Dihedral

[Dazzler]

Why, in general, is the wing dihedral greater in a low wing than an high wing aircraft?

[ZUM]

The dihedral that is incorporated to a larger extent in low wings compared to high wings is the result, I belive, of the action of the relative wind on the fuselage during the sideslip that develops when the plane is disturbed from equilibrium along it's longitudinal axis. In both a high wing and low wing airplane, some force such as a gust, let's say under the right wing, will cause rotation to the left and because of adverse yaw, the nose will lag behind and result in a sideslip to the left. In a high wing airplane, the fuselage acts as a "barrier" toward the relative wind and permits the lowered wing to generate lift to counteract the roll to the left and restore the plane to straight and level. In a low wing situation, there is no structure to "interefere" with this change in relative wind. Dihedral creates an inherent angel of attack with the relative wind in a sideslip that has the same effect as the fusealage for a high wing airplane. That is my understanding anyway.

[Dazzler]

[ QUOTE ]
The dihedral that is incorporated to a larger extent in low wings compared to high wings is the result, I belive, of the action of the relative wind on the fuselage during the sideslip that develops when the plane is disturbed from equilibrium along it's longitudinal axis. In both a high wing and low wing airplane, some force such as a gust, let's say under the right wing, will cause rotation to the left and because of adverse yaw, the nose will lag behind and result in a sideslip to the left. In a high wing airplane, the fuselage acts as a "barrier" toward the relative wind and permits the lowered wing to generate lift to counteract the roll to the left and restore the plane to straight and level. In a low wing situation, there is no structure to "interefere" with this change in relative wind. Dihedral creates an inherent angel of attack with the relative wind in a sideslip that has the same effect as the fusealage for a high wing airplane. That is my understanding anyway.

[/ QUOTE ]

Sounds like you're describing 'keel effect' which helps directional stability. My understanding is that dihedral improves longitudinal stability, but I guess they affect eachother.

[ZUM]

I'm talking about Lateral stability (stability that counteracts the tendency to roll to the left or right). I believe that the surface area of the fuselage is involved to a larger degree in providing lateral stability in high wing vs. low wing because of the greater amount of surface area beneath the wing. It is this surface that aids in re-establishing lift to the lowered wing of a high wing plane.

[Dazzler]

[ QUOTE ]
I'm talking about Lateral stability (stability that counteracts the tendency to roll to the left or right). I believe that the surface area of the fuselage is involved to a larger degree in providing lateral stability in high wing vs. low wing because of the greater amount of surface area beneath the wing. It is this surface that aids in re-establishing lift to the lowered wing of a high wing plane.

[/ QUOTE ]

OK, I meant Lateral stability, not Longitudinal as I originally said. And your response makes perfect sense. Thanks.

[averyrm]

Think about it this way - it's more stable to hang a weight from a support then to place it on top. Low wings need more dihedral because the cg is above the wing instead of below it and there's more tendency to continue the turn.

[flyguy]

The purpose of dihedral is to improve lateral stability (roll tendency). The reason it improves lateral stability (I'll try to explain it without being able to draw a picture which won't be too easy) is due to the vertical component of lift the wing is creating. The lift being created by the wing is directly perpendicular to the wing its self. This means the actual lift vector is going up at an angle with diheral, and this slightly reduces the lift vector acting directly upward opposing weight. However if the airplane enters a roll and one wing drops, that low wing is then more perpendicular to the ground, and therefore has a greater lift vector directly opposing weight wheras the opposite is happening to the high wing. This makes the low wing want to rise and the high wing drop.

Just as averyrm said, the reason we see more dihedral in low wing aircraft it due to the fact that high wing aircraft have a pengelous (sp?) effect which reduces the need for dihedral. In a high wing airplane when one wing rises it is lifting the fuselage below it. The weight of the fuselage then wants to pull that wing back down.

I hope that helps a little, its really hard to explain without being able to draw it but I hope that clears it up a little.

[chris]

See: http://www.av8n.com/how/htm/roll.html

[seagull]

Flyguy

You need to check the link that Chris put up, it's correct, the explanation you got is one of those aviation myths, goes along with the danger of the downwind turn or that lift is a result Newtons 3rd law from the downward deflection of air from the wing (instead of vice versa).

[averyrm]

Huh, never heard that one before

[Raskolnikov]

Wow thanks for posting that link Chris.

[flyguy]

Who wrote that? I'm not sure I fully understand what they are explaining. They are saying dihedral is for directional stability about the vertical axis? I don't know if I buy that, because with or without dihedral the angle of attack does not change as a slip is entered. Also their explanation does not explain why a high wing airplane does not need as much dihedral as a low wing.

Not saying its wrong, just don't think I understand their explanation and any elaboration would be appreciated.

[seagull]

Section 9.2 describes high wing issues. That site appears to be refreshingly correct.

[flyguy]

Yeah, but I still don't see how dihedral changes the angle of attack in a slip. The wing is still meeting the air at the same angle isn't it?

[SteveC]

[ QUOTE ]
Yeah, but I still don't see how dihedral changes the angle of attack in a slip. The wing is still meeting the air at the same angle isn't it?

[/ QUOTE ]

That would be true if there were no dihedral. Imagine you are directly in front of the plane looking at the leading edge of the wings (better yet, use a model plane or even a sheet of paper to simulate a wing). Rotate the plane around the vertical axis and you will start to see the bottom of the wing that rotates toward you, and the top of the wing that rotates away from you. You are now *seeing* the wing's angle of attack during a slip.

[flyguy]

Okay, I think I'm starting to understand. Not sure I'm ready to try that explanation out on an oral though. I'll look more into it.