Some aircraft possess propellers on front of the fuselage, like the Cessna 172, and some aircraft have propellers located at the rear of the fuselage, like the Velocity 173RG-E.

Cessna 172

Velocity 173RG-E

How does the propeller's location affect the aircraft? Why don't we see back-propeller planes more often than front-propeller planes?

  • $\begingroup$ Tradition? Birds have their tails at the back. After the Wright Brothers mid-engine rear twin prop plane, the tails moved to the back for better vertical control with the knowledge designers had at the time. Also more propeller clearance. The props do not move closer to the ground on take off and landing with prop at the front vs at the rear. Also, the props have less disturbed air to bite into at the front vs at the rear. $\endgroup$ – CrossRoads Apr 29 '20 at 17:30
  • $\begingroup$ A rear propeller will put asymmetrical thrust on the aircraft unless it is perfectly in line longitudinally with the center of gravity in a similar fashion that the engines mounted on the wing can do on a multi-engine aircraft. Research the Cessna 336/7 or any other pusher prop with a high-mounted engine. But, that does not necessarily answer the stability question. Going off of memory, there are other pusher aircraft that have had stability issues if the center of gravity got behind or too close to the center of pressure. Having the thrust line behind the COG may have exacerbated things. $\endgroup$ – Dean F. Apr 29 '20 at 17:34
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    $\begingroup$ related $\endgroup$ – Manu H Apr 29 '20 at 17:38
  • $\begingroup$ Cooling the engine is a bigger problem in rear-facing (pusher) engine designs. $\endgroup$ – Ron Beyer Apr 29 '20 at 18:23
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    $\begingroup$ This related question has more info, particularly this answer. $\endgroup$ – Zeus Apr 30 '20 at 1:11

There is a misconception that putting the prop at the back is destabilizing in yaw/pitch, like you were pushing a car as opposed to pulling it. That only applies if the thrust element has a flexible connection to the body it's pushing/pulling.

When rigidly connected, the thrust axis is the same at any location along the plane's length and has no effect on stability. All that matters is how far the thrust axis is from the center of mass where it passes by. If the thrust axis is above the center of mass, power will make it pitch down whether the prop is forward or aft, and vice versa for a low thrust line.

Beyond that, the location of the engine/prop can have inertial effects related to the mass of the engine relative to center of mass, and gyroscopic propeller effects that vary because of the different location of the precession source, and the tail pusher can actually have enhanced stability due to various aerodynamic factors like an effective fin area increase due to the extra surface area of the engine/prop aft, but the actual placement of the thrust element on the longitudinal axis is not part of it.

Engine/propellers are usually on the front because it resolves so many design compromises much more readily. Pushers are tail heavy when unloaded, which is a big problem (tail pusher jets have to have their main gears farther aft than optimal for takeoff and landing so they don't tip back when unloaded, which requires a larger horizontal tail to provide downforce for rotation). The prop is subjected a lot more to junk thrown up by the wheels. Aluminum propeller blades can suffer from corrosion from carbon in the exhaust. All sorts of little problems present themselves, that go away if the engine is at the front.

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    $\begingroup$ That's not all. With props, the thrust line changes with the angle of attack. (Simply speaking, a tilted prop deflects the flow to some fraction of its tilt angle). This creates a fairly strong stabilising effect (in pitch and yaw) for pusher props, and destabilising one for tractors (which is quite counter-intuitive: it doesn't just negate the misconception you mention, but works against it). It's a bit involved to explain the cause of this effect and I don't have time to write an answer now, but this one links to a relevant paper. $\endgroup$ – Zeus Apr 30 '20 at 1:31
  • $\begingroup$ @Zeus Interesting data. Thanks! $\endgroup$ – John K Apr 30 '20 at 3:52
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    $\begingroup$ The side area of the prop also contributes to stability in the same way as a fin or horizontal stablizer. At the front it destabilizes so the tail surfaces have to be bigger, at the back it stabilizes so those surfaces can be smaller. $\endgroup$ – Guy Inchbald Aug 22 '20 at 8:50
  • $\begingroup$ My answer only was intended to address the location of the thrust element when rigidly attached to the airframe; the horse in front of the cart vs behind it. An increase or decrease in weathervaning tendency due to the area and aerodynamic effects of the engine/prop forward is another issue, and in any case is the misconception is the opposite in that the rear engine is thought to be destabilizing (horse pushing the cart). Let me make some revisions. $\endgroup$ – John K Aug 22 '20 at 12:48

A rear-mounted prop stabilizes the aircraft in pitch and yaw, much as an additional tail, but without control surfaces. Especially for a fighter aircraft, this is the opposite of what you want. THAT is why almost all high-powered, singe-engined aircraft have their propeller in the front: Maneuverability!

The stabilizing effect increases in proportion to the propeller surface area and the thrust, of course. Since a regular airplane needs to have basic stability with the engine running at idle, any additional stability change due to propeller placement comes on top. At full power and with the long lever arm of a single pusher prop on a central fuselage (think LearAvia Learfan), the aircraft becomes stiff as a brick. A two-boom layout (think Saab J 21) is better, but creates additional friction and interference drag, so the advantage of the pusher arrangement is reduced. Note, however, that studies of a tractor engine variant (Saab J 23) showed inferior performance to the pusher design.

If you want hard data on that: There is an old NACA report (NACA TN 2586) on this by John L. Crigler and Jean Gilman, called Propellers in Pitch and Yaw.


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