the engine(s) aft improves visibility for the crew. In military aircraft, front armament could be used more easily on account of the gun not needing to synchronize itself with the propeller, although the risk that spent casings fly into the props at the back somewhat offset this advantage. Aircraft where the engine is carried by, or very close to, the pilot (such as paramotors, powered parachutes, autogyros, and flexwing trikes) place the engine behind the pilot to minimise the danger to the pilot's arms and legs. These two factors mean that this configuration was widely used for early combat aircraft, and remains popular today among ultralight aircraft, unmanned aerial vehicles (UAVs) and radio-controlled airplanes. Aerodynamics A pusher may have a shorter fuselage and hence a reduction in both fuselage wetted area and weight. In contrast to tractor layout, a pusher propeller at the end of the fuselage is stabilizing. A pusher needs less stabilizing vertical tail area and hence presents less weathercock effect; at takeoff roll it is generally less sensitive to crosswind. When there is no tail within the slipstream, unlike a tractor there is no rotating propwash around the fuselage inducing a side force to the fin. At takeoff, a canard pusher pilot does not have to apply rudder input to balance this moment. 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 to the fuselage. However, it is usually a minor gain compared to the airframe's detrimental effect on propeller efficiency. Wing profile drag may be reduced due to the absence of prop-wash over any section of the wing. Safety The engine is mounted behind the crew and passenger compartments, so fuel oil and coolant leak will vent behind the aircraft, and any engine fire will be directed behind the aircraft. Similarly, propeller failure is less likely to directly endanger the crew. A pusher ducted fan system offers a supplementary safety feature attributed to enclosing the rotating fan in the duct, therefore making it an attractive option for various advanced unmanned air vehicle configurations or for small/personal air vehicles or for aircraft models. 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