High altitude, high speed, air density and its effect on drag

[Joel]

I'm curious to know what the correlation is with high altitude, lower density and decreased drag. Has anyone done a study and backed it up with figures.
Using myself as the engine, I did once cycle at ( not to! )the north rim of the Grand Canyon. Although I ran out of puff very easily ( I believe the altitude is about 8500feet) I did seem to be able to push the bike along with less effort required.
So although the engines power output suffers, is there a significant reduction in drag which offsets it ?

[Paolo Castellano]

Originally Posted by Joel

I'm curious to know what the correlation is with high altitude, lower density and decreased drag. Has anyone done a study and backed it up with figures.
Using myself as the engine, I did once cycle at ( not to! )the north rim of the Grand Canyon. Although I ran out of puff very easily ( I believe the altitude is about 8500feet) I did seem to be able to push the bike along with less effort required.
So although the engines power output suffers, is there a significant reduction in drag which offsets it ?

Joel, I would just find a 4 mile stretch of straight highway and see how fast you can go. Then compare it to sea level top speed. That should be a pretty good real world test.

[Joel]

Thanks, Paulo, its a bit of an academic question for me now, because high altitude is something that isnt a problem in the UK ! But my recent mile run in Nevada sparked my interest. So really to try to answer my own question and for anyone who is interested I have found out this much.............
There is a ( very approximately ) 0.4 lbs sq in drop in pressure per 1000' increase in elevation. As the pressure drops, density of air decreases. But as the temperature with increasing altitude drops, this increases density. However the net effect is that dropping pressure reduces density more than temp increases it. I spent a couple of hours browsing for actual figures but just couldn't find anything.
The most relevant information was found in the world of cycling, The max distance covered in the hour is around 56km. In Mexico City which is around 7500ft elevation, the increase in speed was seen to be about 1.7kph, or about 3%. Given that the cyclist's output after extended training at higher altitude would be similar to one at sea level....maybe 3% is a rough guide to the reduction in drag at that altitude.
Thats all for tonight. Hope this hasnt put anyone to sleep !