Originally posted by Viking
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.
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.
Hi,
The controlls not always get ineffective, they turn to work in the oposide way, while the elevator in general remain effective for a longer time, but the stickvariations and therefor forces, to counter the beginning "tuck down" was to be to much!
Afaik the main ptoblem for the P38 was early shockwaves around the middle fuselage section(very high relative thickness), which did hit the tailwing and did lead to the rather early loss of manouverability. This in combination with the extreme acceleration did need a experienced pilot, similar like in the german jets, but at a lower speed. This dont make the plane not combat worthy as a escor plane. The escort planes dont had to follow the enemy down, they had to stay high anyway to drag other enemys away and thats what the P38´s did rather successfull, despite their ugly engine problems in cold air.
Aspectratio is a major aspect for compressibility and airfoil as well!!
Most planes with +1mach speed have a very smal aspectratio! This is so, cause while compressibility appear, the airmasses need to get shifted sideard to the wingtip. A smal aspectratio can do this better than a high aspectratio wing.
A high lift airfoil(extreme asymetic) will have the "tuck down" much more than a symetrical airfoil. And the laminar airfoil also work better than a high lift airfoil.
It dont matter why a plane get stiff, it only matter than the pilot cant fight under this conditions. The Me109 got stiff around the roll axe good below compressibility did happen.
And actually i dont wanna say the P47 had a higher critical mach, i only wanna say that the RAE conclusions got proven as wrong!!
The RAE also fount to be the Hurri a better fighter than the 109 and their conclusion was the 109 was obsolete in 1943.
Originally posted by Viking
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.”
Nothing special here, the 109 and FW190 also was planes like the P38. The different to other planes was: They was by easy able to reach +mach 0,75.
I remember a story of a finnish pilot, who couldnt understand why someone create a plane, that can dive so fast without to make it strong enough and manouverable enough and he was talking about the 109 after a +700km/h IAS dive.
The germans also worked on better airfoils and specialy wingforms(shifted wing and/or extreme smal wing aspectratio in the 262, 163 and he162) cause they did encounter exact the same problems, maybe a bit later and with a less fast acceleration, so more time to react.
Originally posted by Viking
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.
This is well known, but the interesting thing is that the P38 in the vertical dive have a much greater mach than 0,7, probably rather mach 0,85, and although its not manouverable anymore, it generally dont break appart.
Originally posted by Viking
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.”
Yes, the different mach with different temperature is known, intersting is, 36000ft is roundabout the service ceiling of the 109G and 190A in 1943 and while a escort mission the planes rarely did fly above 30000ft.
My conclusion still remain:
The P47C/D was a very good high alt fighter in 1943, the problem of a relative low critical mach number was nothing special and was only one (not very) weak point, otherwise it did outperform the 109G and specialy 190A in 30k by easy.
The P38 wasnt that good in high alt in 1943, but mainly due to the engine problems, not cause the low critical mach, which still is good above the Vmax. History show, it was still good enough as a escort plane, to drag the enemy away, while its engine problems and extreme mach limitation dont made it to an air superiority fighter in this altitude(at least not in europe) in 1943.
Greetings,
Knegel
