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If jet engines are smaller in diameter than propellers, is it fair to say that propellers accelerate a lot of air a little while a jet engine accelerates a little air a lot? Basically, a jet engine accelerates the air flowing through it by more than a propeller engine of the same thrust, right?

If so, propellers should be way more efficient because, by accelerating a larger mass of air, they're able to generate more force (thrust) while performing less work (change in kinetic energy of the accelerated air) due to the fact that force is proportional to $\Delta V$ but energy is proportional to $\Delta V^2$, but both are proportional to mass.

So, if this is true, shouldn't propulsion systems try to maximize the mass of air that it accelerates in order to minimize the $\Delta V$ required? This is, after all, the whole concept behind using large wings to stay aloft (they can grab onto a large mass of air).

So how can jet engines be more efficient than propeller engines? What other factors contribute to the efficiency of the propulsion system besides just the mass and velocity of accelerated air? And similarly, why aren't propellers way larger than they typically are? Because by this logic airplanes should have propellers the size of helicopter rotors.

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  • $\begingroup$ Propellers become very inefficient when they cross the sound barrier, this limits tip speed and size... $\endgroup$ – Ron Beyer Jun 15 at 20:54
  • $\begingroup$ Makes sense. So to propell an airliner to mach .85 its propellers would have to spin at supersonic speeds? $\endgroup$ – Jonah Jun 15 at 21:09
  • $\begingroup$ Depending on how big the props are, they can exceed mach 1 with the aircraft standing still on the ground... $\endgroup$ – Ron Beyer Jun 15 at 21:41
  • $\begingroup$ This has got to have a dupe somewhere... there is a huge amount of information out there about the relative efficiencies of props and jets in different flight regimes. Please search around and come back if there’s something unclear in the other questions. $\endgroup$ – Michael Hall Jun 15 at 22:03
  • $\begingroup$ For the most part, yes. The most satisfactory explaination for me was the following: "Another factor is when you fly as fast as the propeller's tip speed—say Mach 0.75 (a Boeing 737 Classic in cruise)—the airfoil of each blade will be flying much faster than your forward speed (compare the hypotenuse to the vertical/horizontal sides in the topmost image)." $\endgroup$ – Jonah Jun 15 at 23:23
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Why aren't propellers way larger than they typically are?

There are some "propellers" these days with a 400 foot disk diameter, known as windmills. But, they are doing their best to extract energy from the airstream and here lies the issue.

"Relative wind" is conveniently used to combine the net direction and force of two aerodynamic effects, but let's look at an old question of: can a sail boat ever sail faster than than wind? This is very easy to explain by separating the "wind" components, rather than combining them. Ice boats can go several times faster than the wind by using wind across the beam as "thrust", and using "wind" from the motion of the boat to create lift.

This is exactly how powered aircraft fly, the ice boat is simply doing it on its side (hence the tiller is the "elevator").

Which brings us to propellers, which can be more accurately described as generating lift than "moving air". But, as the graph shows, net thrust of props decreases as speed increases because disk drag increases with speed.

Turbojets become more efficient at higher speeds than props because they have much less disk drag subtracting from their thrust. Even though they are less efficient at producing thrust, their net thrust out put is much more linear over speed as "ram effect" (increasing amount of air available to produce more thrust) and drag cancel each other out, while the propeller falls victim to its own disk drag.

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