Altitude Compensation

[David M Parrish]

>From:             Mark Pitts <saxon at zymurgy.org>

> The point is even tho' the density drops, if you compress that air to sea level density, there is less oxygen in it, thus you need to lean out at altitude.
> Sorry to go back to my model airplanes, but for the glider tugs (which ran 120cc petrol engines, with electronic ignition) we had to solder an arm on the end of the needle so that we could fit a ser
> o and adjust mixture in flight, otherwise we lost so much power, we couldnt pull the glider any higher. And we are only talking 5000' Like a mile or so up!
> At ground level the tugs would climb out at about 45 degrees, with anything on the back..

No, I don't think so. If you took air from 10K and compressed it to 
sea level, the ratios of the components would be identical or nearly 
identical. The reason aircraft have to lean at altitude is (if not 
alititude compensated) is lower air density and therefore lower O2. 

Mechanical injection systems vary fuel delivery pretty much by 
throttle position, so you have to manually reduce the fuel delivered 
as the pressure (and mass per volume) of air goes down.

(I may be on thin ice here.) Carbs draw fuel based on the velocity of 
air flowing past the venturi, and not the actual mass, so they also 
richen with altitude.

---
David Parrish
Future aviator.
As soon as I finish building my plane.