View Full Version : Turbo Prop Gearing & Free Turbine

28th Aug 2001, 14:01

Could someone help me with information on gearing with turbo prop engines. Are there any turbo props that arn't geared down?

One last question, could anyone help me with a definition and a bit of a description of a free turbine and on what differences are compared to other turbines.

Any information would be much appreciated

28th Aug 2001, 14:59
Regarding the gearing, as the turbine blades rotate at something near 20,000 rpm, if there was no gearing to the prop then ..well it doesn't bear thinking about. The 12ft diameter prop on an ATP for example, rotates at a maximum of 1200 rpm, anything faster and the prop tips would be supersonic and inefficent.

A Free turbine engine, such as the PW126, has no direct connection between the hot part of the engine and the prop. Simply put, the prop is driven by a turbine that sits in the jet pipe, rather than via a gearbox off the engine itself. You can run these engines with the prop stationary and use them as an APU.

28th Aug 2001, 18:49
Just to give another analogy for the 'free turbine'. The free turbine engine has the mother of all clutch mechanism's, and behaves like an 'auto' car. When the engine needs to drive the wheels (rotor) it's clutch is engaged. And, when it is stationary, at the traffic lights say (APU generating elec/air only) it 'free-wheels'. :)

29th Aug 2001, 04:32
The two terms are fixed shaft, and free turbine. They both relate to the form of construction of the engine. On a free turbine engine, the compressor/turbine assembly rotates independantly to the prop/gearbox/turbine assembly. That is, you can spin the prop, and the compressor wont turn. (For example PT6). On the fixed shaft, the compressor turns when the prop turns, because they are on the same shaft (through a reduction gearbox of course)(for example Garett 331). I hope that clutch comment was a wind up! :) Hope this helps.

30th Aug 2001, 04:11
Thanks for the information, it has cleared up some things. I probably worded it badly though, what I meant to ask is are there any turbo props that are designed to run at 1200-2000 RPM without gearing, the approx speed that the prop runs with gearing.

Thanks for the info :D

[ 29 August 2001: Message edited by: Surmount ]

1st Sep 2001, 04:41
A turbine engine is not designed to run that slow. Turbines used for turboprops run somewhere around 40000 RPM where they are most efficient. :p