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B767Driver · December 12th, 2007, 09:32

I went back to an old engine book I had and might have the answer.

Unlike piston powered airplanes, there is a large difference in the actual compressor RPM of many different types and sizes turbine engines. Using "percent RPM" it makes it possible to use approximately the same RPM value for approximately the same power setting on a large variety of engines.

Percent tachometers are designed to read 100% when the tachometers drive shaft is turning at 4200 RPM. To find the actual RPM, divide 4200 by the tachometers drive gear ratio...let's say it's .112. 4200/.112 would yield a compressor RPM of 37,500 RPM when the tach was reading 100%.

So...the 100% value is not a function of engine limitations...it is derived using the tachometer's calibrated value and drive gear ratio.

Hope that helps.

December 12th, 2007, 09:32	#8
B767Driver Old Skool Join Date: Sep 2003 Posts: 1,955	Re: Jet Engine Question I went back to an old engine book I had and might have the answer. Unlike piston powered airplanes, there is a large difference in the actual compressor RPM of many different types and sizes turbine engines. Using "percent RPM" it makes it possible to use approximately the same RPM value for approximately the same power setting on a large variety of engines. Percent tachometers are designed to read 100% when the tachometers drive shaft is turning at 4200 RPM. To find the actual RPM, divide 4200 by the tachometers drive gear ratio...let's say it's .112. 4200/.112 would yield a compressor RPM of 37,500 RPM when the tach was reading 100%. So...the 100% value is not a function of engine limitations...it is derived using the tachometer's calibrated value and drive gear ratio. Hope that helps. __________________ A self described gym rat. "I got next."