How is the total drag variation with velocity related to altitude?

Question

Here in the question given I am confused how to relate the altitude with the given plot of total drag and velocity.

Related: Why is the thrust/drag increase much lower at a lower FL with weight the only variable? — , Sep 22 '19 at 16:48

score 3 · Accepted Answer · edited Sep 22 '19 at 18:59

3

At higher altitude, density is lower. For the same true airspeed and the same drag coefficient, higher altitude will have less total drag.

However, there are factors affecting drag coefficient itself:

Due to the lower density, the lift coefficient must increase for the same TAS. This results in increased induced drag.
Decreasing density also decreases the Reynolds number, which increases the skin friction drag, especially for speeds as low as the ones shown in your graph.

Therefore, it can be expected that the trend will invert below a critical speed.

The answer is h₂ > h₁ from the above rationale.

edited Sep 22 '19 at 18:59

fooot

answered Sep 22 '19 at 16:30

JZYL

I thought the exact same reason... but the next question asking about the stall angle confused me. Now I got it. Thanks. – Shishir Maurya Sep 22 '19 at 17:07

1 Answers1