DESIGN: SynchroLite ~ Power Train - Soft Start Clutch

Outside Helicopters


John Uptigrove, who has Mosquito, is using a Behlen centrifugal clutch that is made for airplanes. It appears that it must be purchased from Hirth. I think that John said that it is a German clutch. I spoke to Matt at Recreational Power Engineering he said that clutch is for 100 hp Hirth engine. It weights 12 lb.


Used a friction clutch and ratchet.


A lever, which is accessed from outside the cockpit, lowers the output shaft end of the engine and thereby put the required tension on the belts to drive the rotors. BJ Schramm also commented on the fact that the pilot must be bent over when performing this function and also during any subsequent inspection. He also said that the pilot must be very alert to other people in the area. The rotor hub is at about 6'-6' elevation and the blade tips could be lower. There is also the obvious danger of the tail rotor. An intermeshing configuration, with blade tips that are even lower at the sides, should probably have a cockpit operated soft- start plus rotor brake.

Re Centrifugal clutch; by Lu Zukerman on PPRuNe

Answer A as applied to Bell clutches would be incorrect. As applied to the centrifugal clutches used by Sikorsky answer A would be correct. Answer B is incorrect for both applications

There are several types. On the original design of what is now known as the Model 47 the clutch was made up of three spring loaded shoes that looked very much like brake shoes. The shoes were spring loaded to the disconnected position. When the engine was started, and it was below X rpm the shoes remained retracted and the rotor was not rotating. As the pilot raised the rpm to a level where the centrifugal force overcame the spring load the shoes would pivot outward and make contact with the clutch drum which resembled a brake drum.

Since this entire assembly was bathed in oil the engine really had to be reved up to make solid contact. As the rotor was spun up the amount of inertia that had to be overcome decreased and the clutch was in a solid drive lock. I don't remember the exact reason but back around the early fifties they took out the springs and the clutch was engaged but slipping just as the engine was fired up and the rotor started turning immediatly. The last Bell model that had this type of clutch was the J2 Ranger. After that, with the advent of the free turbine engines they no longer needed the centrifugal clutch.

On Sikorsky helicopters (HO3-S and HO4-S (S-55 A Model they had a different centrifugal clutch design that had multiple spring loaded shoes that were thrown outward against the spring load making contact with the clutch drum when the engine exceeded 1300 RPM +/-. After that the Sikorsky clutch and the Bell clutch worked the same.


Tension Idler Pulley:

Centrifugal: Items 0158, 0159 & 0230

Both Honda ATV's and Yamaha ATV's have centrifugal clutches.

Locate directly after engine.

Required clutch torque capacity = Maximum engine torque x service factor.

Service factors on Rockwell clutches are from 1.25 to 4.

Could drum be aluminum.

Could and should bearing be bushings; because they are used so seldom.

Thoughts re Soft-Start:

The function of 'soft-starting' is only used once during a flight. It is used at the beginning of the flight, when the craft is on the ground and it is used at reduced torque. Once the rotors are rotating they will never be revolving at a RPM less than that of the engine(s) / [reduction ratio].

Introduction Page | SynchroLite Home Page | Electrotor Home Page | Dragonfly Home Page | UniCopter Home Page | Nemesis Home Page | AeroVantage Home Page:

Last Revised: February 3, 2005