Flying Instructors & ExaminersA place for instructors to communicate with one another because some of them get a bit tired of the attitude that instructing is the lowest form of aviation, as seems to prevail on some of the other forums!
We all know and were told and tell that the high wing in a climbing stall, stalls because it is at a higher angle of attack. The low wing stalls in a descending turn because of the same reason - being a higher angle of attack.
Does anybody have any great analogies to help explain this? Resources with pictures that will help show this?
I have heard of the spiral stair case analogy - outside railing of staircase being a shallow angle and inside being a steeper angle as you travel down it, although if you turn the plane around 180 degrees at the bottom of the stairs and go back up, I feel that analogy falls to pieces as the higher wing is now represented by a shallow angle.
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The spiral staircase analogy works well for both climbing and descending once you are clear that the handrail represents the relative airflow coming towards the wing.
The descending case is straightforward, as you say. The climbing case just needs some visualisation. Now, forgive me some simplification for clarity ...
Imagine the aircraft heading up the staircase. The chordlines of the both wings have the same upwards angle to the earth's surface. Despite the fact that the aircraft is climbing, the relative airflow approaches both wings from underneath the wing.
As the aircraft moves forward, the inside stair rail is coming down towards the aircraft from a higher vertical position than the outside rail. So the angle between the wing (chord) and the inside rail (relative air flow) is small.
The outside stair rail is coming towards the aircraft from a lower vertical position than the inside rail. So the angle between the wing (chord) and the outside stair rail (relative air flow) is greater, and the outside (higher) wing has a greater angle of attack.
Last edited by Unhinged : 28th October 2009 at 11:43.
marvin347, thanks for bringing this up, has been bothering me as well for a while !
You don't say it, but I presume you're talking about a climbing / descending turn.
I'm still in doubt about the "staircase explanation" because of these two practical aspects:
1. Airplanes don't turn "on the spot", they generally make very wide turns. The wing span is tiny in comparison with the diameter of the turn. Wouldn't other factors like dirt on the leading edges, small differences in wing assembly, etc. have a even greater effect ?
2. The climb speed of GA airplanes is microscopic compared to their forward speed. The increased AoA due to the upwards movement of the aircraft must be minimal. Otherwise as above.
I haven't been able to stall myself such that a wing drops. My flight instructor did demonstrate this, but in 1 out of 3 cases the "wrong" wing dropped. I've also read that on C152s, the "wrong" wing may just as well drop.
palou89's input is something I would agree with. I've never noticed what "we all know and are told," only that whether or not the airplane is co-ordinated, will have the predominant effect on which wing drops. And if you want to get picky... a dented wing, other minor dirt/damage will also play a part. Even different angles of incidence would affect it, as one wing would have a (slightly) higher AoA that the other, so would already be nearer to a stalling AoA. Another difference between climbing/descending is the thrust, and slipstream over the wings, also playing a major role in both scenarios. (I'm assuming climbing stall=power ON, and descending stall=power OFF??)
I think that dihedral also has a part to play in all of this, due to the varying vertical lift components... especially in the climbing/turning scenario. But then, for descending, the low wing would appear to have the advantage of a greater vertical lift component. So you could put this down to the outside wing travelling faster (further to go). Also, washout could be a consideration.
Forces in the turn would be another factor.
All this talk of staircases seems too complicated, especially as there are more factors to take into account than just the relative wind... [I initially assumed the handrails (which are level) represented the path of the wings, and not the airflow (relative wind). It's the first time I've heard of this analogy.]
You could have a look through Handbooks & Manuals at the Airplane Flying Handbook, and Pilot's Handbook of Aeronautical Knowledge by the FAA. I don't think they go too far into climbing/descending stalls in turns though??
1. Airplanes don't turn "on the spot", they generally make very wide turns. The wing span is tiny in comparison with the diameter of the turn. Wouldn't other factors like dirt on the leading edges, small differences in wing assembly, etc. have a even greater effect ?
I think this thread is referring to extreme examples of climbing/descending turns which could cause a stall. You are correct about factors such dirt etc having an effect on stall speed however this tends to be evenly spread on both wings.
The fact is if you climb/descend whilst performing a steep turn there is a likely hood you will stall one of the wings of your aircraft. In this scenario your aircraft will not be making "wide turns". The staircase analogy is a good one.
Surely a slight elaboration is required in all this?
In a climbing turn, if the outside (faster) wing literally stalls because it is "at a higher angle of attack", then before it stalled it was generating more lift than the inside wing, and the bank angle would have been increasing, which is not a standard climbing turn.
In practice this well-known over-banking effect is corrected by some up aileron on the outside wing, so the outside wing (as a whole) has a slightly lower AoA than the inside wing, but it is faster so the lift is the same, and the bank angle is stable.
The inner part of the outside wing doesn't get the benefit of this up aileron, and so that is at a high AoA, and so it is just that part of the outside wing which stalls first (theoretically).
Thanks Carrot. That explains the turning while climbing stall. How would you describe the turning while descending stall? How would the rolling tendency factor work in this case?
That is exactly what I am referring to - theoretical. I know there can be dents, bugs, not co-ordinated, angle of incidence, dihedral etc. etc. Let's just say we are in a controlled lab with perfect conditions and a perfectly coordinated robot pilot.
It helps to think of the level turn first. In a level turn, once the bank is established the ailerons are returned to nearly neutral. To maintain the bank angle the AoA of the faster outside wing must be slightly lower than the slower inside wing so there is a very little up aileron.
Now imagine the aircraft rising. The airflow is more nearly parallel to the pitch of the wings, so the AoA is reduced on each wing. The reduction in AoA is the climb speed divided by the wing speed. The outside wing is faster and sees a smaller reduction in AoA and lift. This explains overbanking.
Some up aileron on the outside wing corrects the overbanking, but leaves the inner part of the outside wing at a high AoA, which could then stall when the pilot pitches up to regain lift.
Now imagine the aircraft descending. The airflow is now less parallel to the pitch of the wings, and so the AoA increases. Once again the faster outside wing sees the smaller effect, and there is underbanking.
Some up aileron on the inside wing corrects this, leaving the inner part of the inside wing at a high AoA, which could then stall.
My explanation doesn't exclude the down aileron wingtips stalling. For that we have to look at the size of the forces. Taking any of these turns as an example, and comparing the lift on each wing section to a non-turning manoeuvre at the same g-factor, the inner sections have some asymmetric lift. The outer sections have ailerons applied to cancel out their own asymmetric lift, and then a bit more to counter the rolling moment from the inner sections. But because they are further out, the outer sections have greater leverage, and a smaller asymmetric lift will do the job. So the inner sections have the highest asymmetric lift, and one of them will have the highest Lift Coefficient of any wing section, and (theoretically) it is the only stall candidate.
Personally I never liked the spiral staircase, though I agree with Unhinged that it does explain the change in relative airflow. I think it is more natural to think of the angles made by the ascent/descent speed and the different wing speeds, even if it is the same thing in the end.
Last edited by 24Carrot : 3rd November 2009 at 01:51.
Reason: To exclude wing tip stalling
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