Orsdxy

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?

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

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.

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).

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.