Efficiency: Propeller test
Two blades or three?
Photography by Chris Rose
There are lots of reasons:
- Backcountry pilots using short airstrips at high density altitudes want a “low first gear” that maximizes initial acceleration and provides greater ground clearance.
- Aerobatic pilots want light propellers that are easier on engine crankshafts—especially during gyroscopic maneuvers and snap rolls that impose punishing loads, which can shorten engine life.
- Formation pilots want instant acceleration and deceleration that lets them make quick and precise adjustments.
- Pilots flying in noise-sensitive areas want their aircraft to be quiet.
Now, new materials and manufacturing techniques are allowing lightweight, multi-blade, composite propellers to challenge traditional two-blade, metal props across the full performance range—including top speed and price. When AOPA colleague and Socata TB 30 Epsilon owner Mike Filucci recently replaced a 35-year-old, two-blade aluminum prop ($14,500) with a new, three-blade, carbon-fiber Whirlwind model ($15,200), it gave us the chance to carefully measure some of the performance differences.
The new prop significantly improved the airplane’s climb performance while top speed was virtually unchanged (at both 5,000 feet and 10,000 feet).
“I do lots of formation and aerobatic flying in the Epsilon and I was looking for a prop that would provide quick acceleration and deceleration, and be a little easier on the engine,” Filucci said. “The Whirlwind prop is like a speed brake when you pull the power back. It’s noticeably smoother and quieter—and I really like the way it looks.”
To measure the changes, we topped off the Epsilon’s fuel tanks, loaded it identically, then used the TB 30’s autopilot to perform a series of climbs from 3,000 feet to 10,000 feet using a constant indicated airspeed, and then a constant vertical speed.
During the constant-indicated-airspeed climb, the three-blade prop reached the target altitude quicker (seven minutes versus eight), and it showed higher rates of climb throughout the ascent (1,050 fpm versus 950 fpm at 5,000 feet, and 800 fpm versus 750 fpm at 10,000 feet).
During the constant-vertical-speed climb at a rate of 1,000 feet per minute, the three-blade prop maintained consistently higher indicated airspeeds (148 KIAS versus 128 KIAS at 5,000 feet, and 98 KIAS versus 88 KIAS at 10,000 feet).
The Whirlwind prop, governor, and spinner are 3 pounds lighter than the two-blade metal prop assembly it replaced.
Filucci said he was pleasantly surprised that the new, three-blade prop didn’t significantly reduce his airplane’s top speed. In flat-out speed runs at 5,000 feet and 2,500 rpm, the three-blade prop was one knot faster at 189 KTAS (174 KIAS). At 10,000 feet, the two-blade prop was 2 knots faster at 178 KTAS (153 KIAS).
“Frankly, I was willing to give up some speed on the high end to get the flying qualities I wanted in formation, and what I expect will be longer engine life,” Filucci said. “The fact that the new prop doesn’t really slow the airplane down is a bonus.”
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