Derated Engines?

[Foxcow]

Just to make things a bit more interesting, some aircraft have larger engines installed than they were designed for. These engines are "Flat-Rated" back down to what the airframe was designed to handle. In other words, say for example, an airplane was designed to use a pair of 40,000 LB thrust engines, the aircraft designers might specify a pair of 50,000 LB thrust engines and limit their thrust to 40,000 LBS. Why would they want to do this? Simple, remember that turbine engines are "normally aspirated" and start loosing power the moment they start to climb. By using a larger engine, the aircraft can operate at higher altitudes or temperatures before it runs out of power. The engine never produces more than the “airframe-rated” thrust (in this example 40,000 lbs), it’s just able to do it to a higher altitude.

I hope that this isn't more information than you wanted, but I'm bored. Sorry.

Lead Sled

how is output of the turbine measured for the flat rated engines? I can see a turboprop with torque but turbofan??

 

[Donsa320]

It looks as though no one got back to you. I'll give it a shot.

An engine is "rated" for standard day conditions, ie; Sea level pressure at (29.92 in.hg) and 15 deg C (59 deg. F). Let say you have a 14,500lb thrust JT8D-7 engine flat rated to 80 deg F. ( An older jet engine) Your primary power indication is Engine Pressure Ratio (EPR) and will get that T.O power indication of 1.95 EPR at a given engine rpm, say 92% N1, in standard day conditions. (I don't have the books in front of me so I'm guessing). Your engine is 'Flat Rated" to obtain that 14,500 lbs of thrust right up to 80 deg F at sea level. You therefore are allowed an EPR of 1.95 right up to 80 F. What you will see on a hot day is that the N1 engine rpm will be higher (maybe 96%)and the EGT (Exhaust Gas Temperature) will be higher but it is allowed since it is a flat rated engine. As your ambient conditions change to warmer temperatures still, you cannot get your rated thrust and adjustments in airplane weight and T.O.distances must be made.

If N1 RPM is your primary power indication, as it is in many modern engines, FADEC (Full Authoity Engine Contoller) fuel controls have all the necessary inputs (outside temperature and pressure) to adjust the engine for T.O. power automatically for the ambient conditions. But what you will still see though is that as ambient temperature rises so does engine rpm and EGT temperatures needed to obtain the requested thrust.

With older engines it is possible to obtain more than rated thrust but that would be harmful to the engine in the long run. In an emergency it is there at lower altitudes and cool temperatures and pushing the throttles up to the stops is an option. With FADEC contollers though that would not be an option.

I hope this helped.

 

[Foxcow]

That makes perfect sense. Thanks

 

[Lead Sled]

Donsa320...

I was just getting ready to reply. Good explination and you saved my fingers from cramping up.

'Sled