Originally posted by Benny Moore
Clearly P-38 and P-47 suffered so from this all-American phenomenon.
I’m sure you can get an ointment to help relieve that bruised national pride of yours. The P-38 and P-47
did suffer from compressibility effects, and so do every other plane. However several other fighters, including American, were better suited to deal with the effects. The Spitfire had the highest critical Mach number of any WWII piston-engine fighter, much higher than the P-38 and P-47. The 109 and 190 both had higher critical Mach (but not by as much as the Spit) and a “flying-tail” trim system that allowed them to trim out of dives. The P-51’s laminar-flow wing and high critical Mach number made it an excellent diver.
Originally posted by Benny Moore
Incidentally, to those wondering, it is possible to have only one control surface locked up in a compressibilty dive. Much like in a stall, airflow does not affect all control surfaces equally. The P-38's problem was elevator; trans-sonic shockwaves immobilized it. Trim could overcome it (at risk of breaking up, as did Virden), although under some conditions the wing might not have lift even at very high speed.
Again you show little knowledge of the effects of compressibility and local supersonic airflow. When the airflow gets close to transonic speeds the leading edge of a thick wing (like those found on WWII fighters) acts almost as a snow-plow deflecting the air away from the wing and disrupting the normal flow of air over the wing.
Behind the shock wave the air flow is turbulent, including the boundary layer, and thus renders conventional control surfaces ineffective. The turbulent air flow can cause the control surfaces to vibrate or even oscillate between its extremes of deflection like the ailerons on the P-47 and 109. However, the important fact is that since conventional ailerons and elevators work by changing the curvature of the whole airfoil (wing) they don’t work when the airflow is separated and turbulent. For all intents and purposes the wing is stalled behind the region where the shock impinges on the surface of the wing. You can move the control surfaces to their maximum deflection and they will do nothing. On some planes the controls become lighter after the wing airflow enters compressibility, still the controls remain ineffective.
Originally posted by Murdr
I related what Kelsey's AAR of the event was. He input full ailerons and rudders, something immediately broke on the plane, and he found himself in an inverted flat spin, and was stuck there momentarily with the canopy pinned shut until G force direction changed and pitched him out of the cockpit.
If you didn't want further details from the incident, I guess you shouldn't have brought it up.
If you have this AAR I’d appreciate if you could post it in its entirety. Right now we have one source that says the tail was sheared off by compressibility effects, two guys that assume and speculate that the pilot simply pulled too many G’s pulling out of the dive, and you that say the pilot broke the plane by applying full rudder and ailerons.
Originally posted by Knegel
You realy think they shot each other down at mach 0,73 in 1943?? The 109´s got stiff even more early, there was nothing to fear at this speed than the speed.
At Mach 0.73 there was nothing to be afraid of in a Spitfire, 109 or several other fighters. The controls were heavy in the 109 yes, but not beyond what a normal man could pull with two hands. Like I’ve explained to Benny above the problem with compressibility effects is not that controls get heavy, it’s that the controls become ineffective regardless of control input. The 109 would only be trying to get away from the P-47; the P-47 was the plane that was trying to shoot. To answer your question: no I do not think they shot each other down at Mach 0.73 in 1943. Not often anyway. The shoot-downs would occur at significantly lower speeds where a higher P-47 uses its great mass and HP to out accelerate the 109 and catch it before speed increased too much.
Originally posted by Knegel
The its not only the relative thickness, the aspecratio and airfoil is a majorfactor here. But as i wrote before, the main wing in general had a greater relative thickness anyway, as result the planes started to tuck down before they got absolutly stiff, a uptrimmed plane or a early reaction of the pilot would stop the plane to get into the real stiffness.
Aspect ratio and airfoil are very minor factors in determining the speed at which the effects of compressibility start. However airfoil is a major factor in the severity and nature of the compressibility effects. And again you speak about “stiffness” … as I’ve explained above the problem was not “stiffness”, and now your argument has turned into a “just don’t fly that fast” solution to the P-47’s dive problems.
Originally posted by Knegel
Nowhere i wrote that shockwaves ripped off the tail, the shockwaves made the P38 stiff and i wrote that even the P38 normaly could get recovered in lower alts!!
My mistake, I misread you.
Originally posted by Knegel
I would say the results in war show that it wasnt fragile, but of course fragile is relative!
The P38 was very successfull in the war and you dont will find many combat reports where it crashed due to structural limits. In my book fragile is a plane that get damaged dangerus easy, while normal combat flight manouvers. The P38 was not in this class.
I agree that “fragile” is a relative and subjective term. However I do agree with NASA’s point of view (from NASA Facts):
“The need for transonic research airplanes grew out of two conditions that existed in the early 1940s. One was the absence of accurate wind tunnel data for the speed range from roughly Mach 0.8 to 1.2. The other was the fact that fighter aircraft like the P-38 "Lightning" were approaching these speeds in dives and breaking apart from the effects of compressibility—increased density and disturbed airflow as the speed approached that of sound, creating shock waves.”
Also from p-38online:
“A typical dive of the P-38 from high altitudes would always experience compressibility. Starting from 36,000 ft., the P-38 would rapidly approach the Mach .675 (445 mph true airspeed). At this point, the airflow going over the wing exceeds Mach 1. A shockwave is created, thus breaking up the airflow equaling a loss of lift. The shockwave destroys the pressure difference between the upper and lower wing, and disrupts the ability for the aircraft to sustain flight. As the lift decreases, the airflow moving back from the wing also changes in its form and pattern. Normal downwash aft of the wing towards the tail begins to deteriorate. The airflow across the tail shifts from normal to a condition where there is now a greater upload, of lifting force, on the tail itself. With the greater uploading force applied to the tail, the nose of the aircraft wants to nose down even more, which creates a steeper and faster dive.
As the aircraft approaches the vertical line, it begins to tuck under and starts a high-speed outside loop. At this point, the airframe is at the greatest point of structural failure. When the angle of attack increases during the dive, it also increases for the tail. The resulting effect is that the pilot cannot move the controls because tremendous force is required to operate the aircraft. The pilot is simply a passenger during this period. Shockwaves become shock fronts, which decrease the lift no matter what the pilot tries to do.
Instead of smooth airflow over the wing, it is extremely turbulent, and strikes the tail with great force. The aircraft can only recover when it enters lower, denser atmosphere lower to the ground.
The solution to the problem was in understanding that the speed of sound changes with the altitude. At sea level, it is 764 mph, while at 36,000 ft. it is 660 mph. An aircraft moving at 540 mph at 36,000 ft. is much higher in the compressibility zone. The same speed at sea level results in the aircraft being exposed to lower effects of compressibility, and will respond to pilot controls. The dive recovery flap was a solution to this problem. In the ETO, German pilots would dive out of trouble because they knew the P-38 pilots would not follow. This greatly reduced the effectiveness of the aircraft in normal battle conditions. The NACA tested the flaps in high-speed wind tunnels at the Ames Laboratory. They tried several locations before discovering that when the flaps were positioned just aft of the trailing edge of the wings, it showed definite improvements. The flaps were finally positioned beneath the wings outboard of the booms, and just aft of the main structural beam. The pilots had a button on the yoke, and would simply activate the flap just prior to entering a dive.”
