How much lift is lost by wing dihedral, and can it be fixed?

Question

The loss of lift, though negligible, was mentioned in another thread regarding the angle of dihedral wings. Not knowing the coreect name but will external "ribs" on the wing surface reduce that loss? Also, is that lift loss less or greater with forward swept wings than straight or rear swept? And is that loss due to the dihedral angle of the wings increase with a greater angle?

Answer 1

The amount of lift lost to dihedral is because of a trigonometrical condition, and cannot be fixed by ribs.

Lift is always perpendicular to the local surface, and each wing half develops lift at the dihedral angle. Only the vertical bit is useful for suspending the aeroplane weight.

In order to quantify:

• Cos 5° = 0.9962, so less than 0.4% of lift is not useful.
• Cos 10° = 0.9848, meaning a 1.5 % loss.

Wing sweep has the same roll/sideslip stabilising effect as dihedral does (this answer), so less or no dihedral is required if the wing is swept. But there are many other considerations related to lift and drag with a swept wing.

Answer 2

Dihedral is included in many wing designs because it is very useful in making the plane fly in a stable, comfortable manner. As Koyovis explained, lift loss is minimal, and if more lift is needed, simply make the wings larger (or reduce weight!). This is called wing loading.

Is far as external "ribs", these are called wing fences, and are not usually needed on top of the wing unless you plan to fly near the speed of sound.

However, wing fences underneath the wings, at the wing tips, are very useful for slow flight. These are seen with many Short Take Off and Landing kits. Bottom wing tip fences, along with flaps and slats, help increase the Lift Coefficient and maximum Angle Of Attack of your wing.