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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?

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    $\begingroup$ "if the propeller fails, feathering would be very difficult on the wingtip prop" Why, and how does the feathering mechanism differ based on the spanwise mounting location? This does not compute for me. $\endgroup$ – AEhere Feb 11 at 12:47
  • $\begingroup$ Loosely related: aviation.stackexchange.com/q/13382/2407 $\endgroup$ – RoboKaren Feb 11 at 16:46
  • $\begingroup$ Could you perhaps link to something that clearly shows what you mean? When I google wingtip propeller the first hit is this actual question, and there seems to be conflicting results from a google image search. $\endgroup$ – pipe Feb 12 at 10:46
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    $\begingroup$ If you were clever enough with tip-jets, you could solve some of these drawbacks while introducing many, many others. $\endgroup$ – Roger Feb 12 at 16:08
  • $\begingroup$ @AEhere Sorry, I realize that what I meant was that failure on the wingtip propeller would cause a moment on the aircraft and stability would be difficult. I am a bit new to aircraft desgin and English is not my first language so my apologies for confusion. $\endgroup$ – DumbAeronauticsGuy Feb 13 at 3:19
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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
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  • $\begingroup$ The Flying Flapjack series of experimental aircraft had the engines buried in the wing, at roughly similar locations to a conventional twin. They also used a shaft cross-connect with overrun clutches (much later adopted for the test series leading up to the Osprey tilt-rotor) so that either engine failing would allow normal operation on the remaining engine with balanced thrust. $\endgroup$ – Zeiss Ikon Feb 11 at 15:51
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    $\begingroup$ I'm not saying it can't be done @ZeissIkon, just that there are design considerations. I am familiar with cross connects, failure of cross-connects in the Osprey and its predecessor. has led to at least one fatal crash. $\endgroup$ – GdD Feb 11 at 15:56
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    $\begingroup$ Not sure about the wings. Engines are heavy, but they are not held up but the fuselage. Instead, the wings hold up both the fuselage and the engines. By moving the engines away from the fuselage, the load is spread better over the wing, allowing for lighter wings (except on the ground, but dynamic loads in flight are the design constraint. These can be far more than 1g) $\endgroup$ – MSalters Feb 12 at 9:59
  • $\begingroup$ @MSalters Erm, yeah, but doesn't a longer lever always exert more force when the same mass is applied? The same applies of course to any dynamic force added - like vibration. $\endgroup$ – Raffzahn Feb 12 at 13:41
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    $\begingroup$ @MSalters It it still a lever just because the load distribution on the lever changes, doesn't mean itself is changed. Every engine vibration has now the whole wing length as a lever against the fuselage. And at the same time, the wing needs to be strengthened all the way to the tip to hold the engines weight in flight. After all, lift is (hopefully) distributed all over the wing, while the engines weight is still on the outer end - the one wich has with conventional mounts the least strength. So no matter how it's turned, as farther out the engines are, as heavier the wing has to get. $\endgroup$ – Raffzahn Feb 12 at 14:08
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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.

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  • $\begingroup$ Why would the vibration problem worsen if you move the engines away from the fuselage? The fuselage isn't dampening the vibrations. Instead, it's where the most vulnerable parts are located. Keeping vibration sources away from it is a good thing. $\endgroup$ – MSalters Feb 12 at 10:08
  • $\begingroup$ @MSalters Distance (in this case from wingtip to fuselage) determines the maximum wavelength that the medium (the wing) can support. Since the engine can be seen as one force that can generate vibration, and the fuselage can be seen as the anchor or resisting force, a longer wing means longer waves (more flex), plus more complex harmonics. $\endgroup$ – Michael Teter Feb 12 at 23:35
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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).

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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.

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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.

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