Forward/rearward engine gas loads

[Flight_Idle]

Although not an engine tradesman myself, I have long been curious about the forward rearward gas loads on an engine with reheat.

Most online explanations just mention the change of momemtum resulting in forward thrust, without giving detailed explanations on the resultant areas of gas loads throughout the engine.

I have seen diagrams of a typical old axial flow engine like the RR Avon, with large forward gas loads in the compressor & combustion chamber areas, & lesser rearward gas loads in the turbine area, with the addition of a further rearward gas load in the exit nozzle (Closed position) area.

But the bit that puzzles me is.... When reheat is selected, all you are doing if relieving the rearward gas load in the nozzle area, thus contributing to the total forward thrust, but if the nozzle sticks in the open position without reheat, you get less than normal dry thrust.

Yet I have not been able to find an explanation of the new gas loads (In the unusual non reheat, nozzle open situation)

So, we have relieved a rearward gas load (By having the nozzle stuck open) but we are getting less thrust. So we must have a situation where we have reduced the forward gas loads somehere in the engine. Any idea where?

I have read that the engine should be 'Unaware' if the reheat is active or not. There are some good diagrams of gas loads in the Rolls Royce book, but they do not illustrate the loads in the 'Nozzle open, reheat off scenario'

I hope that makes sense.

Regards, Flight Idle.

[Dodo56]

FI, about a thousand years ago I lived in the same town as you and worked for a well-known engine maker!

When I asked the same question I was told it all depends on the piston areas and pressures within the engine, as the net gas loads on the blading even out. The engine air system allows controlled flows to provide turbine cooling, bearing chamber sealing etc and each area represents a piston area with gas pressure active on it. Some will act forward, others backwards, and as I recall there is no right or wrong way. The intermediate casing then takes the loads and transmits them through to the structure.

Engines can withstand quite major internal loads but from experience it can be almost impossible to calculate or measure them, because gas loads vary with altitide and seal wear. The one thing to avoid is the load passing through the zero point at any time as the main thrust bearings can then skid and self destruct in seconds.

Reheat does not affect the gas loads as the variable nozzle is choked so the engine doesn't see any change. At max dry the nozzle is pretty much running choked anyway.

If reheat fails and the nozzle stick open the fan would overspeed if the ECU didn't fuel back.

[Flight_Idle]

Thanks for such an informative reply Dodo 56, it's put my mind at rest!

Flight Idle.