The propeller, as a blade machine converting rotational motion into translational motion, has been known to mankind for quite a long time.

However, its practical application as a propeller in the air actually began only with the appearance of the first real airplanes.

Of course, a kind of intuitive attempts were made before, starting with primitive children’s toys in the form of small two-bladed “helicopters” set on the rod and rotated between the palms (their existence is known in China before our era), and ending, for example, demonstration model of MV Lomonosov (1754), in which the air propellers with a spring drive was assumed to use for lifting meteorological instruments to a certain height.

We should not forget about Leonardo da Vinci’s “flying propeller”, a kind of prototype of the helicopter. This propeller, however, does not have separate blades, as such, but nevertheless creates traction or, in other words, the force forcing the machine to rise up. The propeller simply “pulls” the rest of the structure with it, making it move in the right direction. The same thing happens in the other examples given here.

In fact, a screw can not only pull, it can also push. It all depends on its location, as a propeller, relative to the rest of the aircraft structure.

However, this is not just about changing the point of force application. It is important that this change is accompanied by a change in the operating conditions of the entire aircraft.

These changes affect aerodynamics, design, weight and alignment, safety and ease of flight and ground operation, and other such things. After comparing all available practical data and theoretical calculations, and often making compromise decisions (which is often the case in aviation), a choice is made in favor of one or another design scheme, a pulling or pushing propeller in our case.

I do not draw conclusions like “what is better and what is worse”. I do not think it is possible for me to do this unequivocally. Let it be something like a small review on “what we have”. And the readers, if they want, will draw their own conclusions.)

Features

The first thing to remember is the efficiency of the propeller, i.e. its efficiency. And here, as, however, in many areas of aviation science, there are certain contradictions associated with aerodynamics.

The design conditions of a push propeller installation on an aircraft can increase its efficiency compared to the efficiency of a pulling propeller operating under the same conditions.

After all, as you know, the efficiency of an aircraft propeller is equal to the ratio of its useful thrust power to the power it consumes (or in other words, the effective power of the engine). Useful thrust power is always less than consumed power, because part of it is spent on other purposes not related to thrust or reducing it.

For example, it is twisting of the air jet thrown off or overcoming of aerodynamic resistance at its interaction with structural elements, which can be considerable, because speed of this jet is much higher than flight speed, and “interact” with various structural elements, in particular with the wing, tail and fuselage (first of all).

As a result, the smaller and smaller in area elements of the aircraft structure are blown by the jet from the propeller, the lower the increase of resistance and, accordingly, the higher effective (or useful) thrust of the propulsion system.

For a pulling propeller, it is impossible to avoid the blowing by the propeller jet on the structural elements. This means that the useful thrust and efficiency will be reduced accordingly. For the pusher propeller, on the other hand, there are, as they say, options.

For flying deck planes of the beginning of the century, the push propeller definitely meant a lot of additional resistance when blowing the truss fuselage and the whole system of draughts and struts.

For modern aircraft, especially those with a “duck” design or with the engine mounted behind the tail, the pusher propeller can be advantageous in this sense. The Rutan Long-EZ aircraft is an example. Here, there are practically no obstacles for the jet behind the propeller.

This tangible reduction in aerodynamic drag can have a positive effect on the aerodynamic quality of the aircraft and its cruising characteristics (range). But we should not forget the downside of this benefit. After all, the pulling propeller blows around the wing, so the aircraft gets some increase in lifting power, which can improve, for example, takeoff and landing characteristics.

The pusher, of course, does not have this, and it is deprived of additional “free” lifting power. However, in this case the tail can be additionally blown (if the propeller is located in front of it), which increases its efficiency. Though, there is a fly in the ointment here as well: this efficiency can appear rather dependent on the mode of engine and propeller operation, can appear superfluous or insufficient, which of course must be somehow considered while making aircraft.

While a pulling propeller usually picks up and “recycles” unperturbed flow, a pushing propeller is largely deprived of this capability. In the plane, washed by the blades of the pushing propeller, it is likely to enter the flow that interacted with the surfaces in front of the propeller, and therefore probably perturbed and containing eddies of varying intensity and volume.

This, in turn, negatively affects the efficiency of the propeller and now already reduces its efficiency, in some cases quite appreciably. Such phenomena can, for example, be noticeably manifested when installing engines on the trailing edge of the wing (Northrop XB-35 (YB-35) and Convair B-36).