Contra-rotation

In a simple propeller, a considerable amount of energy is lost in the swirling motion of the air in the slipstream. Some of this energy can be recovered if a second propeller rotating in the opposite direction is placed just downstream, as shown in Fig. 6.8. The second propeller tries to swirl the air in the opposite direction, thereby tending to cancel the initial swirl.

Contra-rotation also provides a convenient method of increasing the power throughout for a given propeller diameter.

High-powered piston engines produce a considerable torque reaction which tries to roll the aircraft in the opposite direction to the propeller rotational direction. On the ground, the roll is resisted by the runway, but immediately after take-off the resistance is suddenly lost, and the aircraft is liable to start heading rapidly for the hangar. Contra-rotating propellers overcome this prob­lem, as they produce no net torque reaction. Gyroscopic precession effects are also cancelled, and the lack of swirl in the slipstream makes the flow around the aircraft less asymmetric, which further improves the handling qualities. For small aircraft, however, the extra cost and complication of contra-rotating propellers outweigh the advantages. On twin-engined aircraft a similar effect could be obtained by having the two propellers (and hence engines) rotating in opposite directions. For practical reasons this has rarely been adopted. The De Havilland Hornet was one example.

On multi-engined aircraft, the lack of torque reaction reduces structural loads. The experimental engine installation on the Ilushyin shown in Fig. 6.8 used a large diameter multi-bladed contra-rotating propeller unit. The efficiency of the propeller is an advantage for this aircraft which was designed for long range.

The disadvantages of contra-rotation are the extra complexity, the weight of the necessary gearing, and the noise caused by the highly alternating flow as the second propeller chops through the vortex system of the first.