Why Are Wing Mounted Engines Positioned Close to the Fuselage?

[claytonh4]

I've noticed on a/c with only two wing mounted engines that they tend to be closer to the fuselage than the wing tip. There must be some reason for this, and so I guess my question is why? Does having them farther out on the wing create too much stress? I would assume if a/c manufacturers could put the engines closer to the wing tip they would (for cabin quietness/comfort) so it must be a structural thing. If a four engine a/c was forced to run on just two engines (one on each side), would they then rather have the two closest engines running, or the two outward engines?
Thanks for any answers!

 

[Travis_King]

The reasons are mainly stress and flutter concerns (harmonic vibration with a weight at the end of a cantilever). But there are other reasons as well which include minimizing effective drag area, sucking in more air, stuff like that.

 

[AlephZero]

Another issue is the ability to fly a two engine plane with one engine failed. The further the engine is from the aircraft center line, more rudder control input is needed to fly straight, and drag force from the rudder is greater. The plane still needs some capability to make a turn with the working engine on the "inside" of the turn, though turning with the engine on the "outside" is much easier (and without any other contraints, you would probably fly a 270 degree turn the "easy" way rather than making a 90 degree turn the "hard" way).

A four engine plane can be flown on either the inboard or outboard pair of engines. In fact if a plane like a B747 needs to be make a flight with one engine failed (for example to ferry the plane to somewhere with maintenance facilities) the takeoff is usually done on two engines not all three working ones, to keep the thrust symmetrical.

 

[Vadar2012]

To expand on the vibration (harmonic response) part. All structures have a series of frequencies (natural frequencies) at which the displacement is a maximum. For eample, if I vibrate a ruler with one end attached to my desk, different frequencies will result in different deflections. The largest ones are called natural frequencies. The shape of a structure at these frequencies are called mode shapes and they kind of look like sin waves.

So say you've done simulations to determine the natural frequencies of a plane wing. You might find that one of the natural frequencies of the wing is close to the likely operational frequency. For example, 2Hz for a car. One of the ways to overcome this (and an effective one) is to place the motor at one of the node points along the mode shape of this frequency. Picture the node as any point where y=0 on a sin wave.

This is just one of the things to consider when designing a wing.

 

[claytonh4]

Thank you for the answers, everyone!

Another issue is the ability to fly a two engine plane with one engine failed. The further the engine is from the aircraft center line, more rudder control input is needed to fly straight, and drag force from the rudder is greater.

And this is a good point about twins. I hadn't taken into consideration possible single engine ops.