sound barrier. We may discuss on this board Mach speed figures of different planes. The Spitfire could be the example of the prop driven fighter with the highest Mach number of all times. What was the earliest plane to encounter it´s critical Mach Speed? P-38? Spitfire? Me-262? Anything else? (no unmanned designs like Goddarts or v. Brauns supersonic rockets, please)

The critical Mach number for the P-38 was .68/.70 one of the earliest of all the topline fighters in WWII. The wing was designed for Fuel storage and lift at a time before wing shapes like the Laminar flow wing on the P-51. It needs to be remembered that It was only criticle above 20,000ft and even the early P-38s were able to keep the enemy plane in sight through the dive if proper procedures were followed - close throttles and go to flat pitch on the props. The P-38 could always catch up on the level.

As I understand it the eliptical wing on the Spit was able to keep the critical speed up.

edit: Interestingly Warren Bodie in his research for his book of the P-38 (I recomend it for all WWIIaviation buffs) The P-38 had less fatalities due to compresability issues than either the P-51 or the P-47 the P-38 was higher profile and was sent to war over a year earlier with much development done in combat for everyone to see any problems.

[the lancaster kicks ass]

apparently if the spit could be powered to such a speed, the airframe could withstand the stress of mach 1.3...........

[delcyros]

:P
Thanks Wmax! The P-38 had really highly interesting mach figures. I hope you stay in this board, so we can discuss the high speed behavior of the Lighting more in detail. Has anyone informations regarding a P-38 reaching it´s critical Mach number during dives at test flights? At what altitude and what IAS? And when did it happen?

And so to Lancaster. Nice to see you here. The Spitfire had, as you say, an interesting wing with control surfaces, which could -theoritcally- stay in use until Mach 1.3 was reached. The critical Mach number for the airframe (including fuselage and airscrew) was much less. I believe there was a test diving in the mid 50´s, which resulted in the highest recorded Mach number (0.94) for prop driven planes (recoverd safely). But that also was far beyond it´s critical Mach number (..during the dive the Spit was still able to roll a bit thanks to it´s wing). The compressabilty effects of the airscrew reendered the tail control surfaces useles..), the correct term is therefore terminal dive. Anyway a remarkable dive!

Here I have some critical Mach figures of some jet planes:
Me-163A: 0.845 (calculation DFS)
Me-163B: 0,84 (calculation A. Lippsch)
Me-262A : 0.86 (calculations of W. Messerschmidt)
Gloster Meteor: 0.83
Bell X-1: 0.88

Any informations about Fw or Bf-109 or russian planes (or anything else)?

[delcyros]

after a look-araouns I found some additional critical Mach-speed figures:
From Lunatic (compare P-47 vs. F4U):

Spitfir MK IVX: 0.89*
Hawker Tempest: 0.83*
F4U: 0.73 (windtunnel tested)
P-51B: 0.84*
P-38: 0.65
P-47C: 0.69
P-47N: 0.83*

*) All Mach figures, for planes with an airscrew in front of the plane, above 0.80 are highly debatable. It could happen, that a prop driven plane exceed this Mach number but the airscrew would render most controls useles (maybe except the wing). That would result in terminal dive. (terminal means not unrecoverable, but the normal use of controls is out of order. Inverted controls, buffeting, no response and other aspects)For example, the original MiG-9, the soviets first jet fighter had only a critical Mach number of 0.79. Later versions had Mach 0.81.

[wmaxt]

Originally Posted by delcyros

after a look-araouns I found some additional critical Mach-speed figures:
From Lunatic (compare P-47 vs. F4U):

Spitfir MK IVX: 0.89*
Hawker Tempest: 0.83*
F4U: 0.73 (windtunnel tested)
P-51B: 0.84*
P-38: 0.65
P-47C: 0.69
P-47N: 0.83*

*) All Mach figures, for planes with an airscrew in front of the plane, above 0.80 are highly debatable. It could happen, that a prop driven plane exceed this Mach number but the airscrew would render most controls useles (maybe except the wing). That would result in terminal dive. (terminal means not unrecoverable, but the normal use of controls is out of order. Inverted controls, buffeting, no response and other aspects)For example, the original MiG-9, the soviets first jet fighter had only a critical Mach number of 0.79. Later versions had Mach 0.81.

According to Lockheed the P-38 is .68 but evedence suggests that .70 was reachable when conditions were right the P-51D is rated closer to .80.

An interesting fact is that as the Mach is achieved Stabilizer/elevator effectivness is lost. The key to controlable supersonic speed was twofold a) lose the prop and b) an all flying stabilator to eliminate the shock-stall that rendered the elevator useless.

[delcyros]

Interesting. The shockwave effects were not very well understood in the early 40´s. It is a good question how to rate the P-38. 0.65 seems to me a bit to little, too. I will have to check mathmaticly ...
There are two questions remaining: A) Could the wing design sustain the stress and B) had the tail design enough stiffness for a Mach speed of, lets say 0.70?
The loose of control is a problem, too. I think the Brits did got a major breakthrough with their free bending rudder design (look at the Spit for example). I don´t know how the others tried to manage it...

[wmaxt]

Originally Posted by delcyros

Interesting. The shockwave effects were not very well understood in the early 40´s. It is a good question how to rate the P-38. 0.65 seems to me a bit to little, too. I will have to check mathmaticly ...
There are two questions remaining: A) Could the wing design sustain the stress and B) had the tail design enough stiffness for a Mach speed of, lets say 0.70?
The loose of control is a problem, too. I think the Brits did got a major breakthrough with their free bending rudder design (look at the Spit for example). I don´t know how the others tried to manage it...

Yes structuraly the P-38 was fine and if not over stressed (by the recovery methods used) would come out of compressability on it's own at lower altitudes, The trick was to use small turns, retarded throttle position, and a flatter pitch on the props on early models. In later models retarding throttle and opening the dive flaps was enough. .68 is the official rated limiting mach number. Over stressing normaly came from max trim tab use or over controlling, and that sort of thing. Many early aircraft lost were test aircraft that had been stressed to the limit many times in simalar testing. Position of the tails and strength were fine as many tests showed some other aircraft required minor changes. Every WWII top line fighter was affected by compressability and lost aircraft to it. The P-38 was ONLY the first and most public.

The shock wave location was primarily the wing both because (it affected the tail by disrupting flow or by making that flow turbulent) it was affected first (being in front) and the acceleration of the air over the wing to produce lift. The P-38 with it's higher lift "profile" made it prone to shock stall at a lower speed than that of a laminar "profile". The eliptical wing on the Spit varied the location along the wing where the shock-stall formed delaying it's effects.

The Major factor on all WWII fighters encountering the effects of compresability, was simply to go to a lower altitude where the air will slow them down and allow the condition to dissapate.

Originally Posted by wmaxt

According to Lockheed the P-38 is .68 but evedence suggests that .70 was reachable when conditions were right the P-51D is rated closer to .80.

An interesting fact is that as the Mach is achieved Stabilizer/elevator effectivness is lost. The key to controlable supersonic speed was twofold a) lose the prop and b) an all flying stabilator to eliminate the shock-stall that rendered the elevator useless.

Figures I have are 0.84 for for the P-51B, 0.82 for the P-51D because of the slightly thicker wing and the bubble canopy. I also recently found the mach number of the Bf-109G was about 0.78, the 109F was about 0.80.

The P-38 mach number is so low because it has a very thick (by proportions) conventional wing with the maximum chord well toward the front. Also, the shock wave from the wings and fuselage directly impead the function of the tail fin. Basically the P-38 was designed before any knowlege of mach was understood, and they just got unlucky in this one respect. Raising the tail plane to the top of the fin might have relieved a lot of this problem.

=S=

Lunatic

[delcyros]

Thanks Lunatic!

I found the Critical Mach number of the Yak-3 to be rated at 0.76 (OKM tested). The wing of the Mustang has some advantages, no doubt. The elliptical wing of the Spitfire and the swept back wing of Me-262/Me-163 were also very beneficial. I found some interesting calculations of the Ho-229 design: The V1 (unpowered glider) had a critical Mach number of 0.84, but the V-2 has to be redesigned because of the Jumo 004, which did not fit in the airframe. Horten estimated a critical Mach number of 0.79, so it´s recorded top speed of 607 mp/h seems to me suspicious. Depends on the altitude.

[wmaxt]

Originally Posted by RG_Lunatic

Figures I have are 0.84 for for the P-51B, 0.82 for the P-51D because of the slightly thicker wing and the bubble canopy. I also recently found the mach number of the Bf-109G was about 0.78, the 109F was about 0.80.

The P-38 mach number is so low because it has a very thick (by proportions) conventional wing with the maximum chord well toward the front. Also, the shock wave from the wings and fuselage directly impead the function of the tail fin. Basically the P-38 was designed before any knowlege of mach was understood, and they just got unlucky in this one respect. Raising the tail plane to the top of the fin might have relieved a lot of this problem.

=S=

Lunatic

Lockheed built 2 aircraft with raised tails one was at 33". There were no noticable differences in the compressability effects on the aircraft or the stabilizer/elevator. Source: Warren Bodie

33 inches would not be enough. It would need to be something like 60 inches higher than the wing plane to avoid the shockwave.

[redcoat]

going off memory
The piston engined aircraft with the highest Mach speed is a late model Spitfire, which in the hands of a test pilot reached Mach 0.98 in a dive, though the aircraft almost fell apart, and its propellor fell off Fortunately the pilot managed to land OK

Originally Posted by redcoat

going off memory
The piston engined aircraft with the highest Mach speed is a late model Spitfire, which in the hands of a test pilot reached Mach 0.98 in a dive, though the aircraft almost fell apart, and its propellor fell off Fortunately the pilot managed to land OK

Umm, in test trim (no guns, pitot tube, etc.., all such holes covered over) the Spitfire was able to dive to 0.89 mach and make a full recovery. In combat trim this would probably be more like 0.83-0.85.

=S=

Lunatic

[KraziKanuK]

Originally Posted by RG_Lunatic

Umm, in test trim (no guns, pitot tube, etc.., all such holes covered over) the Spitfire was able to dive to 0.89 mach and make a full recovery. In combat trim this would probably be more like 0.83-0.85.

=S=

Lunatic

No pitot tube? It carried a pitot comb 14" wide.

Standard practice was to tape over the gun ports. There was also a pitot added at each wing tip. The P-51, tested at the same time, had its guns and radio removed. It had its normal pitot removed, and as on the Spit, one added at each wing tip. The Spit reached M 0.89 while the P-51, M 0.80.