Since you all know so much

[DipStick]

Very interesting... (taking from Heretik's link)

- The Me 109 was dived to Mach 0.79 in instrumented tests. Slightly modified, it was even dived to Mach 0.80, and the problems experimented there weren't due to compressility, but due to aileron overbalancing. Compare this to Supermarine Spitfire, which achieved dive speeds well above those of any other WW2 fighter, getting to Mach 0.89 on one occasion. P-51 and Fw 190 achieved about Mach 0.80. The P-47 had the lowest permissible Mach number of these aircraft. Test pilot Eric Brown observed it became uncontrollable at Mach 0.73, and "analysis showed that a dive to M=0.74 would almost certainly be a 'graveyard dive'."

And some people gripe about how well the Spit dives.

[Ack-Ack]

Originally posted by TheManx
I'm not sure about this, because in the 38 you can trim clear even though you're in compression. Unless it's just stick force as well. Both planes from my memory seem to lose all control once a certain speed is exceeded, and must be brought back through trimming.

The compressability you experienced in the P-38 is different than the compression you faced in the bf109.  

Compressability is the phenomena where the air flow over the trailing edge exceeds  critical mach.

Found something interesting, there were some models of the Spitfire that faced a similiar problem to compressability.  At high speeds the ailerons could apply more torque than the Spitfire's thin wings could handle, and the entire wing would twist in the opposite direction. This meant that the plane would roll in the direction opposite to what the pilot expected, and led to a number of accidents.  This wasn't noticed until later model Spitfires like the Mk.IX started to appear, because earlier models weren't fast enough. This was solved by adding considerable strength to the wings, and was wholely cured when the Mk.XIV was introduced.

Never heard about that problem with Spitfires before, anyone know if it's true?

But Karnak was right about the bf109.  The controls were too weak and at higher speeds the pilot couldn't  move the controls due to the increased airflow over the flight control surfaces.  A lot in here confuse this with compressability.



ack-ack

[detch01]

Supersonic airflow over the top of a wing and its accompanying shockwave occurs at slower airspeeds  in aircraft with wings that have a higher thickness/cord ratio.  What is commonly referred to as compression is the result of the affects of a supersonic shockwave on wing lift-generating charactaristics (the centre of pressure generally moves aft). The primary reason the Spitfire was able to safely achieve higher mach numbers than most of its contemporaries is that it had a thinner wing (lower thickness/cord ratio).
I don't have the data in front of me to quote but I suspect the B109 series of aircraft, like the P-38 and the P-47 had a relatively thick wing section.



asw

[Widewing]

Originally posted by DipStick
Very interesting... (taking from Heretik's link)

- The Me 109 was dived to Mach 0.79 in instrumented tests. Slightly modified, it was even dived to Mach 0.80, and the problems experimented there weren't due to compressility, but due to aileron overbalancing. Compare this to Supermarine Spitfire, which achieved dive speeds well above those of any other WW2 fighter, getting to Mach 0.89 on one occasion. P-51 and Fw 190 achieved about Mach 0.80. The P-47 had the lowest permissible Mach number of these aircraft. Test pilot Eric Brown observed it became uncontrollable at Mach 0.73, and "analysis showed that a dive to M=0.74 would almost certainly be a 'graveyard dive'."

And some people gripe about how well the Spit dives.

A few facts that will place the above quote into its proper light.

A Spitfire (post war F Mk.22 IIRC) did flirt with Mach .90, but only after it had thrown its prop due to the extreme drag rise. In this case the test pilot landed safely deadstick. I recall that the Spit was damaged beyond repair.

As for the P-47, Eric Brown's "analysis" is clearly faulty. Curtiss Test Pilot Herb Fisher conducted a series of maximum speed dives in a P-47, testing prototype transonic props.

Fisher undertook a long and risky flight test program that incorporated high Mach dives from high altitudes. Typically, Fisher would climb above 35,000 ft. He would then push over into a steep dive, allowing his airspeed to build beyond 560 mph (true airspeed). He would then execute a pullout at 18,000 ft. Several of these dives resulted in speeds of Mach .83. However, that was as fast as the P-47 could go due to the drag rise associated with its propeller.



Here's one of Herb's charts showing a Mach .79 dive, that according to Brown, should have been a "graveyard dive".



My regards,

Widewing

[detch01]

Here's a link for WW's data (interesting stuff) go here

Context is a wonderful thing .

Cheers,
asw

[Murdr]

Originally posted by Ack-Ack
The compressability you experienced in the P-38 is different than the compression you faced in the bf109.  

I might add that the foil shape not only determines when shockwaves form (critical mach) but also where on the foil.  In the 38 they form fore of center causing the tuck under effect.  The 38 dive flaps change the shape and move the formation aft, aiding in recovery.  In the 109 the formation doesnt sound like its as far fore as in the P38, but instead it is located where it distrupts the airfow over the control surfaces.