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It was asserted (tangentially) in this question that airliners use most of their fuel to overcome frictional losses.

  • Is that a true statement?
    • If so, approximately what percentage of fuel is used to overcome friction?
    • If not, what is the major use of fuel? (Beyond the obvious 'to run the engines'.)
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FreeMan
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    By friction, do you mean surface friction based on the roughness of the surface on the aircraft, like rivets, or total drag? – Ron Beyer Dec 18 '15 at 17:36
  • Good question, @RonBeyer, and I'm not really sure. I'm just questioning the statement made in the other question, as it was unsupported, and didn't seem realistic to me. I would suspect that our usual, high quality answers will address both topics. – FreeMan Dec 18 '15 at 17:42
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    @vasin1987 Lift is a force. A force does not necessarily transfer energy, that depends on movement parallel to the force. Once the craft has reached maximum altitude the lift does not contribute any more potential energy to the craft. I don't think long distance flights could turn off their engines once they reach maximum altitude and reach their destination without burning anymore fuel. That energy has to go somewhere, and it isn't going into more potential energy. – kasperd Dec 19 '15 at 13:00
  • @kasperd my comment does not go into detail about force or energy. Too complicate for me. Just suggest that part of fuel burn is used to create lift to overcome gravity. You need some force to overcome gravity. – vasin1987 Dec 19 '15 at 14:16
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    @vasin1987 Force is not energy. You need force to overcome gravity, not energy. – kasperd Dec 19 '15 at 15:04
  • @kasperd - the Force is from midichlorians and you need it to overcome the dark side. ;) – FreeMan Dec 19 '15 at 22:04

5 Answers5

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No, the majority of the fuel is wasted on inefficiency of the engine.

Internal combustion engines like turbine engines are not exactly energy efficient. Most of the fuel is wasted by running the engine. Only about 35% to 40% of the energy from the fuel is converted to propulsive energy. The rest of the energy from the fuel (~ 65%) is lost on heating up the atmosphere directly, excess kinetic energy in the exhaust stream, internal drag & friction of the engine and producing noise.

The remaining ~35% the energy is there to overcome the work done by drag.

This drag can be split into lift induced drag and parasitic drag. It can also be split into pressure drag and friction drag. For simplicity assume that all the lift induced drag is all pressure drag, and all parasitic drag is friction drag.

Aircraft usually cruise near the speed where drag is lowest. In such a situation, 50% of the drag is induced drag and 50% is parasitic drag. In practice cruise speed is a bit above the minimum drag speed, so the parasitic drag exceeds the induced drag. If our earlier assumption is correct, then over 50% of the drag would be friction drag.

That would bring the total estimates to:

  • 65% wasted
  • 20% friction drag
  • 15% pressure drag

For example the GE90, the engine fitted on most Boeing 777s, produces a thrust of 70 kN in cruise flight of 250 m/s at a fuel consumption of 1.08 kg/s.

The propulsive power delivered by the engine is $70 \cdot 10^3 \cdot 250 = 17.5\cdot 10^6 \textrm{W}$.

Jet fuel has a specific heat of $43.15 \cdot 10^6 \textrm{J/kg}$ The energy consumption of the engines is thus:

$1.08 \cdot 43.15 \cdot 10^6 = 46.6 \cdot10^6\textrm{W}$

That gives an efficiency of 37.6%. This may seem poor but it is one of the most efficient engines available today.

Data sourcePDF for thrust, speed & consumption:

DeltaLima
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    Well you are describing "frictional losses"... it's just within the engine, not on the skin. – fooot Dec 18 '15 at 23:34
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    @fooot Friction within the engine is a very tiny part of the 65%, not really significant. – DeltaLima Dec 18 '15 at 23:38
  • Agreed as turbines are very different than internal combustion engines. The air goes through a series of compression and expansion (stators/compressors) as deltalima said the losses are due to thermal losses in the exhaust from the combustion chamber. The energy that is not converted to turning the turbine shaft. This is why oil is not used to lubricate the turbine engine like the internal combustion engine. – Mark Dec 21 '15 at 02:46
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Aircraft like to fly near their optimum L/D ratio, where drag reaches its minimum. At that speed the drag is split evenly between induced (lift-related) and viscous drag. In a very rough approximation, half of the drag is indeed caused by friction.

However, if we look more closely, drag has more sources. Besides friction and induced drag, there is pressure drag due to flow separation at blunt, rear-facing surfaces. More separation could be caused by boundary layer effects, but this is highly aircraft- and angle-of-attack-specific. One could argue that this kind of pressure drag is also caused by friction.

If we now look at specific missions, more differences creep up:

  • An interceptor will try to reach a specific point in the shortest possible time. Here friction drag will easily be the dominant source of drag, induced drag is small and some of the fuel is needed for acceleration.
  • Airliners fly at optimum transport performance, which is faster than what optimum L/D would require. Friction drag here is indeed the biggest source of drag, but contributes maybe 50% to 60%.
  • Observation aircraft which want to optimize flying time will fly slower than what optimum L/D requires, especially if they use propellers. Now induced drag is responsible for the majority of fuel used.
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Peter Kämpf
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For an aircraft in level flight, lift opposes the weight of the aircraft, and thrust from the engines opposes the drag of the aircraft.

One form of drag is skin friction drag. The statement you reference is in the context of skin heating, and the skin friction is the main way that the skin will be heating. Other significant sources of drag add energy to the air instead.

According to this paper, about half of an airliner's drag comes from skin friction. This estimation of cruise drag for a business jet shows a similar breakdown. About a third of the drag is induced drag, which is the side effect of lift.

To address the original statement, friction drag may account for the majority of fuel use, but not by very much. It is the largest source of drag but not always more than half of the total drag.

fooot
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  • I realize that the question is rather vague, there's not much I can do about that, since I'm going off a quote from the other question. I suppose, though, that skin friction drag is the most likely cause of the friction being referenced. Does the majority of fuel use to overcome skin friction drag statement hold up? – FreeMan Dec 18 '15 at 19:03
  • @FreeMan I gave a shot at adding a statement about that. – fooot Dec 18 '15 at 21:30
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That statement simplifies what's really happening to the basics of physics 101. Technically that statement is 100% true. It is also true for cars and ships: your car use most of its fuel to overcome friction.

Newton's first law: An object that is in motion stays in motion.

That means that once your car or an aircraft has reached the desired speed you should no longer need to run the engine and the car/aircraft will continue moving at a uniform speed until you apply the brakes. Is this how you drive your car? Obviously not. You need to depress the throttle all the way (or at least most of the way).

So what gives? Newton says that the car should continue moving and therefore you should be able to turn off your engine for 99% of your trip! The answer is friction, it all its various forms.

Some people may say: "but most of the energy in flight is used to generate lift, not overcome friction". But lift comes from friction. Lift causes drag, specifically induced drag. But this drag is only possible due to friction - the friction of air with itself. We quantify this "friction" as viscosity. Fluids that are completely frictionless have zero viscosity (such things do exist: they're called superfluids) and it's impossible to generate lift in such fluids. Fortunately air is viscous so airplane wings can work in air.

So.. technically it's true - any moving vehicle spends the majority of its fuel to overcome friction. But it's not really useful from an engineering perspective.

There is an exception though: spaceships. Since friction is almost zero in the vacuum of space spaceships burn their fuel only at the beginning of the trip and at the end to slow down. 99% of the time the engines on a spaceship is turned off and the ship merely coast along thanks to Newton's first law.

slebetman
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  • Please do not take the papers on friction and the Kutta condition too seriously. Lift-induced drag has really nothing to do with friction. If those paper authors would had looked at inertial forces, too, they would had realized that they don't need friction to explain the Kutta condition. – Peter Kämpf Dec 20 '15 at 16:34
  • @PeterKämpf: But those inertial forces are mainly possible due to viscosity. Without viscosity there would be no lift. Without lift there would be no lift induced drag. – slebetman Dec 21 '15 at 01:26
  • @slebetman: Why would lift be impossible without viscosity? Imagine an aircraft moving through an idealized inviscid atmosphere consisting of massive molecules that bounce elastically off the perfectly smooth surface of the aircraft but don't interact with each other except at long timescales, to make sure their velocities before the aircraft arrives follow a Maxwell-Boltzmann distribution. If our aircraft flies with a positive angle of attack, it will still be able to bounce molecules downwards, generating lift (and induced drag too) -- but there will be no friction. – hmakholm left over Monica Dec 26 '15 at 00:10
  • @HenningMakholm: Without viscosity, and if the molecules are indeed fluid (instead of ping pong balls) then each molecule will also move up again to its original position after being bounced down (actually this happens even with viscosity). Without viscosity, the amount of up force generated by the molecule bouncing down is exactly equals to the amount of down force generated by the molecule moving back up - generating exactly zero lift. Of course, real-world superfluids may not exactly generate the same amount of force both ways so physical superfluids do generate lift but not much. – slebetman Dec 26 '15 at 14:03
  • @slebetman: Which force or interaction would make each molecule "move up again to its original position"? – hmakholm left over Monica Dec 26 '15 at 14:55
  • @HenningMakholm: Non viscosity. The molecule moving down creates an absence of molecules above it. This creates a vacuum. In a viscous fluid some of the molecules from above will be sucked in to this region adding to total lift. In non viscous fluids most of the molecules that have moved down will instead move up to fill the void because moving down creates a high pressure region that pushes on the molecules back up. – slebetman Dec 26 '15 at 16:06
  • @slebetman: I'm specifying that the molecules don't interact on short timescales, so there's nothing that can change the velocity of each molecule after it has bounced off the wing and started moving downwards. More importantly, though: Bouncing molecules downwards from the wing means that downwards momentum is being transferred from the aircraft to the molecules in the gas. If, as you claim, no lift is being generated, that momentum will have to be returned to the aircraft. How does that happen? – hmakholm left over Monica Dec 26 '15 at 16:19
  • @HenningMakholm: Then that's not a fluid. So OK. Under your specification airfoils can certainly create lift when flown in an atmosphere of pingpong balls. You yourself have failed to account for conservation of momentum. In a non-viscous superfluid, molecules that have bounced downwards from the wing will move back up to its original position. How does the wing gain momentum? – slebetman Dec 26 '15 at 16:23
  • @slebetman: The wing gains momentum because more molecules hit it on the underside than on the top, due to moving through the air at a positive angle of attack. What happens to those molecules afterwards is irrelevant to lift generation -- it doesn't influence what the wing experiences anyway. – hmakholm left over Monica Dec 26 '15 at 16:33
  • @HenningMakholm: So far we've been arguing theories. I submit an actual physical experiment of measuring lift on airfoils in a superfluid helim "wind tunnel" that found lift to disappear at low velocities and extremely small even at higher velocities. This is not theory, it's simply how nature behaves: http://thesis.library.caltech.edu/455/1/Craig_pp_1959.pdf – slebetman Dec 26 '15 at 16:35
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A jet engine is a Brayton cycle device and has fundamental limits on the performance. From the diagram in the source looks like for the ideal theoretical engine with no friction at all and pressure ratio about 30 (Boeing 747 engine), the efficiency limit is somewhat 60 %.

This means that about 40 % of the energy dissipates as the heat, the engine cannot convert it. It is not lost due friction. This is significant percent but less than a half.

About some WWII jet bomber with pressure ratio about 3 it is possible to say that the major energy loss is because of the engine thermodynamics, not because of the friction.

h22
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