Geared Continental GO-300 175 hp Six-cylinder Engines
By: Norm Goyer
I have flown many geared-engine aircraft, including the C-175, Twin Bonanza, Golden Eagle, Helio Courier and dozens of geared Rotax two-and four-stroke-engine powered homebuilts. My experience boils down to some simple advice; read the owner’s manual, and follow the engine operating parameters exactly. The Cessna C-175, which was basically a slightly modified C-172 with a geared 175 hp engine and a different cowling, was a very nice aircraft. Unfortunately, pilots tried to operate the engine along the same guidelines as the non-geared O-300 in the C-172. How, or why, did they do that? I know that I am going to get some 172 pilots upset with me, but many of thrm shouldn’t be allowed to drive a car in Fargo, North Dakota, on a very slow day. A large number of less-than-natural pilots I have known seemed to be C-172 owners. Now, in my opinion, this plane was an excellent choice; the easy-flying Skyhawk probably kept them alive. I often had nightmares about selling a pilot the wrong airplane, thereby causing an accident. Before I would sell a new plane to a prospective owner, I would have our Chief Pilot fly with the person and evaluate his true flying capabilities. If my retired Colonel, a retired Vietnam F-100 and L-19 pilot had doubts, I would not sell the aircraft. On two occasions, the person went elsewhere and bought a similar aircraft and within a year had a fatal accident. One was a F4 POW pilot who wanted a Super Cub; another, a Cessna pilot who purchased a Cessna Aerobat.
Operator troubles with the Continental GO-300 started with imbedded sound and vibration instincts. Pilots were used to the sound that an O-300 made at a 2400 rpm cruise. Those vibration levels seem to be stored in their memory. The geared GO-300 engine was designed to cruise at approx 3,200 rpm. The gearing would reduce the propeller rpm to 2400. The lower prop rpm was very efficient. The engine, including the oil pressure, was designed to work best at a steady engine rpm of 3,000 to 32000 rpm. But the engine didn’t sound right to the O-300 pilots, so they brought the throttle back to what they remembered as being the sound and vibration levels of the smaller engine. This produced a lugging-effect on the geared engine and possible resulting low pressures. Failures occurred constantly until Cessna was forced to pull the engine from their lineup. These same pilots bad mouthed the engine as a piece of junk, when it was their errors that caused the problems. Meanwhile, geared engines, which were being operated correctly by experienced high-performance pilots, racked up hours of trouble-free flying.
The slower propeller rpm was a very distinct advantage, and the ability to swing a longer-diameter prop was another. We had a Beech B-55 Twin Bonanza on the line and its high-legged landing gear was needed due to the Lycoming GO-480 geared engines which were swinging huge two-bladed props for increased efficiency. Cessna also used two geared Continental GTSIO-520 six-cylinder engines in their top-of-the-line C-421 Golden Eagle cabin-class twin. These engines were geared down, so the props could operate in the 1600 to 1900 rpm range. The lower rpm greatly reduced cabin noise. Early versions of this engine did have quality control problems with the gear boxes. Later versions had a reputation of being very reliable.
One of the most popular light four-cylinder engines currently powering a huge number of Experimental, and now the Certified LSA aircraft, is the Rotax four-stroke liquid cooled 912 and 914 engines. This is a geared engine operating in the 2,000-to-5,600 rpm range with the prop swinging in the normal 2000-to-3000 rpm range. They are reaching TBOs once reserved for well-cared-for Lycomings O-360 engines. These engines can also use auto fuel for increased savings. The gearbox is rated as 1.2 to 2.275 reduction ratio.
Many of the Cessna 175s have had their GO-300 engines replaced with Lycoming O-360 180 hp engines. This is a highly desired conversion.