How is the power/thrust of a turboprop measured, and what do the numbers mean? I know thrust lbs and horsepower for piston and jet aircraft, but not the TP's
shaft horse power ? SHP
Yes, it is both. They can be measure in either but I don't know what the dirrerence it between them.
A turboprop is a superfancy propellor bolted to a turboshaft jet engine.
The most common is the Pratt & Whitney PT-6, but the Allison 250 is a much easier engine to understand, so here is a link:
You see at the bottom of the engine, the power output(N2), on that particular engine, it's outputting 6,600 rpm, geared down from the output turbine spinning 32,000 rpm. The jet engine's governor holds that at a VERY constant speed, so a turboprop is different from a reciprocating propellor assembly which adjusts propellor speed to change thrust. The governor adds fuel or subtracts it to maintain output speed depending on load.
The N2 shaft is rated on that engine at 650 hp continuous or 714 take-off horsepower, which means it can make 714 hp, but it will overheat after about 5 minutes at that power setting.
Now, the propellor is where the magic of the operation happens. The prop angle changes to make the thrust, and the jet engine adds or subtracts fuel to maintain it's speed, so the propellor will ALWAYS spin at the same speed. It's usually geared down to about 2,500 rpm, I think. The propellor is rated just like any other, with lbs of thrust.
Some people think that the jet exhaust also helps to move a turbo-prop through the air, but that is not true, because turboshaft engines do not make excess power, they make exactly enough to keep the N2 turbine spinning at that magic 32,000 rpm. There is negligable thrust exiting the exhaust, it's mainly just heat, but a whole lotta heat.