Anhedral Dihedral?

Question

How does anhedral wings improve maneuverability? Is it because they lower the center of lift to counter the pendulum effect of overstability? How does dihedral overcome lateral stability?

Answer 1

There is no such thing a pendulum stability.  A pendulum is a large weight that is suspended below a pivot point.  In an aircraft, the C of G IS the pivot point.  The stabilizing effect you refer to is the tall tail effect and is a result of a low C of G and a tall tail (especially T-tails).

It is one of the four factors that give lateral stability (the others being swept wings, dihedral, and fuselage interference).  Lateral stability is a funny thing, because, unlike pitch and yaw where they use the airflow, the aircraft doesn't really have a reference point (if you are in coordinated flight, the plane can't tell where down is).  But if a gust catches your plane and rolls it (which it generally does since roll is the least stable axis) it also induces a slip.  We use these four factors to create slip-roll coupling, which will tend to level the wings.  This is also why when you side slip, you have to keep an input into the ailerons to keep the wings from leveling.  

All of these four features (dihedral, tall tail, fuselage interference, and swept wings) are complimentary and make the aircraft laterally stable.  However on an aircraft like the C-5 or C-17, you have a plane that has a low C of G and a tall T-tail, swept wings, and high mounted wings which cause fuselage interference giving an excessive amount of lateral stability.  Therefore the wings are bent down in an anhedral to REDUCE the stability and make the aircraft more controllable.

If you want to make the aircraft more maneuverable in roll, skid the aircraft.   Even though the roll is uncoordinated it will be faster due to these effects aiding the roll inputs by the ailerons.  Make sure you have enough airspeed and altitude to do this.

This article explains it the best:

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

Answer 2

Great reference to answer this question Cal.....!!

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"See How It Flies,A new spin on the perceptions, procedures, and principles of flight."

http://www.av8n.com/how/

Answer 3

Military fighter aircraft often have near zero dihedral, or anhedral. This reduces inherent stability, but increases maneuverability.  Pronounced anhedral is also often seen on aircraft with a high mounted wing, such as the BAe 146, Lockheed Galaxy and others. In such designs, the high mounted wing is above the center of mass which confers roll stability due to the pendulum effect also called the Keel effect, so additional dihedral is not required. In fact, such designs can be excessively stable, so the anhedral is added to cancel out some of the roll stability to ensure that the aircraft can be easily maneuvered.

Dihedral is the upward angle from horizontal in a fixed-wing aircraft or bird wing from root to tip, as viewed from directly in front or behind the aircraft. Downward angled wings have negative dihedral, or anhedral. Wings with local dihedral angles that change along the span are polyhedral.

The purpose of positive dihedral is to confer stability in the roll axis. Most aircraft in the civilian or transport sector use dihedral for roll stability. The dihedral angle is usually greater on low-wing aircraft, compared to an otherwise similar high-wing aircraft.

Simple explanation-

If a disturbance causes an aircraft to roll away from its normal wings-level position, the aircraft will sideslip in the direction of the down-going wing. This creates an airflow component along the length of the wing from tip to root called the relative wind. The dihedral angle can be seen as presenting a positive angle of attack to this lateral flow, hence generating some additional lift. It is this lift which restores the aircraft to its normal attitude.