View Full Version : Drag/speed relationship

10th Apr 2006, 07:33

in regard to why modern airlines fly at high altitude one book states "Since drag, in terms of speed, varies only with EAS, and since TAS in relation to EAS increases with altitude, it follows that the higher the altitude the higher the TAS for a given EAS and drag. Hence more miles are covered for the fuel expenditure necessary to produce the thrust to equal the drag". In reference to the drag equation, some books define velocity as TAS (some even use CAS). If TAS is the governing speed within the drag equation, how does this coincide with drag being only a function of EAS and not TAS?

Thanks in advance for any input.

10th Apr 2006, 10:13
doesn't your equation using TAS, use density as well?

If so, there is your answer...

10th Apr 2006, 10:50
Using the drag formula

Drag = Coefficient of drag*(1/2air density*V squared)*surface area.

The part of the formula I have put in brackets is IAS.

As altitude increases air density decreases.

Therefore assuming Cd and surface area remain the same, for the same amount of drag, as air density decreases the V squared must increase to keep the formula balanced. V in this case is TAS.

So for an increase in altitude at a constant IAS and the same drag, the TAS increases.

Old Smokey
10th Apr 2006, 12:04
Methinks that this is an oversimplified discussion. The Low speed drag polars are related to EAS, true, but at higher levels the same EAS will be beyond Mcrit, and total drag will then be the sum of the low speed and the high speed drag polars.

Below the Pressure Height where the selected speed schedule is below Mcrit, the previous arguments hold true, above that, typical flight is at a Mach Number about .03 to .04 above Mcrit, with decreasing CAS/EAS as Pressure Height increases.


Old Smokey