Warbird Information Exchange

... and how come, only the P-38 seem to have it? Is the p-38 the only warbird that has a great diving speed? What about the corsair, or the dauntless, or the stuka - which were pretty fast in a dive, right?


cheers!

People with more technical knowledge re: aerodynamics, will provide you with the adequate answer.

A dive, was not made at full speed. As a matter of fact, the Stuka dove at slower speeds in order to be accurate in the delivery of the ordnance.

Same would go for the Dauntless.

Compressibility, when it came to the P-38, but also the Mustang and other WW-II fighters, was encountered when the diving speed was so high that the mass of air being encountered by the airplane, could not "part" fast enough for the fuselage to move through it, due to the shape of the airframe.

If I am not totally mistaken in this, the compressibility came usually very close to the zone where the airplane would have gone supersonic (which of course, varies with altitude and speed of the dive).

Structurally, some airplanes survived these experiences, but there were clearly limitations as to the operating speeds, in order to avoid precisely, these problems.

Would the experts please, provide the why and how?

Saludos,


Tulio

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All aircraft have this issue...it is a matter of fluid dynamics, rather than one particular aircraft.

Simply put, as these aircraft get closer and closer to M=1, the air molecues reach their maximum density and start forming shockwaves over the top of the aircraft surfaces. These shockwaves could go so far as to deny/blank out airflow over the control surfaces.

I think you'll find that all the major fighters of WW2 have this issue (I have personally read reports from Mustangs and Jugs, but that's just off the top of my head), and this is the root of the 'flying tail' design (the all-moving horizontal stabilizer, rather than just a moveable elevator surface) that is on F-86.

The Stuka, BTW, was not fast in a dive....

I was at War Eagles outside of El Paso once upon a time and who should walk in but Scott Crossfield ! We were discussing NACA P-51 dive tests as one of the museum Mustangs was in the NACA markings. Mr. Crossfield told me he flew some of those dive tests and he doubted the claims that were made regarding supersonic dives during the war. He told me that he could actually see the shockwave forming off of the wingtip in the transonic range but couldn't quite get beyond it and the instrumentation in the aircraft bore that out.

I later gave him a tour of the A-26 they have there, inside and out. He had never flown the type and was impressed by the plane's capabilities.

Made my day !

Thanks for clearing that up....

For a while there, seeing that the P-38 was the only one to be outfitted with "special" dive flaps (in europe) in the middle of the war, I thought that the problem was isolated to them.



cheers!

Later P-47s also had compressibility dive flaps. Some aircraft like the Wildcat had a structural dive limitation that was faster than they could achieve pointed straight down at full power (so I am told). The Dauntless and Stuka had dive flaps to prevent them from going too fast in a dive- not that compressability would likely be an issue on those aircraft due to their higher drag configurations (Stuka had fixed landing gear for instance). The dive flaps slowed the dive enough to give sufficient time to aim and gave the pilots a repeatable dive speed so they could consistantly hit their targets.

The P-38 would start a tuckunder at high mach and the pilot was unable to pull out. Some pilots found that they could push out of it but I gotta believe that was brutal ( not as bad as the alternative ). The dive flaps fitted to the P-38 were more of a speedbrake that dropped out of the bottom of the wing.

http://p-38online.com/levier.html