Here is a story about thrust testing two engines from our time at the Edgewater hangar. This work was the start of our efforts with turbos on Corvairs. The testing below lead directly to flying a turbocharged 2700cc engine in our Stitts Skycoupe test aircraft the following year. The StolGlass aircraft had a very nice 912S rotax in it, with an inflight adjustable prop on it.
If legends, hangar flying stories and old wives tales were to be mistaken for testing and data, the Rotax would have delivered great numbers, which it didn’t. Few people understand that if the gearing on the Rotax was for maximum performance, it would be in the range of 1.8:1, but the gearing the factory selected is far higher, 2.54:1, and this is driven by the need for the engine to meet very stringent European noise restrictions.
Turbo Corvair and 912S Thrust Testing
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. Our previous tests have 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, 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.