wing bending relief

[john_tullamarine]

PaxBoy,

Shocks are not an appropriate analogy ....

try these thoughts ...

(a) Simple analogy .... but not quite kosher ...

The wing is, if you like, holding up the aeroplane and working not much at all (during a bunt), moderately (during straight and level flight), and hard (during a pull up).

An analogy which is not too far off the mark (if you don't put too fine a line on the distinction between a load and a moment) is your goodself down at the gym pressing weights ...

Here you are lying on your back pressing a bar loaded with, say 50lb. Being a strong lad, no problem. (bunt)

Now you load the bar to 200lb. Still no problem (straight and level flight)

Then you load the bar to 400lb. (pullup) Not a problem for a while (as you are a strong lad) but after a while it starts to hurt a bit (fatigue damage) and you are starting to wilt a bit ....

Now, brothers Fred and Bill Engine, both good mates of yours, see that you are in difficulty and come over to see how you are going.

Each positions himself at either end of the bar you are holding with very, very sweaty hands.

At last they get the picture and each takes a hold of his respective end of the bar and lifts up with a force of 50lb. Now, with Fred and Bill lifting a total of 100lb, this leaves you to lift only 300lb ....

Believe me ... I used to do a lot of weights .... that is (bending) relief ....

(b) A more appropriate analogy (a bit like one halfspan of a wing upside down .....)

Now you set up a bar with a single weight on one end.

Pick up the bar at the OTHER end and hold it out level. If the bar has a very small load on the end, this is not too bad ...

Now try it again with a heavier weight ...... gets harder and so on...

This is quite like an aeroplane where the wingroot or carrythrough structure has to stop the wingtips clapping hands above the fuselage .... which is similar to your letting the side down, letting the bar fall and rotate ... .and skinning your shins ...

Now Fred Engine comes along and takes a hold of the bar at the midpoint and takes some of the load .... feel better ? .. that is pretty similar to bending relief ...

What Fred is doing to help you .... is what the wing mounted engine is doing for the wing structure ..... and, just as Fred relieves the (cantilever) load you are taking ... so does the engine relieve the bending load which the wing experiences ....


(Hopefully Genghis, John Farley, Mutt, OverRun, or any of the other good engineers in the sandpit won't peer-review this rigorous explanation ......)