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Race Engine Technology Magazine

(1) This is perhaps THE most important point of this entire discussion:

PSRU’s, propellers, engine conversions, and engine mounts are COMPLEX devices. PSRU’s are the incarnation of EXPERIMENTAL. There is no significant body of accumulated experience behind most of them, and many of them have a disturbing history of fatal experiences. Treat them as such.

(2) Understand the complexity of your project.

The life expectancy of your PSRU and propeller (and by extension, you, your passengers and your aircraft) depends on a complex mixture of factors including the dimensions, stiffnesses, materials, heat-treatments, and quality control of critical components, the metallurgical goodness of the gear material (AGMA GRADE), the accuracy (AGMA QUALITY) of the gears, the reduction ratio, the J_m (mass moment of inertia) of the propeller, the talent of the designer(s), and other factors.

(3) TEST STAND OPERATION IS NOT THE SAME AS FLIGHT.

That fact has been demonstrated far too many times in real life.

(4) DO NOT be misled by the often-quoted-yet VERY false belief that "if you cannot FEEL vibration, then none exists".

The reality is that destructive vibration can (and often DOES) occur at frequencies that are far beyond the range that humans can detect.

(5) Be SUSPICIOUS of:

1. any PSRU that allows the use of a light flywheel,
2. any PSRU that has a torsionally-rigid coupling between the engine crankshaft and the PSRU drive gear, or
3. any PSRU that which can be bought without a drive coupling system engineered for THAT PSRU and YOUR engine.

(6) RUN (don’t walk) away from anything with an alleged vibration-killing one-way clutch in the propeller drive system, or with magical-sounding (but unexplainable) properties.

Be careful and be cynical.

(7) Ask Hard Questions.

Before buying a PSRU, have the seller satisfy you (with data, not allegations) that the SYSTEM will operate correctly with your selection of engine, propeller, and gear ratio. Don’t be satisfied with marketing drivel.

(8) TBO CLAIMS !

Remember that, unlike the experimental world, TBO numbers in the realworld are established on the basis of substantial experience and documented evidence, both practical and scientific. Far too many TBO's in experimental aviation are no more than the product of some peddler's wishful thinking.

(9) Ask to talk to the designer.

Don’t be afraid to ask him the relevant questions (see FAQ's). Maybe the designer knows about gears but didn’t do the vibration analysis. Maybe the designer has no engineering knowledge at all, but has money enough to buy CAD software and CNC machining equipment, and can talk faster than you. Maybe the "...thousand hours this PSRU has in service" came from 10 hours each on 100 airboats. Ask about previous litigations involving their products. Speak to actual customers.

(10) Don’t accept one of the countless variations on the following theme:

THIS PRODUCT WAS DESIGNED BY THE (best, most experienced, most aviation-knowledgeable) EXPERT IN (the world, the USA, Arkansas, Cleveland) BUT HE (is too busy, is undercover for the CIA, has been abducted by aliens, only speaks Swahili) AND CAN’T TALK TO YOU.

(11)Be cautious of most race-car experts.

There are a FEW brilliant race-engine guys out there, but they work in NASCAR / INDYCAR / F1 shops and you can't get at them. Keep in mind that a race-car-guy's perspective is often quite different than yours in terms of longevity. The purpose of a race engine is to deliver the most power possible within the rules structure, and to survive at that output level / RPM only as long as the end of the race. "Long Life" for a race engine is all relative. For a NASCAR Monster Enegy Cup, Xfinity, or Camping World engine, long life is 5-7 hours.  Long life for a typical hi-performance ski-boat engine is around 100 hours.   Long life for a dirt sprint car engine is around 10 hours.  ( Some race-car-guys think long engine life is 15 seconds, measured in 5-second chunks.)

REMEMBER: Lots and lots of Lycomings and Continentals live thousands of hours at 75% power without a failure. In fact, a great deal of the grief with "LycoNentals" comes from DISUSE.

(12) FAR's Don't Apply.....?

While it is well recognized that the Federal Aviation Regulations defining design and testing standards (PARTS 23, 33 & 35) do not apply to experimental aircraft, nevertheless those regulations provide a rich source of MINIMUM DESIGN STANDARDS which a prudent aircraft designer would be well advised to study, especially with regard to structural and reliability issues.