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originally posted by ben91069
You can do a simple intuitive experiment to allow you too see why. Take a length of metal coathanger and bend it into a V or chevron shape. Hold it by its ends and see which way it "wants" to rotate to. The center of gravity will try to rotate lower than the ends.
The positive V is like a marble sitting inside a bowl, but the Negative V is like trying to balance a marble on the same bowl inverted or its convex surface (just an analogy).
Originally posted by Desert Dawg
Your V is known as dihedral - pic #1 of the airliner - and anhedral, pic #2 of the Harrier.
Dihedral is used as a stabilizing device when the CG is above the wing.
The fuselage will remain level due to the dihedral effect until a control force is input.
Same thing with anhedral for the plane in the pic.
The CG is below the wing and the fuselage remains level due to weight until a control force is input.
Some aircraft, like the F4 Phantom have a fairly flat wing with dihedral out toward the wingtips.
The horizontal stabilizer has anhedral.
Anhedral on the stabilizer because in almost all cases the stabilizer imparts a downward force and not a lifting force like most think.
Lose the horizontal stabilizer and you get a nose down pitch.
The fairly extreme anhedral on the F4 may have to do with getting the horizontal stabilizer into "clean air" much like the T tail does.
Aircraft with no dihedral such as the WW1 Spad were a bitch to fly due to the lack of leveling effect from either dihedral or low CG.
Aileron input was a continual requirement for the pilot.
It did make for a very maneuverable aircraft, but the normal biplane fighters of the era had dihedral and were about as maneuverable.
Originally posted by Valhall
Okay, as you start into a roll (banking turn) you get slip (sideways movement) which causes two things:
1. The dropping wing will experienced a faster airflow than the lifting wing, so you get increased lift on the lower wing relative to the upper wing. That causes the plane to want to level out.
2. The slip introduces a sideways velocity vector onto the wing. For the rising wing this sideways air movement will wash across the top of the wing causing a downward force vector (working against lift, therefore decreasing apparent lift on that wing). Again, these differences in lift on the two wings result in a rolling moment that wants to make the plane level out - which means it is less maneuverable (takes more control force to keep it banked).
So if your wings are angled up (as in my picture) the sideways moving air actually starts impinging on the bottom of the lower wing instead of just moving across it (even more lift on that wing) and starts impinging on the top surface of the rising wing more (even less lift on that wing) so the rolling moment to level out is even greater - even more control force to keep banking.
BUT, with an anhedral wing, the sideways air movement impinges on the TOP surface of the lower wing (a downward force on that wing) decreasing the rolling moment that wants to level the plane, so it takes less control force to stay in a bank.
No, I wasn't talking about both (you can't have both). I was just trying to show you how a dihedral angle makes a plane more stable in a banked turn, and a anhedral makes it less stable. For a fighter with swept wings you want to have anhedral so that you can stay maneuverable.
[edit on 7-9-2006 by Valhall]
Originally posted by waynos
I think talking about manouverability is slightly misleading though (but I'm no expert myself). It is not just 'stability' in general that is affected by anhedral/dihedral, but specifically lateral stability. Being laterally unstable does not impart highter manmouverability, it merely makes the plane want to roll over onto its back while flying in a straight line, therefore shoulder mounted anhedral wings tend to dictate a larger than usual fin and rudder in ordeer to counteract this tendency. Its true that the C-5 etc are easier to turn if they are easier to bank but this is not the same thing as being 'manouverable' FIN's question in this regard higher up the thread conjures up wierd images in my mind of C-5 pilots trying to do Red Arrows impressions
Lateral instability doesn't equate to wanting to flip on your back in level flight