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Jet engines are by their very nature push-engines, however, most propeller airplanes use pull-engines. Is there an inherent advantage to using pull-propellers except for the increased airflow over the fuselage and tail (with its rudder and elevator)?

Twins generally have their engines on the wings, and the tail is no longer directly behind it, does that mean the choice of a pull-engine is not as advantageous?

If there isn't an inherent disadvantage, why are pusher configurations so rare? If there is one, why do they exist at all? Disregarding designs where the choice is obvious, like powered parachutes where you simply don't want a propeller in your face.

The Convair B36 is one notable multi-engine aircraft with engines in pusher configuration, as is the Piaggio Avanti. And the Cessna Skymaster is a push/pull configuration (If you get a multi-engine rating in a Skymaster, your ticket will be limited to multi-engine aircraft with in-line thrust). Single engine aircraft are even more uncommon, and pretty much all I could find except the Lake Buccaneer are all kit-planes (e.g. Velocity, Rutan), ultralights (Quad City), military, or experimental.

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You can make a "puller" jet, but no one does. Look at the airflow of the PT-6 turboprop engine. Notice the intake below the exhaust, the back-to-front combustion path and the exhaust pointing backward almost at the front of the engine. Remove the prop and gearbox, optimize the engine for pure-jet, and you have a puller jet engine. Should be obvious that the other designs are more efficient. Early rockets were built in the puller configuration, one production jet close to a puller would be the Pegasus, used in the Harrier - it has no rear-facing exhaust, using nozzles to vector the thrust. –  paul Apr 10 at 10:26
    
@Federico you were right. Note that turbojet engines are not real push engines. The force is not generated at the rear of the engines. Turbofans are very much puller engines with most of the thrust generated by the compressor. Pure jet engines like in the Tornado are perhaps a bit more push than pull, but still it has a lot of pull in it. Take a look at the strain in the main shaft. For pull props, the shaft is tension loaded, for push props the shaft is compression loaded. For all jet engines, the shaft is tension loaded; the compressor pulls the aircraft ahead. –  DeltaLima Apr 10 at 10:28
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@Federico no thrust is generated in the turbine, on the contrary: the turbine extracts energy from the internal airflow to drive the compressor thereby creating drag. The combustion chamber provides most of thrust in a pure jet engine. –  DeltaLima Apr 10 at 10:42

6 Answers 6

There are many disadvantages, they seem to outweigh the advantages.

Here are two:

  • A pusher prop is working in a disturbed airflow, causing increased vibration and noise
    If the propeller is fitted behind a wing, each propeller blade is passing through the separated boundary flow twice each rotation. These cycles create additional noise and lower the efficiency of the propeller. The vibration makes the propeller blades more susceptible to metal fatigue.

  • Propeller clearance at takeoff
    Due to the pitch up at take-off, the propeller gets close to the ground. Therefore the diameter needs to be reduced (loss of efficiency) or the landing gear struts need to be made longer (added weight). Since the propeller is behind the landing gear, it is susceptible to debris kicked up from the gear, increasing the need for added blade protection (increased weight, loss of efficiency)

Wikipedia has a list of additional disadvantages.


Edit:

Your statement that

Jet engines are by their very nature push-engines

is not entirely true.

In turbofan engines, most of the thrust is generated by the fan and compressor stages. Even in a pure jet engine, a lot of thrust is generated by the compressor. Therefore the shaft of a jet engine is tension loaded, just like a propeller shaft in puller configuration.

thrust distribution in a jet engine Source: Rolls Royce - The Jet Engine

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I noticed this list as well. So why are there pushers at all? –  falstro Apr 10 at 7:55
    
@falstro you can have an unobstructed view out front, they also are quieter since the engine is at the back. they may be safer as well, since they are difficult to stall if they have a canard configuration: youtube.com/watch?v=H50zFi11OMU (just to name a few) –  MikeFoxtrot Apr 10 at 8:21
    
@falstro Just above that list is a list of advantages. That are the reasons people built pushers. And once they are built, they are around for a while. Although you don't see them often. –  DeltaLima Apr 10 at 8:36
    
The interference with the wing wake is a serious issue, but only when the prop axis is in the plane of the wing. Once it is lifted above or lowered below, the prop blade will slice through it gradually than hitting it over the whole span, making noise and dynamic loading tolerable. Please take look at the Piaggio Avanti: This is an excellent design. –  Peter Kämpf Apr 10 at 20:34
    
@DeltaLima I'm not convinced the turbofan or jet engines are either push or pull just because of loading on the shaft; helicopters often have turbine engines, yet it's the fact that the rotor is on top pulling it up that makes it a puller configuration, isn't it? Having the rotor on the bottom, would make it a pusher, which of course would be insane. Not impossible though, space shuttles would be an example of a vertical take-off craft in pusher configuration. –  falstro Apr 11 at 6:39

Having been reading up on 1930s and 1940s aircraft, I can think of a few major problems that dogged the design back then.

Pushers had advantages of pushing the wings and control surfaces through undisrupted air and that gave enough advantages that the a lot of pusher designs were put forward. A pusher would have superior cruising speed and better wing loading because it flew through undisturbed air.

The most successful was probably the XB-42 mixmaster which would have been deployed had the war gone on. (As it was, it it eventually deployed as America's first jet bomber.)

The B-36 used pushers precisely to give its wings a clear airflow. It was actually designed in 1942.

But they had big tradeoffs.

A major problem with pusher designs of that era was cooling the engine on the ground.

With a tractor (puller) you have this giant fan running blowing air over the plane and the engine (air cooled radial) and/or radiator (water cooled.) With a pusher, the prop just blew air behind the plane, leaving a powerful engine with nothing but passive cooling.

During a crash, a tractor engine in front served as a crumple zone for the rest of the plane, particularly the cockpit and could plow through obstacles making the plane come to a halt slower. Conversely, with a pusher, the engine was behind the pilot and not only didn't offer any protection but tended to tear loose and ram through the rest of the fuselage like a pile driver.

The pilot could not see and therefore visually inspected the engine e.g. if you start loosing oil in a tractor, you know right away. If the engine is behind you, you might not notice until you see the gauge.

Center of thrust was more difficult to balance and that made center of gravity tricky as well.

Still, with all these disadvantages, XB-42 and B-36 showed that the advantages could be come out on top with enough good engineering. And in the end, jets work at least half by pushing. If jets had be delayed a few years or the war had started back in the mid-30s, we probably would have seen more pusher designs in service.

With jets, the military need for pushers disappeared and in civilian aircraft, there is seldom much need for an increase in performance that justifies fighting all the tradeoffs.

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The pusher design is more efficient, because the suction forward of the prop reduces flow separation, and the accelerated flow behind it is not streaming around the fuselage (or wing), where it would create additional friction drag. In case of the Do-335 (see picture below), the single-engine top speed was 30 km/h higher with the rear engine running than with the front engine (both were DB-603s with identical power rating).

Do-335

On the other hand, the puller prop will help to maneuver the plane on the ground (this is a big benefit for taildraggers - note how many two-engined, taildragger airplanes have an H-tail (two rudders as endplates of the stabilizer). They were placed in the prop wake and this gave much better directional control at low speed on the ground. Also, the prop wash helps to increase the lift from flaps.

The main disadvantage for a single-engined aircraft, the reduced tail clearance, has already been mentioned. If you cannot really rotate, takeoffs and landings are high-speed affairs. But there is another disadvantage to a rear-mounted prop: It stabilizes the aircraft, 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 Lear Propfan), the aircraft becomes stiff as a brick. A two-boom layout (think Saab 21, pictured below) is better, but creates additional friction and interference drag, so the advantage of the pusher arrangement is lost.

Saab-21

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 (http://naca.central.cranfield.ac.uk/reports/1952/naca-tn-2585.pdf)

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stability depends on overall design, not just engine placement. If you use a twin boom tail like the D.XXIII or Cessna Skymaster used, that problem largely goes away. But yes, it can be a factor. And of course if you use such a design, you're more than halfway towards a design with enough space to put both engines in the wings, by extending the twin booms forward to become engine pods, like in the P.38. The push/pull design however makes for far better engine-out handling, as there's no off axis thrust vector. –  jwenting Apr 11 at 6:33
    
I've read that the stabilizing effect is so strong that pusher-prop driven flying wings didn't need vertical tail planes/rudders e.g. the Northrup flying wings. When they converted to jets, they had to add them. –  TechZen May 28 at 17:07

For single engine aircraft at least, having a pusher prop makes egress in flight much more dangerous (a.k.a. bailing out, ejecting). During WW1 I believe (when pusher props were far more common) more than a few pilots were seriously injured or killed by their props when having to jump out of a burning aircraft.
For this reason the Germans fitted (or planned to) an ejection seat in their Do.335 push/pull design, and Fokker planned to do the same in his D.XXIII.
The B-36 was large enough that pusher props would be far away from the fuselage, getting rid of that problem.
An advantage I can think of of having the props in the front that stems from the increased airflow is that you get free extra cooling of the engines.
Having the engine in a single engine design in the rear with a pusher prop, you end up with a more complex air inlet and tail design than simply letting the engine suck in air that it bites into already. You need ducts and stuff, adding weight and complexity.

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Why are push-propellers so rare

Other answers have covered this.

yet they are still around?

A front mounted propeller limits the field of view of the pilot or payload.

Consider a surveillance drone or UAV. There's no pilot whose view would be obscured but there is almost certainly a great advantage for the radar, optical and other forward-looking systems.

Surveillance missions can benefit from a relatively slow-moving, low-altitude platform. This favours the use of propellors rather than jets and the pusher arrangement can be beneficial as noted above.

Photo of MQ-9 Reaper drone in flight MQ-9 Reaper

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Pusher props offer a real advantage to small amphibians like the Lake Buccaneer and the Republic See Bee where pilots need to be able to jump out and catch mooring buoys, lines or jetties without fear of striking a spinning prop blade.

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It's also easier to keep the prop out of the water without placing it right over the cockpit. –  TechZen May 28 at 17:03

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