How dissimetry of lift is managed in Bensen type gyroplanes?

Help Support


Well-Known Member
May 25, 2015
West Valley City, UT
I understand that the rotor is windmilling all the time and the pilot can only adjust the orientation of the rotor, without being able to modify the blades pitch in flight.
Oriol, there's a lot going on in these controls, and some of it is subtle. You have this first part exactly backwards. The rotor on a Bensen weighs about 50 pounds, is 23-25 feet in diameter and spins at 350 RPM or more. The light forces on the control stick can't possibly move this gigantic spinning mass. They change the cyclic pitch of the blades, which then leverages aerodynamic force to move the disc to its new position.

However what intrigues me the most is, how Bensen does to avoid dissimetry of lift given that the blades can not flap, since they are bolted together in a single piece, and can not change pitch indistinctively either to overcome dissimetry of lift?
They do indeed flap - the need for flapping is equal and opposite on the two blades, so having them attached to each other rigidly works out just fine. To deal with the dissymmetry, the advancing blade rises as it turns to reduce its AoA, while the retreating blade falls to increase its AoA. At the same time, because they're rising and dropping in unison and by the same amount, they're changing their effective diameter (compared to level) by the same amount, so they have to speed up and slow down by the same amount as they move around the circle, making lead/lag hinges unnecessary.

The rotor is basically on a big U-joint, so moving the stick won't directly tilt the disc. Moving the stick forward increases the AoA on one blade while decreasing it on the other, and precession manifests the effect 90º later. It's all pretty elegant, and results in a very low parts count for what's going on.