Efficiency: Perfect pitch

Unmanned aircraft and airboats would seem to have little to offer general aviation, but growth in the two industries is spurring advancements in propeller design and other technologies—with positive implications for aircraft owners and pilots. Sensenich Propeller has long produced wood, carbon fiber, and metal propellers for UAVs, airboats, and GA. And lately, substantial investments in specialized tooling and tough, lightweight materials for UAVs and airboats have led to a variety of new aircraft propellers—and lower prices.

“We’re producing large numbers of carbon
fiber propellers for airboats and the UAV industry, and those economies of scale allow us to bring down the cost of our aviation products,” said Don Rowell, president of the Sensenich manufacturing division in Plant City, Florida. “Without those business lines, the cost of producing carbon fiber props would be far higher.”

One of the new Sensenich products is a hollow-core, ground-adjustable propeller for Experimental aircraft that actually costs less than fixed-pitch aluminum models ($3,500 for a new carbon fiber prop versus $4,145 for aluminum) and improves both climb and cruise performance.

I recently tested one of the new propellers on my Experimental category Van’s Aircraft RV–4, and the difference between it and the fixed-pitch metal prop was apparent from the first takeoff roll. Acceleration was much quicker with the carbon
fiber propeller, and the rate of climb at 100 KIAS increased to 1,500 fpm from 1,200 fpm.

The reason for the improved performance was clear. The blades were set at a finer pitch, and that resulted in higher propeller and engine rpm—and greater horsepower output. At full throttle the carbonfiber prop turned about 2,400 rpm in the climb, or 90 percent of the Lycoming O-320’s full rated power of 150 horsepower at 2,700 rpm. By contrast, the fixed-pitch metal prop only turned 2,200 rpm at the same power setting, or 83 percent power. Level at a pressure altitude of 6,000 feet and wide-open throttle, the carbon fiber prop reached its 2,700-rpm redline at 154 KIAS (171 KTAS), five knots faster than the metal prop, which topped out at 149 KIAS (165 KTAS) under the same conditions. Once again, the new prop at higher rpm had the advantage of more horsepower (87 percent compared to 83 percent) because of the metal prop’s higher pitch and lower rpm (2,500).

At an economy cruise setting of 65 percent power at an altitude of 6,000 feet, the speed difference was negligible, but the
carbon fiber propeller was noticeably smoother. Sensenich’s Rowell said stainless-steel leading edges make the carbon fiber propellers stand up to rain better than wood or aluminum, but aluminum is superior for rough strips.

Changing the blade angle of the new prop on the ground is simple and just about foolproof. Loosen the prop bolts, insert a metal key, rotate the prop blades until the internal indexing pins contact the key, then tighten the bolts. As long as you use the same key for both blades, the prop will be set correctly. Sensenich says it takes five minutes to change settings, and I found that to be true—excluding the 10 minutes it took to remove and reattach the spinner.

The carbon fiber prop also came with a four-inch prop extension from Saber Manufacturing, of Granbury, Texas, and the solid aluminum casting is such a work of art that it’s a shame it’s hidden under the engine cowling.

On my airplane the best all-around “Goldilocks” setting for the propeller turned out to be number five (on a scale from two to seven), but that can change based on the purpose of any particular flight. For formation flying and aerobatics, I’ll switch to a finer pitch for quicker acceleration and deceleration. For long cross-countries, I’ll move to a coarser blade angle for optimum efficiency.

Sensenich has collected data on carbon
fiber propeller performance and durability from its work in UAVs, airboats, and Light Sport aircraft, and the company will consider pursuing FAA certification if the Part 23 rewrite now under way makes it economically feasible. The company also is exploring three-blade, carbon fiber props for Experimental airplanes (and already is making three-, four-, and even six-blade props for airboats and UAVs).

Rowell said he is convinced that ground-adjustable props are the best option for many Experimental aircraft. “Experimental airplanes tend to vary widely in performance based on engines, weight, aerodynamics, and other factors,” Rowell said. “Any fixed-pitch prop is going to be a compromise, but we think ground-adjustable props are the way to go because they give pilots the flexibility to optimize their propeller’s performance—whether they’re flying from a field elevation of 6,500 feet or sea level.”

Rowell said it’s easy to imagine many more benefits for GA as a result of new technologies in the burgeoning UAV market.

“There’s a great deal of research and development taking place in areas such as heavy-fuel engines, propellers, traffic systems, and autopilots that have direct applications for aviation,” Rowell said. “We’re investing in new tooling and materials, and there’s much more on the horizon.”

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