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Race Engine Technology Magazine

This page presents the factors to consider when deciding on the direction of engine and propeller rotation. One of the main decisions is whether or not the aircraft designer is willing to build an aircraft with a propeller which rotates in the "wrong" (for American aircraft, but "right" for European aircraft) direction.

The aircraft in which most of us ( in North America ) learned to fly have propellers which turn clockwise as viewed from the cockpit. Most pilots are unwilling (or unable) to reverse all those hard-learned synapses which cause automatic application of the correct rudder pedal in climb, stall, and high-gyroscopic-moment maneuvers. (This observation, of course, does not apply to pilots who use the rudder pedals only for a footrest.)

For the purposes of this discussion, let’s assume that a counter-clockwise (from the cockpit) rotating propeller (on a single-engine airplane) is not a desirable option. It is generally accepted that most engines considered suitable for conversion turn clockwise as viewed from the cockpit ("front" of the engine). Therefore, If one wishes to retain that clockwise direction of rotation on the propeller, one could select a two-mesh PSRU, or a PSRU using belts, chains, or certain planetary arrangements, in which the output shaft rotates in the same direction as the input.

A gearbox which contains a single idler gear ( "two-mesh" ) requires special attention with respect to gear fatigue. The teeth on the PSRU input and output gears are subjected to severe fatigue loading: zero to maximum and back to zero, every revolution. However, the teeth of a single idler which meshes with both the input and output gears are subjected to the most severe form of fatigue loading: fully-reversing load. They are fully loaded in one direction when they mesh with the input gear and fully loaded in the opposite direction when they mesh with the output gear. In the single-idler case, the allowable working stress level is about 2/3 of the unidirectional-load value, and tooth load limits are usually established by the idler at that reduced working stress.

Some two-mesh gearboxes use an idler which has two different gears on a common shaft, one meshing with the driver and one meshing with the driven gear. That arrangement has two advantages: (1) because of the compound reduction, it can implement a large reduction ratio with relatively compact gears, and (2) it eliminates the additional fully-reversing fatigue scenario experienced by a single idler gear. However, that arrangement adds weight and cost, and the second idler gear (the one that meshes with the outout gear) must be designed for substantially greater torque than the idler gear that meshes with the driver gear.

If, on the other hand, one decides to use an externally-toothed, single-mesh, offset geared-PSRU on a clockwise-rotating engine and wishes to retain the clockwise propeller rotation, one can either: (1) reverse the engine rotation and use a single-mesh reduction attached to the flywheel-end of the engine or (2) retain the original engine rotation and drive the single-mesh PSRU off the wrong end of the crankshaft (a VERY bad choice). Here is an expanded discussion of each option.

1. Reversing the engine (covered on another page of this site) so that it will drive the propeller in the conventional direction has some interesting advantages. An obvious one is that, all other things being equal, a single mesh gearbox will weigh less than one with an idler gear. EPI has designed propeller reduction gearboxes in both configurations (with and without an idler). One model for an engine producing up to 600 lb.-ft. of torque weighs 73 pounds in the two-mesh (idler configuration), including an integrated gear-drive for the prop governor. The single mesh version of that same EPI PSRU weighs just 59 pounds including the same integrated prop-governor drive. A fourteen pound weight reduction can be hard to find on a 500 pound powerplant.
2. A less obvious, but very significant advantage is that the gyroscopic moment produced by the engine can reduce or cancel the gyroscopic moment of the propeller with respect to the loads the engine mount applies to the airframe. In fact, when EPI designed the engine mount system for the Orenda conversion of a popular high-performance twin, the analysis showed that the gyro-moment of the counterclockwise-rotating engine significantly reduced the gyro-moment of the large metal propeller. That reduction leads to a more lightweight engine mount structure that is appropriately stiff and strong.
3. There are several good reasons not to drive off the wrong end (opposite the normal output flange, the flywheel-end) of the crankshaft, including (a) lack of crankshaft structural strength to support the torque output stresses, and (b) torsional vibration issues potentially destructive to the crankshaft. The folly of this practice has been proven time and time again in several classes of boat racing, but still it happens....    A further discussion of this particular example of Wishful Thinking Engineering is beyond the intended scope of this presentation, but be assured the arguments are both valid and persuasive.

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