The following looks at Dihedral and Washout design concepts on the most basic level, and the functional reasons they are incorporated into the construction of aircraft wings. Dihedral angle is the upward angle from horizontal of the wings or tailplane on a fixed wing aircraft. Dihedral generates a stabilizing roll torque due to the difference in angle-of-attack experienced by the left and right wings during a sideslip. A sideslip is when the aircraft is not pointed in the same direction it is moving through the air. Wings with more than one angle change along the span are described as polyhedral, and many sailplanes, especially entry level models, are polyhedral. Andy Lennon explains what happens when a sailplane with polyhedral is side-slipping:
“Thermal gliders have polyhedral—typically 5 degrees from root to 3/5 of the semi-span, with an increase of 3 degrees from the polyhedral joint to the wingtip. On this type, when rudder is applied, the model yaws. Air strikes the wing at a slight diagonal. For the wing on the outside of a turn, the wind that strikes the wing at any given point on the leading edge exits from the trailing edge at a point slightly closer to the fuselage. Because of the dihedral, there is an effective increase in AoA. This situation is reversed on the opposite wing. Both cause the model to roll. It is important that such models have good spiral stability.” From Chapter 10, Roll Control Design, in Andy Lennon’s BASICS OF R/C MODEL AIRCRAFT DESIGN, published by Air Age Media.
Washout is a spanwise twist in a wing that reduces the angle of attack at the tips in relation to the wing roots. In simplest terms, washout means that the wing tips are angled down at a greater angle than at the root. This helps avoid tip stalls in turns and when flying near stall speed, because the wing root will have a more positive angle of attack so that it stalls before the tips achieve a stall angle. When an aircraft stalls in the center instead of at the wingtips, it tends to nose down instead of falling to one side, and with sufficient altitude will recover it’s flying speed.
The first three illustrations illustrate dihedral in models. The last three show the Klingberg Wing, a pure flying wing (no vertical surfaces) that uses washout to promote center stalls—note how its wingtips are angled downward. Photo of LIAM electric courtesy of Graphite13.com.
By Tom Atwood