dihedral and swept back lateral stability
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dihedral and swept back lateral stability
when one wing is lower than the other, the lower wing produces more lift bcoz of higher AOA to the sideslip velocity of air.
I'm unable to understand how is that the AOA of the lower wing increases wrt relative wind coming from sidewards(lower wing)?
I'm unable to understand how is that the AOA of the lower wing increases wrt relative wind coming from sidewards(lower wing)?
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I think you'll find things easier to grasp if you don't mix dihedral, wing position (vertical location on the fuselage) and sweep effects. Each of them can be considered in isolation and then, once you've got the basics sorted out, you can think of their effects in combination.
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Dihedral is the angle between the wing and the horizontal plane, viewed from front/back. Position on the fuselage is where the root chord is relative to the fuselage.
So, an aircraft can have dihedral or anhedral, and can be low, mid or high wing, and the two are in principle independent choices (although there tends to be a correllation, it is not fixed and there are exceptions). Certainly the effect of dihedral can be discussed independent of the effect of wing position.
I referred to not mixing the three concepts because the OP talks of dihedral, sweep and "one wing being lower" and so mixes up the concepts its hard to grasp at the intent of the question (at least, hard for me)
So, an aircraft can have dihedral or anhedral, and can be low, mid or high wing, and the two are in principle independent choices (although there tends to be a correllation, it is not fixed and there are exceptions). Certainly the effect of dihedral can be discussed independent of the effect of wing position.
I referred to not mixing the three concepts because the OP talks of dihedral, sweep and "one wing being lower" and so mixes up the concepts its hard to grasp at the intent of the question (at least, hard for me)
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But I think I agree with your comment.
The end result of either dihedral on a 'low' wing' (Jodel) or positioning a 'plank on top of the fuselage' (F-27, the 'SAAB box') in the end is the same: adequate lateral stability. Just the basic formulae and explanations tend to be different.
CJ
http://en.wikipedia.org/wiki/Dihedral_(aircraft)
While wikipedia isn't always a great source, the article at the link does a decent job of explaining how the dihedral effect works, and the distinction between dihedral angle and dihedral effect. The illustrations are helpful.
Perhaps you should start at this point in the article
"Using dihedral angle to adjust dihedral effect"
and read from there. I think it will answer your questions, in particular figure 2.
Also, if you draw yourself a sketch from top view of the aircraft, draw one with the nose into the relative wind (zero side slip) and one with the nose cocked off slightlly, which is a non zero yaw (nose not aligned with relative wind). This may help understand the sum of the forces acting upon the wing, and their direction.
If I remember correctly, the act of slightly masking (via the fuselage) the wing in the direction being slipped into, whereas the wing being slipped away from is not masked, provides the difference in total force via air flow over the wing to make the difference in lift leading to roll toward the slipped towards wing.
.
Memory fails, but depending upon the wing itself, a slight side slip may influence airflow over the wing in terms of increasing "span wise flow" which is not as effective in lift production as chord wise flow ... but I apologize, it's been a few years.
While wikipedia isn't always a great source, the article at the link does a decent job of explaining how the dihedral effect works, and the distinction between dihedral angle and dihedral effect. The illustrations are helpful.
Perhaps you should start at this point in the article
"Using dihedral angle to adjust dihedral effect"
and read from there. I think it will answer your questions, in particular figure 2.
Also, if you draw yourself a sketch from top view of the aircraft, draw one with the nose into the relative wind (zero side slip) and one with the nose cocked off slightlly, which is a non zero yaw (nose not aligned with relative wind). This may help understand the sum of the forces acting upon the wing, and their direction.
If I remember correctly, the act of slightly masking (via the fuselage) the wing in the direction being slipped into, whereas the wing being slipped away from is not masked, provides the difference in total force via air flow over the wing to make the difference in lift leading to roll toward the slipped towards wing.
Dihedral effect of an aircraft is a rolling moment resulting from the vehicle having a non-zero angle of sideslip. Increasing the dihedral angle of an aircraft increases the dihedral effect on it. However, many other aircraft parameters also have a strong influence on dihedral effect. Some of these important factors are: wing sweep, vertical center of gravity, and the height and size of anything on an aircraft that changes its sidewards force as sideslip changes
Memory fails, but depending upon the wing itself, a slight side slip may influence airflow over the wing in terms of increasing "span wise flow" which is not as effective in lift production as chord wise flow ... but I apologize, it's been a few years.
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Originally Posted by MFS
"one wing being lower"
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when one wing is lower than the other, the lower wing produces more lift bcoz of higher AOA to the sideslip velocity of air.
I'm unable to understand how is that the AOA of the lower wing increases wrt relative wind coming from sidewards(lower wing)?
I'm unable to understand how is that the AOA of the lower wing increases wrt relative wind coming from sidewards(lower wing)?
With sideslip, a swept wing will also present a longer effective wingspan to the airflow.
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If the question is about dihedral effect - i.e. Cl-beta (or lv in proper British notation) - and how that varies with configuration, then the roll attitude of the aircraft is irrelevant. As long as there is no sideslip present, the contribution to rolling moment due to the dihedral effect will be precisely, necessarily, zero. The aerodynamics acting on the wing have no knowledge of, or care for, the local gravity vector.
The dihedral effect becomes associated in pilot's minds with a banked condition because the banked condition is "trimmed", in the sense that the aircraft is in equilibrium, in most cases discussed. In order to obtain an equilibrium condition in banked flight it is typically necessary to impose some amount of sideslip on the aircraft, and it is that sideslip which triggers the dihedral effect. But the sideslip is, in principle, independent of bank angle, unless additional constraints are imposed.
The dihedral effect becomes associated in pilot's minds with a banked condition because the banked condition is "trimmed", in the sense that the aircraft is in equilibrium, in most cases discussed. In order to obtain an equilibrium condition in banked flight it is typically necessary to impose some amount of sideslip on the aircraft, and it is that sideslip which triggers the dihedral effect. But the sideslip is, in principle, independent of bank angle, unless additional constraints are imposed.
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Dear rajeshmarndi,
Sweep-back produces a dihedral effect.... (recalled from my fast jet days).
Just as mentioned ... better to understand each concept seperately.
However, they have similar result on lateral stability vis-a vis sideslip/ roll.
You have to imagine this.... sideslip on swept-back inner lower wing makes it a straight wing....thus lift increases on that wing, resulting in a tendency to come out of the roll/ sideslip.
I hope I am making some sense..
Sweep-back produces a dihedral effect.... (recalled from my fast jet days).
Just as mentioned ... better to understand each concept seperately.
However, they have similar result on lateral stability vis-a vis sideslip/ roll.
You have to imagine this.... sideslip on swept-back inner lower wing makes it a straight wing....thus lift increases on that wing, resulting in a tendency to come out of the roll/ sideslip.
I hope I am making some sense..
taken from http://www.b2streamlines.com/EffectiveDihedral.pdf
If the wing has some dihedral (wing tips higher than wing roots), the sideslip creates a situation in which the lower wing, which is moving into the sideslip, meets the oncoming air at a greater angle of attack than the higher wing. This generates a restoring force. It should also be noted that the lower wing, because it is operating at a greater angle of attack, is generating more drag than the higher wing. This creates a yaw to the left, thus reducing the sideslip. The result of all of these effects is to both reduce the sideslip and restore the wings to level.
If the wing has some dihedral (wing tips higher than wing roots), the sideslip creates a situation in which the lower wing, which is moving into the sideslip, meets the oncoming air at a greater angle of attack than the higher wing. This generates a restoring force. It should also be noted that the lower wing, because it is operating at a greater angle of attack, is generating more drag than the higher wing. This creates a yaw to the left, thus reducing the sideslip. The result of all of these effects is to both reduce the sideslip and restore the wings to level.
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If the problem is simply one of trying to understand the fact that dihedral causes the angle of attack of a dropped wing to become greater than that of the raised wing, a demonstration may be sufficient.
Take an A4 sheet of paper and fold it down the centre so that the two short edges come together. Now unfold it so that it looks like a pair of wings with dihedral. We will use this to represent our aircraft. It works best if you use a large dihedral angle (about 30 degrees).
Grasping the sheet at the tail end of the centre crease, hold it up about a foot in front of your face so that the crease is pointing directly at your right eye. Hold the paper so that it is in a non-banked attitude, and raise the front end of the crease to represent a nose up pitch attitude.
Close your left eye and look at the paper with your right eye. The view that you see is what the approaching air would see (assuming of course that air could see at all). You should be able to see the underside of both wings and the view of the left wing should be a mirror image of that of the right wing. This indicates that both wings are set at the same angle of attack.
Now rotate the front end of the crease so that it points at your (still closed) left eye and slightly drop the wing that is closest to your right eye. The view from your right eye now represents an aircraft that has dropped its left wing and is side slipping towards you. Once again the view that you have of each wing represents the view that the approaching air would have. You should be able to see more of the underside of the dropped wing and less of the underside of the raised wing. This means that the angle of attack of the dropped wing is greater than that of the raised wing.
If this difference is not obvious gradually increase the bank angle. You will eventually get a situation in which you can see the bottom of the dropped wing and the top of the raised wing.
The above process will not explain why the angles of attack change, but it should at least convince you that it is true.
Take an A4 sheet of paper and fold it down the centre so that the two short edges come together. Now unfold it so that it looks like a pair of wings with dihedral. We will use this to represent our aircraft. It works best if you use a large dihedral angle (about 30 degrees).
Grasping the sheet at the tail end of the centre crease, hold it up about a foot in front of your face so that the crease is pointing directly at your right eye. Hold the paper so that it is in a non-banked attitude, and raise the front end of the crease to represent a nose up pitch attitude.
Close your left eye and look at the paper with your right eye. The view that you see is what the approaching air would see (assuming of course that air could see at all). You should be able to see the underside of both wings and the view of the left wing should be a mirror image of that of the right wing. This indicates that both wings are set at the same angle of attack.
Now rotate the front end of the crease so that it points at your (still closed) left eye and slightly drop the wing that is closest to your right eye. The view from your right eye now represents an aircraft that has dropped its left wing and is side slipping towards you. Once again the view that you have of each wing represents the view that the approaching air would have. You should be able to see more of the underside of the dropped wing and less of the underside of the raised wing. This means that the angle of attack of the dropped wing is greater than that of the raised wing.
If this difference is not obvious gradually increase the bank angle. You will eventually get a situation in which you can see the bottom of the dropped wing and the top of the raised wing.
The above process will not explain why the angles of attack change, but it should at least convince you that it is true.
