Max altitude of turboshafts?
I'm trying to understand if turboshaft engines have any design constraints that limit their maximum altitude. For example, the GE CT7-8 turboshaft has an operational limit of 22,000 ft. Yes, I know helicopters rarely fly so high, but I'm curious on technical grounds specifically about the engines and not the aircraft.
Is the engine altitude limitation for fluid mechanics type constraints (e.g., the compressor surge margin erodes at higher altitudes from the lower Reynolds number) -- or thermal constraints (e.g., oil cooling becomes inadequate above a certain altitude) -- or other?
My best guess is that it has to do with the surge margin, because GE also limits the CT7-8 inlet ram pressure ratio to 1.1.
It's just odd that turbojet and turbofan engines can operate at 51,000' and turboprops at 45,000', yet many turboshaft engines are limited to half these altitudes. Ditto re the ram pressure ratio -- I calculated the CT7-8 limitation of 1.1 is only Mach 0.58, which would be unacceptably low for a turbofan or even turboprop engine.
I also wonder if these engine limitations are even that real. For example, Honeywell lists an operational limit of 31,000' for their TPE331-14 turboprop engine, yet the Cheyenne 400LS with this same engine had a certified ceiling of 41,000'.
Any turbine engine gurus know the reason for these limits?
Last edited by Mach Stall; 15th Aug 2014 at 08:00.
Reason: clarity