Singing Propellers' Praises

Unlike many of us who suffer from chronic gotta-fly disease, most people under age 75 envision a jet when someone says “plane.” For most of them, a business jet is a “small plane.” I guesssome vaguely know there are also really small planes that aren’t jets. But many seem to think that any airplane with a propeller must be left over from World War II. 

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Contrary to popular understanding, propellers offer some inherent advantages over jets, even today. Their big fan-like blades generate far more thrust from a standing start, enabling takeoff from much shorter runways than a jet needs. That’s especially true for propellers that allow pilots to control the angle at which the blades cut into the air. Similar to the gears of a bicycle, they can be set to a “fine” pitch (like a fine-thread screw) for takeoff and climb, and “coarse” for cruise. A jet takes a while to spool up to speed, though it gets up and goes much faster once it does.

Combining a propeller with a turbine engine—a turboprop—offers much of the reliability of a jet plus the short-field performance of a propeller. Though improvement in piston engines has mostly matured out, turbine engines continue to be refined with new materials and configurations, so turboprops are getting faster, quieter, and more fuel efficient. 

Today’s lighter and stronger composite blade materials and computer-based design have enabled manufacturers such as Hartzell, McCauley, and MT to reduce weight and tweak the aerodynamic shape of propellers. The latest composite swept-tip “scimitar” props eke out maximum thrust and efficiency, especially at higher speeds, while placing less demand on the engine. They also look really badass.

Scimitar tips address one of the biggest challenges in propeller design. As the propeller slices through still air at great speed, the tips generate lots of micro-turbulence. That creates annoying noise and energy-sapping drag, which the shape of the scimitar tips minimize, similarly to the way winglets reduce drag at the ends of wings.

While twin-engine turboprops remain a staple, the most dramatic developmental strides have come in single-engine models. Pilatus’s PC-12, Daher’s TBM series, and Piper’s M600 compete in a crowded market. And it’s about to get even tighter with the entry of Textron Aviation’s Denali, expected to make its first flight early this year. But recent changes to European rules have enlarged this market segment by enabling single-engine aircraft to be used for charter operations there.

Interestingly, the configuration of jet engines is morphing closer and closer to turboprops. The “fan” of a jet turbofan consists of the large-diameter series of vanes you can see when you look inside the front cowling (cover) of a Boeing or Airbus jet engine. The percentage of air that passes through the “fan,” as opposed to entering the engine’s combustion chamber, is known as the “bypass ratio.” The fan is essentially a big propeller surrounded by the cowling and generating much of the overall forward thrust.

Older jet engines had almost no bypass ratio. They were like blowtorches that got all their thrust from the exit of jet blast from the high-pressure combustion section. Over the years, manufacturers have learned that the higher the bypass ratio, the more efficient and quiet the jet engine. So, while they’re still classified as jets, turbofans with larger fans and ever-higher bypass ratios are edging closer and closer to essentially being turboprops with multiple vanes taking on the role of the prop blades. 

Another reason turboprops are getting better is that they are getting easier to fly. Where the pilot used to be responsible for monitoring engine and propeller settings, computers now perform those duties; and they do so with far greater precision than is possible with old-style controls. Just as today’s cars’ software manages engine settings and transmission performance, modern turboprops have software that maximizes performance and efficiency. The pilot need only push a “power” lever into a series of preselected detents, leaving more time and awareness for other flying chores.

So, if your mission takes you to a smaller airport, or you want to carry more people and luggage than a small jet can carry, don’t turn up your nose at a turboprop. After all, even though a jet can get you there faster, that may not matter if “there” is a big airport that’s a two-hour drive from where you really want to go.