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What difference does horsepower make? If the engine can spin the propeller fast enough, why does it need power behind it?

kevin
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qwertyuiop
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    I feel like there is a similar (but not duplicate) question around here somewhere. If I recall, it answered the question title quite well. – dalearn Nov 29 '19 at 23:44
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    It wouldn't make any difference in vacuum and with perfect bearings. – Eric Duminil Nov 30 '19 at 10:45
  • @dalearn Maybe this one: https://aviation.stackexchange.com/q/43121/18733 – PerlDuck Nov 30 '19 at 10:53
  • @PerlDuck yes, indeed. – dalearn Nov 30 '19 at 13:50
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    to understand this, just put your hand in a bath of water and try moving the water around quickly. it surely takes some power! – Fattie Nov 30 '19 at 15:18
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    If the engine can spin fast enough it must already have enough power to do so. – user207421 Nov 30 '19 at 22:54
  • @EricDuminil A propeller in vacuum would be quite pointless. – Mast Dec 01 '19 at 14:16
  • @Mast sure. And this fact should help to answer the question. – Eric Duminil Dec 01 '19 at 14:30
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    This is not a duplicate of the other question. The answer may rely on the same concepts as the answer of the other question, but it doesn't make them a duplicate, and I doubt very much that the answer to the other question will help resolve this question for the person who is asking it. – Daniele Procida Dec 01 '19 at 20:07
  • I'm voting to reopen. If someone doesn't know that you can't produce RPM without power, then saying "you can't produce power without torque" isn't going to help. – Tanner Swett Dec 01 '19 at 21:45

5 Answers5

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You ask:

If the engine can spin the propeller fast enough, why does it need power behind it?

Good question. However, the answer is in the if of your question.

A propellor does work - it pulls (or sometimes pushes) the aircraft through the air. In order to move the aircraft at a useful speed (or even at all) it has to spin pretty fast.

The faster it spins, the more work it has to do. That work requires power. If the engine doesn't have enough power, it can't spin the propellor fast enough.

So yes, though the power and the speed of the spin are not the same thing, you won't get the spin without the power.

If you consider a car, the engine has to spin the wheels fast enough to move it along at the desired speed. And what makes it possible for it to spin the wheels fast enough is power. Otherwise, the wheels won't turn fast enough (or at all), and the car won't move fast enough (or at all). It's the same in the case of an aeroplane.

Daniele Procida
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    The key here is that if you lift the car off the ground, it takes almost no power to spin the wheels as fast as you like. It's easy for someone unfamiliar with the physics of it to think that it must be similarly easy to spin the propeller - after all, it's about the same size, right? The difference to note is that the propeller is designed to push a lot of air while spinning, while the average car wheel experiences relatively little air resistance against its rotation. A fairer comparison is to put the car back on the ground so that the wheels have something to push against. – anaximander Nov 30 '19 at 08:50
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    Maybe a boat with a prop is an easier to understand scenario? – Tim Nov 30 '19 at 09:34
  • Consider spinning a flat disk, for instance the platters in a disk drive. Takes very little power to turn them at high speeds - 7200 RPM or more - because they're not pushing air around. If you take that disk and make it into blades, like a simple household fan, it starts pushing air, which takes work. Propellors are designed to push a lot of air, so it takes a lot of work to turn them. – jamesqf Nov 30 '19 at 18:56
  • @anaximander Why on earth would anyone lift a car off the ground?! – Daniele Procida Nov 30 '19 at 21:47
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    @DanieleProcida: A mechanic in a repair shop using a car lift? – Jonathan Dec 01 '19 at 11:55
  • @Jonathan Or anyone else changing tires. They don't last forever, after all. – Mast Dec 01 '19 at 14:17
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    You don't even need to lift the car off the ground, just try starting on an icy road. Especially if you're pointed uphill :-) – jamesqf Dec 01 '19 at 18:14
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Most propeller-driven airplanes use constant-speed props. That means they use a governor to vary blade pitch (and thus resistance) so the engine always spins at the selected speed.

The more power you have turning the prop, the higher the blade pitch will go, biting deeper into the air and thus generating more thrust, and therefore the faster the plane will travel.

The main exception are very low-powered aircraft, such as used in primary flight training, which can barely keep the prop spinning at a decent speed even at a very low, fixed blade pitch.

StephenS
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  • "the more power you have, the higher the pitch will go" wouldn't it be better to swing your prop at optimal AOA, and match its size and the engine properly? Lots of power tends to "spoil" engineers, we should not forget efficiency. "Very low-powered aircraft" are speed envelope limited due to their fixed pitch, but a larger, slower variable pitch prop with produce more thrust for a given amount of power because "moving a lot of air a little" more efficiently generates thrust per horsepower. – Robert DiGiovanni Nov 30 '19 at 09:13
  • @RobertDiGiovanni Prop size is limited by ground clearance and tip speed. At some point, you have to add blades to keep the prop AOA below stall, which then introduces new problems. – StephenS Nov 30 '19 at 18:26
  • The Wrights had a pretty good idea adding two 2 bladers to 1 engine rather then 4 blades to one prop. – Robert DiGiovanni Nov 30 '19 at 18:51
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A prop is just a wing going in a circle.

If you have a wing going straight through the air, you need force propelling it along, a thrust source, and that force is used to make the wing redirect air down by operating at an angle of attack, creating lift, which is vertically oriented thrust. It takes energy to do this. In a single engine airplane the wing is being driven forward in a straight line by a thrust source on the fuselage. Unless it's going downhill, it has to have the thrust source.

A prop, or helicopter rotor, is the same wing being driven around a central axis instead of linearly. Being driven around an axis, instead of linear force being applied to the wing, it's rotational force, or torque, which comes directly from the engine.

In the end, it's still force applied to generate aerodynamic thrust forces, but in the prop's case the output is horizontal thrust instead of a vertical thrust force (lift).

So in an airplane, the stronger the thrust force pushing or pulling the plane along, the higher angle of attack the wing can operate at for a given speed, driving air down harder, and more lift (vertical thrust) is created. Or it can stay at the same angle of attack and just get pushed through the air faster.

With the propeller, the stronger the rotational force, the higher the propeller blade's AOA can be and/or the faster it can go, driving air aft harder, and more thrust (horizontal lift) is created.

So the more propeller thrust you want to make, the more horsepower (torque x velocity) is required.

John K
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Because spinning the propeller to make thrust creates friction and drag. Using a piston engine as an example, when we start the motor and add fuel, the motor will spin the propeller faster and faster until the friction of the pistons moving, plus the energy required to move fuel/air mixture in and exhaust out, plus the friction of the bearings, plus the drag of the propeller through the air, equals the expansion force released by the fuel burning in the cylinders.

Once maximum rpm is reached, this is called "steady state". The average force of the fuel being burned and pushing the pistons is equal to all friction and drag forces. If you designed your engine well and it does not overheat, you get maximum THRUST until it runs out of fuel.

So for a given prop, the more horsepower you have the more rpms you get.

Sizing props with engines can take some time to learn, but it is generally better not to overspeed the engine by trying for very high rpms and risk overheating it. A larger prop swung more slowly will be more efficient, but you still need to burn enough fuel per unit time to get it going.

Robert DiGiovanni
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  • I'm no an expert in aeronautics or fluid dynamics, but I'm pretty certain that the "friction" aspect of it is pretty nearly trivial. It's the lift induced drag that's the main factor. It takes power to turn the prop because the turning prop moves a huge mass of air. – Solomon Slow Nov 30 '19 at 20:13
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    @Solomon Slow I recently learned at higher rpm, pumping the fuel/air in and exhaust out of the engine actually becomes significant, and piston/bearing friction also take a part of the power pie. It gets interesting to "load" the engine with various props to get optimal performance. Pitch and prop length can be tested. For example, a 10/5 or 10/6 prop may work very well for takeoff, but a 10/7 is better in cruise. – Robert DiGiovanni Nov 30 '19 at 23:40
  • OK, I wasn't thinking about friction in the engine itself. I saw "horsepower" in the question, and I only thought of shaft horsepower. – Solomon Slow Dec 01 '19 at 00:15
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Newton famously (is quoted as saying) "For every action there is an equal and opposite reaction.

Propellors propell by forcing air in one direction (action) and in turn generate a force against the propellor (reaction) which drives the air forward.

The classic drag equation

Force = 0.5 x Air_density x Cd x A x V^2 ... (i) & the related
Power = Force x V ... (ii)

  • 0 < Cd <= 1 is the drag coefficient. Cd=1 is the best case in this application.
  • A = area (of prop circle in this case),
  • V = velocity of air through prop,

when analysed these equations are found to link the thrust, and the power imparted to the air which passes through the propellor.

ie to get air of a certain velocity, or a certain amount of thrust you need to 'plug in' appropriate figures into the above equation. As soon as the propellor starts moving air it will generate thrust and thrust is linked to the power consumed (or delivered) by the 2nd equation.

Russell McMahon
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