Dihedral Design on Lateral Stability
If its the free stream, then I don't agree and I don't agree with the parallax demonstration by Mr Williams either. The reason the parallax arguement doesn't work is the free stream presents itself to the wing from a direction which is parallel to the trajectory of the aircraft. So if you close one eye then look from a point half way down one wing you are assuming that the free stream is approaching both wings from one point. Which it clearly isn't.
keep both eyes open and position the paper so that it is side slipping towards your eyes. Now look directly ahead so that the distance between the two points at which your eyes are focused (one on each wing) is the same as the distance between your eyes. The line of sight from each eye is now parallel to the trajectory of the aircraft.
As before the angle of attack will be greater for the leading (dropped) wing. Taking it to extremes if you yaw the paper to give a side slip angle of 90 degrees you will see the bottom of the leading wing and the top of the trailing wing. This means that the leading wing will have a positive angle of attack and the trailing wing will have a negative angle of attack.
My only reason for suggesting that one eye be closed is that some people experience blurring and double vision keeping both eyes open.
In one of your earlier posts you have stated the following:
The lift vector is perpendicular to the wing. So if there is dihedral, the vertical component of the lift vector is greater from the lower wing which tends to roll the aircraft back level.
Last edited by keith williams; 31st Jul 2013 at 14:13.
Busser is right. In a steady turn, the Angle of attack is the same.
The lift vector is perpendicular to the wing.
I've got an ATPL
To keep most aircraft in a turn you have to hold a small amount of aileron - if you centralise the yoke the aircraft will tend to roll back to wings level.
I have no idea what lightenings diagram is showing.
I think they want to know if, in a balanced turn, the angle of attack is the same for both wings.
Anyone out there can explain to me why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?
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Chaps, in true PPrune fashion, there seems to be an unpleasant edge to these responses. Especially from Clandestino who seems to think its important to have a poke at my qualifications which wasn't a point I raised. But I find that edge really annoying.
I believe what the original poster was asking was to do with an aircraft in a steady state, coordinated, balanced turn. He wanted to know why the inner wing (that is the lower wing, the one on the inside of the turn) has a greater angle of attack than the outer wing and was asking why dihedral caused this. My suggestion is that dihedral doesn't cause this and unless your airspeed is very low indeed there is no discernable difference in the angle of attack of the two wings. It is the result of the very slightly different airspeed as the outer wing describes a larger turn radius but its only of theoretical interest until you are operating at very low airspeeds very close to the stall. At normal operating speeds it is of no concern at all.
The effect whereby dihedral will give a restoring couple is as follows. When an aircraft tips from wings-level into a bank, this causes the flight path to start to curve. To keep the aircraft's nose aligned with the actual direction of the flight path and relative wind at any given moment, the aircraft's yaw rate must increase. Until this happens, the aircraft's nose will point toward the outside of the turn in relation to the actual direction of the flight path and relative wind at any given moment. The aircraft's yaw polar inertia in yaw will tend to cause the aircraft's heading to remain constant as the flight path starts to curve. This is a sideslip. The resulting sideways component in the relative wind will interact with the aircraft's dihedral geometry to create a difference in angle-of-attack between the two wings, which will create a roll torque in the direction of a restoring couple.
But this effect has nothing to do with a steady state, coordinated, balanced turn. And if it does, I'm happy to hear about it because I haven't before.
Except that I'm tired of the underlying abusive tone on PPrune from contributors like Clandestino. I just don't care enough to carry on being interested in trying to contribute to intelligent debate to have someone have a pop at my ATPL - what does it even mean, perform it twice?
I believe what the original poster was asking was to do with an aircraft in a steady state, coordinated, balanced turn. He wanted to know why the inner wing (that is the lower wing, the one on the inside of the turn) has a greater angle of attack than the outer wing and was asking why dihedral caused this. My suggestion is that dihedral doesn't cause this and unless your airspeed is very low indeed there is no discernable difference in the angle of attack of the two wings. It is the result of the very slightly different airspeed as the outer wing describes a larger turn radius but its only of theoretical interest until you are operating at very low airspeeds very close to the stall. At normal operating speeds it is of no concern at all.
The effect whereby dihedral will give a restoring couple is as follows. When an aircraft tips from wings-level into a bank, this causes the flight path to start to curve. To keep the aircraft's nose aligned with the actual direction of the flight path and relative wind at any given moment, the aircraft's yaw rate must increase. Until this happens, the aircraft's nose will point toward the outside of the turn in relation to the actual direction of the flight path and relative wind at any given moment. The aircraft's yaw polar inertia in yaw will tend to cause the aircraft's heading to remain constant as the flight path starts to curve. This is a sideslip. The resulting sideways component in the relative wind will interact with the aircraft's dihedral geometry to create a difference in angle-of-attack between the two wings, which will create a roll torque in the direction of a restoring couple.
But this effect has nothing to do with a steady state, coordinated, balanced turn. And if it does, I'm happy to hear about it because I haven't before.
Except that I'm tired of the underlying abusive tone on PPrune from contributors like Clandestino. I just don't care enough to carry on being interested in trying to contribute to intelligent debate to have someone have a pop at my ATPL - what does it even mean, perform it twice?
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It is indeed a little more complicated than what pilot students are told (even ATPL)...
To make it simple, for a wing the angle of attack at a certain "rib" (aerodynamically, nor structural), so at any section with a rob chord parallel to the longitudinal axis of the aircraft and perpendicular to the wing plane, is defined by the angle between the local arflow projected into the rib plane and the rib chord. Any flow component perpendicular to the "rib" does not influence AOA.
Local airflow is a combination of the forward speed of the aircraft (the free stream component parallel to the longitudinal axis), the vertical speed of the aircraft (in the plane of symetry) and the lateral speed of the aircraft due to dideslip (perpendicular ro the plane of symmetry) and any local speed due the rotation of the aircraft. In a steady turn this means a pitching and a yawing rotation. It is different in any location of the aircraft, resulting in locally different AOA. Different "rib" orientation due to Dihedral/Anhedral will have an influence as well.
To make it more clear, a 90° angled V-Tail on an aircraft in a steady 45° bank turn mens one stabilizer is vertical, one is horizontal. As the Aircraft rotates around an vertical axis perpendicular to the ground in a turn, only the outer stabilizer (the vertical one) is affected by this rotation, the innner stabilizer (the horizontal one) is not affected at all. On the other hand, vertical speed (as indicated on the VS indicator) will only affect the inner (horizontal) stabilizer, while the outer (vertical) one will not be affected.
Its all about trigonometry...
To make it simple, for a wing the angle of attack at a certain "rib" (aerodynamically, nor structural), so at any section with a rob chord parallel to the longitudinal axis of the aircraft and perpendicular to the wing plane, is defined by the angle between the local arflow projected into the rib plane and the rib chord. Any flow component perpendicular to the "rib" does not influence AOA.
Local airflow is a combination of the forward speed of the aircraft (the free stream component parallel to the longitudinal axis), the vertical speed of the aircraft (in the plane of symetry) and the lateral speed of the aircraft due to dideslip (perpendicular ro the plane of symmetry) and any local speed due the rotation of the aircraft. In a steady turn this means a pitching and a yawing rotation. It is different in any location of the aircraft, resulting in locally different AOA. Different "rib" orientation due to Dihedral/Anhedral will have an influence as well.
To make it more clear, a 90° angled V-Tail on an aircraft in a steady 45° bank turn mens one stabilizer is vertical, one is horizontal. As the Aircraft rotates around an vertical axis perpendicular to the ground in a turn, only the outer stabilizer (the vertical one) is affected by this rotation, the innner stabilizer (the horizontal one) is not affected at all. On the other hand, vertical speed (as indicated on the VS indicator) will only affect the inner (horizontal) stabilizer, while the outer (vertical) one will not be affected.
Its all about trigonometry...
I think the wikipedia drawing is slightly better than Lightning Mate's (no offence meant, professor) in that it shows the three-dimensional aspect of what goes on:
http://upload.wikimedia.org/wikipedi...Dihedral_3.png
A side-slip means the aircraft axis is slightly twisted relatively to the flight path, and it is that twist, or divergence from parallel between the relative wind and the airplane centerline, that (if the wing has a dihedral angle) presents the lowered wing to the air in such a way as to increase angle of attack to the oncoming (forward movement) airstream, while reducing AoA for the raised wing.
http://upload.wikimedia.org/wikipedi...Dihedral_3.png
A side-slip means the aircraft axis is slightly twisted relatively to the flight path, and it is that twist, or divergence from parallel between the relative wind and the airplane centerline, that (if the wing has a dihedral angle) presents the lowered wing to the air in such a way as to increase angle of attack to the oncoming (forward movement) airstream, while reducing AoA for the raised wing.
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I previously posted this explanation elsewhere on this forum. Perhaps not the classic explanation of dihedral effect, but it works for me:
Consider the flow over an unswept, untapered wing with no washout. Let’s imagine a streamline (the path followed by an air 'particle') starting at some point on the leading edge of the starboard wing. Let’s further assume that the point is roughly mid span, so we can ignore ‘end-effects’. If the aircraft is slipping neither left nor right, the streamline will have no spanwise component; if we are looking down from on top of the aircraft, the streamline will appear aligned fore and aft.
Now consider what happens if the aircraft slips to the left. Our right wing streamline will now meet the wing trailing edge further outboard than it did with no slip. If the wing has dihedral, the trailing edge ‘seen’ by the streamline will therefore be higher (relative to the leading edge) than when there was no slip. So, in a left slip, the wing surface profile followed by our streamline will appear to be ‘nose-down’ relative to the profile it followed with no slip.The right wing streamline therefore sees a reduction in effective incidence. The equivalent streamline on the left wing leaves the trailing edge further inboard and therefore sees an increase in effective incidence. So the right wing lift decreases and the left wing lift increases, rolling the aircraft such as to oppose the slip.
Note that the above argument is independent of the attitude of the aircraft or the direction of gravity.
Consider the flow over an unswept, untapered wing with no washout. Let’s imagine a streamline (the path followed by an air 'particle') starting at some point on the leading edge of the starboard wing. Let’s further assume that the point is roughly mid span, so we can ignore ‘end-effects’. If the aircraft is slipping neither left nor right, the streamline will have no spanwise component; if we are looking down from on top of the aircraft, the streamline will appear aligned fore and aft.
Now consider what happens if the aircraft slips to the left. Our right wing streamline will now meet the wing trailing edge further outboard than it did with no slip. If the wing has dihedral, the trailing edge ‘seen’ by the streamline will therefore be higher (relative to the leading edge) than when there was no slip. So, in a left slip, the wing surface profile followed by our streamline will appear to be ‘nose-down’ relative to the profile it followed with no slip.The right wing streamline therefore sees a reduction in effective incidence. The equivalent streamline on the left wing leaves the trailing edge further inboard and therefore sees an increase in effective incidence. So the right wing lift decreases and the left wing lift increases, rolling the aircraft such as to oppose the slip.
Note that the above argument is independent of the attitude of the aircraft or the direction of gravity.
Last edited by Flash131; 2nd Aug 2013 at 08:34.
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HALCYON DAYS.
Back in the dawn of history, our excellent non-commissioned aerodynamics instructor used to say the 4 ways to ensure lateral stability were - the 2 Chinamen and the 2 Welshmen.
Hi Fin and Hi Wing, Dai Hedral and Sweep Bach!!
This was before the digital computer took all the fun out of aircraft design and flying! Sweet memories, and a great way to remember when it came to the exams.
Apologies to our Eastern and Taff colleagues, this was NOT a racist rant.
However, it must have involved sideslip of some sort, and I'm alarmed at how quickly AoA and the position of probes etc crept into the discussion...............!
Back to basics anyone??
Hi Fin and Hi Wing, Dai Hedral and Sweep Bach!!
This was before the digital computer took all the fun out of aircraft design and flying! Sweet memories, and a great way to remember when it came to the exams.
Apologies to our Eastern and Taff colleagues, this was NOT a racist rant.
However, it must have involved sideslip of some sort, and I'm alarmed at how quickly AoA and the position of probes etc crept into the discussion...............!
Back to basics anyone??
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none of that addresses the original posters question.
He is asking about the angle of attack during a stable, balanced, banked turn.
Not yawing, not slipping, not skidding, just turning.
Can't be that hard.
He is asking about the angle of attack during a stable, balanced, banked turn.
Not yawing, not slipping, not skidding, just turning.
Can't be that hard.
The original question was
It included absolutely nothing about
You introduced all of those additional conditions.
why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?
during a stable, balanced, banked turn.
Not yawing, not slipping, not skidding, just turning.
Not yawing, not slipping, not skidding, just turning.
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When your aeroplane is banked is it normally side slipping, skidding, or in a balanced turn? What's the normal, everyday situation? So if someone asks about angle of attack during a bank do you think it's sensible to assume, given that he's not mentioned slipping or skidding, that we are talking about a steady state, balanced turn.
What question do you think the op was asking?
What question do you think the op was asking?
The OP was asking about dihedral. You made assumptions about his question that makes his question irrelevant. The others here are making assumptions, or adding information, that keeps his question relevant. I think the second set of assumptions are more reasonable, don't you? Or do you really think the OP will be satisfied with the answer "it doesn't"? Don't you think he is after information about dihedral, and that providing situations where dihedral is relevant is a better way to answer?
So if someone asks about angle of attack during a bank do you think it's sensible to assume, given that he's not mentioned slipping or skidding, that we are talking about a steady state, balanced turn.
why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?
Instead of answering this question, you decided to add further conditions.
What question do you think the op was asking?
Who does that suggest has got the wrong interpretation?
Last edited by keith williams; 4th Aug 2013 at 12:36.
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HOW TO MAKE LIFE DIFFICULT?!
Oooooooh dear!
If this is the standard of "instruction" or the current way of "imparting knowledge", then gawd help us all.
A simple posting on a simple aerodynamic quality descends into a personal and adverserial series of criticisms of others attempts to answer the OP.
Tommoutrie, I hope you are not a training pilot or ground instructor of any type as I can see the lessons being excruciating and counter-productive.
If that's personal/adverserial then so be it, I hold my hand up and admit it, but I can hardly seee how your contributions are helping.
Maybe stay your hand on this one and allow the discussion to proceed productively?
If this is the standard of "instruction" or the current way of "imparting knowledge", then gawd help us all.
A simple posting on a simple aerodynamic quality descends into a personal and adverserial series of criticisms of others attempts to answer the OP.
Tommoutrie, I hope you are not a training pilot or ground instructor of any type as I can see the lessons being excruciating and counter-productive.
If that's personal/adverserial then so be it, I hold my hand up and admit it, but I can hardly seee how your contributions are helping.
Maybe stay your hand on this one and allow the discussion to proceed productively?
The majority of the posters in this thread appear to have had no difficulty in interpreting the question in the manner intended by the OP.
So arguing about the intent of the original question is somewhat academic - we'll probably never know what was meant ...
