"De-Rated" vs. "Flat Rated"? (Turbine engines)

[msw]

Can anyone explain the difference in these terms? I understand the concept, and I suspect there may be a subtle difference in the definitions of these two phrases relating to turbine engines........ but no one (yet) has been able to explain it satisfactorily to me. I am hoping one of you smart guys out there can!

 

[SpiraMirabilis]

De-rating just operates the engine at a lower HP rating. For example, the PW123D engines on the Dash-8 produced I think something like 2150HP but were derated to something like 1990HP because it reduces the wear and tear. There is a ~100HP gap in between Max Rated Power and what is available to you in normal operation.

Flat-rating means that the engine will always produce X horsepower at the prop/shaft/whatever, but as you climb altitude the engine itself starts working harder to maintain this flat rating. You can consider it de-rated at sea level, and then it starts eating into that gap between normal power and max rated power as you climb in order to keep the power output the same. I actually think it is based on temperature rather than altitude but that is a good way to explain it.

 

[Polymox]

De-rating just operates the engine at a lower HP rating. For example, the PW123D engines on the Dash-8 produced I think something like 2150HP but were derated to something like 1990HP because it reduces the wear and tear. There is a ~100HP gap in between Max Rated Power and what is available to you in normal operation.

Flat-rating means that the engine will always produce X horsepower at the prop/shaft/whatever, but as you climb altitude the engine itself starts working harder to maintain this flat rating. You can consider it de-rated at sea level, and then it starts eating into that gap between normal power and max rated power as you climb in order to keep the power output the same. I actually think it is based on temperature rather than altitude but that is a good way to explain it.

Good explanation.

 

[Patrick]

It is the difference between the thermodynamic potential of the engine to make power, and what the engine controls are set to deliver. De-rated and flat rated will both produce less than the engine's thermodynamic ability. The beauty of flat rating is the ability to hold takeoff power all the way to much higher altitudes for better climb performance and cruise speeds. A common example would be comparing the -21 or -28 PT6 on the King Air 90 vs the -135 on the 90 GT. They are all flat rated to 550HP, but a -21 90 will true at 230-240 (on a good day) knots, while the -135 powered 90 GT will true 270 and provide significantly better climb rates on the way up.

Also consider that on the Piagio, their PT6 will make takeoff power all the way up to somewhere around FL250. This is part of the reason that they can get to FL410.

 

[msw]

It is the difference between the thermodynamic potential of the engine to make power, and what the engine controls are set to deliver.................

So.... is one of those flat-rated and the other de-rated? Which is which?