Hi there, I have a "homework/test" question which I'd like to ask, but don't think the aviation stackexhange site would be suited for it. Is it okay to post it here? In the math stackexhange site they have a chat room where anyone can post any math related question, and I've found it very convenient!

My question is of course aviation related.

@schn go ahead

@DanHulme Thanks :) I had this question on a test. In level flight, if you fly against the wind and turn 180 degrees (so you're flying with a tailwind), how will this affect the reading on the indicated airspeed if RPM remains constant? Correct answer: it'll remain unchanged. My answer: the IAS will initially decrease and then settle at a constant value.

My reasoning: since the pitot tube measures the dynamic pressure (plus static pressure, although this definitely remains unchanged), and dynamic pressure is proportional to airspeed, I was thinking that the airspeed must be greater when flying against the wind than with it. I was visualising the air molecules hitting the pitot tube harder when flying against the wind than the other way around.

I was also looking at this question, but couldn't see why my answer was incorrect.

On the other hand...maybe one should think of the airspeed, i.e. the TAS, as relative to the air but independent of the air's speed?

@schn yes

the airspeed is faster in a headwind if the groundspeed stays the same

but the groundspeed isn't staying the same here

almost everything aerodynamic (lift, drag, airspeed) is relative to the air

@DanHulme Yeah, I think I understand it mathematically, but still have a hard time shaking off the idea of the air molecules possibly hitting the pitot tube "harder" in a headwind (and thus generating a greater dynamic pressure than what they would if the aircraft flew with a tailwind).

But then again dynamic pressure is independent of the air molecule's speed...

@schn yes, they do hit harder in a headwind, because your IAS is greater

(for the same groundspeed)

@schn Your question would be fine on the main site. Homework questions are well accepted if you explain why you don’t understand. Go ahead and post it if Dan’s response didn’t clear it up for you.

Although it might be marked as a dupe of that other question.

You have to detach from the ground :) You move through the air at some IAS, but the air itself is moving (the wind) and both of those combine to give you speed over the ground.

Thanks for the replies, I think I understand it now.