ATPL Performance Question

[Mr Irons]

As static temperature increases at the same altitude with N1 held constant:

A: TAS increases and SFC decreases.
B: TAS and SFC both increase.
C: Neither TAS nor SFC will be affected.
D: TAS decreases and SFC increases.

Does anyone know the answer and why?

Thanks

[Sepp]

SFC is the quantity of fuel consumed (by weight) to produce one unit of power in one unit of time. In piston engines, the SFC is equal to the mass ratio of fuel to shaft horsepower. In gas turbine engines, the SFC is equal to the mass ratio of fuel to thrust. It is 'best' when its value is minimum, and the value of the SFC decreases with height and in colder temperature conditions - or if you prefer, SFC will increase with temperature. So it ain't A or C.

Other things being equal, if temperature increases then density will decrease and TAS will increase. So it ain't C or D.

Only B satisfies both conditions.

[DFC]

N1 is not a parameter used in piston engine operation?

There are quite a number of issues in this question.

First the simple one - TAS.

At a constant pressure and increase in temperature will cause a rise in TAS provided that the CAS remains constant.

Next the turbine engine.

First, Specific Fuel Consumption = fuel flow (divided by) thrust.

The thrust produced is affected by a number of variables;

1. Engine speed -

the most obvious but not applicable here since N1 is held constant.

2. Aircraft speed

as speed increases then since exhaust velocity is effectively fixed thrust will decrease.

However, increasing the speed at high speed causes the air to be compressed and thus increases the density and hence the mass airflow through the engine and this can cause an increase in thrust which will at times counteract the loss due to increased airspeed. Yes there is a temperature rise (decreases thrust) along with a pressure rise (increases thrust) but the overall result is generally an increase in thrust.

3. Ambient pressure - not applicable here since the pressure is constant.

4. Ambient temperature - As the temperature increases, the thrust will decrease.

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So we have a situation where the N1 and pressure are constant but the temperature increases by an amount.

If the rpm is constant then the fuel flow is constant (very general view) and since the temperature has increased then the thrust has reduced.

thus you are getting less thrust for the same fuel so SFC has increased.

Thus the simple answer to the question is that both TAS and SFC increase - B

i.e. if it is warmer at the chosen level you are going to get there quicker but it is going to be more expensive.


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However, the increase in speed as mentioned above would in theory cause an increase in thrust.

and/or

The loss of thrust due to the increased temperature would cause the aircraft to slow.

It all depends on the specific design. The net effect could be zero.

[Sepp]

True, but as I said in my reply: "In gas turbine engines, the SFC is equal to the mass ratio of fuel to thrust."

[Sepp]

DFC, you have given me good cause to think a bit more about the Q (never a bad idea, I suppose). Thank you for that.

True, N1 does not relate to piston engines. Is, then, the question asking about the effect of temperature on SFC? On reflection, probably not, because jet engines are considered to burn fuel by mass, not volume, and (thrust)SFC is therefore temperature-independent.

If the question really is, then, asking about the effect of holding a constant N1 as temperature rises, then TAS and SFC will decrease*. Sadly, this is not one of the given answers. Deciding what happens to SFC will require one to know whether the questioner considers SFC to be consumption per unit thrust over time (effectively, consumption per mile) - or time/distance to consume a given amount. It ought to be the former, but if the latter, then answer D would (just about) fit the bill. Blind alley, or poorly worded? I leave you to decide.

What would I have answered? B, obviously, as per my original post, and not having the benefit of DFC sitting behind me. I think I still would.

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*Example - C56X at 18000lbs and 18000 ft, and N1 at 85.5%:
At ISA -10, TAS is given as 396 kt, with fuel flow 1758 lb - and nm per 100 lb in still air as 22.5.
At ISA +10, TAS is 388, ff 1598 and nm/100lbs is 24.3.
SFCs are therefore 4.44 lb/nm and 4.11 lb/nm, respectively.

[DFC]

Sepp,

We crossed.

Clearly we are thinking along the same lines.

If you look at a real manual the problem is that they are not set up for a constant N1 - they are usually set for a chosen Mach number or other parameter eg long range cruise i.e. the things of interest when flying.

My comment regarding what you said about piston engines was merely pointing out an important instructional technique - stick to the question asked. This is the instructor forum after all :ok:

Can the original poster let us know the source of this question? - Is it in the JAA question bank or is it an Oz question?

[Sepp]

DFC - very true, on all counts! I stand duly admonished.

As you say, the perf tables do not reference "set" N1s, and in fact I had to look long and hard to find the example I gave. It is one of the few cases in the perf figs where the same N1 is actually referenced at the same alt/mass, at two different static temperatures, and so I guess it satisfies the parameters of the original Q.

Same discussion running in Tech Log - wonder how long it'll be before they're merged...?