In-game the P51D30 starts losing HP at 5500m which is surprising for me considering it was built for high-altitude escort. I haven’t checked the other variants but I assume they start losing HP at high altitudes too with the exception of the P51H variant probably. In my opinion, it is even more outclassed by BF109 in the usual characteristics other than top speed at altitudes above 6000m. Is this realistic?
All aircraft do. Thinner air means less oxygen to get into the cylinders.
For aircraft with turbochargers/superchargers designers have to design a charger for a specific altitude as well, too high and engine performance at lower altitudes suffers.
Also the less dense the air the less air a prop has to move in order to provide thrust to pull forwards or push. Depending on config of aircraft.
less air for the supercharger to suck in and less air for the props to push against. Yes its realistic.
Superchargers don’t completely counteract the power loss at high altitude, they just lessen it. It’s less “We don’t lose power at high altitude” and more “We lose less power than the other guy”.
This is for the P-51H and is from http://www.wwiiaircraftperformance.org/mustang/p-51h-na-8284-pg12.jpg
You can see where the supercharger stages kick in - high altitude is on the horizontal axis, which is a little confusing 1st tiem you look at it!