Is it a better idea to put propellers on the wings than the nose?

Question

Most propeller aircraft have either a single propeller at the nose or two propellers on the wings. Nose propellers have a big disadvantage as the big fuselage can obstruct the airflow of the propeller and reduce its thrust. Wing-mounted propellers are only obstructed by the engines (which are much smaller) and the wings. Although the airflow over the wings causes some drag, it also increases the lift, while nose propellers don’t have this advantage. Nose-mounted single engine aircraft are still widely used because internal combustion engines are very big, heavy and expensive. When a single engine is sufficient, it will save a lot of cost compared to twin-engine aircraft.

Electric aircraft don’t have this problem. Electric motors are very small, very cheap and need little maintenance. As a result, it doesn’t cost much to increase the numbers of propellers. Because electric motors are very small, the engine pylons can be made much smaller to minimize the interruption of the airflow. As a result, wing-mounted electric motors should better leverage their size advantage and become the standard solution of electric aircraft. However, many experimental electric aircraft (e.g., Pipistrel Velis Electro, eFlyer 2, and eFusion) still have a single nose propeller instead of two wing propellers. So I am wondering if I overestimated the advantage of wing propellers.

Answer 1

The chief disadvantage to wing mounted engines and propellers is loss of control due to asymmetric thrust in the event of an engine failure. If one engine goes, the thrust line is offset from center of mass, resulting in a yawing moment. At low speeds, the rudder may not be sufficient to counteract this moment, resulting in loss of control of the aircraft ie the dreaded and fatal ‘Vmc roll’ known to multi engine pilots. Multi engine airplanes have been produced with centerline thrust designs with an engine in the nose driving a tractor propeller and an engine aft of the cabin driving a pusher propeller. Probably the best known and most successful of these designs is the Cessna model 337 Skymaster. These arrangements do not suffer from the thrust asymmetry problems described above in the event of an engine failure, but they do result in fuselage space consumed by engines and limitations on the configuration of the design due to structures, controllability and center of mass issues.

You are correct that it’s very easy to mount a distributed electric propulsion system in small pods along the leading edge of the wings, each one housing an electric motor driving a propeller. Because of the asymmetric thrust issues, this design is not favorable for an initial pilot trainer, which is why designs like the E flyer or Pipistrel‘s Alpha Electro favored a nose mounted motor offering conventional centerline thrust.

Answer 2

Mounting of propellers is strongly related to aircraft function design.

If possible, thrust is generated most efficiently by one large prop. Drag is also reduced through smaller tail surfaces if the engine and prop are mounted in front, on the nose, which places CG forward.

Once the design limitation of prop arc is reached, the next best thing is 2 on the wings. Accelerated airflow from wing mounted prop blast add a bit more lift, reducing the required wing area, which saves weight and drag. Span loading the wing also increases its tolerance to manuvering G forces. This is why wing mounts were very popular for long range prop driven aircraft, and remain with jet airliners.

One draw back on wing mounts is that a sudden increase in throttle will create more of a nose-up pitching tendency than a nose mount (by way of greater increase of lift). Mounting the engines with a down angle solves this but results in a loss of efficiency (downward thrust component). Nose mounts are easier for aircraft such as trainers, where throttle changes are much more common than with long distance cruisers.

Answer 3

As usual in the aerospace world, also the placement of the propeller(s) is the result of compromises among conflicting requirements. For some of the most common configurations a list of pros and cons is given. Note that some pros of a configuration can be cons of another one and viceversa. Feel free to expand each point.

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Front tractor propeller

• pros: the propeller works with undisturbed airflow which gives a better efficiency (note that this is actually the opposite of what is supposed in the question); the engine gets also fresh air for cooling purpose; better accessibility for maintenance.
• cons: blocks the front vision; the wake affect the aerodynamic of the whole airplane; noise and vibrations "flow" in the cabin.

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Aft pusher propeller

• pros: the propeller sucks air in, helping against flow detachments from the aft fuselage lowering the drag; better visibility; less noise in the cabin.
• cons: weight shifted backward reducing the leverarm of the tailplane which has therefore to be bigger giving more drag and weight; propeller less efficient; longer landing gear to avoid collision with the ground during the rotation phase.

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Wing mounted

• pros: the weight of engine+propeller compensate for the lift relieving the wing's structure which can be lighter.
• cons: unbalance in case of OEI (one engine inoperative) which has to be compensated by a bigger vertical tailplane giving more drag and weight; nothing safety-critical (pilot included) should be on the trajectory of a brocken propeller; in case of a low-wing design, longer landing gear are required; mutual aerodynamic interference between wing and propeller (note that also this is actually the opposite of what is supposed in the question).

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Also other less common configuration exist, some of them are shown in the next pictures (all pictures Wikimedia).

Burt Rutan's Voyager, a push-pull configuration with a pusher and a tractor propeller, optimised for very low consumption.

Burt Rutan's Boomerang, a double propeller asymmetrical, designed asymmetric to mitigate OEI operation

Over the fuselage propeller, configuration normally used for amphibious airplane to get propeller+engine as far as possible from water (Icon A5 depicted here)

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That's for the propeller(s). Obviously the electrical engine has different features/requirements than a heat engine that have to be taken into account as well.