I can't think of any advantages, but some disadvantages would be:
-Much more reinforcement required in the wing to carry the weight of the engines as well as the thrust produced that far out (heavier wing)
-Asymmetric thrust would be greatly amplified in an engine out scenario
-More likely to ingest foreign objects when on the ground such as when taxiing up to a stand

Of course, I'm not a pilot or aeronautical engineer, but as a mechanic these are what stand out to me. Good question though, I've never even thought of something like that although I can envision it, maybe look like very large tip tanks.

My guess is that VMCA would be higher than the top speed of the airplane. You'd have a thrustline all the way outboard in one direction, and a ton of drag and weight outboard in the other direction. I'll bet you'd spin as soon as an engine failed.

Electric motors are not all that heavy, so that gives you more options for placement. The reason they're put at the wingtips on designs like the Eviation Alice is to reduce tip vortices by imparting a torque on the air stream as they spin.

Engines are typically placed farther outboard to relieve wing bending caused by lift thus reducing structural weight, but it’s all trade-offs. Props ate the wingtip are a horrible idea for conventional, versus, tilt rotor, due to crosswind considerations. Vmca and Vmcg are issues, but centralized engines with complex drive trains can compensate, see tilt rotor design.

Overall, as we’ve never seen one, I doubt there’s any advantage.

GalaxyFlyer wrote:

Engines are typically placed farther outboard to relieve wing bending caused by lift thus reducing structural weight, but it’s all trade-offs. Props ate the wingtip are a horrible idea for conventional, versus, tilt rotor, due to crosswind considerations. Vmca and Vmcg are issues, but centralized engines with complex drive trains can compensate, see tilt rotor design.

Overall, as we’ve never seen one, I doubt there’s any advantage.

Never say never in aviation design.

Behold the Vought V-173 "Flying Pancake"!

Good God!

Interesting Wiki entry and link to Air and Space Magazine. Centrally mounted engines driving both propellers to deal with OEI case, forerunner to tilt rotors. Lindbergh flew it and liked it.

Starlionblue wrote:

Behold the Vought V-173 "Flying Pancake"!

Nice gem

Still there are many more examples. MH22 obvious (also reasoning behind why wingtip powerplant are obvious), or CL-400 Suntan. There was also a VJ101, also intended as vertical takeoff.

If anyone wonders why they're not popular - see the wing. Needs to be structurally reinforced, and that means worse wing parameters (airfoil etc) in addition to extra weight. You lose not once, but twice or triple (I believe wing vortex increasing also adds up to negative effects).

It's just a bad idea unless you have no other solution. That's why it's usually on vertical takeoff planes only.

Cheers,
Adam

gloom wrote:

If anyone wonders why they're not popular - see the wing. Needs to be structurally reinforced, and that means worse wing parameters (airfoil etc) in addition to extra weight. You lose not once, but twice or triple (I believe wing vortex increasing also adds up to negative effects).

Incorrect. It provides wing bend relief, making the wing lighter, not heavier. On modern airliners it's the upwards bending motion while it is generating lift, that you need to strengthen the wing for. If the wing isn't strong enough, it would effectively bend upwards and snap. It is one of the reasons why the A340-300 was so much more capable than the otherwise identical A330-300. Having engines mounted further outboard reduces the wing bend, allowing it to be built lighter or carry more.

But for control reasons, it is a horrible idea. If you lose a wingtip engine at takeoff, the aircraft will be virtually impossible to fly.

Starlionblue wrote:

GalaxyFlyer wrote:

Engines are typically placed farther outboard to relieve wing bending caused by lift thus reducing structural weight, but it’s all trade-offs. Props ate the wingtip are a horrible idea for conventional, versus, tilt rotor, due to crosswind considerations. Vmca and Vmcg are issues, but centralized engines with complex drive trains can compensate, see tilt rotor design.

Overall, as we’ve never seen one, I doubt there’s any advantage.

Never say never in aviation design.

Behold the Vought V-173 "Flying Pancake"!
]https://www.usni.org/sites/default/files/styles/embed_large/public/19610120000a.jpg?itok=S4XoQuC0

Barnes Wallis also had wings with engines at the edges with his Swallow design. Not only were they at the edges, but the wings were variable geometry and the engines would pivot. Seemed very complicated - perhaps even for today, let alone when he designed it. As far as I know it never went further than design drawings, a huge desk model he had and possibly wind tunnel models.

I don’t want to think of what would occur with an engine failure - or worse, two failures on the same side since they were stacked one above the other on each wing edge.

There are plenty of resources for further reading on the Swallow design, but they are on rival sites to this one so cannot be linked.

I believe that this discussion should come in the context of hybrid and multiple propeller aircraft designs, not necessarily with the same power or diameter. I understand that judicious projects are looking for new possibilities in terms of aerodynamic, structural and propulsive efficiency for short distance trips.

cpd wrote:

Starlionblue wrote:

GalaxyFlyer wrote:

Engines are typically placed farther outboard to relieve wing bending caused by lift thus reducing structural weight, but it’s all trade-offs. Props ate the wingtip are a horrible idea for conventional, versus, tilt rotor, due to crosswind considerations. Vmca and Vmcg are issues, but centralized engines with complex drive trains can compensate, see tilt rotor design.

Overall, as we’ve never seen one, I doubt there’s any advantage.

Never say never in aviation design.

Behold the Vought V-173 "Flying Pancake"!
]https://www.usni.org/sites/default/files/styles/embed_large/public/19610120000a.jpg?itok=S4XoQuC0

Barnes Wallis also had wings with engines at the edges with his Swallow design. Not only were they at the edges, but the wings were variable geometry and the engines would pivot. Seemed very complicated - perhaps even for today, let alone when he designed it. As far as I know it never went further than design drawings, a huge desk model he had and possibly wind tunnel models.

I don’t want to think of what would occur with an engine failure - or worse, two failures on the same side since they were stacked one above the other on each wing edge.

There are plenty of resources for further reading on the Swallow design, but they are on rival sites to this one so cannot be linked.

Thanks for the info. That's quite the striking aircraft!

VSMUT wrote:

Incorrect. It provides wing bend relief, making the wing lighter, not heavier.

I would disagree here. Sure, at cruise and stable - you are right. But you get three extra difficulties there:
1. Thrust forward at the tip (not discussing vtol here, just a wingtip engine) - wing needs to keep the shape regardless the thrust (or thrust change).
2. At ground and no speed, you have a heavy engine at wingtip. So basically, you have the tradeoff from lift bending at cruise, to point-heavy engine on ground. Since the lift at the wing end is quite low, I'd say it requires quite a lot of strenghtening there.
3. Aero at tip becomes much more difficult to handle.

However, this is just my opinion. If you have math to support your thesis, I'll be happy to read.

Cheers,
Adam

gloom wrote:

VSMUT wrote:

Incorrect. It provides wing bend relief, making the wing lighter, not heavier.

I would disagree here. Sure, at cruise and stable - you are right. But you get three extra difficulties there:
1. Thrust forward at the tip (not discussing vtol here, just a wingtip engine) - wing needs to keep the shape regardless the thrust (or thrust change).
2. At ground and no speed, you have a heavy engine at wingtip. So basically, you have the tradeoff from lift bending at cruise, to point-heavy engine on ground. Since the lift at the wing end is quite low, I'd say it requires quite a lot of strenghtening there.
3. Aero at tip becomes much more difficult to handle.

However, this is just my opinion. If you have math to support your thesis, I'll be happy to read.

Cheers,
Adam

2. The weight of the engine on the wing on the ground is trivial compared to the holding the aircraft aloft in the air. 2-5 tones of engine vs 60-400+ tonnes of total aircraft.

Starlionblue wrote:

cpd wrote:

Starlionblue wrote:

Never say never in aviation design.

Behold the Vought V-173 "Flying Pancake"!
]https://www.usni.org/sites/default/files/styles/embed_large/public/19610120000a.jpg?itok=S4XoQuC0

Barnes Wallis also had wings with engines at the edges with his Swallow design. Not only were they at the edges, but the wings were variable geometry and the engines would pivot. Seemed very complicated - perhaps even for today, let alone when he designed it. As far as I know it never went further than design drawings, a huge desk model he had and possibly wind tunnel models.

I don’t want to think of what would occur with an engine failure - or worse, two failures on the same side since they were stacked one above the other on each wing edge.

There are plenty of resources for further reading on the Swallow design, but they are on rival sites to this one so cannot be linked.

Thanks for the info. That's quite the striking aircraft!

Yeah, and here is the man himself with the remarkable forward-thinking Vickers Swallow design, here with wings fully extended:

https://c8.alamy.com/comp/GBY36X/transp ... GBY36X.jpg

And the desk model, which I believe was powered and the various features were able to be controlled for better effect:
http://4.bp.blogspot.com/_8DAL7gPYBiM/T ... wallow.jpg

Those engines even pivoted along all three axes, serving in place of a rudder, ailerons and elevators. I don't know about you, but to me and for the time that sounds very ambitious and tricky to do.

NASA even took some interest in it:
http://webcache.googleusercontent.com/s ... =1&vwsrc=0

gloom wrote:

VSMUT wrote:

Incorrect. It provides wing bend relief, making the wing lighter, not heavier.

I would disagree here. Sure, at cruise and stable - you are right. But you get three extra difficulties there:
1. Thrust forward at the tip (not discussing vtol here, just a wingtip engine) - wing needs to keep the shape regardless the thrust (or thrust change).
2. At ground and no speed, you have a heavy engine at wingtip. So basically, you have the tradeoff from lift bending at cruise, to point-heavy engine on ground. Since the lift at the wing end is quite low, I'd say it requires quite a lot of strenghtening there.
3. Aero at tip becomes much more difficult to handle.

However, this is just my opinion. If you have math to support your thesis, I'll be happy to read.

Cheers,
Adam

It is basic ATPL Principles of flight stuff taught at flying school.

1. Along the thrust-line the wing is strong enough, although you obviously wouldn't be able to do razor-thin wing tips like the 787 or A350. But that leads us to your 3rd point:

3. Aerodynamics would actually benefit from having the engine at the tip. Wingtip devices of all sorts benefit the aerodynamics in the same way a winglet does. This applies to tip tanks, engines, weapons pylons etc.

2. As Starlionblue pointed out, the engine might weigh a few tons, but lift in flight amounts to the entire weight of the aircraft and then some to compensate for turbulence. For the A350-900 that means 280 tons of upwards bending force. Downwards force you have the engines at 14 tons and up to 47 tons of fuel.

If you notice during the infamous wing-bending tests when a new aircraft type gets certified, they always bend the wings up, not down:
https://www.youtube.com/watch?v=ET9Da2vOqKM

VSMUT wrote:

2. As Starlionblue pointed out, the engine might weigh a few tons, but lift in flight amounts to the entire weight of the aircraft and then some to compensate for turbulence. For the A350-900 that means 280 tons of upwards bending force. Downwards force you have the engines at 14 tons and up to 47 tons of fuel.

Sure, but the arm of force would likely be 3 times longer. Center of weight would be at 1/3rd approx in case of wing tanks, vs tip on engines.

I still feel it's not that easy, since all wingtip engine planes seem to have an extraordinary thick airfoil, but since I have no evidence other than what I presented, it's EOT for me.

Thanks for some good points, though.

Cheers,
Adam

Another reason engines and especially propellers on the tips is a bad idea is crosswind performance. It's difficult to bank into the wing with the propellers out there.

gloom wrote:

VSMUT wrote:

2. As Starlionblue pointed out, the engine might weigh a few tons, but lift in flight amounts to the entire weight of the aircraft and then some to compensate for turbulence. For the A350-900 that means 280 tons of upwards bending force. Downwards force you have the engines at 14 tons and up to 47 tons of fuel.

Sure, but the arm of force would likely be 3 times longer. Center of weight would be at 1/3rd approx in case of wing tanks, vs tip on engines.

I still feel it's not that easy, since all wingtip engine planes seem to have an extraordinary thick airfoil, but since I have no evidence other than what I presented, it's EOT for me.

Thanks for some good points, though.

Cheers,
Adam

It still doesn't amount to the upwards bending force the wing is required to take. For certification requirements, it has to take 150% of the MTOM, meaning for an A350 the wing has to be designed to take 420 tons of upwards pressure.

But that's probably a mute point, because electric engines are going to be much lighter.

Looking up the Eviation Alice, it has a MTOM of 6350 kg and the Magni250 motors weigh 71 kg each. We are talking a wing that has to take 4760 kg of upwards pressure with just a 71 kg engine to weigh it down. Even considering the 7-8 meter arm, it doesn't come anywhere close to the upwards force.

A number of helicopter designs used tip jets to rotate the blades, the main advantage was no need for a tail rotor.