ORIGINAL: av8tor1977
Engines lose about 2 to 3% power per 1000' of altitude. But what is actually important is what is called "Density Altitude". This is true, or actual altitude corrected for temperature and barometric pressure differences, and is a better indicator of air density and consequently the effect it has on engine power.
Higher altitude, higher temperatures, lower barometric pressure, and high humidity all LOWER air density and LOWER engine performance. In all full size aircraft operating manuals, the actual power percentage loss is stated. Due to both the loss of power AND the loss of effectiveness of the propeller and wings due to the thinner air, a take off at about a 7000' density altitude will be twice as long, and the airplane will climb half as fast.
Short of supercharging, the only help is higher static compression ratio, more nitro, or more cubic inches... Even these things will not improve the losses suffered by the prop and wings however.
AV8TOR
Well I was going to mention density altitude, but that sometimes blurs the discussion for folks...
But yeah it makes a HUGE difference when you figure in especially ambient temperatures. I remember the first time I suggested trying to figure density altitude to my buddy who owned an NHRA car. I explained what the deal was and although he was initially unconvinced, I got an old altimeter from the airport so we could get the pressure altitude and we used an inexpensive set of household type guages for temps and humidity. Then we started figuring the altitudes and setup the engine jetting and timing for that figure as opposed to the actual track elevation. When combined with the data we already had on altitiude changes, it worked great. At that time everyone in the pits made fun of us, now you won't find a good racing team anywhere who doesn't do it.
For any of you who might be interested, full scale stuff is rated at Sea Level, 59 degrees F and 29.92 on the barometer. That applies for both the engine and airframe since as AV8TOR mentioned, density alitutde affects the prop and wing performance as well. Any deviation from those numbers and aircraft performance will change.
Ralph, thanks for the tidbit on temps and retarding the timing, gonna have to play with that a bit more....[8D]