Originally posted by paladinsfo
The aircraft construction was all metal, but because of the flutter problem the control surfaces were metal frames covered with fabric and doped, as on a piper cub....the P-40 and the P-51 all had fabric covered control surfaces....
Several folks tried to fully metalized the aircraft by covering the surfaces with aluminum...the planes were either static displays or may have broken up in flight, but invariably the fabric stayed.
The P-38 was one of the first fighters to be build with an all metal flush riveted skin. If you notice from this picture you'll see that the flight controls are aluminum sheet metal.
Do I need to show more? Oh..why not, showing that you're a fake is fun. Here are some more.
OK, now to address your other assertion...
On the P-38....not leading edge conpression..in a power dive...
Initially all fighter aircraft are run through flight testing before being accepted for combat and the purchase is complete. Part of the test is a power dive from a specified altitude and a defined angle of dive. There is a cruise speed for aircraft and a never exceed speed. In the tests the P-38 was so clean aerodynamically that at 80% power the aircraft could easily exceed the design speed and actually broke the sound barrier, and cause control reversal. Initially two pilots were killed and a third barely escaped before losing the aircrft during flight tests..He ...said that when he tried to pull out of the dive the controls would not respong and the harder he pulled back, the steeper the plane dived....At a later date the same thing happened to the test pilot and for some unexplained reason he pushed forward on the controls and the aircraft climbed out of the dive. Prior to breaking the sound barrier he described the aircraft as shaking so badly that he thought he would break up in mid air then suddenly all was quiet, with virtually no sound in the cockpit....The air speed indicator was pegged full scale. When he recovered from the dive and was suddenly climbing then as he fell below the speed of sound the aircraft again reacted as though it was going to fail.....It was quite a while before they realized exactly what happened..The final write up was still top secret at the end of the war, and became public in the early 50's.... [/B]
The P-38 was one of the first airplanes fast enough to encounter "compressibility" (more properly called shock stall) problems in high altitude, high speed dives. The basic problem was that in a sustained dive from high altitude, speed quickly built to the point that the airflow over parts of the airplane (such as the upper surface of the wing) reached supersonic speeds.
Not that the airplane itself was breaking the sound barrier, but the airflow in certain places was. A shock wave forms. This destroys the lift over that part of the wing. It also causes the air flowing off the wing to affect the tail in an unusual manner: it increases lift at the tail (Which is normally negative--an airplane is balanced by the weight in front of its wings, a down force; the lift of its wings, an up force; the negative lift of its tail, a down force--imagine a teeter/totter).
This loss of lift from the wings, coupled with increased lift from its tail, causes the nose of the airplane to go down. The increased dive angle causes the speed to increase farther. And so on, in a vicious and often fatal circle. The natural response of the pilot is to pull back on the yoke, which normally causes the elevators at the tail to increase the down force at the tail, and brings the nose up to pull out of the dive. But something terrifying happens. As the pilot tries to pull the stick back, the up force on the tail increases. No matter how hard he pulls, the aerodynamic force on the tail pushes harder. The controls have been described as feeling as if they were set in concrete. At this point the airplane is totally out of the pilot's control; there is literally nothing he can do.
The P-38 was not the only airplane to encounter this effect in dives from very high altitudes (where the air is thin), the P-47 and F4U both suffered the same problem. But the P-38 was different. The big radial engine fighters would dive uncontrollably toward the earth until they reached the thicker air at lower altitudes. There two things happened: 1. The speed of sound increases as an inverse function of altitude (that is, the speed of sound goes up as the altitude gets lower); 2. The increased drag of the thick air on their large frontal surfaces would tend to limit further speed increases.
The result was that when the speed of sound went up as the airplane got lower, the shock waves started to dissipate (the airflow over the wings began to fall back below the increased speed of sound), and as the increased drag started to affect the airplane, the speed of the airflow also decreased, and the shock waves dissipated more. Finally the pilot would begin to get some control back, and still pulling back as hard as he could on the stick, would wind up in a screaming zoom climb (unless he was unfortunate enough to have begun the process over mountains high enough to intrude before he reached the thicker air of lower altitudes).
The way in which the P-38 differed was in its extremely "clean" (streamline) design. Its drag was so low that the thicker lower air often (not always, some pilots did survive compressibility dives in P-38's) did not have enough effect for the pilot to regain control in time: the P-38 just dove straight into the ground like an arrow. The problem was magnified by a "flutter" (increasing amplitude vibration) set up in the tail by these excessive speeds, which often caused the tail to come off.
Lockheed and the Air Corps lost a number of test pilots and aircraft trying to understand and solve these problems. The P-38 had taken them into flight regimes unknown (or at best poorly understood) at that time.
A harrowing series of test dives, at progressively steeper angles, was required to plot the boundaries of these effects. The eventual solution included counter balancing and raising the tail of the airplane some 30 inches, and developing high speed dive flaps to control the rate of descent.
And one more fun fact...the F-90 penetration fighter was the first Lockheed aircraft to break the sound barrier – during a 60-degree dive at 900 mph.
ack-ack
