Can this vane prevent/delay wing stall?

Question

Given that a stall results from the airflow over the top of the wing separating from the wing at high angles of attack, would it be possible to prevent, or at least ameliorate, a stall by deflecting the airflow downwards into the "dead zone" behind the wing using a curved/angled vane located just above the leading edge of the wing?

A rough illustration of what I'm thinking about:

Is this structurally viable? And even with the deflected air, would it retain a sufficient velocity to maintain lift anyways? Interesting in theory but I've been doing some work on my own on boundary layer airflow and you'd be surprised how much these surfaces slow down and create secondary flows, which I GUARANTEE you will eliminate effective lift. — Jihyun, Sep 18 '18 at 03:51

Pilothead · Accepted Answer · 2018-09-20T15:46:38.893

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This is a very good idea and has already been done. It is called a slat and is positioned ahead of the wing where it will do a better job of turning the airflow and not causing the slat to stall, as shown on your diagram. Slats allow a 10-12 degree higher angle of attack before the wing stalls. Fixed versions in this more forward position have lower drag in cruise.

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The reason a more forward placement is superior can be seen in the streamlines of the airfoil at high AoA. The inlet gap between the slat and wing is located right at the airstream stagnation point. This is where air pressure is greatest, enhancing the operation of the slat.

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edited Sep 20 '18 at 15:46

answered Sep 18 '18 at 04:27

Pilothead

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That climb/cruise diagram seems to confuse pitch with AOA. While climbing you would expect a lower AOA than cruise at equal IAS. – Sanchises Sep 18 '18 at 05:52
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@Sanchises The AoA when climbing will be lower that in S&L flight only if the attitude of the plane is kept constant during the climb; but we usually pitch up the plane when climbing, and that increases the AoA significantly... – xxavier Sep 18 '18 at 07:04
I mean, constant and the same as when flying S&L... You're flying level, then you open the throttle and keep the attitude constant. You'll climb, and the AoA will decrease... However, you could also pitch the plane up, give more gas and you'll be climbing at the same IAS, but with an increased AoA... – xxavier Sep 18 '18 at 07:20
However, you could also pitch the plane up, give more gas and you'll be climbing at the same IAS, but with an increased AoA That's incorrect. In climb, the thrust vector has a vertical component, which reduces the lift required from the wings. At the same IAS, this means a reduced AOA (except during the transition from S&L to climb, but that's only a few seconds) during climb. See also https://aviation.stackexchange.com/a/12720/4108 – Sanchises Sep 18 '18 at 07:31
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Also, most large aircraft have them. – Cpt Reynolds Sep 18 '18 at 15:19
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@Sanchises Climbing at cruising speed is an enroute activity where changes in AOA are a fraction of a degree. That is not the intent of this diagram. This is from a STOL aircraft manufacturer whose products climb and descend steeply. This type of climb requires power and takeoff speed are low, so there is neither the possibility or desire to climb at cruising speed. Slow takeoff, climb and descent use high AOA to get the necessary lift. The situation is similar for airliners but the climb and cruise speeds are multiplied by some large number. No aircraft takes off or lands at cruising speed. – Pilothead Sep 18 '18 at 16:09
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@Sanchises When climbing with a pitch-up attitude, the AoA is higher the more pronounced that pitch-up attitude is. The fact that the prop thrust may reduce the necessary lift from the wings has nothing to do with the obvious fact that the relative wind strikes the wind at a much higher AoA. That's due to the pitch-up attitude... I use to fly a 'Savannah' STOL and know that situation first-hand. – xxavier Sep 18 '18 at 18:35
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I'm just saying that the diagram would be better labelled "high AOA" instead of "climb". Fact is, the "climb" situation in this diagram applies just as well to a short field landing. – Sanchises Sep 18 '18 at 18:59
@xxavierv That just doesn't make sense. I fully agree if you say the low takeoff speed gives a high AOA, but saying a high pitch attitude results in a high AOA just doesn't add up. Extreme example: what do you think the AOA is in a fully vertical takeoff? – Sanchises Sep 18 '18 at 19:02
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@Sanchises "I'm just saying that the diagram would be better labelled "high AOA" instead of "climb"." I agree. The thing that caused all the comment was the "climb at cruise speed" which is not commonly done. Most aircraft don't have excess power and have to reduce speed (and drag) to get the power to climb. So while your thrust contribution to lift argument is technically possible, the effect for normal aircraft is minuscule even in theory and not achievable in practice due to that power limitation. – Pilothead Sep 18 '18 at 19:54
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@Sanchises You're wrong, probably because you have no flying experience at all. If you could fly a STOL, where these things are more pronounced, and hence more clear, you would perhaps understand them better. But I'm not going to insist. Period. – xxavier Sep 18 '18 at 20:29

Can this vane prevent/delay wing stall?

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