At the hangar, we do testing all the time. It's not a special process, but rather integrated at every
opportunity. In these photos, you'll see two tests that we ran during the summer. The photo above shows a
direct drive 164cid Corvair engine we used as a test mule for our simple turbo setup. The
June Thrust Testing Page has more photos of this same engine, but here we're testing a
72" two-blade Warp Drive propeller. In this photo you'll clearly see that this is not a rebuilt engine. We
used the engine as is to confirm the initial sizing of the turbo. At this point, we did not have it heavily
instrumented. Without an accurate EGT gauge, it's quite easy to harm a test motor when initially developing
a turbo installation. However, I had no worries here. This particular engine, nicknamed "Old Greasy," was
purchased running for $200, putting a very low cap on my potential loss. Notably, the engine ran through all
the tests with flying colors, and never broke anything.
Above, Dave is holding the digital optical tach and the pressure gauge. If you look closely, you'll see
the engine is turning a wood prop, the thrust output here is about 360 pounds. This is an appropriate prop for
a 180mph aircraft. When this propeller was replaced with the 72" Warp Drive, a prop appropriate for a plane
with an 85-100mph cruise speed, the thrust shot up to 470 pounds. This is roughly equivalent to the static
thrust available from an O-320 powered Cessna 172. The main difference between the two props is primarily the
pitch, not the diameter. Lower pitched props appropriate for aircraft with lower cruise speeds produce
significantly more static thrust than props with higher pitch. The 72" prop and the turbo is a combination we're
looking into for STOL airplanes. My line of thought: The 20 pound turbo setup is lighter than any gearbox or
belt reduction, comparitively immune to torsional vibration, and a whole lot less expensive. More testing to
follow, but the few runs we made here already exceeded my expectations.
Above is a line of airplanes outside our hangar. The Cessna 120 belongs to Gus, and it's thrust tested on the
June 2004 Thrust Testing Page. The Taylorcraft is Grace's BC12D (C-85 powered).
The Corvair powered KR2 belongs to Steve Makish. Of interest here is the StolGlas in the foreground. This is a
factory built aircraft from South America. It is imported by CR Aviation in Miami. It is a popular aircraft in
South America, and is now being brought to the U.S. as a kit/LSA. Steve Critelli of CR Aviation brought the
aircraft to our hangar to explore the possibility of re-engining the aircraft with a Corvair. When we tested it
for a baseline, it had its factory installed Rotax 912S 100hp powerplant and a 3-blade, 72" diameter, in-flight
adjustable Ivoprop. The engine and propeller were in first class condition with 140 hours on them.
The results of the test were surprising to say the least. Let me start by acknowledging that the Rotax is
a good engine, it's known to make its rated power, and it is something of an industry standard for experimental
engines in its class. Although it's a small motor, barely more than 1,300cc, it's heavily geared, 2.58 to 1.
Common consensus holds that a combination like this should be capable of producing a lot of thrust.
We carefully rigged the airplane for thrust testing to make allowances for the thrust line of the aircraft,
and also to protect the airframe.
After several full power runs, carefully checking the propeller's low pitch setting, and confirming WOT,
the engine pulled 340-345 pounds of thrust. The propeller rpm was about 2,200. With the gearbox, the engine rpm
was near 5,700. This amount of thrust was far less than expected if old wives' tales of low rpm propeller
efficiency are to be believed. Compare this with direct drive Corvair powerplants we have built turning 68" props
at 2,800rpm. The Corvair powerplant easily produces 10-15% more thrust. This is contrary to what most people
have been led to expect. I've been teaching people for many years that higher rpm props are better right up to
the point where the tip goes supersonic, and that low rpm props with low tip speeds are actually a disadvantage.
The time to climb capabilities of aircraft like Pushy Galore are graphic presentations of my point. So
why did Rotax gear this engine down this far? The most plausible explanation is for noise abatement. Although not
yet a design consideration in the United States, European engines are required by very strict laws to meet
extremely stringent noise restrictions. It is illegal to operate engines which don't meet these standards throughout
much of Europe. The Rotax engine is a European product designed to meet these standards. While the Corvair engine
is not particularly loud by American standards, it would be hardpressed to match the Rotax. Having worked for
the German firm MT-Propeller, I can attest to the great efforts European manufacturers go to in order to meet their noise standards.
There's nothing wrong with the Rotax, but there's certainly no magic in its gear reduction when it comes to
thrust output. Of course a 1,300cc powerplant needs some type of a reduction to be a viable 100hp aircraft engine.
But this testing has shown just as obviously that a 2,700cc engine does not need a reduction to more than
match the smaller engine's thrust output. While theories have their place, testing in the real world has far
greater value for people who want to build and fly airplanes, not just talk about them.