What happens to specific range if temp rises

[verbc]

Hi,


I'm a bit confused on the following subject, I would be pleased to get the correct explanation:

What happens to specific range if temp rises?

SR = NM/lb fuel thus TAS/FF. And TAS rises as temp increases, FF decreases with increasing temp thus SR will increase.

Is this correct???

[Keith.Williams.]

For maximum specific range we require best airframe efficiency and best engine efficiency at the same time.

Fuel flow in propeller aircraft is proportional to power required, so best airframe efficiency means getting the best TAS to Power ratio. This occurs at Vmd.

Fuel flow in jet aircraft is proportional to thrust (which is equal to drag in constant speed level flight). So best airframe efficiency means getting the best ratio of TAS to drag. This occurs Vmrc which is about 1.32 Vmd.

For best engine efficiency we need to get the best ratio between power output and fuel flow in prop aircraft, or between thrust output and fuel flow in jet aircraft. This ratio is called the Specific Fuel Consumption or SFC.

SFC is the cost per hour in Kg of fuel for each unit of thrust or power produced.

Jet SFC = Fuel Flow in kg/hour / Thrust

Prop SFC = Fuel flow in kg/hour / Power.

For maximum range we need minimum SFC.

SFC varies with a number of factors including the temperature of the incoming air. The colder the air, the lower will be the SFC.

So if the air temperature increases, then the SFC increases and this causes the specific range (and the range) to decrease.

If you are studying for the JAR ATPl Perf exam then the answer you require is that SFC is directly proportional to air temperature.

[verbc]

Thanks for the reply.

I got this question on my theoretical entry exam with Jetairfly. Next week I have been invited for the interview so I suppose I will be asked for an explanation.

So in response to your answer are this assumptions correct:

- higher temp means higher density thus less airframe drag and longer range
- higher temp means more thrust required thus more FF thus shorter range.

So which of the two is the limiting factor

[Keith.Williams.]

Satement 1
"higher temp means higher density thus less airframe drag and longer range".

No, this is not true.

Increasing density will not decrease drag.

The higher temperature causes a decrease in density. This reduces power and thrust output at any given RPM, so you must increase RPM.

Increasing RPM increases fuel flow.

If you were initially in ideal conditions for maximum range, then the increased RPM would increase SFC.

This would increase fuel consumption rate and decrease specific range and range.

Although your TAS at any given CAS would have increased, this benefit would be outweighed by the increased SFC.


Statement 2
"higher temp means more thrust required thus more FF thus shorter range."

No, this is not true either.

But as described above, the increased temperature will increase the SFC.

The increased SFC means that each unit of thrust cost more in terms of fuel consumption rate. So to maintain the same thrust you are bruning the fuel faster.

It is this which causes the range to decrease.

[verbc]

Thanks,

Finally it's getting clear to me. Problem solved