Another example of a free turbine turbo prop is the Bristol Proteus engine, which was developed in the early 1950s and produced about 4000 HP
The danger of a free turbine was if the free turbine shaft broke then the turbine would be
off loaded and go rapidly into overspeed. However should the compressor shaft fracture then h it's turbine would be offloaded and run to overpeed, but in this case the compressor will slow down so the energy would be removed from it's turbine.
To prevent the free turbine overspeeding and coming apart an overspeed device was fitted to the engine which should sense an approach to an overspeed and would close the fuel High Pressure cock and so remove the energy
The free turbine shaft run inside the main engine's shaft and drove the prop via a reduction gear
The Proteus had a number of teething troubles, which delayed it's introduction into service, but it did become a very good engine, though too late to become a huge success
|Quoting CptSpeaking (Reply 1):|
The concept of a free turbine means that in theory, you can stand next to the plane and hold the prop while the engine is starting, and (up until a certain point), it won't move until you let go.
On the Britannia this was not just theory but practice, as you had to release the prop brake prior to starting engines and should you have a tail wind then the prop and it's turbine would start to rotate the wrong way which would result in a slow and hot start. To prevent this a person would hold the prop during the intial start and only release it when he felt it trying to turn in the correct direction.
I think "health and Safety " department would have anight mare at this today