Aircraft propellers are very efficient, with around 80-90% of input horsepower being applied to axial flow. However, that raises the question of where the remaining 10-20% of energy goes. I'm specifically wondering about the percentages that go into specific things, such as:

  • Noise (vibration of the air)
  • Heat (random vibration of the air)
  • Centrifugal flow (impulse applied orthogonal to the propeller axis)

On that last note, I'm curious about how much of centrifugal flow is "blade chasing" (following the movement direction of the blade that created it), and how much is instead span-wise along the blade

I know this is a rather strange sounding question, but any nonrandom motion is at least theoretical able to have some energy extracted. I know this is part of how contra-rotating propellers gain increased aerodynamic efficiency over traditional propeller systems, as the second propeller can utilize some of the centrifugal flow of the first to generate additional thrust. My theory is that, by mounting propellers close to the tip of a wing with top blade moving away from the center line, some of this centrifugal flow could be exploited to "pump" against span-wise flow. But even before the obvious practical concerns, this theory is dependent on there being enough unused energy in the system to run it without additional power load being required

  • $\begingroup$ Heat, as everything that goes wasted. This would be a nice question for physics.se $\endgroup$
    – sophit
    Commented Mar 13 at 15:01
  • $\begingroup$ Some of it, yes, but I highly doubt a whole 20% of the applied power is generating heat $\endgroup$ Commented Mar 13 at 15:04
  • $\begingroup$ Could it be drag? BTW heat would not apply to brake horsepower, only to engine efficiency delivering that horsepower from fuel burned, right? $\endgroup$ Commented Mar 13 at 15:14
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    $\begingroup$ @RileyRosell: why? There are not much options. Sound also go to heat (and I do not think it will consume too much energy, but you can estimate: think dB of an airplane and of an amplifiers, and the Watt of the amplifier). Some *wind" power, but also I do not think too much energy. I doubt it will break some molecules, radiation is possibly just from heat. Maybe some push backward (which requires so additional force, and so we should double count energy), but would it just create heat? $\endgroup$ Commented Mar 13 at 15:16
  • $\begingroup$ @RobertDiGiovanni: drag is finally heat too 😉 $\endgroup$
    – sophit
    Commented Mar 13 at 15:21

1 Answer 1


Eventually, it all ends up as heat. It always does.

Initially, there is some that is held in the swirl velocity of the propeller slipstream. Over time, this velocity difference dissipates due to the friction within the air -- which generates heat.

There have been many studies on wingtip propulsors as a means of either increasing propulsion efficiency or decreasing induced drag. It has many complicating factors, but it is something that others have studied from time to time.

  • $\begingroup$ Eventually, yes, but my question is more about immediately behind the propeller disk $\endgroup$ Commented Mar 13 at 16:49
  • $\begingroup$ @RileyRosell: it goes in aerodynamic drag which eventually get dissipated on heat due viscosity. Some power is also dissipated in the gearbox connecting the propeller to the engine: in this case the dissipation is due local structural deformations which eventually also transform in heat. Is this what you're looking for? $\endgroup$
    – sophit
    Commented Mar 13 at 17:07
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    $\begingroup$ @RileyRosell I addressed this -- there is a swirl velocity. This can be recovered to an extent. People have studied placing propellers at the wingtip. There is a measurable benefit to doing so. $\endgroup$ Commented Mar 13 at 18:58
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    $\begingroup$ Sorry about that, didn't mean to sound snippy $\endgroup$ Commented Mar 13 at 19:14

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