I was speaking to a group of pilots a few years back when one of them asked about a story I wrote a decade before on the London-to-Sydney Air Race. Back in 2001, I was fortunate to help position the winning airplane, a Super 700 Aerostar, from Coeur d' Alene, Idaho, to Biggin Hill, U.K., the starting point of the race.
The Aerostar has always been my favorite twin, and I jump at the chance to ferry one from anywhere to somewhere else. It's not a perfect machine, but it's the fastest piston production airplane with any number of engines. It outhandles even most singles, and it certainly doesn't hurt that it's one of the sexiest shapes in the sky.
On this trip, I was to fly the Atlantic with another pilot. For reasons that only insurance companies understand, Cathay Pacific Airlines Chief Pilot Mike Miller, the Aerostar's captain for the London-Sydney race, was told he needed 10 hours in Aerostars before he could act as PIC. At the time, Miller had more than 22,000 flight hours and a dozen or so type ratings in everything up through Boeing 747s, but he had no time in Aerostars.
That meant he needed someone who knew a little about Aerostars to accompany him on the 5,000 nm positioning flight from Coeur d' Alene to Biggin Hill, just outside London. Since I knew as little as anyone about Ted Smith's marvelous, twin-engine hot rod, I was nominated. We split legs on the three-day trip, and Miller turned out to be a brilliant aviator who had practically memorized the Pilot Operating Handbook. I doubt if I taught him much about the Aerostar, but I certainly learned a lot from him about flying the big iron.
This Aerostar, dubbed, the Spirit of Kai Tak after the famous Hong Kong Airport that had only recently been closed, was sponsored by Cathay Pacific and accordingly, it was definitely different, at least in appearance. In honor of China's semi-independent, breakaway republic, the airplane was painted bright red with a gold dragon on both sides of the fuselage. The Spirit certainly looked fast, and as it turned out, it was.
Before we launched for England, I talked to Jim Christy, vice president of Aerostar Aircraft, the company that had prepared the airplane for the race. Christy mentioned that they had done several things to make the airplane as fast as possible, all within the rules, tricks that any Aerostar owner could perform.
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.
Adding blades gets mixed reviews in improving efficiency. An extra blade can be a deficit rather than a benefit. The late Lyle Shelton's racing F8F Bearcat, Rare Bear, mounted an 18-cylinder, Wright R-3350 engine that first flew with a four-blade, Douglas DC-7 prop. Shelton later switched to a wider, three-blade tractor borrowed from a turboprop, specifically, a Navy P-3 Orion. That proved to be the better, more efficient propeller for Shelton's race plane. (The prop was so big that Shelton and all other Rare Bear pilots were forced to land the airplane three point.) Although many World War ll fighters fly behind four-blade props, few general aviation airplanes operate with four-blade propulsion, as the higher thrust doesn't offset the extra drag.
Except when it does. Many years ago, Mooney tried switching from a three-blade to a longer two-blade prop in search of better cruise speed. In fairness, the resulting airplane did fly about a knot or two faster, but the reduction in climb and the longer takeoff and landing roll convinced Mooney to switch back to the three-blade model.
Blade design has evolved considerably in the last 20 years. Curved, semi-scimitar blades with significantly modified airfoils have gradually displaced the older straight blades that were the rule until the dawn of the 21st century. These days, many of the new-generation propellers feature a blending of airfoils and changing pitch along the entire length of the blade. Shorter blades with optimized angle of attack are common at stations closer to the hub. Moving the point of optimum AOA closer to the hub allows for better efficiency since the rotational speed at the revised station is slower and can often generate more thrust.
Today's props are about 85% efficient. Back in 1903, the Wright Brothers' eight-foot diameter two-blade pusher propellers were 81% efficient in converting their engine's 12 hp to thrust. (In contrast, my airplane's motorized tug develops six hp.) While a mere four percent improvement may not seem like a major technological advance in 110 years, it should serve as a reminder of how simple most propellers are.
But propellers need love, too. Attend to any nicks or chips as soon as you spot them, maintain the props per the manufacturer's recommendations, and never pull or push on the prop tips. If you need to use the prop as a handle to move the airplane, push or pull near the hub, and remember the words of Max Conrad, a famous long-distance pilot and A&P mechanic. "Treat every prop like a pistol, and always assume it's loaded."