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GEnxsux
21st Apr 2008, 15:59
Having a bit of a mental block,

Why do high thrust rated engines (e.g GE90-115B) have increased ranges when compared to lower rated engines on the same airframe?

I'm looking at the range equation but can't see a connection....

TyroPicard
21st Apr 2008, 17:48
?Because they can lift more fuel with the same payload?

hawk37
21st Apr 2008, 18:30
The bigger the engine is, generally the lower the TSFC, the Thrust Specific Fuel Consumption. TSFC is lbs of fuel of fuel burned, per lb of thrust produced, per hour. IIRC, expect a minimum of .57 for this type of engine. I seem to remember the TSFC was in the range equation, but could be wrong.

What can complicate things here is the TSFC will go up as speed increases (holding pressure altitude etc constant), and contrary to what many may think, it will also go up with pressure altitude (speed etc held constant). IE, you burn more fuel per lb of thrust produces as speed increases, or as altitude increases.

hawker750
21st Apr 2008, 19:06
Think of your car. If you could modify the engine to produce more power for the same RPM and throttle setting without adding any weight then your car will use less petrol Voila!

enicalyth
23rd Apr 2008, 11:11
In simple form Range = (speed/tsfc)*(lift/drag)*natural log (Initial weight/final weight).

tsfc enjoys a curvilinear relationship with thrust. Typically there is a point on the operating characteristic where it falls to its lowest value with increasing thrust and then begins to rise.

Another way of looking at it is to say that within a family of engines and operating characteristics the law of cussedness of nature means that you can't always get what you want. You might want to trade-off a slightly worse tsfc for longer engine life for example. But if everyone involved in the whole design and buying process is in sympathy with everyone else, the harder you work the engine at its optimum altitude, the better performance you get... within reason.

http://s28.photobucket.com/albums/c220/enicalyth/tsfc_jdb.jpg

and "Gas Turbine Performance" by Walsh and Fletcher, pub Blackwell and ISBN 063206434X page 356 onwards might give further insight.

chornedsnorkack
23rd Apr 2008, 11:30
But I somehow doubt that TSFC increases freely as the engine is increased. Say you were to install a GE90-115 on really small plane... Cessna Citation would be definitely too small, maybe Dassault Falcon.

When GE90-115 is running on idle, the thrust is very small. The fuel burn would be less than in normal cruise or takeoff, but I think still appreciable - and the TSFC would be high on idle engines. Right?

IFLY_INDIGO
24th Apr 2008, 10:54
it depends how much efficient a higher thrust rating engine is at a lower rpm than the normal rpm of lower rating engine for same speed, wt, airframe, oat etc... I am sure all higher thrust rating engines are not equally efficient.. therefore, it is a matter of specific higher thrust rating engine...

cheers..