Swept wing stalling
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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...?
"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...?
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Air flows spanwise because of the way the upper surface pressure distribution behaves on a swept wing. Refer to the diagram:
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.
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.
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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?
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?
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Always wondered why the Boeings had flat topped, no camber on the upper surface aft of about the quarter chord. Guess this is why.
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.
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Reading this thread has helped me. (I think
)
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)
Please help me
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...)
Again. Please help if I am seing it wrong.
)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)
Please help me
Swanie vbmenu
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...?
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...?
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...)
Again. Please help if I am seing it wrong.
Last edited by BGRing; 27th Jul 2007 at 00:25.
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stalling
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
BHaskar
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
BHaskar
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)
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)
