Hi all,

I have the following KDR which I'm struggling to answer:

Effect of changes in flight conditions on total drag - Part 61 MOS Unit 1.3.2 CADA, 2.3.2
2.3.2 State the effect on total drag resulting from changes in IAS, aircraft weight and height.

I am confident in answering the effects from changes in IAS and Weight but I am unsure in regards to height, am I correct in saying that the total drag decreases with height due to the reduction in density?

I have tried to research online but have found conflicting and confusing information. Could someone help me out?!

Many thanks,
Carly

The total reaction and therefore lift and drag depend upon the IAS. For a given IAS the associated TAS will be higher if altitude increases. Therefore the same drag at high altitude will be accompanied by a higher TAS. It's really exactly the same argument when considering lift at high altitude.

If I may add a comment to Bob's answer.

Although a bit beyond PPL/CPL level, it is worth keeping in mind that Cd and Cl also involve two other non-dimensional quantities which, in general, are ignored for low level, low speed flight.

These quantities are Mach Number (which you would have heard about) and Reynolds Number (which you probably haven't). There are some straightforward explanations in the NASA websites, eg

www.grc.nasa.gov/www/k-12/airplane/dragsphere.html

www.grc.nasa.gov/www/k-12/airplane/dragco.html

You ought not to see any questions about these at this level, but it is useful to keep in the back of the mind if you end up flying jets.

Thanks Bob! That fixes the issue I was having with TAS and IAS with increased height

And John.Heddles thanks for the additional info, I've heard of the terms but not familiar with the meanings, I'll have a read!

Cheers