Plane & Pilot
Tuesday, August 20, 2013

It’s What’s Up Front That Counts

Consider for a moment the job of the lowly tractor propeller (with apologies to pusher drivers)

The props were basically stock Hartzells, but Aerostar had discovered that cutting them down from 78 to 72 inches resulted in seven knots better speed with practically no loss in climb. That's a huge improvement, especially in the speed regime of the Aerostar. The prop deice boots had also been improved by fairing them at the rear to reduce the small, abrupt step behind the rubber boot on the standard deice system.

There were a few other changes, again within the limits of the rules, but all intended to reduce weight, smoothen the flow of air and maximize speed.

The results were spectacular. Miller flew the airplane at full throttle on each of the 16 legs. The result was an average cruise speed of 279 knots between London and Sydney. In fact, the Spirit of Kai Tak not only won the race, it outpaced the competition on each leg of the race, winning all 16 segments. (That's all the more surprising considering that the second fastest airplane in the race was a turboprop, a King Air C90B, that averaged 253 knots.)

The Aerostar experience reminded me of how critical prop selection and maintenance are in translating power to thrust. We lavish great care and attention on our engines and avionics, but the props often receive short shrift. As long as there are no recurring ADs and no obvious nicks or other damage, we give them an often cursory inspection during preflight, then, fly off blindly into the sunset, confident that our propeller(s) won't let us down.

The good news is that they usually don't. In almost 50 years of flying, I've had a grand total of two props run away (both on twins), and one that suffered grievous dings on a gravel runway in Baja, but still managed to get me home. There have also been a few governor malfunctions, but again, they never brought me down. Props have generally been faithful to me.

Trouble is, propellers are easy to take for granted, because they're such obviously simple devices. That's a warning sign—in aviation, anything that seems ridiculously simple often isn't. As mentioned above on the Kai Tak Aerostar, longer blades weren't necessarily better. Propellers become progressively less efficient as tip speed approaches Mach 1.0, 662 knots at sea level or 573 knots at 36,000 feet. When the prop tips exceed about .90 Mach, blades lose efficiency and begin to lose thrust. Turn a shorter blade at slower revs, and you may actually increase thrust and improve speed.

In general aviation terms, a 76-inch prop turning 2,600 rpm is generating a "pure" tip speed of roughly 511 knots. Forward speed is additive and increases the effective tip velocity, but I'll leave that calculation to aerodynamicists. Keeping tip speed within bounds helps make prop-powered airplanes better neighbors. Another method of reducing noise level while maintaining optimum thrust is utilizing Q-Tip design. This bends the tips 90 degrees aft and often results in props that can generate the same thrust on shorter diameter, thereby reducing tip velocity and noise level.


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