Pusher Configuration - Disadvantages

Disadvantages

Structural and weight considerations

A pusher design with an empennage behind the propeller is structurally more complex than a similar tractor type. The increased weight and drag degrades performance compared with a similar tractor type. Modern aerodynamic knowledge and construction methods may reduce but never eliminate the difference.

A remote (buried) engine requires a drive shaft and its associated bearings and supports, special devices for torsional vibration control, increasing mechanical requirements, weight and complexity.

Center of Gravity (c.g.) and landing gear considerations

To maintain a workable CG position, there is a limit as how far aft an engine can be installed. The forward location of the crew may balance the engine weight and will help determine the CG. As the CG location must be kept within defined limits for safe operation load distribution must evaluated before each flight.

Due to a generally high thrust line (needed for propeller ground clearance), negative (down) pitching moment and sometimes absence of prop-wash over the tail, higher speed and longer roll is required for takeoff compared to tractor aircraft. Main gear located too far aft (aft of empty aircraft c.g.) may require higher takeoff rotation speed) or even prevent the rotation. The Rutan answer to this problem is to lower the nose of the aircraft at rest such that the empty c.g. is then ahead of the main wheels.

Due to the center of gravity often being further back on the longitudinal axis than on most tractor airplanes, pushers can be more prone to flat spins, especially if loaded improperly.

Aerodynamic considerations

Due to the generally high thrust line (aft propeller/ ground clearance), a low wing pusher layout may suffer pitch changes with power variation (pitch/power coupling). Pusher seaplanes with especially high thrust lines and tailwheels may find the vertical tail masked from the airflow, severely reducing control at low speeds, such as when taxiing.

The absence of prop-wash over the wing reduces the lift and increases takeoff roll length.

Pusher engines mounted on the wing may obstruct sections of the wing trailing edge, reducing the total width available for control surfaces such as flaps and ailerons.

When a propeller is mounted in front of the tail changes in engine power alter the airflow over the tail and can give strong pitch or yaw changes.

Propeller ground clearance and foreign object damage

Because of pitch rotation at take off, propeller diameter may have to be reduced (with a loss of efficiency) and/or landing gear made longer and heavier. Many pushers have ventral fins or skids beneath the propeller to prevent the propeller from striking the ground at an added cost in drag and weight.

On tailless pushers such as the Rutan Long-EZ the propeller arc is very close to the ground while flying nose-high during takeoff or landing. Objects on the ground kicked up by the wheels can pass through the propeller disc, causing damage or accelerated wear to the blades, or in extreme cases, the blades may strike the ground.

When an airplane flies in icing conditions, ice can accumulate on the wings. If an airplane with wing-mounted pusher engines experiences icing the props will ingest shedded chunks of ice, endangering the propeller blades and parts of the airframe that can be struck by ice violently redirected by the props.

In early pusher combat aircraft, spent ammunition casings caused similar problems and devices for collecting them had to be devised.

Propeller efficiency and noise

The propeller passes through the fuselage wake, wing and other flight surface downwashes - moving asymmetrically through a disk of irregular airspeed. This reduces propeller efficiency and causes vibration inducing structural propeller fatigue and noise.

Prop efficiency is usually at least 2-5 % less and in some cases more than 15 % less than an equivalent tractor installation. Fullscale wind tunnel investigation of the canard Rutan VariEze showed a propeller efficiency of 0.75 compared to 0.85 for a tractor configuration - a loss of 12 %.

Pusher props are noisy, and cabin noise may be higher than tractor equivalent (Cessna XMC vs Cessna 152).

Propeller noise may 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.

Engine cooling and exhaust

In pusher configuration, the propeller does not contribute airflow over the engine or radiator. Some aviation engines have experienced cooling problems when used as pushers. To counter this, auxiliary fans may be installed, adding additional weight.

The engine of a pusher exhausts forward of the propeller, and in this case the exhaust may contribute to corrosion or other damage to the propeller. This is usually minimal, and may be mainly visible in the form of soot stains on the blades.

Propeller and Safety

In case of propeller/tail proximity, a blade break can impact the tail or produce destructive vibrations leading to a loss of control.

Crew members may strike the propeller while attempting to bail out of a single-engined airplane with a pusher prop. At least one early ejector seat was designed specifically to counter this risk. Modern light aircraft, may have a parachute system that saves the entire aircraft, so there is no need to bail out, though baling out is no longer as common as it once was.

Engine location in 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. 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.