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EK4457
20th Mar 2011, 21:42
Quick question for all the aerodynamicists out there;

During the ATPL exams I was taught that swept wings suffer from tip stall. Nearly every book I've read has agreed with this. From speaking to other pilots, this seems to be 'common knowledge'. Thinking about it though, I've never really seen a convincing explaination as to why.

However, I have recently read 'Handling the Big Jets' by D. P. Davies. He seems to say that a modern airliner's wing stalls at the tip due to taper rather than sweep.

He argues that the taper causes a lower surface area at the tip than the root. This gives rise to a higher wing loading at the tip and thus a higher stall speed.

I suppose in terms of AOA, the taper causes the tip to reach Cl max at a lower AOA than the root.

Is this correct? Do we confuse this issue because swept wings are nearly always tapered and assume that the sweep gives rise to tip stall?

I suppose it would be good if any former Lightning chaps could let us know the stall characteristics of a wing with sweep but no (or little) taper.

Thanks,

EK

Jane-DoH
21st Mar 2011, 06:27
I'm not an expert here. I think it's both taper and sweep

Tarq57
21st Mar 2011, 06:33
I'm no expert, but have thought that, at least partly, it's also a function of the span on a swept wing having a chord component, equal to the cosine of the sweep angle. (Or is it the sine?)

Sciolistes
21st Mar 2011, 07:26
I'm a bit of an aeroduffer, but I think it is almost entirely due to taper due to the narrower portion of the being simply unable to generate as much lift. I don't see why sweep would mean a great CLmax at root or tip unless the wing were tapered.

Nepotisim
21st Mar 2011, 08:45
It is called wash out. It is a twist in the wing where the angle of incidence changes between the root and the tip.

I think the angle of incidence is greater at the root thus, on a swept wing aircraft, causing the root to stall first and a nose down tilting moment. Assisting with the recovery. Whilst at the tip the outboard ailerons are still working.

Hope this helps.:ok:

QJB
21st Mar 2011, 09:42
Taken from aerodynamics for naval aviators:

Sweepback applied to a wing planform alters the lift distribution similar to decreasing the taper ratio. Also, a predominating influence of the swept planform is the tendency for a strong crossflow of the boundary layer at high lift coefficients. Since the outboard sections of the wing trail the inboard sections, the outboard suction pressures tend to draw the boundary layer toward the tip. The result is a thickened low energy boundary layer at the tips which is easily seperated.

I believe in general that stall patterns are a result of varriations in downwash, and thus induced angles of attack along the span.

J

dClbydalpha
21st Mar 2011, 10:32
From my understanding, and stated within QJB's post, the disadvantage of rearwards sweep is that you end up with a spanwise flow. This thickens the boundary layer towards the tip. The result is earlier separation at the tips, and therefore earlier stall. The use of fences on swept wings was there to prevent this flow and help relieve this condition.

The consequences of a tip stall on a swept wing design can be more dramatic in terms of pitching moment. This means that not only are swept wings more susceptible, but also the need to prevent it is greater.

EK4457
21st Mar 2011, 21:56
So,the general consensus seems to be that both contribute to the tip stall.

Sweep causes a greater span wise flow which leads to thicker boudary layer at the tip. This seperates at a lower AOA.

Taper for the reasons stated earlier.

Right?

EK

Mad (Flt) Scientist
22nd Mar 2011, 00:33
More or less.

Where you have to be very careful is that statements about the relationship between sweep and increased likelihood of tip stall, or taper and increased likelihood of tip stall, both assume that all other variables are held constant.

Since, as mentioned above, the consequences of tip stall, especially on a swept wing, are undesirable, the designer will do a lot to avoid that stall, using many additional tricks - including washout and variation of the section along the span, tuning of inboard devices to prompt an early inboard stall, and so on. It is therefore dangerous to simply compare, say, the sweep of a number of aircraft and their stalling characteristics without being aware of the details of the wing design. Add in leading edge devices across part of the span and the story becomes even more complex.