A345 - engine mounting
 

Question from an enthusiast...

At LHR tonight, and seen the Arik A345 taxi by. It may have been an optical illusion, but the two inboard engines seem to be canted slightly forward in comparison to the outboard engines. Checked a few pics on the web, and the same effect appears to be present. Am I seeing thngs or is this correct? Does this correct in flight as the wing lifts?

Just curious...

My, you have good eyesight :)

Not sure exactly what you mean by 'canted forward' but the inboard engines are about 1.5 degrees nose down relative to the outboards when on the ground. In flight the wing twist (which goes with the bending) brings them into line.

Thanks Owain,

Eyesight one of the few things still working!

Seems counter-intuitive that the inboard engines are the ones that 'drop' on the ground. I can see how the wing lifting might pull up the outer engines, but it seems strange that the inboards would lift while the outers don't...but that's maybe why I'm an electrical rather than structures engineer!

The key statement is in brackets. The moment of the wing twists the wing tip "nose down". But having a positive wingsweep, any upward bending results in an even more significant "nose down" twisting of the wingtip, simply because in the coordinates of the wing, the leading edge of the wingtip is more inboard than the trailing edge, resulting is a different amount of up-bending. As wings are much softer in bending than in twist, it is mainly the second effect which is important.

They don't! The inboards are mounted on the stiffest part of the wing and their in-flight deflection is peanuts. The outer wing bends upwards and takes the outer engines with it, but that is not the direct cause of the twist.

On a swept wing the lift of the outer bits lies aft of the flexural axis of the inboard bits. Consequently the outer wing exerts a torque on the inner bits tending to produce nose down twist. The inboard half (say) of the wing is so stiff that this twist is pretty small, but as you move outboard the bending and torsional stiffnesses of the wing get smaller and more liable to bending and twisting deflections. Adding winglets makes it worse.

http://www.smugmug.com/photos/i-x8VG...-x8VGt26-M.jpg
Owain, yes, good eyes indeed...when this picture was taken I hadn't noticed! So the outboard engine's "angle of incidence", (so to speak!) reduces as the wing bends, to become approximately the same as the inboard engine's? Would there still be a minor difference between the two depending upon take-off weights?

PJ2

Yes that's right. There would indeed be a minor difference depending on TOW or, for that matter, as fuel is burned off in cruise, but those are small differences.

Now I come to think of it, the downward bending on the ground due to fuel in the outboard tanks would presumably have the opposite effect ....