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)


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.
The Aerostar has always been my favorite twin, and I jump at the chance to ferry one from anywhere to somewhere else.
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."



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