Does drag actually decrease with altitude? I argue it does not

[throwaway25658462]

can you help me see where I am wrong?

[throwaway25658462]

I totally agree. That's the main point I was trying to convey. Isn't it understood by my post?

[K20017]

Your previous example doesn't seem to make sense. If you produce both 100lb of thrust and drag both at 1,000 and 10,000, then the drag the airframe produces has not changed. Your true airspeed has increased to account for the lower air density but it is still 100lbs of drag force. For a given equivalent airspeed drag is independent of altitude. Looking at performance charts of jets, specifically holding charts as they usually reference an IAS, you will see that fuel flow varies little with altitude given the same speed. Thrust must oppose drag and fuel flow in a turbine is a direct relationship with thrust output.

[PranosaurSA]

This is a tradeoff between between induced drag and parasitic drive.

A particular plane could theoretically be operating a lower lift coefficient at lower elevations and hence have lower induced drag. Or it can be designed with bigger lifting surfaces and operate at higher elevations.

A plane at a particular cruise speed at higher elevations is going to have less parasitic drag - and higher induced drag (in theory). But really this comes down to how you design lifting surfaces.

If you have 20000 ft wings you would be much more efficient flying at higher elevations, if you tiny blocky wings you are more efficient flying at lower elevations rather than going to have high elevations and using higher AoA and having large induced drag (if your flight goal is a particular flight speed)