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HEALY
13th Jul 2007, 08:00
Hi

Currently getting ready for an interview and having difficulty understanding a concept about wing tip stalling. Although this has been done to death I find many conflicting ideas about how it occurs.

Firstly it is my understanding that due to the outboard wing section trailing the inboard section a suction is created whereby the boundary layer moves towards the wing tip causing it to be thicker in this region. This means more susceptibility to boundary layer breaking causing the stall.

Thats fine and I generally understand this except for two things

1) Why is a suction created by the outboard section relative to the inboard section?

2) Again its my understanding that due to spanwise flow the airflow over the top of the wing is deflected inwards, so how can the boundary layer movement outward "due to suction" appose the spanwise flow tendancy inward?

Have I just confused myself?

Swanie
13th Jul 2007, 10:48
I also would like a clear explaination as to why.

I do know that the inboard section generates the most lift, and is therefore the area of lowest pressure, which sets up the spanwise flow towards the fuselage

Not confident in saying this, but may it have something to do with this altering the "relative airflow" more directly over the chord, reducing the Mcrit benifits swept wings are supposed to achieve...?:confused:

W.R.A.I.T.H
13th Jul 2007, 12:00
on a swept back wing occurs only outwards, as far as i can remember, and grows with distance from the wing root. Since lift is only generated by the straight LE-TE airflow component (chordwise), the spanwise component, the bigger it is, the more it eats into the amount of lift generated by the wing at that particular location. I wouldn't read too much into it except this, however I'm ready to be corrected :ok:.

Swanie
13th Jul 2007, 13:06
I understand what you mean that lift is due to the component of the airflow that flows perpendicular from LE-TE, but don't see how the spanwise flow could flow in-out since;

1) I thought that spanwise flow was due in part to the voticies that form at the tips, flowing bottom to top, which tend to push the upper surface air towards the root.

2) The lift formula, L=CL1/2pV^2S
- The inboard section has a greater surface area (S)
-Flap causes a greater increase in camber at the root VS. the tip (CL)
-Spanwise flow, causes some slight compression due the the fuselage resisting this flow, which increases the local density (p)

1+2= inner section producing more lift that the tip, and High to Low = inwards spanwise flow...
Please let me know if this is :mad: but I havn't been caught out using this reasoning in exams

still not sure about the tip stall..:ugh:

13th Jul 2007, 14:08
At higher angles of attack the lift distribution normally changes to more outwards loaden wing. As such the local lift coefficient os higher outwards and - as far as I know - the local pressure is also lower.

The wing tip vortices are less the reason than the result. The whole airflow over the wing has some kind of rotation.

13th Jul 2007, 14:22
I could've sworn wings were designed with washout, a gradual reduction of incidence towards the tip, so as to cause the inboard section of the wing to stall first. This causes a rearward shift of the centre of lift and therefore a nose down pitching moment which aids with the stall recovery. It also means that the ailerons, which are usually towards the wing tips, remain more effective at high incidences as it is the wing tips which should stall last.

Regards. :ok:

13th Jul 2007, 14:33
I could've sworn wings were designed with washout, a gradual reduction of incidence towards the tip, so as to cause the inboard section of the wing to stall first. This causes a rearward shift of the centre of lift and therefore a nose down pitching moment which aids with the stall recovery. It also means that the ailerons, which are usually towards the wing tips, remain more effective at high incidences as it is the wing tips which should stall last.

Regards. :ok:They are indeed, so that stall behavior is normally quite gentle. I think some early high performance aircraft had problems, along with some surprises about the aerolasticity of wings.

HEALY
14th Jul 2007, 03:38

You are absolutely correct in what you are saying about washout aiding in the wing root to stall first. I see this as a interesting question in terms of an interview response.
If the question is along the lines of "does the nose pitch up or down in the stall of a swept wing aircraft" the response of "down" can be justified by explaining such design features as washout. Mr Airbus and Boeing have designed the wings for this reason.
In taking the washout out of the equation I am still stuffed in terms of the correct way to explain what is actually going on.

Swanie
14th Jul 2007, 06:31
Swept wings pitch up at the stall due to the tip stalling first (for what ever reason..), therefore the outer section of the wing is no longer generating lift and so the center of pressure moves towards the root and slightly forward because of the sweep.
Could this be a reason for stick pushers...?:confused:

FE Hoppy
14th Jul 2007, 07:37
http://ntrs.nasa.gov/search.jsp?R=39197&id=1&qs=Ne%3D35%26Ns%3DArchiveName%257C0%26N%3D123%2B4294966789

It's not that the tip stalls first but that the root stalls last!!

pipertommy
14th Jul 2007, 08:25
I remember reading that the spanwise flow moves in the direction of the tip due to the rearward sweep.Part of the airflow continues over the wing and some is influenced by the sweep to move in the direction of the tip along/just behind the leading edge area(think its called rams horn).This is one of the problems at low speed,causing a reduction in the lift from a swept wing and high AoA on approach.Hence wing fences ect.

411A
14th Jul 2007, 12:29
Swept wings pitch up at the stall due to the tip stalling first (for what ever reason..),

Oh gosh, do they really?
Well, having actually fully stalled a swept wing aeroplane (Lockheed TriStar) while a Lockheed test pilot was in the RHS keeping an eye on progress, strange as it may seem to most here, the nose pitched gently down...not up.

Maybe Lockheed, in their infinite wisdom, got it wrong.
All this would have slipped passed the FAA, not to mention the UKCAA, as TriStars were on the British civil register as well.

Me thinks many here had better have a rethink about the whole situation..:rolleyes:

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.

John Farley
14th Jul 2007, 22:20
Healey

I hope that the quote from Handling the big jets has clarified it all for you.

If not and you feel you may have to explain or justify what you say in answer to the original question then in your postion I would say:

1 It varies according to the particular type

2 A simple swept wing with no twist and a constant aerofoil from root to tip will inevitably stall at the tips first due to spanwise flow hence the aircraft will pitch up

3 However many design features are used today to change this simple situation. These include washout, fences, leading edge notches and in particular a variation in the type of aerofoil used at the root compared with the tip.

4 Only with a knowledge of all these issues can one conclude nose up or nose down.

5 From a certification point of view only an aircraft nose down pitch is acceptable. This may have to be achieved artificially by using a pusher.

In general beware of questions that ask for a simple yes/no or black/white answer when the topic is complex. By agreeing to such an answer (by guessing for instance) then you may well be showing that you do not understand the topic. "I don't think there is a simple general answer to that question that would apply to all types so I would need more information before even hazarding a guess" can never get you into trouble and should result in a rather less general and more specific question that you will be happier with. They just want to get you to talk after all. Questions are for exams questions at interview are to get you talking.

If you want to be a pilot NEVER be afraid to say "I don't know. I would need help with that"

extreme P
14th Jul 2007, 22:35
Flight Theory and Aerodynamics (by Charles Dole and James Lewis) is basically an updated HTBJ. Well worth a look for interview prep. GENERALLY SPEAKING T tails pitch up, low tails pitch down. Be prepared for the "super stall" question surely to follow as the adept interviewer begins to spin his web...

Swanie
15th Jul 2007, 02:33
"Well, having actually fully stalled a swept wing aeroplane (Lockheed TriStar) while a Lockheed test pilot was in the RHS keeping an eye on progress, strange as it may seem to most here, the nose pitched gently down...not up."

Well I can't argue with what you say you've observed first hand, and as I've never even flown a swept wing aircraft, I can't post my experiences, my posts only come from my CASA theory, supposedly based on the B767.

A quote from the AFT systems notes;

"A disadvantage of sweepback is the tendency to stall tip first (especially if combined with wing taper) due to strong spanwise flow at high angles of attack. This can cause a pitch up at the stall as the CP moves forwards and in. The poor stall characteristics of a plain swept wing often necessitates the use of features such as washout, flow fences, slats and leading edge flaps to modify the stall pattern. Artificial stall warning devices (stick shakers/pushers) may be required"
"A forwards CP movement generates a nose up pitch moment which can help to accelerate the aircraft into a more thoroughly stalled condition."
I'm sure the stalling characteristics vary with aircraft:ok:

411A
15th Jul 2007, 06:55
The poor stall characteristics of a plain swept wing often necessitates the use of features such as washout, flow fences, slats and leading edge flaps to modify the stall pattern.

Indeed so, Swanie, the devil is certainly in the details.

Basic theory is one thing, the finished product quite another.
Washout (for example) has been used for many many years on a variety of aeroplanes, both straight wing and swept designs.

The DC-4, DC-6, and DC-7 aircraft all used the same basic wing (airfoil) design, the most significant change between the types (besides structural) was washout.

Brian Abraham
15th Jul 2007, 07:44
The certification regs detail the characteristics that an aircraft must pocess. For transport category aircraft (eg 411A's favourite Tristar) FAR 25 applies.

§ 25.201 Stall demonstration.

(d) The airplane is considered stalled when the behavior of the airplane gives the pilot a clear and distinctive indication of an acceptable nature that the airplane is stalled. Acceptable indications of a stall, occurring either individually or in combination, are—

(1) A nose-down pitch that cannot be readily arrested;

(2) Buffeting, of a magnitude and severity that is a strong and effective deterrent to further speed reduction; or

(3) The pitch control reaches the aft stop and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated.

§ 25.203 Stall characteristics.

(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. (My bolding) The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.

(b) For level wing stalls, the roll occurring between the stall and the completion of the recovery may not exceed approximately 20 degrees.

(c) For turning flight stalls, the action of the airplane after the stall may not be so violent or extreme as to make it difficult, with normal piloting skill, to effect a prompt recovery and to regain control of the airplane. The maximum bank angle that occurs during the recovery may not exceed—

(1) Approximately 60 degrees in the original direction of the turn, or 30 degrees in the opposite direction, for deceleration rates up to 1 knot per second; and

(2) Approximately 90 degrees in the original direction of the turn, or 60 degrees in the opposite direction, for deceleration rates in excess of 1 knot per second.

§ 25.207 Stall warning.

(a) Stall warning with sufficient margin to prevent inadvertent stalling with the flaps and landing gear in any normal position must be clear and distinctive to the pilot in straight and turning flight.

(b) The warning must be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight. However, a visual stall warning device that requires the attention of the crew within the cockpit is not acceptable by itself. If a warning device is used, it must provide a warning in each of the airplane configurations prescribed in paragraph (a) of this section at the speed prescribed in paragraphs (c) and (d) of this section.

(c) When the speed is reduced at rates not exceeding one knot per second, stall warning must begin, in each normal configuration, at a speed, VSW, exceeding the speed at which the stall is identified in accordance with §25.201(d) by not less than five knots or five percent CAS, whichever is greater. Once initiated, stall warning must continue until the angle of attack is reduced to approximately that at which stall warning began.

(d) In addition to the requirement of paragraph (c) of this section, when the speed is reduced at rates not exceeding one knot per second, in straight flight with engines idling and at the center-of-gravity position specified in §25.103(b)(5), VSW, in each normal configuration, must exceed VSRby not less than three knots or three percent CAS, whichever is greater.

(e) The stall warning margin must be sufficient to allow the pilot to prevent stalling (as defined in §25.201(d)) when recovery is initiated not less than one second after the onset of stall warning in slow-down turns with at least 1.5g load factor normal to the flight path and airspeed deceleration rates of at least 2 knots per second, with the flaps and landing gear in any normal position, with the airplane trimmed for straight flight at a speed of 1.3 VSR, and with the power or thrust necessary to maintain level flight at 1.3 VSR.

(f) Stall warning must also be provided in each abnormal configuration of the high lift devices that is likely to be used in flight following system failures (including all configurations covered by Airplane Flight Manual procedures).

It's true that the swept wings inherent tendency is to pitch up, but it is the aerodynamicists role to tweak the design by way of fences, notches etc to provide the desired handling qualities. If he fails for any reason the engineer will have to resort to a stick pusher. It's interesting that the C-130J requires a stick pusher where the previous models had acceptable aerodynamic qualities at the stall. Influence of the new 6 bladed props on aerodynamic qualities I presume.

HEALY
15th Jul 2007, 07:54
Thanks for all the responses, this does appear to be a subject that can certainly provided different ideas and concepts all of which to appear correct. It seems that different authors have made slightly different cases to justify their answers.

To keep the ball rolling I still have a issue explaining the idea of spanwise flow having in this case. The reason for this is (as my understaning goes) span wise flow will create wing tip vortices. This in turn creates a downwash rear of the trailing edge. This causes a resultant airflow ahead of the wing to be "slightly angled downwards". With this in mind surely this reduces the effective angle of attack of the wing tip and in a round about way protect it from reaching the critical stall angle prior to the wing root.

All answers so far have been a wealth of knowledge I am just digging further so I dont have to bluff my way thru any questions

BOAC
15th Jul 2007, 09:39
In simple terms, the vortex is caused by high pressure air under the wing arriving at an 'open end' (wing-tip) where it naturally wishes to make friends with the low pressure air on top. Hence the improved efficiency due to winglets which modify the 'open end'.

If I recall correctly, the boundary layer thickening you mention is only a concern for the upper surface of the wing. I think any BL thickening under the wing is either non-existent or insignificant?

It may be of interest to your thoughts, but a combat jet spends a significant amount of time with the wing near the stall in manoeuvre, so it is vital that it does not 'pitch-up', otherwise control could be lost. I'm sure JF will correct my failing memory, but I recall the Mk 2/6 Lightnings used a leading-edge 'notch' to reduce boundary layer outflow and a wider chord at the tips (extended leading edges). Both of these improved the manoeuvre characteristics of the type which spent a lot of time in stall buffet:) (including in the circuit).

Swanie
15th Jul 2007, 09:48
Not exactly sure how credible this is as it's from "howstuffworks.com" but I'll post it anyways;

"When a swept-wing travels at high speed, the airflow has little time to react and simply flows over the wing. However at lower speeds some of the air is pushed to the side towards the wing tip. At the wing root, by the fuselage, this has little noticeable effect, but towards the tip the airflow is pushed sidewise not only by the wing, but the sidewise moving air beside it. At the tip the airflow is moving along the wing instead of over it, a problem known as spanwise flow."

"The lift on a wing is generated by the airflow over it from front to rear. As an increasing amount travels spanwise, the amount flowing front to rear is reduced, leading to a loss of lift. Normally this is not much of a problem, but as the plane slows for landing the tips can actually drop below the stall point even at aircraft speeds where stalls should not occur. When this happens the tip stalls first, and since the tip is swept to the rear of the center of lift, the net lift moves forward. This causes the plane to pitch up [corrected by features already discussed above], leading to more of the wing stalling, leading to more pitch up, and so on. This problem came to be known as Sabre dance in reference to the number of North American F-86 Sabres that crashed on landing as a result."

Now it says the sweep causes the air flow to "slide" towards the tip.
I'm at a loss to see how the net force can be "tip ward", for the reasons posted earlier (in #4 I think... ) there is also a flow towards the fuselage.
I can see that both flow directions are possible but which is more pronounced, I doubt they cancel, but am sure there are features to strengthen one to balance the other...?:hmm:

15th Jul 2007, 13:41
Air flows spanwise because of the way the upper surface pressure distribution behaves on a swept wing. Refer to the diagram:

http://img464.imageshack.us/img464/8932/airfoiltx9.jpg

At each section of the wing, the pressure distribution along the chord looks like the upper diagram. The front of the upper surface has a favourable pressure gradient - that is, the local pressure drops as one moves aftwards.
This means that there's no real obstacle to the boundary layer moving aftwards - in fact, the pressure distribution encourages this.
But, after the suction peak is passed, the pressure distribution is adverse for aftwards motion - the local pressure is increasing as the air moves aft. The boundary layer is also thickening, so there's less energy to overcome the adverse gradient.

On a straight wing, the pressure distribution is pretty much the same spanwise, so the air "has no option" but to continue trying to move aft; a trailing edge stall/separation will occur if it just can't overcome the pressure gradient.

But, on a swept wing, there's an option - spanwise. On the lower diagram of a swept planform I've drawn in two lines, roughly corresponding to the 1.0 Cp point on the upper chart. The arrows indicate the adverse gradient. If our air molecule has reached the aft red line, and has 'run out of energy' to move aft, it can move outwards instead - where the pressure gradient is not adverse, since moving spanwise actually means moving forward relative to the local chord.

Brian Abraham
15th Jul 2007, 14:23
HEALY, You may find the following of use http://www.hq.nasa.gov/pao/History/SP-468/ch10-4.htm
Towards the bottom of the page you will find a section headed "Stalling of Swept Wings".

Is indeed a complex subject as the following shows from http://adg.stanford.edu/aa241/drag/sweepncdc.html
"Near the wing tip the flow around the tip from the lower to upper surface obviously alters the effect of sweep. The effect is to unsweep the spanwise constant pressure lines known as isobars. To compensate, the wing tip may be given additional structural sweep. It is at the wing root that the straight fuselage sides more seriously degrade,the sweep effect by interfering with curved flow. Airfoils are often modified near the root to change the basic pressure distribution to compensate for the distortions to the swept wing flow. Since the fuselage effect is to increase the effective airfoil camber, the modification is to reduce the root airfoil camber and in some cases to use negative camber. The influence of the fuselage then changes the altered root airfoil pressures back to the desired positive camber pressure distribution existing farther out along the wing span."

Always wondered why the Boeings had flat topped, no camber on the upper surface aft of about the quarter chord. Guess this is why. Mad (Flt) Scientist?

411A
15th Jul 2007, 18:47
Always wondered why the Boeings had flat topped, no camber on the upper surface aft of about the quarter chord. Guess this is why.

Be very careful reading too much into the Boeing design.
If you look closely at the Lockheed design wing on the TriStar, you might notice just the opposite.
Now, having said this, the L1011 is quite unique...in many many ways.

BGRing
27th Jul 2007, 00:51

I think I understand what is going on. (re span wise flow from root to tip)

Please Educate me if I have it drastically wrong or even just plain wrong.

is it to do with the air changing direction (Horizontaly out,not just up verticaly) before it gets over the wing, and by the time the air is at the LE of the wing tip (slightly after a horizontaly parallel air partial is already over TE the wing root) it has already had time to accelerate some more on a horizontal Plane..
Not sure I have managed to explain my concept. but if I have to I might draw a diagram to illustrate. (it will not be as good a diagram as Mad (Flt) Scientist's but might help)

Not exactly sure how credible this is as it's from "howstuffworks.com" but I'll post it anyways;

"When a swept-wing travels at high speed, the airflow has little time to react and simply flows over the wing. However at lower speeds some of the air is pushed to the side towards the wing tip. At the wing root, by the fuselage, this has little noticeable effect, but towards the tip the airflow is pushed sidewise not only by the wing, but the sidewise moving air beside it. At the tip the airflow is moving along the wing instead of over it, a problem known as spanwise flow."

"The lift on a wing is generated by the airflow over it from front to rear. As an increasing amount travels spanwise, the amount flowing front to rear is reduced, leading to a loss of lift. Normally this is not much of a problem, but as the plane slows for landing the tips can actually drop below the stall point even at aircraft speeds where stalls should not occur. When this happens the tip stalls first, and since the tip is swept to the rear of the center of lift, the net lift moves forward. This causes the plane to pitch up [corrected by features already discussed above], leading to more of the wing stalling, leading to more pitch up, and so on. This problem came to be known as Sabre dance in reference to the number of North American F-86 Sabres that crashed on landing as a result."

Now it says the sweep causes the air flow to "slide" towards the tip.
I'm at a loss to see how the net force can be "tip ward", for the reasons posted earlier (in #4 I think... ) there is also a flow towards the fuselage.
I can see that both flow directions are possible but which is more pronounced, I doubt they cancel, but am sure there are features to strengthen one to balance the other...?:hmm:

After reading this post and also checking Wiki. I saw they were the same....

As for the Question and the Part here i highlited in red.

(Again Guys and Gals, corect me if I am wrong)

I can see the air moveing root to tip before it reaches the wing, and then After the wing it has a Down flow. (Not sure if it is going slightly inward at this point ) but i can see an outflow again after the down flow in line with Vortex currents.
Ok I am being very general here (Not beiing specific to different tip designs and washout and winglets etc etc...
just a plain swept wing no washout etc etc...)

29th Jul 2007, 13:53
HI!

also the dihedral and varrying angle of incidence allows the tip to stall last so that the effectivity of the aileron remains to come out of the stall

Shot Nancy
30th Jul 2007, 05:18
Tip Stalling
The wing of an aircraft is designed to stall from the root to the tip.
This:
a. induces buffet over the tail surface thereby providing advice of the stall,
b. retains aileron effectiveness up to the critical angle,
c. avoids the large rolling moment that would arise if the tip of one wing stalled before the other;
d. reduces the downwash behind the root and may provide a stable nose down moment.
A rectangular wing will usually stall from the root because of the reduction in effective angle of attack at the tips caused by the wing tip vortex. If washout is incorporated to reduce vortex drag, it also assists in delaying tip stall. A tapered wing on the other hand will aggravate tip stall due to the lower RN at the wing tip.
Wing tip stalling can be prevented by:
a. Washout. A reduction in incidence at the tips will result in the wing root reaching its critical angle of attack before the wing tip.
b. Root Spoilers. By making the leading edge of the root sharper, the airflow has more difficulty in following the contour of the leading edge and an early stall is induced.
c. Change of Section. An aerofoil section with more gradual stalling characteristics may be employed towards the wing tips. i.e. increased camber
d. Slots and Slats. Used on the outer portion of the wing will increase the stalling angle of that part of the wing.
Effect of Sweepback on Stalling. When a wing is swept back, the boundary layer tends to change direction and flow towards the tips. This outward drift is caused by the boundary layer encountering an adverse pressure gradient and flowing obliquely to it over the rear of the wing. Initially when the boundary layer flows rearwards from the leading edge it moves towards a favourable pressure gradient, i.e. towards an area of lower pressure. Once past the lowest pressure however, the component at right angles to the isobars encounters an adverse pressure gradient and is reduced. The component parallel to the isobars is unaffected, thus the result is that the actual velocity is reduced (as it is over an unswept wing) and also directed outwards towards the tips
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 separated.
Slots, slats, and fences tend to reduce spanwise flow.
(I know a picture is worth a thousand words but I couldn't paste the accompanying picture)