Helo Performance - Effect of Ambient Temp Formula?

[The Ferret]

Here is a performance question for all to ponder? Does anybody have a simple formula?

I am trying to establish a very simple layman's way of explaining the effect of ambient temperature of the performance of a gas turbine operated helicopter. I am aware that there will be many variables that will effect density altitude calculations etc. However, I am trying to obtain a figure that very simply illustrates the effect of higher temperatures on fuel usage.

Therefore the exam question is to establish "x" assuming that pressure remains a constant (or any other constants that need to be assumed):

"For every 1 degree C of temperature rise x% more fuel is required to retain the same performance"

I look forward to your solutions.

The Ferret

[Shawn Coyle]

I don't know of any simple rule for this very complex subject.
But you are to be commended for even thinking about it.
PM me for more info.

[topendtorque]

Therefore the exam question is to establish "x" assuming that pressure remains a constant (or any other constants that need to be assumed):

"For every 1 degree C of temperature rise x% more fuel is required to retain the same performance"

Complicated it is, and yes, I think I know what you are asking, but the part of your question as above retains an ambiguity.
That is the pressure does not remain k when the temperaturse rises.

If you could relate it back to any easy way of calculating the weight of the air mass, then the weight of the fuel would vary directly as the mass increasers, or decreases. That is of course as long as your donk's fuel system will always deliver the correct % to give the "same performance".
Perhaps you should ask the engine manufacturers just how they work it out.

The function of the engines' density controller will possibly reveal this dep dark secret.

There are two major problems with assuming too much here.

1) Does the engine manufacture its full rated power at sea level which will always decrease as pressure decreases, or does it have a heap in reserve which allows production of rated power for the helicopter components to be not limited until the engine reaches its 'full throttle' (for want of a better expression) pressure setting? Look at the 206 A model beside the 206B III

2) Many helicopter rotor systems decrease in peformance in less dense air. There are som notable examples. The jetranger blades at altitude compared to the bell 47 blades is one. to see that simply put a soloy 47 up a mountain beside a 206 each with the same engine and see what happens

[Mel Effluent]

Helicopter performance figures (including fuel consumption) are generally related to density altitude rather than pressure altitude. The classically simple method of determing this is the formula:

DA=PA plus or minus 120T

where T is the difference between actual temperature and that expected in the International Standard Atmosphere (ISA). Thus, if you have a pressure altitude of 1000' (where the ISA temperature would be +13 degrees because of the lapse rate of 2 degrees per 1000 feet) and an actual temperature of +20 degrees, the DA would be:

1000 + (120x7) = 1000 + 840 =1840'

In this case, the adjustment is positive because the actual air temperature is hotter than the ISA value and therefore less dense.

The Flight Manual should then give you the fuel consumption figures for this DA.

[singesavant]

2) Many helicopter rotor systems decrease in peformance in less dense air. There are som notable examples. The jetranger blades at altitude compared to the bell 47 blades is one. to see that simply put a soloy 47 up a mountain beside a 206 each with the same engine and see what happens

I am quite curious, what would happen? Do you have any explanation?

Thanks

Davy