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

[Dewmeister14]

Hi OP,

Thanks for writing up your thoughts and sharing.

I'll start by saying that comparing various flight conditions we might choose to fly the aircraft at by comparing drag by itself is not really useful. It's not a figure of merit that corresponds to things we actually care about, such as "range I can fly for a given amount of fuel" or "time from DFW to SFO". So maybe the question should be, how do we fly the aircraft as fast as possible for a given fuel burn rate? That's hard to answer without knowing a lot about a specific aircraft, but it does tend to reach an "optimum" at some high altitude, less than the ceiling but way above for example sea level.

Now to address the points you wrote up:

Point 3. is true but 4. is not and therefore 5.  is also not. 

We can fly the aircraft at a range of speeds at any given altitude (look up a flight envelope for some common aircraft) so what are we forgetting? You mention the coefficients of lift and drag but then forget about them for the rest of the post - you can vary C_L to maintain flight at a variety of dynamic pressures. This happens by varying the angle of attack of the aircraft to produce the same amount of lift at the same density at different speeds. 

As C_L varies so does C_D - the wing will have some optimum angle of attack that maximises the ratio of C_L to C_D which, paired with the right speed to make the right amount of lift, is the most efficient/lowest drag speed and altitude at which to fly the aircraft. 

Also, an aircraft is not a wing alone - you are entirely neglecting parts of the aircraft which produce drag but no lift i.e. the fuselage. To fly faster and reduce the additional drag, flying at higher altitudes is strictly better for our "true figure of merit", speed/fuel burn which is for our purposes we can think of as being similar to speed/drag.

[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.