Is this paragraph about the dihedral effect in the FAA's Glider Flying Handbook correct? I've been reading the FAA's Glider Flying Handbook, 2013 (FAA-H-8083-13A). In chapter 3 ("Aerodynamics of Flight"), the book is discussing stability. On page 3-12, it states:


  Dihedral is the upward angle of the wings from a horizontal (front/rear view) axis of the plane. As a glider flies along and encounters turbulence, the dihedral provides positive lateral stability by providing more lift for the lower wing and reducing the lift on the raised wing. As one wing lowers, it becomes closer to perpendicular to the surface and level. Because it is closer to level and perpendicular to the weight force, the lift produced directly opposes the force of weight. This must be instantly compared to the higher and now more canted wing referenced to the force of weight. The higher wing's lift relative to the force of weight is now less because of the vector angle. This imbalance of lift causes the lower wing to rise as the higher descends until lift equalizes, resulting in level flight.


That doesn't sound right to me.

This paragraph says that there's more lift on the lowered wing and less lift on the raised wing. That's not true, is it? The amount of lift only depends on a wing's airspeed and angle of attack, not on the wing's bank angle.

The paragraph then explains that the lowered wing produces more upward lift than the raised wing. I think this is true, but it's not relevant, because upward lift isn't the only lift which contributes to rolling moment. Rolling moment depends on the total lift (and the direction of the lift).

All in all, I think that dihedral can't produce a stabilizing rolling moment in the way that the book says that it does. Any stabilizing effect must come from a difference in airspeed or angle of attack between the two wings.

Is the book's description of dihedral correct?
No, the explanation is not correct.

Tanner, you are right when you say that lift does not depend on bank angle. Lift is caused by a pressure difference, and pressure can only act perpendicularly to a surface. Therefore, the lift on each wing and its lever arm to the center of gravity won't change with bank and no "correcting" rolling moment is created.

Instead, what does happen is a side force from the wing's bank angle which will accelerate the airplane sideways. This in turn will result in a sideslip condition, and only now will a dihedral effect show up: By changing the angle of attack on each side differently, the sideslip in combination with dihedral will create a correcting rolling moment.

As you correctly observe, a stabilising effect can only come from a difference in angle of attack between both wings, and that only occurs in a sideslip.