Resolved. German jets were a waste of time and effort

[red admiral]

We installed filters on our Blackhawks because of the and and particals flying into the engines. They worked fine...

We had 0 incidents or accidents or lost any aircraft because of engine problems or failure caused by the injesting of sand or particles.

Engine life drops quite a bit though.

 

[gumbyk]

Just the inherent 'probable' location of the helicopter engine (close to rotor shaft/cg of airframe) seems to always dictate an axial flow rather than centrifugal type engine...

Most helicopter engines use a combination of axial flow and centrifugal flow stages:
RR250 series
Honeywell LTS101 series
Turbomeca Arriel and Arius series engines.

This is because a centrifugal desgin will have a higher compression ratio per stage, so you can get away with one centrifugal stage where you might need 6 or 7 sets of axial blades to get the same compression. Also, the reliability of the centrfugal compressor. the only catastrpohic failures that I have seen have been as a result of damage to the axial stage blades.

 

[DerAdlerIstGelandet]

Engine life drops quite a bit though.

We were replacing engines between 250 hours and 500 hours. Of course there were cases where we had to replace them earlier, but we had that back in Germany as well. The engines were not lost either. They would be overhauled and eventually put back into another aircraft.

 

[gumbyk]

Adler,
What sort of filters did you have on them? Barrier, or just particle separators?

 

[drgondog]

I'm not sure I follow that line of thought. Helicopter engines are pretty low powered giving fairly small engines compared to the size of the aircraft. Space isn't a big factor for hanging things on the outside.

It wasn't a 'space available comment'.

You should prefer to place the engine and transmission as close to the Cg as possible, particulalry for the 'non aerodynamic condition known a 'auto rotation'.. second factor is the engine to shaft to transmission relationship - short driveshaft usually far better than long drive shaft for transmitting torque more efficiently from the engine to the transmission.



Engine design is a complicated issue, one of the things raised here is the lower sfc offered by axial types. That only comes with higher pressure ratios, and then you need to increase temperature as well. IR signature is a major driver in recent designs. Helicopter engines are operating at much lower temperatures than contemporary turbofans. You could change the materials and cooling arrangements to bring about a couple of hundred hp extra (even quite a bit more than that in some cases) but IR signature balloons (radiation proportional to T^4).

Current operations in Iraq and Afghanistan require you to put a lot of sand and dust through your engine every time you touch down. Axials don't tend to like that very much and it's driving a lot of future requirements - as well as the power drop for hot/high operations.



Probably useful to point out that axials are more finnicky to manufacture as well. The UK was still having problems in the early 1950s manufacturing enough quality blades to limit axial production. The German wartime press/fold method leads to rather dodgy quality blades.

My experience and 'feel' for the application of centrigugal flow engines to Rotary wing applications could be written on a matchbook cover with a magic marker. At Bell we occasionally had input to the pwerplant but the USA, USMC and USN had their preferences - and for me it was all axial flow types in the late 60's and early 70's

 

[drgondog]

I'm not sure I follow that line of thought. Helicopter engines are pretty low powered giving fairly small engines compared to the size of the aircraft. Space isn't a big factor for hanging things on the outside.

It wasn't a 'space available comment'.

You should prefer to place the engine and transmission as close to the Cg as possible, particulalry for the 'non aerodynamic condition known a 'auto rotation'.. second factor is the engine to shaft to transmission relationship - short driveshaft usually far better than long drive shaft for transmitting torque more efficiently from the engine to the transmission.



Engine design is a complicated issue, one of the things raised here is the lower sfc offered by axial types. That only comes with higher pressure ratios, and then you need to increase temperature as well. IR signature is a major driver in recent designs. Helicopter engines are operating at much lower temperatures than contemporary turbofans. You could change the materials and cooling arrangements to bring about a couple of hundred hp extra (even quite a bit more than that in some cases) but IR signature balloons (radiation proportional to T^4).

Current operations in Iraq and Afghanistan require you to put a lot of sand and dust through your engine every time you touch down. Axials don't tend to like that very much and it's driving a lot of future requirements - as well as the power drop for hot/high operations.



Probably useful to point out that axials are more finnicky to manufacture as well. The UK was still having problems in the early 1950s manufacturing enough quality blades to limit axial production. The German wartime press/fold method leads to rather dodgy quality blades.

My experience and 'feel' for the application of centrigugal flow engines to Rotary wing applications could be written on a matchbook cover with a magic marker. At Bell we occasionally had input to the pwerplant but the USA, USMC and USN had their preferences - and for me it was axial flow types in the late 60's and early 70's for the UH1 series (IIRC). I know we used both Lycoming in UH-1a through D and Pratt's in the twins (AH-1J (USMC twin), UH-1H, etc). I can't remember which engine the first AH-1 (army) and the Uh-1E had.