Type Half Prop-Reduction

Rev - Nov 15, 2017

By Jan Zumwalt (EAA #66327)

See Also

For additional information on this topic see VW Type 1 & 2 Prop-Reduction.


T he use of a reduction unit is common in the construction of experimental homebuilt aircraft when automotive engines may be used. These engines, in addition to their lower cost, are customarily smaller in cubic inch displacement and develop peak torque at high revolutions per minute (rpm), typically near 4,500 rpm.

Traditional aircraft engines, where the propeller most commonly is fastened directly to the engine crankshaft, develop peak power at a safe and efficient speed for the propeller - about 2,500 to 3,000 rpm. This speed is considered the typical maximum rpm for a aircraft propeller due to the need to keep the propeller tip speed below the speed of sound.

Why Not 1/2 VW?

P ower pulses and rpm keep a 1/2 VW from efficiently using a re-drive. The 1/2VW, being a 4-stroke, has one power pulse per revolution of the crankshaft (each cylinder fires every other revolution). 2-stroke engines have two power pulses per revolution (each cylinder fires once each revolution).

The 1/2 VW lacks enough inertia in the way of a flywheel on the crankshaft, to suppress or tame these power pulses. The broad range of frequancies being transmitted on each beat promote fast fatigue of parts. The closer a part is connected to the crank (in this case the redrive) the harder these pulses would be.

Also, since HP is torque times rpm and the VW has roughly half the rpm of a typical 2-stroke, you can see that the power pulses are 4 times as hard as a 2-stroke. This beats on the redrive and causes intense torsional vibration. A redrive for a 1/2VW would have to be four times as strong (so probably 4X as heavy and bulky) as a redrive for the same 2-stroke HP.

What if I Want to Try It?

T he engine shown here is a Cuyuna 40hp snowmobile engine converted for aircraft use. The same technique could be easily adapted for a VW engine. The engine in these pictures has 250hrs with this installion and has been periodicaly inspected and shows no abnormalities.

I f you do not mind the weight, you can build a 4 groove 3v type belt redrive for low cost using an electric motor pulley with QD type bushings. Lightning holes can be drilled in the larger pulley to reduce weight. A belt drive with a spring loaded idler wheel should provide damping for the prop and engine pulses. A jackshaft and two mounted bearings can be used for the large pulley, and a QD type chain sprocket as the prop hub.

The chain sprocket acts as the "prop hub" and is a very lowest cost way to have a good prop mount. A sprocket was already on hand for this installation but other alternatives are available. The sprocket has a QD type tapered bushing which seizes on the shaft. 6 holes were drilled in the sprocket to match the prop bolt pattern. The prop bolts go through the prop cover plate, the prop, another cover plate and then the sprocket. When the bolts are tightened it tightens both the prop and the sprocket bushing.