So many reasons to use a P.S.R.U.

Nov 20, 2022
Prop speed reduction units have been around for a while now, but a lot of ultralight builders choose not to use them, mostly because of weight. But it just might be worth ditching that heavier wood prop, switch to a lighter “Ultra Prop” style and gaining the weight back in the form of a PSRU.

Please remember that this isn’t an article about the exact weights but about benefits. In the early days they were mostly 2:1 ratio and some times called torque multipliers. Now you can find 1.5:1 to 2.5:1 ratios. Even more for custom builds. Besides the obvious, there are many more reasons.

One is the use of a bigger propeller.

Second is that most units generally place the prop up higher giving you more ground clearance, some planes even handle better with a higher point of thrust.

Third but I think the most important is the side load on a PSRU is transferred through a roller/needle thrust bearing(s) that no only can handle rotation but side loads also caused by the prop wanting to pull away from the engine. When a prop hub is attached directly to the crankshaft all of th side load is applied to the small surface of the thrust side of a plain bearing. You are basically relying on a thin film of oil to keep from regrinding your crankshaft. Professional shops will machine down the crankshaft just enough to fit in a small needle thrust bearing when building a motor for aviation. But I have found that machine shop bill alone is the cost of a PSRU. Most engines are happiest spinning with the side load on the crank. You also have safety factors built into a PSRU, multiple belts, redundancy. Also if you have an idler pulley( only on the slack side) will eliminate prop breakage crank snout snapping of and most vibrations because it can’t transfer resonance to the motor. Ever watched one spinning and see the idler pulley bouncing around at idle, that’s the killer resonance. Chain drives even with idler can still resonate. I am sure that there are some more reasons, like they just look cool, that I am not mentioning, but I hope that this is helpful and not a start for an argument with the prop hub “purist” thank you for taking the time to read, no responses necessary.
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