Turbine performance question

[Fox6]

On the AS350BA the flight manual calls for reducing the MTP and MCP .05 percent for each degree Celsius above Standard temp. For example the Max continuous is usual 98 percent Ng (N1) but on a 100 degree + day it is reduced to around 97%. The Ng is the percentage of compressor rpm (around 51800 I believe - don't have the book handy) My question is this: why then, when the air is thinner and the engine would need to move more volume through by turning faster (increasing rpm) does the FM call to decrease the limit??

Thanks

[NickLappos]

Fox6:

The reason the power must be dropped is actually to control the engine combuster temperature, but since the engine company asks you to govern everything by Ng value, you are told to control Ng downward. As you do, you will be reducing the temperature, too, but TM likes to portray everything in terms of Ng.

In reality, the Ng must be reduced as temp rises because the gas producer turbine blades are getting softer as they are heated up (the metal creeps more), also the higher speed creates higher centrifugal forces on them, and the greater gas pressure pushes on them to make them deform more. This triple effect of higher stress and softer material is reflected in reduced power available as the OAT rises. An abused engine that is run to excessively high temperatures shows blade rubs from the creep, and warped buckets from the gas pressure.

Remember, Ng is not really power at all, it is just an easy thing to show you so that you keep the engine limits. Power is actually torque times output RPM.

[Fox6]

Nick,

thanks for the reply, I figured that you would have the answer. I have one more question on your reply. You said that it was a reduction in Ng to control the combustor temperature correct? I am assuming that this is not the t4 limit you are talking about (it is measured near the free turbine) because I have never seen the engine get anywhere near the t4 limit. Why wouldn't a critical combustor temp show up on the t4 guage as a limit?, Just because they wish to use Ng for everything? or am I missing something here.

The rest makes perfect sense to me now,
Thanks for clearing this up.

F6

[NickLappos]

Fox6,
The issue is really how the usage of the engine is figured. A limit on the temp gage is designed to flag a one time occurrence that requires maintenance action. The power limit that you obey through reduction of the Ng reduces the amount of blade creep for the long haul, more of a way to assure continued safe operation throughout the life of the engine.

In other words, you are reducing the temperature below its absolute limit, but are preserving the engine's long term viability.

Generally, a given engine can have several possible sets of limits, depending on how the manufacturer and customer want to trade service life against available power. If one raises the absolute temperature, the power goes up but life goes down. If you keep the temp lower, and reduce it further during high stress conditions, the engine will last longer.

This is why some engines have multiple models with different certificates and different power limits, but virtually identical parts. Because the parts undergo different operating temperatures and speeds, they might have different part numbers (but be physically identical) so that they don't get confused and mixed during overhaul or field maintenance.

[Fox6]

Nick
thanks for the answers. It makes more sense now.. The flight manual did not really address any of this in detail, it just said to do it.. Now I know why.

Thanks

F6