Max altitude of turboshafts

[Mach Stall]

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?

[Brian Abraham]

The tcds for the engine does not list a maximum altitude. My guess would be the altitude limit quoted is for reasons other than engine, that is it's a limit of the airframe in which the engine is installed. The 331-10 is used in the Predator drone which goes to 45,000.

[rigpiggy]

could be that turboshafts are mostly helicopter usage, and you would start having mach issues much above that

[pattern_is_full]

One should not forget that "certified" limits often mean "tested" or "demonstrated" limits - a test pilot took the device to such-and-such a speed or such-and-such an altitude, without problems.

It might work fine a thousand feet higher or 10,000 feet higher or 50 knots faster - but no one knows for sure; no one actually tried it.

So a given engine might have been tested as a helicopter turboshaft only to 22,000 feet (and certified to that altitude) while the same powerplant configured as a fixed-wing turboprop may have been flown to 28,000 feet (and certified to THAT altitude in THAT configuration).

So there may well be no technical or aerodynamic reason for a certified limit. It can simply be "We didn't test it beyond this limit, so we aren't certifying it beyond this limit. It may work fine, but if you exceed that limit - baby, you're on your own."

[barit1]

The CT7 vs T700 story may have other issues as well. The inlet separator (a vortex inertial system) was part of the helo mission requirement for the US Army - early 1970's design. But it had a gear-driven blower to make it more effective. This cost some useful output power, but extended the engine useful life in sand and dust.

The CT7 turboprop was designed a few years later, the vortex system and the blower were removed and replaced with an eductor in the exhaust. More HP became available.

Seems to me this has some effect on what inlet conditions the engines could accommodate.

[GipsyMagpie]

Isn't an APU a turboshaft engine? I don't think they have a similarly low ceiling.

[barit1]

Agreed - merely changing the nameplate is hardly reason to place an altitude limit.

e.g. The T64 (late 50s design) had both helo and turboprop (CT64) applications, with hardly any changes in the core engine. It ran up to 45K or so PA in a test rig; and installed in the DHC-5 Buffalo (http://en.wikipedia.org/wiki/DHC-5_Buffalo), set a couple time-to-climb records for that class.

[etudiant]

Check out the Grob 520 recon turboprop, powered by a TPE331-14F, max altitude 53,000 feet.
Grob G 520 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Grob_G_520)