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The P-80 had its top speed at sea level, but the Me 262 achieved its top speed at 6km altitude. What were the reasons for this? Was it that the engine was designed for such an altitude to deal with high altitude B-17 raids? And if so, how specifically did they tune the engine for altitude?

from USAAF Wartime Tactical Planning Characteristics & Performance Charts (Page 44 of the PDF) P-80 top speed

from post-war British test flights of Me 262 (Page 5 of the PDF) me 262 top speed

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  • $\begingroup$ Where do you see referenced the P-80's top speed occurring at sea level and the Me-262's at 6km? Neither of those sounds intuitively correct, but I'd be interested to see what source they're from. $\endgroup$
    – Ralph J
    Commented Oct 21, 2023 at 22:25
  • $\begingroup$ @Ralph J I updated the post to include sources $\endgroup$ Commented Oct 21, 2023 at 22:48
  • $\begingroup$ Are the quoted speeds CAS, EAS, or TAS? Are the two data sets using the same airspeed? It could be the difference. $\endgroup$
    – Gerry
    Commented Oct 22, 2023 at 2:13
  • $\begingroup$ @Gerry They're both definitely TAS. No doubt about it. $\endgroup$ Commented Oct 22, 2023 at 2:25

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It has to do with the engine design, as follows.

The higher a plane flies, the faster it can go for the same engine power. This is because as the air thins out, drag decreases. However, as the air thins out, the amount of oxygen available to burn the fuel decreases and so the engine puts out less power the higher it flies.

There comes a crossover point where the drag decrease with increasing altitude is cancelled by the power decrease with increasing altitude, This is called the critical altitude. If you trim the plane to fly higher than this, you go slower and at some point you get boxed in by the stall speed and you cannot fly higher.

For a turbojet, you add altitude capacity by adding compressor stages to jam more air into the engine, which pushes the critical altitude up. So the performance difference between those two planes boils down to how much the compressor stage of each engine boosts the pressure in the combustors.

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  • $\begingroup$ Thank you. If you may, there is one question I still have. Even though you can increase altitude performance by increasing total pressure ratio, the general shape of the alt vs thrust curve would stay the same, wouldn't it? Being max at sea level and gradually decreasing up to altitude? Adding compressor stages would only shift the curve to the right, wouldn't it? $\endgroup$ Commented Oct 22, 2023 at 4:33
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I am no expert, but this is how I understand it:

All air breathing engines lose power as you increase altitude due to decreased air density. This also means leases drag at high altitudes, but the loss of power was greater than the loss of drag.

Early jet fighters like the P-80 and F-86 had their max speed at sea level because of this.

Engine design eventually changed so that the rated power was limited or restricted at lower altitudes. This is called a “Flat Rated” engine. Aircraft with flat rated engines could maintain this artificial rated max power to higher altitudes which meant their max speed was no longer at sea level.

The Me 262 may have had “flat rated” engines, but I suspect there may have been some other limitations that resulted in slightly less speed at lower altitudes.

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