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

[C-3PO]

Here is what I think:

SFC = FF/Thrust, since thrust is a function of air density which itself decreases with increasing temperature, then a higher SAT= less thrust which leads to an increased SFC.

TAS is a function of air density as well but in the form 1/ density's square root, so a higher SAT leading to a decrease in air density will accordingly cause an increase in TAS

So I guess your answer is B

[kuksinhyd]

The answer is B

[enicalyth]

I can help you choose your answer. The examiner appears to subscribe to a commonly held view that sfc varies as the ratio of N1 divided by the square root of absolute temperature at the compressor stator outlet. The examiner appears to me at least to be doing his best to suggest that everything else remains constant besides N1. Therefore if his premise is correct, at least within the confines of the exam, sfc varies inversely with the square root of absolute temperature only. If only someone had told the engines to always react with such simplicity!

Good Luck and always remember the examiner is correct, regardless...

[Keith.Williams.]

Specific Fuel Consumption (SFC) is the mass of fuel that is used per hour to produce each unit of thrust (for a jet engine) and each Brake Horsepower (for a piston engine).

SFC is at its minimum value when:

a. The engine RPM is within an ideal band (typically 90% to 95% for jet engines.
b. The throttle is fully open (for piston engines).
c. The air pressure and density are high.
d. The air temperature is low.

So increasing ambient air temperature will increase SFC.


Increasing ambient air temperature will reduce the air density. This will have the following consequences for the engines:

a. Mass airflow rate through the engine at any given N1 will decrease.
b. Reduced mass flow will decrease thrust and power available.

It may be tempting to think that the IAS will remain constant, in which case the deceasing air density will cause TAS to increase. To investigate this idea we need to look at power available and power required.

To maintain constant TAS in level flight we must have power available = power required.

But as stated above the decreased air density will decrease power available.

And

Power Required = Drag x TAS.

Drag is proportional to TAS squared, so power required is proportional to TAS cubed.

So if our TAS increases then our power required would increase with the cube of the TAS.

But our Power Available is actually decreasing because of the reduced air density.

So our TAS will actually decrease.

So the answer to this question is TAS will decrease, SFC will increase. (option D).

[HazelNuts39]

TAS is not immediately affected by a sudden increase of static temperature. However CAS decreases, therefore AoA must increase to restore Lift=Weight to maintain altitude. With N1 held constant N1/root(T1) is reduced, and hence thrust decreases.

Due to the thrust deficit the aircraft decelerates at constant altitude, hence TAS decreases.
SFC has a minimum somewhere between idle and max. thrust. For low power settings sfc increases with decreasing thrust, for high power settings SFC decreases with decreasing thrust.

In most practical cases (airspeed higher than max. range speed) TAS and sfc both decrease, so none of the answers is correct.

regards,
HN39

[MD83FO]

can this be proven in one of the NASA website applet simulators?

[enicalyth]

Tom Benson's applets attempt to mediate much more information than any applet can handle. I have tried to write applets many a time and eventually the toolkit fails to deliver. So I think the answer is a no. I teach my university students fluid mechanics, thermodynamics of propulsion and computer programming and for a spell away from flying I worked for one of your favourite engine manufacturers. Sometimes an applet will hit the nail on the head but that isn't grounds for extrapolating the success to other instances. The other thing I have against applets is the time and effort they can consume when a higher degree of accuracy can be achieved quicker and with an accessible set of instructions using an ordinary computer. But there again one of my most succesful students chose a desktop flightsim for his honours thesis instead of some grand engineering conundrum. Not only did it make the marketplace but I have to concede it's quite good and the cheeky humour of such a project did get him a place within a "proper" simulator company. That's one in the eye for me and one up for him so I may be a grouchy OF.

Cheers

The "E"

[rudderrudderrat]

Hi Mr. Irons,

As temperature increases at a given FL, Air Density decreases.

The Fan (N1) produces most of the thrust. The lower the density, the less power is needed to rotate it at the same N1 therefore fuel flow decreases - SFC has decreased. With the same IAS, TAS has increased because of the lower density.

I'd have gone for A.
That's why we fly as high as we can (low air density) for best TAS & fuel economy.

[blackmail]

hello all,
checking all the different opinions & theories, all correct, there seems to be no consensus on what is the correct answer to this question.
so let's analyse two different scenarios : 1) aircraft is stabilized at cruise IAS/MACH, with a certain cruise N1, Fuel flow, Thrustlever position, resulting in correct thrust output in accordance with actual outside conditions of Temperature & Pressure. now the aircraft enters a warmer airmass. what is happening ? due to the increased Temperature & corresponding reduced density a thrust deficiency develops & the aircraft keeping altitude constant will have a tendency to slow down. now the automatics(or pilot intervention) are kicking in to keep above parameters constant. so up(forward) go the thrustlevers, N1 increases, Fuel Flow increases, thrust output goes back to original setting. we have now a situation where SFC = (FF/F) is higher & TAS is also higher, due to reduced density. this would correspond to answer B) & most probably would happen in the real world. BUT that is NOT the question asked here. so let's go for scenario 2). aircraft again enters a warmer airmass. Autothrottle is off. initially, due to reduced density, N1 & TAS increases only slightly trying to temper but not sufficiently the Thrustloss, which is exacerbated by reducing N1 back to original setting(pilot intervention or because N1 was already at a limit) . now FF will be slightly lower but F will be much lower, resulting in SFC increase. due to the thrust deficiency the aircraft IAS/M, will slow down. TAS will reduce at a slower pace than IAS, due to reduced density, but it(TAS) will reduce. this scenario correspond to answer D) TAS decreases & SFC increases.hope this reasoning makes sense but i am open for any correction
greetings to all,
bm

[HazelNuts39]

Just an afterthought to my post above:

The end result will be decreased TAS, CAS and Mach, and (at speeds above 'green dot') increased specific range (NM/kg of fuel). For speeds above 'green dot', NM/kg decreases with increasing Mach, and is independent of ambient temperature. Also, while NM/kg is of practical use for a pilot, sfc is merely of academic interest. Perhaps the question confuses specific range with fuel mass flow per unit thrust (sfc)? In that case the correct answer is obviously D. I'd bet on that.

regards,
HN39