thrust specific fuel consumption

[GTP]

Can any propulsion engineer explain this to me and does it improve with altitude on a jet or remain the same??
please

[popay]

Hey GTP, you can check it out here. It looks like the TSFC is increasing with the ALT for turbojet maintaining the constant speed or mach. I have tried both with speed and mach and every time went from 0 ft with the steps of 10000 ft and the TSFC has increased slightly, due to significant loss of thrust, which is compared to fuel flow reduction is much higher. Mathematically, TSFC is a ratio of the engine fuel mass flow rate mdot f to the amount of thrust F produced by burning the fuel:

TSFC = mdot f / F


http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
Well thats, what I have found out.
CHeers.:8

[enicalyth]

engineer-pilot-engineer, and back again.

Good answer popay! Mind if I add a point?

In practice we are more immediately concerned with weight (which is a force) than with mass. So the daily units of tsfc in flight are not fuel mass per unit force per second (gm/N/sec) but rather fuel weight per thrust force per hour i.e. lb/lb/hr or kg/kg/hr. As long as fuel and thrust are measured in the same units of weight such as lbf or kgf the figure is the same... 0.55lb/lb/hr is 0.55kg/kg/hr.

Tsfc varies in an engine depending on what you are asking of it. Engineers often talk of 'on-axis' performance and 'off-axis' meaning how close the engine is running with regard to optimum conditions..... for the engine. A graph of tsfc closely resembles a plot of Lift/Drag. No surprise there because if you divide weight by L/D the result is the thrust force required to maintain equilibrium. But TSFC varies with rotational speed, with the inverse of sqrt(abs Temp) and with a lot more besides which latter bit is where the great Tom Benson and his uproarious applet go gloriously tits up despite the NASA sticker. It all gets too complicated to cram into one applet, so... he leaves the difficult bits out and is happy for the wrong answer to pop out, sometimes disguised with a real engine's name. Shame Thomas!He employs more effort in constructing the message than in the content and therefore commits the classic teaching blunder which is - to condone erroneous teaching because it is easier for the pupil to understand and the teacher to teach. If you can unpick the clangers, then fine. But if you can't.... the harm is done.

Tsfc is wonderfully low at sea-level on take-off, perhaps 0.35kg/kg-hr ISA. If your aircraft weighs 200 tonnes and its lift to drag ratio is 10:1 it needs 20 tonnes thrust to remain in equilibrium. At sea-level tsfc 0.35kg/kg-hr that's a fuel flow of 20000 x 0.35 = 7000kg/hr. But if the aircraft magically cleaned up its act, pulled in all extruberances and staggered up to FL290 it might have a L/D of 20:1 and supposing it still weighed 200 tonnes the specific fuel consumption would have risen to about 0.65kg/kg-hr. Do the sums and the fuel flow is 6500kg/hr for equilibrium. Later on having burned off some weight and settled at FL370 the tsfc improves, getting more "on-axis" to mebbe 0.55kg/kg/hr but push your luck to FL390 and tsfc is beginning to go a bit "off-axis" performance-wise again and tsfc tends towards 0.58kg/kg/hr say. All this for my example aircraft. Pick another aircraft and its different.

Obviously on take-off and climb the aircraft is not in equilibrium, it is accelerating and consumes more fuel than "cruise" conditions.

These graphs may help. Think of a large, new, medium-sized twin aircraft with very large by-pass ratio engines and you're not wrong Narel.

http://i28.photobucket.com/albums/c220/enicalyth/tsfc_1.jpg

http://i28.photobucket.com/albums/c220/enicalyth/tsfc_2.jpg

They may be a bit blurry so if you want the full resolution versions just pm.

Hope I have helped. Again like Tom Benson's applet it is difficult to cram everything in I'd like. So if you want more info, pm and I'll reveal as much as I dare and still keep my new job.

Merry Xmas

The "E"

[popay]

enicalyth, thanks mate my respect to you!!! I guess out your explanation, that's why we need to stick to optimum ALT for commercial aircraft with large turbofan.
CHeers.