I stumbled across a good photo of the helicopter I built in ’74. There was never any intent to sell anything so doll up was not an issue.
Power was by a Canadian Kohler 440 cc snowmobile engine of ~30 hp without the exhaust system.
Empty weight was ~300 lb. using 3 cutdown Hughes OH-6 rotor blades.
It was a floating hub system without flap or drag hinges; the blade feathering bearings were connected to the apices of a triangular aluminum plate, laminated from 4 layers of structurally bonded 0.093 sheet that was in turn connected to the rotorhead via rubber bushings. This permitted the hub to self-align with the rotor tip plane, eliminating the need for individual flap and drag hinges.
The rod that can be seen sticking through the rotorhead was tilted for cyclic and raised and lowered for collective. It pivoted on a ball bushing at the hub center.
The tail rotor shaft can be seen to be curved; that’s to permit it to operate above critical speed. A small diameter straight shaft will encounter whip if operated above the critical speed which is determined by stiffness and unsupported length for a given shaft weight.
This thing was built over a period of 3 months, already having the rotor system left over from another project.
The most time consuming part was development of the tail rotor. I made up a test stand driven by a 10 hp electric motor with the tail rotor gearbox mounted on trunnions arranged so that torque could be measured. I had overlooked the loss due to centrifugal air pumping; at the intended speed of 2400 rpm, zero thrust consumed 2 hp. Plugged the blades and power dropped to ½ hp as it should have been.
In this hovering shot, the left skid is low; that’s because the rotor must be tilted to counter sideways push from the tail rotor and the tight coupling between rotor and airframe causes the tilt.
Tight coupling between rotor and airframe requires that the CG be as near to the rotor center as possible to minimize gyroscopic effects.
Power was by a Canadian Kohler 440 cc snowmobile engine of ~30 hp without the exhaust system.
Empty weight was ~300 lb. using 3 cutdown Hughes OH-6 rotor blades.
It was a floating hub system without flap or drag hinges; the blade feathering bearings were connected to the apices of a triangular aluminum plate, laminated from 4 layers of structurally bonded 0.093 sheet that was in turn connected to the rotorhead via rubber bushings. This permitted the hub to self-align with the rotor tip plane, eliminating the need for individual flap and drag hinges.
The rod that can be seen sticking through the rotorhead was tilted for cyclic and raised and lowered for collective. It pivoted on a ball bushing at the hub center.
The tail rotor shaft can be seen to be curved; that’s to permit it to operate above critical speed. A small diameter straight shaft will encounter whip if operated above the critical speed which is determined by stiffness and unsupported length for a given shaft weight.
This thing was built over a period of 3 months, already having the rotor system left over from another project.
The most time consuming part was development of the tail rotor. I made up a test stand driven by a 10 hp electric motor with the tail rotor gearbox mounted on trunnions arranged so that torque could be measured. I had overlooked the loss due to centrifugal air pumping; at the intended speed of 2400 rpm, zero thrust consumed 2 hp. Plugged the blades and power dropped to ½ hp as it should have been.
In this hovering shot, the left skid is low; that’s because the rotor must be tilted to counter sideways push from the tail rotor and the tight coupling between rotor and airframe causes the tilt.
Tight coupling between rotor and airframe requires that the CG be as near to the rotor center as possible to minimize gyroscopic effects.
Attachments
Last edited: