AOA and relative wind

[LongRoadBob]

Every time I think I'm getting it, I think a little more and it seems like I'm not.

As a student, I imagine there are others like me that read the definition of AOA, and that the ONLY factor in stalls is AOA, and think "ok, but..." and get the feeling it is a symantics thing. I think I do understand that aircraft can stall at any speed, at any angle, but here I go again...confused.

I think the problem is in all the definitions I'm reading, they are very basic definitions. I have trouble figuring out in different situations what the relative wind is, where it is coming from.

In a recent thread here about wind sheer, someone linked to a really helpful site that talked about damping during the beginning of a roll. That the low wing in a roll is moving forward AND down, so it is increasing its AOA because relative wind is back and up, with the opposite happening on the high wing. But after that? If one keeps holding the ailerons to bank, where and what is relative wind doing then? From my theory, I believe the low wing is slowing down and has more drag than the higher, and this turn the plane. Though the ailerons are not two whole wing, the up aileron on the down wing means less AOA which I thought meant less drag, the down aileron on the upside wing should mean more lift and so drag as well??

During climb or descent, what is the relative wind (if it is an increase or decrease in thrust, but level attitude?) .

Maybe my questions can give a clue to experienced and knowledgable pilots here as to where I'm getting off track?

I see it stated that the relative wind is parallel to the flight path. Is this a constant?
Also just realized that the wing is slightly twisted by design and that AOA is sometimes considered to be at the base of the wing. Any and all help appreciated.

 

[dtuuri]

Every time I think I'm getting it, I think a little more and it seems like I'm not.

...the low wing in a roll is moving forward AND down, so it is increasing its AOA because relative wind is back and up, with the opposite happening on the high wing. But after that? If one keeps holding the ailerons to bank, where and what is relative wind doing then? From my theory, I believe the low wing is slowing down and has more drag than the higher, and this turn the plane. Though the ailerons are not two whole wing, the up aileron on the down wing means less AOA which I thought meant less drag, the down aileron on the upside wing should mean more lift and so drag as well??

Relative wind is caused by the motion through the air--not moving, no relative wind, move forward, wind rearward, etc. Same for rolling action. Ailerons go up further to somewhat offset the drag from the down moving one, but still doesn't completely balance the adverse drag. This causes side-slip. The airflow across the fuselage in a slip is disturbed as it reaches the trailing wing root and induces a stall there despite the fact that wing has a lower angle of attack. Some analysts focus on the downward deflected aileron as the culprit and make the case that the wing is really at a higher angle of attack because of it. I don't buy into that theory because dihedral increases the forward wing's angle of attack and reduces the trailing wing's angle of attack in a slip which I believe to have the greater overall effect. Plus, I've neutralized the ailerons prior to the stall break in a slipping stall and there has been no difference in the stall behavior, thus ruling out the aileron deflection as cause. Interference drag from the fuselage is the only plausible explanation. It is also William Kershner's conclusion as set forth in his Advanced Commercial Pilot handbooks.

EDIT: During a level climb and level descent the relative wind comes from a different positive angle of attack. The wing must make the same (roughly) amount of lift in each case for unaccelerated flight, so, since the climb itself in a level attitude would cause less positive angle of attack, at the lessened AoA the airspeed would need to be increased to make the same amount of lift. Alternatively, the speed would have to be slower in a level descent at the increased positive AoA caused by going downward. Thrust would be the control input to achieve these conditions.

dtuuri

 

[LongRoadBob]

Thanks. You put a lot of good info into a small space, and I'm going to go over it a few more times at least. It made sense to me, thanks.

 

[dtuuri]

Thanks. You put a lot of good info into a small space, and I'm going to go over it a few more times at least. It made sense to me, thanks.

Could you tickle my "Like" button?

dtuuri

 

[LongRoadBob]

I'd like to add what I see as my weakest point, I cannot seem to get a good mental picture of the wind mass movement..tailwind effect and headwind.
I had it almost (I thought) and was able to answer quiz questions about airspeed indicator in a tailwind as opposed to headwing (I belive my answer was that the airspeed indicator would read the same given the same thrust in a HW as TW. IAS) and I currently have a mental model of prevailing winds I guess it is called as being somewhat like a canoe on a river, or a plane stuck in jello where you can move the jello around forward, back, or to the side but the plane moves through the jello exactly as before. So, it's only "interesting" with regard to the ground which is stationary, the plane is moving through the medium (in my case jello) no matter the direction?

So I get to the point then when I think that ok...IF you lose thrust because the motor/propeller stop, you still have inertia, at the moment it stops thrust=drag, lift=weight. Say a tailwind of 55 knots (is that a storm? I don't even know but say 55 knots is stall speed with flaps up, etc.). The wind mass the airplane is in is moving forward at 55 knots, but the plane itself just lost thrust and IN that mass is slowing down. Depending on the angle of attack, you now have a glide path? You may want the maximum glide path (Vy?) but whatever it is the plane will descend as you hold that AOA, and it would descend the same if you had the 55 knot headwind, except in that case you wouldn't have the same Ground Speed, wouldn't cover as much distance, etc.

Is any of what I said above correct?

 

[FastEddieB]

Read "Stick and Rudder".

And/or check out this thread:

http://www.pilotsofamerica.com/community/threads/stick-and-rudder-moments-redux.79699/

 

[FastEddieB]

You may want the maximum glide path...

Is any of what I said above correct?

As to this part, speed for best glide distance is higher in a headwind and lower with a tailwind.

Reductio ad absurdum, imagine your best glide is 50kts and you're attempting to make a runway only 20 feet away - but with a 50kt headwind! Best glide, you come up short, since you're descending vertically. But if you glided faster, you'd make it easily.

Does that help?

 

[dtuuri]

Read "Stick and Rudder".

And/or check out this thread:

http://www.pilotsofamerica.com/community/threads/stick-and-rudder-moments-redux.79699/

I second the motion that you read "Stick and Rudder". Until you get a copy you can imagine being in a hot air balloon, drifting with the wind. Do you feel a breeze when you're drifting within the moving mass of air? No. Suppose your balloon is equipped with, say, an electric propeller and you then switch it on. Would your balloon be pulled in the direction the prop is pointed? Yes. So, an airplane is the same--it leaves the ground and drifts with the wind except for the relative motion caused by the pull of the prop. If you flew over a cloud deck, you would not see any evidence of drift at all. The clouds would all move fore to aft. Fly over an opening in the clouds and you might be surprised to see yourself flying sidewise or backward even with respect to the ground.

dtuuri

 

[LongRoadBob]

Read "Stick and Rudder".

And/or check out this thread:

http://www.pilotsofamerica.com/community/threads/stick-and-rudder-moments-redux.79699/

Thanks, I had read it (prior to starting ground school) and also searched here for tailwind and found your link and read it. Well written.)
I did get the general concept and plan on rereading Stick and Rudder (even before ground school I found it SO well written, so descriptive and educational, but I suspect rereading now after going through the aerodynamic part of ground school will give a lot more to me.
The one single biggest thing I took away from my first read through was AOA and when stalling get the nose down.

Also the idea that you are in an environment where reference to the ground is only "interesting" to flight when thinking of navigation and ground speed.

I liked very much your points from S&R which also brings it more together. The parts of S&R that confused me a little I believe now had to do with landing glide and being on "the back side of the power curve" but I need to think and study a little more about that as well.

Just to mention, "ground school" for me consists of nine (plus some additional) main textbooks that I read through, answer the quizes at the end of chapters, etc. and also after certain number of chapters take a small "test" online to answer. If I understand correctly, when I am done with that there is required ground school classroom where I believe we go through it all one more time but in a class with a teacher. Seems like a good idea to me. And then if we pass their tests, try for the actual written test. I explain so that if you see lots of holes in my knowledge, I haven't "completed" any ground school until I have done the classroom bit, even though I am on to Meteorology/weather book now.

 

[LongRoadBob]

I second the motion that you read "Stick and Rudder". Until you get a copy you can imagine being in a hot air balloon, drifting with the wind. Do you feel a breeze when you're drifting within the moving mass of air? No. Suppose your balloon is equipped with, say, an electric propeller and you then switch it on. Would your balloon be pulled in the direction the prop is pointed? Yes. So, an airplane is the same--it leaves the ground and drifts with the wind except for the relative motion caused by the pull of the prop. If you flew over a cloud deck, you would not see any evidence of drift at all. The clouds would all move fore to aft. Fly over an opening in the clouds and you might be surprised to see yourself flying sidewise or backward even with respect to the ground.

dtuuri

That was my point about the plane being in a cake of "jello" which can be moved any which way but only the relative movement (i.e. same thing no matter what with refernce to the jello it is embedded in). For some reason jello helps. I used to think of water, as in a canoe where you still have to paddle faster than water moving with you to make "headway" with reference to the water but I like a big cake of jello more because I can think of it moving any which way, not moving, all the same to the plane with regards to its speed IN the mass.

I think I get that part. It's the resulting other aspects that I get a little thrown off about until I integrate the idea better

 

[LongRoadBob]

As to this part, speed for best glide distance is higher in a headwind and lower with a tailwind.

Reductio ad absurdum, imagine your best glide is 50kts and you're attempting to make a runway only 20 feet away - but with a 50kt headwind! Best glide, you come up short, since you're descending vertically. But if you glided faster, you'd make it easily.

Does that help?

yeah, it definitely helps. I am thinking that this is the "boundary layer" point. What I mean is even though the plane is still in the air and thus flying is only relative to the air is it in, when you are thinking about gliding down to a landing you have to then start looking at what the actual prevailing air movement is doing to your touchdown target?

Like coming up on a mooring point beside a river, you have to have control in the oncoming stream, and adjust to hit a stationary point at the relative speed and location you pick.