Dave Siciliano (ADS)
(Top Gun)
09/20/08 03:42 PM
Re: Turbine P-Baron


When Hawker/Beech re-engined the C90 King Air to make the C90GT, they replaced 550shp PT6A-21 engines with 550shp PT6A-135A engines. Why bother, you say? Because the -135A engines are derated; they are thermodynamically capable of 750shp. So they can maintain full rated power of 550shp to a much higher altitude without overtemping. This make the C90GT almost 30 kt faster, and much better climb performance.

Thanks Mase! Robert, if you have the same amount of power; you'll true out faster higher up.
According to Conklin & de Decker:

Up to about 14,000 feet at ISA, the PT6A-21 delivers its full take-off torque. At recommended cruise, this is 1,315 Ft.- Lbs per engine. Above this altitude, the engine loses torque and at 26,000 feet produces only 862 Ft.-Lbs of torque. This yields 235 KTAS for a mid-weight King Air C90A.

Just to compare the -135s: and I quote again:
At normal cruise, the PT6A-135A produced 1,520 Ft-Lbs. up to 18,000 feet. At 26,000 feet is still produces 1,170 Ft-Lbs. torque - 36% more power than the PT6A-21.

So, constant torque on the -21 up to 14,000, then, it declines. For the -135s, torque remains constant up to 18,000 feet; then, declines. This is primarily do to a better compressor.

Did this answer your question? Sorry to get into the new engine, but it shows how the same power is made to a higher altitude.



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