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# Temperature penality on turbine heli

## Temperature penality on turbine heli

(OP)
Does anyone know the weight penalty for a one degree outside temp increase if at max take off. Especially a sk76. If not, any other reference such as a chart or equation. Thank you very much.
Replies continue below

### RE: Temperature penality on turbine heli

lepage (Aeronautics)
Why don't you do a web search on density altitude?
B.E.

### RE: Temperature penality on turbine heli

(OP)
Thanks for your reply Berkshire. Yes, I have put a search but only results I can get is basic calculations for density Alt. I did find out that a combustion engine can lose as much as 12 percent of its power on hot and humid day versus around 3 percent for a turbine which the sk76 is turbine of course. Anything else you may know or suggest? Thanks again.

### RE: Temperature penality on turbine heli

I don't know for the Sk76 particularly, but generally gas turbines are "flat-rated" to a certain temperature and pressure altitiude.  This means that no matter what the air temp or altitude within the flat-rating boundary, you will always get the same power.  The fule control system just chucks more juice in to get it.  The downside is a reduction in life, particularly of the hot-end components.  This is all taken into account in the engine lifing algorithms.  Beyond the flat-rating limits, there is a drop on power delivered according to pressure altitude and temperature, but this is surely laid down in the pilots notes (both the flat-rating limits and the lapse rates)?  I know for the civil airlines, it is very clearly laid down.

### RE: Temperature penality on turbine heli

I fly the S-76 for a living but it's not easy to answer this because the aircraft, in common with all others, cannot be operated over max gross weight. I think the data you need isn't available in the Flight Manuals. The published performance graphs only relate to certified safe limits, taking into account the possibility of a single engine failure. The pilot must fly the aircraft to a particular profile (as given by the manufacturer) to ensure the graph is relevant.
Also, which model do you mean? The (twin engined) SK-76 has been around a long time. The maximum certificated gross weight of 'A' models is 10,800 lbs whereas on the  later SK-76s (B, C, C+) it is 11,700 lbs.

The limiting factor is usually transmission torque (100/100%), limited by the pilot watching the Tq gauge and moderating his input on the collective lever. The engines automatically give whatever power is required to achieve the torque output.

If it helps, the C+ can maintain guaranteed safe single engine performance at max gross weight of 11,700 lbs up to an outside air temperature of 30C at sea level for takeoff over a clear area. Above that, max gross weight must be reduced in order to guarantee certified performance. The max. ambient temperature limit is plus 50C, towards which the gross weight should be reduced incrementally to 10,200 lbs.

PEW

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