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This question already has an answer here:

I have read the post where it describes about the push and pull configurations of the engine, advantages/dis-advantages in them but not the purpose.

I have seen images of a figter where the propellers are placed both at the front as well as back, such as the Dornier Do 335.

enter image description here
(Image Source: WikiPedia - Author: Ad Meskens)

Later there are some downsized planes which are remotely controlled vehicles like the MQ-9 Reaper where the propellers are placed at the rear, Miniature UAV have the props in the front (sometimes even the direction of the propeller also vary)

Is it the size which makes the difference or the purpose?

Also what was the scenario due to which the designers had to go in for a varying propellers why not stick to the simple front facing propellers (which are placed either on the wings or in the front why to go for propellers with backward design/rear placed)?

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marked as duplicate by Jan Hudec, Federico, SentryRaven, vasin1987, mins Mar 18 '15 at 9:38

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ @user285 Was your edit and removal of the example image I provided intended or just a mistake as your edit overlapped mine? $\endgroup$ – SentryRaven Mar 18 '15 at 7:13
  • $\begingroup$ @SentryRaven I had edited at the same time as yours so it overlapped $\endgroup$ – user285oo6 Mar 18 '15 at 10:36
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The biggest factor is familiarity: What has worked before?

Other factors are:

  • Propeller size: Slow-turning propellers driven by powerful engines have a big diameter. This is desirable because it improves efficiency. However, now the propeller cannot be placed behind the wheels if the rotation angle should not be restricted.
  • Engine location: Driveshafts cost weight and run the risk of resonance problems, so the propeller should be directly in front or at the back of the engine. The engine location is dictated by the desired mass distribution, cooling, the pilot's field of vision, and space demands.
  • Interference: This includes all factors like prop wash and swirl. Putting the propeller ahead of control surfaces or flaps improves their effectiveness at low speed. Conversely, the increased flow speed increases friction drag on the surfaces wetted by the propeller's slipstream.
  • Stability: The propeller acts like an additional small wing and creates lift and side force when not exactly perpendicular to the inflowing air.
  • Engine-out performance: Multi-engined aircraft should still be flyable with one dead engine. This is an interaction between propeller location and control surfaces, and to minimize drag, those control surfaces should be as small as possible. Thus, the propeller should be close to the center of gravity.

The rear propeller is more efficient, because it helps to prevent separation on the surfaces ahead of it, and its slipstream will not increase the friction drag. However, integrating a propeller at the back of an aircraft is hard, so few designs (apart from those in this answer) made use of it. Notable examples are:

  • A range of British fighters during the early years of WW I, when only the Germans had mastered the technology of synchronized machine guns.
  • The Zeppelin airships. Their gondolas had rear-mounted propellers.
  • The Convair B-36. The pusher location in the wing was selected for better efficiency, but created cooling problems for the engines.
  • The Learfan, started during the first oil shock and designed for efficiency. Two PT-6 drove a single pusher propeller.
  • The Piaggio P-180 Avanti, which was designed a few years later and much for the same reason as the Learfan.

Also, Claude Dornier had used two engines in one engine pod, driving a propeller each at the front and at the back, as early as 1915, when he worked for Zeppelin on a giant metal flying boat. This became a common theme on many Dornier aircraft, especially the flying boats, and on the Zeppelin Staaken giant bombers. The last example of this range is the Dornier Seastar, designed in the late 1970s.

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It is the purpose. Each configuration has it's own advantages and disadvantages. This is already explained in Why are push-propellers so rare, yet they are still around? and this answer in particular.

In case of Do335, the reason is that they wanted two engines, but keep the engines on the centreline. This results in lower drag due to lower frontal area and better manoeuvrability due to lower moment of inertia as the engines are closer to centre of gravity. The pusher propeller itself reduces the manoeuvrability somewhat though.

In case of MQ-9 it simply matter of providing space and clear view for the radar and cameras in front while the reduced manoeuvrability does not matter much in this case.

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