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Old Jul 11, 2004, 09:03 PM
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fuel usage : miniature jet engine vs real one

How come a model jet engine can only be run 8- 10 min before fuel runs out, while a real aeroplane can fly for 10 hrs+ without refueling?
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Old Jul 11, 2004, 10:26 PM
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Fuel tank size. Full size military jets can only fly a couple hours before they need to refuel by either landing or aireal fueling. Commercial airliners have 3 huge fuel tanks; one in each wing and one under the passenger compartment. So, remember next time you fly in a 737, you are sitting right above a whole bunch of jet fuel.

If you use larger fuel tanks (such as having some in the wings) in your R/C jets you'll get longer flight times. If you add drop-tanks, then you'll extend flight time as well.

-Frank
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Old Jul 12, 2004, 12:10 AM
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say a typical mini jet engine uses 250ml per minute. If this engine were to power a 8hr flight it will need 120L of fuel. Of course no model airplane can carry this much fuel.

1 typical real jet engine is say, around a few thousand times bigger than these model jets, if it was just as fuel efficient as the model jet engines we play with, it will consume I would think a few olympic size swimming pools of fuel. A passenger plane has 2/4 of these engines.

why is it that these model jet engines are such fuel guzzlers for their size?
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Old Jul 12, 2004, 02:00 AM
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First, let's take a look at scale dimensions. The scale volume of a model is the cube of the length-scaler of the model. So a 1/10th scale model has a volumetric scale of 1/1000. If a 1/10th scale model aircraft is capable of carrying 5 liters of fuel, then the full scale aircraft should easily be able to carry 5000 liters of fuel. A Boeing 777 carries 171,160 liters of fuel. A 1/12th scale model would have a wingspan of 5 meters and could theoretically carry 99 liters of fuel (providing you used the proper scale fuel tanks). That would lead to a 3.3 hour flight by the numbers you provided (250 mL/minute). However, those fuel consumptions figures are probebly at full throttle and with proper throttle management longer flights should be doable.

A passenger aircraft doesn't typically use the high thrust to weight ratios a model has. Many sport jet models seem have a 1:1 thrust to weight ratios. The more thrust you create the more fuel you use. The Boeing 777 uses two engines that produce about 95,000 lbs of thrust each, for a total of 190,000 lbs of thrust. The maximum take off weight of the 777 is 660,000 lbs (although the plane is probebly rarely fully loaded). That leads to a thrust to weight ratio of 0.29:1 The less thrust you use, the less fuel you use as well.

The reason why model aircraft flights are so short is because large enough fuel tanks aren't used. If modelers used the wings and excess fuselage space to store fuel, then longer flights would be possible.

Again, full scale military fighter aircraft (which seem to be the most popular to model) typically only fly for a couple hours before refueling is needed.

I hope this isn't confusing.

-Frank
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Last edited by FrankW; Jul 12, 2004 at 02:02 AM.
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Old Jul 14, 2004, 12:07 AM
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Model turbines are not efficient. There is a lot of energy just thrown away. When you are having fun, a few gallons of kerosene is cheap. High bypass turbofans in current passenger aircraft are much more efficient. When fuel cost to operate a fleet of aircraft is millions of dollars the need for efficient engines emerges and suppliers are more than ready to develop them.

Efficiency improvements probably wouldn't even halve the fuel consumption in models, but the cost would be substantial. So it's much easier and cheaper to just burn fuel.

Greg
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Old Jul 14, 2004, 01:08 AM
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Chiang Rai, Thailand
Joined Sep 2001
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And don't forget that model turbine engine is actually a centrifugal flow turbojet engine.Even the full size turbojet engine(which is much more efficient than its tiny cousin) has the SFC(specific fuel consumption)around one lb.of fuel per one lb. thrust per hour and it gets even worse if you light up that afterburner.The modern high bypass turbofan engine has SFC around 0.3lbfuel/1lb thrust/hr.As you can see, you just can't compare the inefficient model turbojet engine to the big high bypass turbofan.Does that make any sense?

Best regards

Tom
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