What are some drawbacks to having a wingtip propeller on an aircraft?

Question

I was reading about aircraft concepts that involved wingtip propeller design and was wondering what the drawbacks of such a technology would be. I could not access the full paper but came across this: https://arc.aiaa.org/doi/pdf/10.2514/3.44076

For me, one of my concerns is that if the propeller fails, feathering would be very difficult on the wingtip prop. I am also trying to decide on optimal placement for aircraft propellers in general. Would the drawbacks from having a wingtip propeller outweigh the benefits in high-lift generation and drag reduction?

Answer 1

Feathering isn't really an issue, if you can feather a prop on an engine further inboard you can do it on the tip too. There are 3 major drawbacks that come to mind:

1. The wing structure has to be stronger: engines are heavy, the further out they are the beefier the structure has to be to hold them. Stronger wings mean more weight and possibly a thicker cross section. Neither are good traits in a wing
2. Decreased roll rate: the farther the engines are the greater the moment arm and the slower your roll rate will be. Think about skaters spinning around, the farther their arms from their body the slower they spin. The same principle is at work here, so you need bigger ailerons to give you maneuverability, so more weight, cost and complexity
3. Safety in a single engine failure scenario: engines on the wingtips will cause more yaw in a single engine failure than engines further in board, so you'll need a bigger rudder to counteract it. A bigger rudder means more weight and cost, and there are also limits - eventually you will get to the point you can't counteract the force effectively and an engine failure will cause a loss of control. Yaw onset will be faster as well, giving a pilot less time to react, and there's nothing you can do about that; a bigger rudder doesn't help with reaction time. Mechanical cross-connections could be used to share power across the wing in the case of a single engine failure, like the V-22 Osprey, Chinook helicopter, however these increase weight, cost and complexity. Also, these systems aren't perfect, a single engine failure is still a possibility

Answer 2

Another non-flying attribute that having a wingtip-mounted propeller can drive is landing gear length. Since there are minimum clearance distances for propeller tip to ground during taxiing, and the wings may sag or dip during a turn while taxiing, you may end up having to change your gear length (which can cause other issues in turn).

Answer 3

Moving the thrust (and additional weight) to the wingtips creates more drawbacks than benefits.

Yaw - because of the increased moment of inertia (compared to having the engines be closer to the fuselage - the center line of the mass), it would be harder to initiate yaw changes as well as harder to stop or reverse them. Left/right thrust differentials could be used, and that would certainly increase yaw change rate, but then you have to consider the time cost of changing the force of each engine quickly. And it becomes a very serious problem if you have a failure on one side, leaving you with only one wingtip generating all of the thrust. Depending on the geometry of the aircraft and the size of the vertical stabilizer, it might not even be possible to counter the yaw force generated by the one engine producing enough thrust to keep the aircraft flying.

Roll - Similar to the yaw problem, the roll rate would be reduced the further the weights were moved away from the center line.

The V-22 Osprey is an example of this design. The wings are kept short to minimize the increase to moment of inertia, but the operational requirements of the vehicle (VTOL) required it to have large propellers (rotors), so the wings had to be long enough to keep the prop tips from hitting the fuselage.

Additionally, vibration and external (turbulence) effects on the wing structures would have to be considered. Even in normal operating conditions, the wings would be subject to increased vibrations that could create structure failures in some complex compound wave situations. Aircraft designers already deal with this and model these scenarios, but the complexity increases (I suspect exponentially) as the vibrational force is moved further toward the wingtip.

Answer 4

To me a showstopper disadvantage of wingtip propellers is that they are potentially disastrous in crosswind or landings, or landings with significant turbulence. Any pilot who has flown long enough has had landings where wingtips came quite close to the ground. In fact in a cross wind landing you should have your wing on the wind side a bit lower than the other wing, which will be in partial shadow from the fuselage. With wingtip propellers now you have less clearance for surprise gusts but more importantly the consequences are far worse. You could damage a prop and suddenly have asymmetrical thrust at the worst possible time.

A second disadvantage is collisions on the ground. Wingtip scrapes are relatively common in all sizes of aircraft used by all types of operators. They are nearly always inconsequential. With a wingtip propeller these incidents would cause a lot more damage to both craft and are more likely to cause injury due to flying debris.

And of course as others have mentioned, loss of a motor implies that the center of thrust will be very far from the center of mass and aerodynamic center of the aircraft. On takeoff particularly this is not what you want. It is hard enough to manage with regular twins, which have the engines as far inboard as possible. It's just a bad idea.

Answer 5

The Navy V-173 "Flying Flapjack" was an example of this design, flying in the mid 1940s.

Development of electric technology has brought this concept to light again for wingtip propeller mitigation of vortex drag so we again return to the question of efficiency.

Aerodynamicly, this is a question of drag for a given weight at a given speed. Wingtip motors would require additional strengthening of the wings and larger roll control surfaces for the same performance, adding weight to the design. Adding weight adds to drag, canceling any savings in drag from tip vortex reduction.

Propulsive efficiency must also be considered. Breaking 2 propulsion units down to 4 or 8 also results in a loss of efficiency. Indeed, the trend is opposite, as seen with the Boeing 777.

In summary, keeping 2 propulsion units at the wing tips (like the Flapjack or the Osprey V-22) will result in a lower aspect and/or heavier air craft, while splitting propulsion units increases fuel consumption per unit thrust.

Where are the gains? Somewhere between airplane and helicopter, which is where the Osprey is for military purposes. Practical, cost effective commercial applications remain to be seen. Lower cost fuel could be a driver of development.

Answer 6

Wrapup of smaller things:

This could affect the minimum runway width required. Wings will pass over grass okay, but props/engines could risk blowing dirt/dust/plants about and inhaling them causing FOD. Any obstruction beside the runway could have consequences.

Slight increase in fire risk from any sparks from the engine because the spark might drop into shelter rather than dropping onto the hard tarmac of the runway.

Increased risk to first responders in the event of an incident/accident because the moving parts may force crash tenders to stand back a bit further slowing the quench time of fire.

Engines over grass/soft ground could make landings and take offs slightly quieter as a benefit. I'm unsure if the passengers would find it quieter or louder.

Answer 7

First of all, not all but most of the answers you have received are based in psychological inertia, loss aversion, confirmation bias, availability cascade and shoehorning of scenarios, and so they are subjective, poor and based in government sponsored petrochemical industry ICE piston or jet engine obsolete technology, that has hijacked aviation technology for so long with inefficient aerodynamics and propulsion systems, that in this case has limitation in propulsion placement because of the whirl created by all those engines's moving pieces and other factors (the unlikes of this post prove that fundamentalism that has nothing to do with aviation, technology or science, but with fake technology advocacy that avoids and tries to suppress any challenge to the technological status quo. as today there have been 3 or 4 unlikes that have countered the 2 or 3 likes of this answer and keep this answer as negative. this type of behaviour is a plage in several pseudo technological aviation or boat design forums created by biased people hoping for advertising revenue or even wiki pages, which i do not recommend to enter).

With other types of propulsion as battery electric, hydrogen electric, solar electric, solar stirling or isothermal compressed air ICA propulsion ( https://www.reddit.com/r/AerospaceEngineering/comments/pkmdz6/stable_flying_wing_design_setup_propelled_by/ ) there are no barriers to propulsion placement and so there can be several configurations as distributed electric propulsion DEP with boundary layer suction or ingestion and wing tip propellers as nasa x-57 (https://www.nasa.gov/aeroresearch/X-57/technical/index.html) and other publications (https://www.researchgate.net/publication/329036195_Wingtip-Mounted_Propellers_Aerodynamic_Analysis_of_Interaction_Effects_and_Comparison_with_Conventional_Layout).

Rather than a simplification or eliminating discourse of centre propeller vs wing tip propellers, they can be a complement for the main aircraft propulsion and can play a role in swept angle tailless aircrafts by moving part of the propulsion further aft (tailless aircrafts normally use push propulsion for that effect), which are far more efficient that today's petrol aircrafts with cylindrical fuselage and short aspect ratio wings.

Wing tip propellers can increase lift that is very important for STOL development, thrust and reduce induced drag by rotating counterwise to the wingtip vortex (nasa x-57) and they can increase stability of the aircraft by placing them off-axis (NEW EDIT: this is the real publication: Wingtip-mounted propellers installed at a nonzero yaw angle 2021 Thesis of Yannick Chance https://repository.tudelft.nl/islandora/object/uuid:7aceebd5-4221-41f0-8657-3b61b152cad0) what can open the door to fly-by-wire 3d gyrating or 2d rotating wing tips propellers as stability assistance during adverse weather conditions instead spoilers or ailerons, what can increase also roll rate and manoeuvrability.

Also in the case of general aviation aircrafts, by being placed in the tips the propellers create less parasitical drag compared to a pull centre propeller that creates additional turbulences and parasitical drag trough all the fuselage, a drag rate that increases exponentially with the speed.

As propellers and propeller arrays have several uses as take-off thrust, climbing or soaring thrust, cruising thrust or gliding assistance thrust, in the end wing tips propellers can help to reduce drag of sailplanes at small rpm or reduce drag during cruising in different aircrafts with additional thrust, besides increasing the mentioned directional stability of aircrafts and improve STOL performance.

there are several trade-offs of wing-tip propulsion, and it will vary by the type of propeller or engine and also from aircrafts with only wing tip propulsion to with aircrafts that use additional propulsion besides wing tip propulsion, and also from tailless aircrafts to tailplanes, and push or pull configurations, and cruising speed as well:

-As with DEP there is wing and whirl flutter and so the need of additional structural reinforcement, however the weight and whirl of electric motors are a small percentage of a combustion engine and even less in the case of pistonless rotary air engines, and also flutter can be further reduced by using a small or partial percentage of the total aircraft thrust in the wing tips.

-Width increase and propeller crash on landing can be a problem by using fixed propellers, however as today there are propellers that can be folded so this issue can be totally avoided.

-single engine failure will be a problem in aircrafts that only use wing tip propulsion, and compared to twin engine aircrafts because it still will be an advantage compared to an engine failure in general aviation single ICE engine aircrafts.

-yaw and roll performance in the case of aircrafts using only wing tip fixed axis propulsion, however an aircraft that uses gyrating or rotating propellers in the wing tips will have better yaw and roll performance than today's single or twin propeller tailplanes that depend from spoilers, alieron and rudder besides having a greater stability control, and for tailless aircrafts this means a gamechanging technology that is currently in development or to be developed.