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TO PUSH OR PULL





It’s a growing lament among automotive design buffs that sedans of today all look increasingly alike. Airplane afficionados must consider themselves lucky, then, as they never seem prone to utter similar complaints. High-wings, mid-wings, low-wings, standard configuration or canard, single-engine or twin, T-tail or not: the retinue of design options allows for flexibility and creative expression in aircraft design that car designers don’t have. As layout architecture goes, one of the variables available to aircraft designers is the choice of where to put the engine and propeller. To push the airplane through the air, or pull it: why chose one over the other?

A “tractor” arrangement has the propeller in front of the engine and, thus, pulls the aircraft through the air. A “pusher” installation has the propeller behind the engine and, thus, pushes the aircraft through the air. Cast your eyes up and down any flight line and you’ll be hard pressed to find too many pushers (if any): tractors dominate airfields. But if you consider that the aircraft that kick-started powered flight, (i.e. the Wright Flyer), was a pusher, you must consider that it had, (and still has), a blend of advantages and disadvantages that make for an interesting design equation.

Pushers are more commonly found amongst homebuilts and kits. One of their foremost advantages is that their layout allows their airframes to fly in undisturbed air. By contrast, the airframes of tractors fly in the turbulence created by the wake of their own propellers. A tractor’s propeller flies in undisturbed air. The prop on a pusher, on the other hand, suffers reduced efficiency because it’s working with disturbed airflow coming off its fuselage, wings and tail. A prop hanging off the back is also more exposed to damage due to over-rotation on take-off, or from rocks thrown up by the landing gear.

A fuselage-mounted pusher – as opposed to pod-mounted (like the Lake Amphibian), or wing-mounted (like the Piaggio Avanti) – usually has a reduced wetted area owing to a shorter fuselage. The inflow of air to the propeller permits a tighter, “coke-bottle”, fuselage closure shape that doesn’t suffer from airflow separation than might otherwise be the case. A tractor installation, however, tends to shorten the front of the aircraft, thereby permitting flexibility in the design of the empennage that can lead to maximizing a front-engined aircraft’s stability. Given that it’s breaking into the on-coming air, a tractor’s engine is easily cooled by this in-coming airflow which can be readily directed to the engine. Engine cooling for a pusher, by contrast, is not so easily sorted out. On the ground, tractors are better cooled as air gets blown directly into the engine’s air intakes. Pushers, simply, cannot do that. Cooling air intakes on pushers are at the back of the fuselage where fuselage-induced aerodynamic effects make boundary layer air thicker and slower moving. To overcome this drawback, pushers tend to use large scoops to direct air into their engine bays which, while providing the required cooling, are detrimental as aerodynamic drag.

With the engine located in the back, pushers tend to have better forward visibility. The threat of fire, smoke or carbon monoxide getting into the cockpit is minimized too. Pushers also have reduced cabin noise, mostly because the engine exhaust is behind the cockpit area, and the cabin’s windscreen isn’t being hit by a turbulent propwash as is the case in a front-engined aircraft. Vibration in the cabin of a pusher is usually less too. The smaller frontal area offered by a rear-engined layout makes for a more efficient air-penetrating package. However, engine power lost to cooling is often greater in a pusher configuration. A rear-mounted engine also alters the aircraft’s CG, making for different stability challenges that designers of pushers must cleverly overcome.