The other end of the design scale, the smoothest “propeller” configuration, is found in high-bypass fanjet engines. With modern
turbofans, more than five times as much air passes through the fan
as goes into the core combustion chamber, so it is really acting as a
large, multi-blade (the Rolls-Royce RB211 has 34 fan blades), ducted
propeller. And it’s very smooth and quiet. But a single blade and a
turbofan are the extremes. Most light airplane propellers have two
or three blades. The principles do apply to choosing which of those
two options is best for your airplane. Two blades are generally more
efficient and weigh less (by about 22 pounds in a Top Prop comparison); three blades are quieter and smoother.
Hartzell offers a selection of more than 50 Top Prop options for
several high-performance singles and light twins—mostly pistons.
They include Beech Bonanzas, Barons, and Travel Airs (also T-34s);
too many Cessnas to list, starting with 170s and up to the 400-series
turboprop twins, including the 207 Caravan series of turboprop
singles; Commander 112s; Diamond DA40s (two-blade props in
aluminum or composite to replace the original three-blade MT
composite); Fairchild Metro IIs; most of the Mooney M20 series;
several Piper models, ranging from early Apache twins to the latest
Mirage/Matrix models; and three-blade conversions for Socata’s
TB20s and TB21s.
For some airframe/engine combinations, there is a choice of two
or three blades. For example, owners of Beech Bonanzas with
Continental IO-470 engines can choose a two- or three-blade Top
Prop option. Price is $7,350 for the two-blade kit or $9,500 for the
three-blade kit. As horsepower gets into the 280 to 310 range, said
Trudeau, more blades are needed to absorb the engine power without making the blades so long that their tips approach supersonic
speeds. Most of the “blat-blat-blat” noise you hear when a North
American T- 6 takes off, for example, comes from a series of small
sonic booms from the two-blade prop tips at low pitch and high rpm,
not from engine exhaust. Kits include a polished spinner and, of
Hartzell’s Top Prop series has applications available for several homebuilt designs,
including most of Van’s RVs.
course, the supplemental type certificate paperwork. Depending on
the application, there are also options for propellers with or without
Hartzell’s latest best-sellers include the swept-tip scimitar
designs, which specifically target lower noise, but also are gaining
favor for their distinctive appearance. Trudeau said the challenge in
meeting external noise specs began with Appendix G of FAR Part 36
in 1988. It changed the noise-measuring criteria from “fly-over”
(at 1,000 feet) to measuring noise at a point 8,200 feet from the
beginning of the takeoff roll, with a maximum of 78dB(A) permitted.
“There are a lot of older propellers that could not meet that
spec today,” he said. But they are still legal under their old certification status.
So new propeller designs have to not only improve performance
but also meet the more demanding noise limits. The limits are even
more restrictive in Europe and could be getting even stricter.
Interestingly, there is no standard for measuring interior noise levels
in aircraft, probably because there is no FAR setting standards, said
Trudeau. But Top Prop conversions are known for not only lower
cabin noise levels but also lower vibration. And that translates to less
wear and tear on airframe and engine parts, resulting in greater
longevity. Not to mention more comfortable conditions for pilot
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5500 Sullivan St., Cashmere, WA 98815
1-888-356-7659 • (1-888-EKO-POLY)