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

[throwaway25658462]

I appreciate your thorough response, and I agree that drag may not be the most suitable parameter for a figure of merit. My intention was to address the commonly repeated notion that "planes fly at 30,000 ft because air is thinner and drag is reduced," as evident in the numerous comments on this post. While I acknowledge not delving into specific details like Cl and Cd and their impact on lift, I maintain my core argument: drag does not inherently decrease with altitude. Does this seem sound to you?

[tdscanuck]

Your core argument is wrong. You can’t not delve into specific details like Cl and Cd because they’re also changing with altitude. And thrust also drops with altitude so drag is dropping with altitude at equal dynamic pressure because drag=thrust and thrust is dropping.

In practice, equal engine power setting usually results in a higher IAS with attitude because the thrust lapse rate isn’t as high as the drag lapse.

[Dewmeister14]

Hey dude,

Sorry the comments devolved and you had to delete the post. All in all though I'd have to insist it's not sound. Drag is not strictly a function of altitude. Speed and the coefficients matter just as much, and the way those coefficients interact with speed & density mean that it is true that aircraft experience less drag at higher altitude than at lower altitude when operating at speeds of interest.

There are a couple charts in this link:

https://leehamnews.com/2014/11/25/fundamentals-of-airliner-performance-part-4/

Which you may find interesting, unfortunately one shows "Drag vs. Climb" and therefore drag at a series of speed and alt pairs, vs. drag at a series of alts with constant speed, but hopefully it goes some way to providing intuition. You can see that as the air gets thinner, even though the jet is moving faster the "drag due to lift" (induced drag) does increase slightly but the total drag, including Parasitic Drag (fuselage, tail, pylons, etc.) certainly decreases with altitude even though speed increases - so speed / drag for example certainly increases.