Stall on B738
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Stall on B738
Hi friends,
Question on stalling for B738,:if stall happens, which part enters stall firstly, the wing-root or wing-tip? and why?
Question on stalling for B738,:if stall happens, which part enters stall firstly, the wing-root or wing-tip? and why?
From an old topic:
http://www.pprune.org/archive/index.php/t-283827.html
HotDog
14th Jul 2007, 13:29
From handling the big jets, by D.P.Davies.
Quote:
The effect of the wing planform characteristics (sweep). In practice the whole wing does not stall at the same instance. A simple swept and tapered wing will tend to stall at the tips first because the high loading outboard, due to taper, is aggravated by sweep back. The boundary layer outflow also resulting from sweep reduces the lift capability near the tips and further worsens the situation. This causes a loss of lift outboard (and therefore aft) which produces pitch up. A lot of design sophistication is needed, including the use of camber and twist, leading edge breaker strips, fences, etc., to supress this raw quality and get an inboard section stalled first so that the initial pitching tendency is nose down. However, when a highly developed swept wing is taken beyond its initial stalling incidence the tips may still become fully stalled before the inner wing in spite of the initial separation occuring inboard. The wing will then, therefore, pitch up.
http://www.pprune.org/archive/index.php/t-283827.html
HotDog
14th Jul 2007, 13:29
From handling the big jets, by D.P.Davies.
Quote:
The effect of the wing planform characteristics (sweep). In practice the whole wing does not stall at the same instance. A simple swept and tapered wing will tend to stall at the tips first because the high loading outboard, due to taper, is aggravated by sweep back. The boundary layer outflow also resulting from sweep reduces the lift capability near the tips and further worsens the situation. This causes a loss of lift outboard (and therefore aft) which produces pitch up. A lot of design sophistication is needed, including the use of camber and twist, leading edge breaker strips, fences, etc., to supress this raw quality and get an inboard section stalled first so that the initial pitching tendency is nose down. However, when a highly developed swept wing is taken beyond its initial stalling incidence the tips may still become fully stalled before the inner wing in spite of the initial separation occuring inboard. The wing will then, therefore, pitch up.
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Not quite Beech Boy; that reason is not why the tip must not stall first. Read DP Davies again.
Recall also that loss of aileron authority is not the end of the world in a swept wing jet, due to the extensive reliance on spoilers.
Recall also that loss of aileron authority is not the end of the world in a swept wing jet, due to the extensive reliance on spoilers.
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Hi Tubby Linton,
I don't understand the last two sentences in the quote from D.P. Davies. Why would the airflow on the inner wing reattach when the angle of attack is increased? I do accept that the initial pitch down effect is lost once the tip is also stalled.
I don't understand the last two sentences in the quote from D.P. Davies. Why would the airflow on the inner wing reattach when the angle of attack is increased? I do accept that the initial pitch down effect is lost once the tip is also stalled.
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@Gusbrecht
Davies is not saying the inner wing reattaches. He says that the tip may stall FULLY before the inner wing stalls FULLY.
Generally it's hard to get the inboard to stall before the tip - everything is favouring the tip to go first. So the designer biases it. But there's a penalty to be paid, in performance/efficiency, in doing so. So you can't afford to totally kill the inboard wing.
So you twist and dress the wing to force an inboard stall to start. But maybe it only stalls right where you put the trip, or right at the end of the slat, or whatever; the REST of the inboard wing is still very robust. keep increasing AOA and now the tip goes; increase it more, and the tip stall develops and spreads, while the inboard still has only a partial stall.
Its all because the tip stall is "natural" and the inboard stall is "artificial" - the tip is really where the wing "wants" to stall, and the designed-in inboard tendency gets overcome by the natural behaviour.
I'd also caution that to read basic design principles - such as "don't let the tip stall first" etc. - and to ASSUME that any given type slavish follows that behaviour is dangerous. The ultimate pass/fail is whether you can comply to the relevant regs (at time of cert) - not some theoretical design goal. If I have a small tip stall first, but it doesn't grow, then that might be fine. For example, I don't think anyone deices a winglet, so chances are with ince on the airframe, the winglet (and perhaps the last few feet of the wing) stalls long before the protected portions. Not in and of itself an issue - if it passes the regs, it's ok.
edit to add that the above is intended as generic comments on the general wing design statements being made; i don't have specific knowledge of the B737 wing design nor its stall characteristics
Davies is not saying the inner wing reattaches. He says that the tip may stall FULLY before the inner wing stalls FULLY.
Generally it's hard to get the inboard to stall before the tip - everything is favouring the tip to go first. So the designer biases it. But there's a penalty to be paid, in performance/efficiency, in doing so. So you can't afford to totally kill the inboard wing.
So you twist and dress the wing to force an inboard stall to start. But maybe it only stalls right where you put the trip, or right at the end of the slat, or whatever; the REST of the inboard wing is still very robust. keep increasing AOA and now the tip goes; increase it more, and the tip stall develops and spreads, while the inboard still has only a partial stall.
Its all because the tip stall is "natural" and the inboard stall is "artificial" - the tip is really where the wing "wants" to stall, and the designed-in inboard tendency gets overcome by the natural behaviour.
I'd also caution that to read basic design principles - such as "don't let the tip stall first" etc. - and to ASSUME that any given type slavish follows that behaviour is dangerous. The ultimate pass/fail is whether you can comply to the relevant regs (at time of cert) - not some theoretical design goal. If I have a small tip stall first, but it doesn't grow, then that might be fine. For example, I don't think anyone deices a winglet, so chances are with ince on the airframe, the winglet (and perhaps the last few feet of the wing) stalls long before the protected portions. Not in and of itself an issue - if it passes the regs, it's ok.
edit to add that the above is intended as generic comments on the general wing design statements being made; i don't have specific knowledge of the B737 wing design nor its stall characteristics
