Smooth operator

They are considered exotic by some, essential by others, and the future of prop blades by a growing number of users. Among them is former U.S. Air Force fighter pilot and airline first officer Scott Perdue.

Several years ago, Perdue produced a pair of detailed videos on FlyWire, his YouTube channel, comparing the performance of metal props on his Beechcraft Bonanza F33 and Aviat Husky with MT-Propeller composite props.

In well-documented flights, noise levels inside and outside the airplane were halved with the MTs. Departure roll was 14 percent shorter in the Bonanza. Climb rates were better in both airplanes—20 percent in the Husky—but cruise speeds were similar. The MTs were lighter, stronger, quieter, immune to corrosion, and have no life limits on the blades. As Perdue said, they are also “orders of magnitude smoother. The vibration differences are considerable. The anecdote you often hear about these props is they’re turbine smooth. They are, and it’s amazing.”

In Germany, the first natural composite propeller was created in 1928 from reinforced, laminated wood. Because metal was precious and wood was widely available, propellers created using this method found early, widespread use on World War II fighters such as the Messerschmitt Bf 109, Supermarine Spitfire, Hawker Hurricane, and many others.

After the war, Germany was restricted from many industries and focused on rebuilding its cities. Civil aviation didn’t really get rolling again until 1955. That’s when Hoffman Propeller started producing props that took design cues from the World War II blades.

Technical designer Gerd Muehlbauer joined the company in 1966 and became the lead engineer. In 1981, he founded MT-Propeller, which claims to be the current global leader in composite propeller sales. Although it produces metal and fully synthetic composite blades, its most popular products feature sophisticated, highly engineered wood cores that greatly dampen vibration.

Wood seems old fashioned, but many people call it nature’s perfect shock absorber. The robust, interlocking fibers in select beech and spruce wood MT uses are dramatically enhanced when injected with special epoxies. Cross-layered and fused together under high pressure and heat, they are then shaped with computerized milling machines. Highly trained craftspeople envelop them with protective carbon fiber layers enhanced with special resins and paint. The leading edges are electroformed nickel that’s five times stronger than aluminum.

The other leader in the certified aircraft market is Ohio-based Hartzell Propeller. It made its first aviation prop in 1917 when Orville Wright and Robert Hartzell collaborated. The company’s advanced structural composite designs feature completely synthetic cores. Like MT, the exteriors are protected by aerospace-quality carbon fiber and precisely engineered nickel leading edges.

Hartzell believe its distinct foam core and carbon sock technologies, introduced in 1978, provide the highest degrees of material consistency and performance.

MT’s 285 employees in Germany and the United States are similarly confident in their materials and processes. The engineered wood substrates they produce are ultrasound tested multiple times and have proven their mettle in more than 30,000 props, and the world’s toughest aerobatic competitions.

MT’s and Hartzell’s elegant creations are many pounds lighter than metal, vibrate less, shed ice better, and come in a wide variety of configurations for mission optimization. All those attributes can benefit pilots, engines, avionics, and airframes. But those benefits come at a cost. Composite props typically cost more than aluminum. Many pilots sell their old props or seek trade-ins to help offset the expense.

Like their metal counterparts, composite propellers have recommended overhaul schedules and are included in annual inspections. However, unlike aluminum blades that eventually go out of spec from having nicks and leading edges dressed over the years, composite blades aren’t life limited. MT’s handbook allows for field repairs that owners can do with recommended epoxies like JB Weld. If nickel leading edges get damaged, they can be replaced at authorized repair stations. When more complex maintenance is required, both companies have service center networks across North America supporting owners.

Composite props are an obvious choice for seaplanes since water won’t corrode them on takeoffs and landings. On aluminum props, water damage from droplets is highly abrasive. The leading edges take on a frayed appearance that leads to re-profiling, reducing the prop’s size and lifespan.

Dennis Warner of Warner Propeller in Tucson, Arizona, sells and services most makes and models of propellers. He says, “Aluminum props are almost like tuning forks. If the dimensions or vibrational frequency of the metal prop interferes with the engine, you can have adverse issues. I used to fly a Mooney with a Lycoming engine that had an rpm restriction on its three-blade Hartzell. I don’t recall any composite props having such restrictions.”

He added that his grandfather attended the Boeing School of Aeronautics in 1932 and was taught to take care of aluminum props. For nearly a century, that’s been true for most A&Ps. They are less familiar with composites, but that’s gradually changing.

Warner added that even though engine teardowns are still mandated by the FAA after a prop strike, the amount of force transmitted through a crankshaft when a composite prop hits the ground is significantly lower than a metal prop.

Issues surrounding prop mass, momentum, and physics bring us to renowned aerobatic pilots Sean D. Tucker and Kirby Chambliss, and legendary aircraft designer Walter Extra. They were all early adopters of composite props because of performance and safety benefits.

Tucker broke eight different crankshafts that had metal props over a four-year competition period. As he remarked from his Central California base, “I want the friggin’ prop and crankshaft working together on the front of my airplane, not flying off into space.”

Chambliss flew a Pitts whose crankshaft cracked after flying with a metal prop. He said, “One more snap roll and it probably would have been gone. Compared with lighter weight composites, metal props put too much load on the crankshaft during snap rolls and gyroscopic maneuvers.”

Extra, who is based in Germany, recalled hot-shot aerobatic “cowboys” in the 1980s making bad landings that clipped off prop tips. They kept flying since the prop mass was so low balance issues weren’t catastrophic. That wasn’t prudent airmanship, but it’s telling that the airplanes were still manageable in flight.

Composites have made it possible to create computer-aided designs with blade shapes and weight distributions that generate far more thrust or braking action when needed. They are also far easier on the engine, airframe, electronics, passengers, and pilots.

Over the past 45 years, increasingly sophisticated composite props are being used on military transports, Beechcraft King Airs, high-performance turbine singles, every single- and twin-engine certified piston airplane, and many experimentals. MT and Hartzell have been leading the charge for decades, with McCauley dipping a few toes in this water. Catto and Sensenich are largely focused on the experimental community.

“If you’re a retired guy on a fixed income and you only use the airplane to go chase a hamburger, I would probably stay with a metal prop,” Prop expert Warner said. “They’re cheaper and get the job done. But if you want the best performance, smoothest ride, less noise, and maximum longevity, nothing beats a composite prop.”

Kevin Knight is an instrument-rated pilot from Texas who owns and flies a Cessna TR182.