Use of Anahedral for Lateral Stability

[aviationluver]

I was always taught that lateral stability was a result of dihedral. If that's the case, why do some heavy jets, such as the C5 Galaxy and AN124, use Anahedral? Also, does use of Anahedral in these planes produce lateral stability? If so, how?

[SMOC]

High wing A/C are more stable than low wings this is why you then get dihedral to increase stability in low wings and anhedral to decrease stability in high wings.

The Dutch Roll tendency described above is exacerbated when an aircraft has too much lateral stability. Generally pilots prefer to fly aircraft which exhibit neutral static lateral stability, or very slightly positive.
As a result aircraft with both swept and high wings often are too stable. This can be "fixed" by incorporating anhedral (negative dihedral.) The BAE-146 to the left is an example of an aircraft with anhedral.


http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page5.html

[Mad (Flt) Scientist]

1. There's no such thing as "lateral stability". Aircraft are neutrally stable in the roll axis except when either subjected to artificial augmentation or a novel design. What this means is that an aircraft does not tend to stabilise at any given roll angle. It's no harder to keep an aircraft at any single roll angle. The fact that the Spiral mode exists is partly due to this.

2. There is a stability derivative which is usually referred to as the "dihedral effect" - notated as either Lv (where the v is a subscript) or Cl-beta (where the l is a lower case subscript and the beta is the Greek letter and a further subscript) or Cr-beta (where r for "roll" is used to avoid CL vs Cl confusion).

This term relates to the rolling moment generated due to sideslip, and is a powerful term in determining the lateral-directional characteristics of an aircraft. It's affected by many things, including wing sweep, position of the wing relative to the fuselage, and, you guessed it, dihedral/anhedral.

Because Lv is such an important term, there is a lot of effort to get the "right" value for a given aircraft - bigger isn't always better, it's possible to have "too much" dihedral effect, ruining one of the handling characteristics.

It so happens that for most - but not all - low wing aircraft, a small amount of positive dihedral gives the best overall aerodynamic characteristics. But for most high wing aircraft, there is already "too much" dihedral effect for good handling, so the designer introduces some anhedral (=negative dihedral) to achieve the desired overall effect.

There can, of course, be non-aero requirements driving the dihedral value chosen (outrigger ground contact or OB nacelle ground clearances, say) and if these are overriding then the aero designer has to do something else to "fix" the right dihedral effect, of course.