An aircraft constructed with a pusher configuration has the engine mounted forward of the propeller - which faces in a rearwards direction - giving an appearance that the aircraft is "pushed" through the air. Sometimes the propeller is situated at the rear of the fuselage - more often at the rear of a nacelle - rotating between tail booms. Wing mounted pusher propellers are typically situated behind the trailing edge of the wing. This means that the airframe has a stress applied to it in compression from the rear rather than in tension from the front.
[edit] History
Many early aircraft were pushers, including the Wright Flyer, and the Curtiss biplane used by Eugene Ely for the first ship take-off. In the early years of the First World War pushers were favoured by the British and French because they enabled a forward-firing gun to be used without being obstructed by the arc of the propeller. Such aircraft included the Vickers F.B.5 Gunbus, the Royal Aircraft Factory F.E.2 and the Airco DH.2. (Germany did not have the same requirement due to the early development of Fokker's interrupter gear.)
Single-engine pushers usually had the engine mounted on the centreline at the rear of the aircraft's nacelle. Such aircraft had no fuselage, the tail section being mounted on a framework that cleared the propeller.
With the widespread adoption of interrupter gear, most benefits of the pusher configuration were lost and the tractor configuration was favoured. Pushers did not become extinct after the war but were a minority of new aircraft designs. The 1930s Supermarine Walrus was a seaplane with a single pusher engine. Large multi-engine aircraft, such as the Short Singapore, continued to be built with a push-pull configuration, combining the tractor and pusher configuration (that is - with one or more propellers facing forwards and one or more others facing back). Possibly the most extreme example of the type is the Convair B-36, the largest bomber ever operated by the United States, which wing-mounted six 3,800 hp Pratt & Whitney Wasp Major radial engines in a pusher configuration, augmented in the B-36D by four General Electric J47 turbojets. The Saab 21 was also initially built as a pusher since jet engines were not available. The Quad City Challenger and Cirrus VK-30 have pusher configuration.
There is a revival of pusher configurations on the Unmanned aerial vehicles with propellers such as Boeing ScanEagle, RQ-1 Predator, MQ-1C Warrior, RQ-2 Pioneer, RQ-5 Hunter, RQ-7 Shadow, MQ-9 Reaper, RQ-11 Raven and RQ-15 Neptune.
[edit] Advantages
- Efficiency can be gained by mounting a propeller behind the fuselage, because it re-energizes the boundary layer developed on the body, and reduces the form drag by keeping the flow attached. However, this effect is not nearly as pronounced on a small airplane as it is on a submarine or ship, where it is quite important due to the much higher Reynolds number at which they operate.
- Wing efficiency increases due to the absence of prop-wash over any section of the wing.
- Rear thrust is somewhat less stable in flight than with a tractor configuration. This has the potential to make an aircraft more maneuverable.
- Visibility of a single-engined airplane is improved because the engine does not block forward vision. Consequently, this configuration was widely used for early combat reconnaissance aircraft, and remains popular today among ultralight aircraft.
- The propeller of a single-engined airplane can be placed closer to the elevators and rudder. This increases the speed of the air flowing over the control surfaces, improving pitch and yaw control at low speed, particularly during takeoff when the engine is at full power. This can be beneficial while bush flying, especially when taking off and landing on airstrips bounded by obstacles that must be avoided while the airplane is moving slowly.
- The engine is mounted behind the crew and passenger compartments, so fuel does not have to flow past personnel, any leak will vent behind the aircraft, and any engine fire will be directed behind the aircraft (however, this arrangement puts the empennage at greater risk, if there is one -- but this is less of an issue if the fire occurs on, or as a consequence of, landing). Similarly, propeller failure is unlikely to directly endanger the crew. Also, the engine's metal mass behind a cockpit increases rear armoring for military aircraft.
[edit] Disadvantages
- It is claimed that the pusher configuration might endanger the aircraft's occupants in a crash or crash-landing. If the engine is placed behind the cabin, it may drive forward under its own momentum during a crash, entering the cabin and injuring the occupants; however there is no case where this has been reported to have occurred (in the US and UK accident records). Conversely, if the engine is placed in front of the cabin, it might act as a battering ram and plow through obstacles in the airplane's path, providing an additional measure of safety (however, fuel and oil in the engine area is more likely to be a fire hazard if the engine hits the ground first, and high-energy pieces of propeller may be flung into the cabin area).
- Due to center of gravity often being further behind on longitudinal axis than on most tractor airplanes, the pushers can be more prone to flat spin, especially if loaded improperly.
- Crew members may strike the propeller while attempting to bail out of a single-engined airplane with a pusher prop. This scenario may be part of the reason that pusher props have rarely been used on post-WWI fighters despite the theoretical increase in maneuverability. Modern lightweight aircraft, however, often have a parachute system that saves the entire aircraft, so there is no need to bail out.
- A less dire but more practical concern is foreign object damage. The pusher configuration generally places the propeller(s) aft of the main landing gear. Objects on the ground kicked up by the wheels can find their way into the propeller, causing damage or accelerated wear to the blades. However, pusher configurations are used by thousands of ultralight aircraft on grass airfields, with few problems. A few centreline pusher designs (such as the Rutan Long-EZ pictured above) place the propeller arc very close to the ground while flying nose-high during takeoff or landing, possibly making the prop more likely to strike vegetation when the airplane operates from a turf airstrip.
- When an airplane flies in icing conditions, a layer of ice can accumulate on the wings. If an airplane with wing-mounted pusher engines experiences wing icing and subsequently flies into warmer air, the pusher props might ingest pieces of ice as they shed, posing a hazard to the propeller blades and other parts of the airframe that can be struck by chunks of ice flung by the props.
- The propeller increases airflow around an air-cooled engine in the tractor configuration, but does not provide this same benefit to an engine mounted in the pusher configuration. Some aviation engines experience cooling problems when used as pushers.
- Propeller noise might increase because the engine exhaust flows through the props. This effect may be particularly pronounced when using turboprop engines due to the large volume of exhaust they produce. Similarly, vibration may be induced by the propeller passing through the wing downwash, causing it to move asymmetrically through air of differing energies and directions.
- The absence of prop-wash over the wings can reduce the airflow across the wing flaps, making them less effective. Wing-mounted pusher engines block portions of the trailing edges of the wings, reducing the total width available for flaps and ailerons.
- Placement of the propeller in front of the tail can have a negative side effect: strong pitch and yaw changes may occur as the engine's power setting changes and the airflow over the tail correspondingly speeds up or slows down.
[edit] Gallery
| | F.E.2b pusher (1914). The propeller is just behind the wing. | | |
| | Experimental fighter XP-55 (1943) | Experimental fighter XP-56 (1943) | Experimental flying wing Northrop XB-35 (1946) |
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