Why are push-propellers so rare, yet they are still around?

falstro
  • Why are push-propellers so rare, yet they are still around? falstro

    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. 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.

  • 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.

  • 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

  • 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, 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).

    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. 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) is better, but creates additional friction and interference drag, so the advantage of the pusher arrangement is lost.

    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)

Related questions and answers
  • 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... 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. Single engine

  • The Soloy Dual Pac apparently allows two engines to rotate one propeller -- here's a picture of it on an Otter: Is this recognised as a centreline thrust twin engine aircraft, a "standard" twin engine aircraft or just an aircraft with a single engine for FAA certification? What about for pilot licensing?

  • How do flying wings, like the B-2 Stealth bomber, actually keep themselves from yawing out of control without a vertical stabilizer? For the record, I assume this has to be a simple mechanics process. Why? Well flying wings go all the way back to the 30s. One of the earliest (and my personal favorite) is the N-9M, which was a scale model of the XB-35, a prototype bomber for the allies during... speeds, well, that and the ground effects were pretty strong. But, no mentions of going into flat spins when going into hard maneuvers (that I recall). So how do they control that Y axis on flying wings

  • I've been told that the best kinds of planes to train in are very small ones, like Cessna 150s and 152s. But I've never been clear as to why. I know they are cheaper to operate, so is operation cost the only thing? Or are there aerodynamic properties that 152s have that make them "easier"? What makes for a good training aircraft?

  • Recently, the crew of an Indian airline performed a short choreographed dance sequence mid-flight on the occasion of Holi. This is, a not so rare practice amongst low-cost Indian carriers, who organize such dance sequences to celebrate special festivals (since festivals are a huge part of Indian culture, plus publicity for the airline afterwards). Here is a YouTube video. One of the pilots can be seen recording the dance on his camera. SpiceJet specially planned this event, and had extra cabin crew on-board the flight as a precaution. Also, during the dance, one of the pilots

  • My only detailed experience with carburetors is in aircraft. I'm pretty familiar with the principles behind float-type carbs, but I recently saw a schematic for a "downdraft carburetor" with a choke valve. This got me curious, so I did a little research and found that what I'm used to is an "updraft carburetor", and that (according to wikipedia) they fell out of fashion in the automotive industry in the 1930s. Why is the updraft carburetor design so prevalent in aviation? Does an updraft carb actually have a choke valve? Images below to provide a little context for those of us who

  • I know that for land aircraft and seaplanes that they require separate endorsements to fly them. However, for the case of amphibians, what do you need to fly one? Do you need to have another, completely different endorsement, or just a seaplane and land endorsements? What about if you always fly it on water or land?

  • See Wikipedia:Drag polar and Wikipedia:Polar curve (aviation) for example. These curves are not on a polar coordinate system. Why are they called polars?

  • I was watching an episode of MythBusters where they were trying to break glass windows and cups using a sonic boom generated by a F/A-18 Hornet, flown by the Blue Angels. In summary, they were..., the plane isn't going any faster, so perhaps the surface area of the wave would be larger (because of the larger plane) but you wouldn't actually have any more energy at any given point? Anyone know if a bigger plan than the F/A-18 would have a higher energy sonic boom? And could that cause glass to break? Bonus point: A mathematical formula showing why, I love those things. For those

  • Most GA piston aircraft still use dual magnetos for their ignition system, but there are some STC kits available to add electronic ignition to common piston engines, and new aircraft often come with FADEC systems. Are there particular FAA requirements for electronic ignition systems? If so, what are they and how can you demonstrate compliance?

Data information