Rotol Contra-Prop

With ceilings for airplanes constantly going up under pres- sure of wartime needs, the ever- persistent problem of converting power into thrust is more and more occupying the attention of designers. One answer to this problem has been advanced in the shape of a counter-rotating propeller.

The contra-prop received some attention in this country by Curtiss-Wright, which tried out an adjustable pitch counter-rotating propeller, consisting of two two-bladers, on a P-36A in 1939.

In England, the Rotol Airscrews, Ltd, has produced a counter-rotating propeller which basically is a normal pair of 3-bladed hydraulically operated, full-feathering, propellers. Mounted in line on a common center at a spacing of 15", the Rotol contra-prop is driven by two concentric shafts revolving one within the other. Both shafts are driven from the same engine by means of suitable reduction gear.

Although weight is naturally increased in the contra-prop, it is found, according to the British, to be only 10 percent approximately for any particular installation. To this must be added the weight of the reduction gear, between 70 and 100 lb, which, including the weight of the contra-prop, does not bring the total weight of the installation above .30 lb/bhp.

The only other disadvantage besides the slight weight increase, is the need of greater maintenance because of increased complication. This is outweighed by the advantages which include: elimination of swing at takeoff, because slipstream is straight on the tail unit; elimination of swing upon sudden opening or closing of the throttle in dogfights, thus enabling the fighter pilot to keep the enemy in the sights even when shutting off the throttle to avoid overshooting; improved handling characteristics because of the elimination of torque and straightening of slipstream; reduction in propeller diameter, in landing gear height and spacing of engine nacelles; absorption of power at high altitudes.

This short article is excerpted from the "Aviation Engineering" column in the October, 1941, issue of Aviation magazine, vol 40, no 10, p 127.