wing bending relief

[1515Blue]

can someone explain exactly how this concept works... and how it is that wing mounted engines help in 'wing bending relief'

thanks

[john_tullamarine]

A jargon phrase used by the stressman fraternity.

Imagine you have a rule (ruler, straightedge according to your jargon) representing the wing.

Apply thumb and finger loads to represent the fuselage (zero fuel) and semi-span wing lift loads .. and the rule starts to bend ... and as the loads increase the amount of bending increases.

If the bending gets too great, either the compressive (top "wing" surface) or tensile (bottom "wing" surface) stresses (a bit like trying to squash or tear the material apart, respectively) get to a critical point where the material starts to give up and the structural properties deteriorate or, in the case of plastic fantastics, perhaps let go with a bang and not much warning.

So .. we get the notion that only so much loading can be permitted to harass the wing. These loads are prescribed in a generic way in the design standards loading envelope. The loads analysis works out what the wing is doing, where the loads are, and where they are being transmitted in the structure.

So what ? ... we end up with a maximum load (typically expressed as the max ZFW).

Now, if we put some weight (OK, mass for the purists) out in the wing somewhere (and we might do this by unbolting the engines from the fuselage and glueing them onto some wing nacelles) and repeat the exercise, we find that, for the same useful fuselage load (ZFW consideration) and g-load (manoeuvring or gust loads), the wing bending is LESS because the out span mass is doing its bit to REDUCE the extent of bending. This observation is what leads to the expression "wing bending relief" .. for given fuselage and wing lift loads, the addition of the out span mass reduces the bending and the reduced bending reduces the critical structural stresses in the wing.... if you like the reduction in stress (loading) levels can be thought of as providing some structural "relief" to the wing as in the wing doesn't have to work as hard (for the same structural design).

This is of great importance in, for instance, after market gross weight increases. So, for example, a passenger aircraft converted to a freighter might see its MZFW increased by imposing a requirement to tanker fuel in the outboard tanks (this was done for the Lockheed Electra). There is a bunch of other things to worry about as well, but you will get the basic idea ....

Similarly, if one looks at two aircraft, one with aft fuse mounted or buried wingroot engines and the other with wing mounted nacelle engines, for the same sort of loads the latter should be expected to have a lighter wing structure as both the bending and aeroelastic properties can be improved when compared to the former. Again the term wing bending relief is appropriate.

[Flight Safety]

JT, I'd like to rephrase what you said to see if I understand this concept correctly, and please correct anything that's not right.

Lets say an aircraft has a fuselage with 2 engines that together have an all up mass of xxx kgs. Let's say it also has wings of yyy sq. meters area designed to lift the total airframe mass, plus a full fuel load. Let's also say that we can "build" the aircraft to 2 different configurations, either with the engines on the fuselage near the tail, or the engines mounted on the wings with nacelles.

So the real difference is that with the tail mounted engine config, the wings must lift the entire mass of the airframe AND the engines, from the very END of each wing (the end part mounted to the fuselage), thus causing a greater bending moment for each wing. However with the pod mounted engine config, the wings must still lift the fuselage from the end of the wing, but at least the engine weight is now being lifted from a more "mid-span" location, thus reducing the amount of "end-lift bending" the wings must be subjected to. In other words, lifting the engine weight from a "mid-span" location on the wing causes less bending stress on the wing than lifting the engine weight (when attached to the fuselage) from the end of the wing, since engines are quite heavy.

I suppose then that distributing the fuel more evenly over the wings, reduces the bending stresses on the wing when it lifts the total fuel load also. I can understand that even moving the fuel load more "outboard" on the wing can sort of counter balance the load that's on the inboard end of the wing in flight, and can result in more "lifting" being done towards the outer end of the wing also, thus reducing the bending stress.

This sounds a little like taking a pencil and tying a one kilo weight to it with a string, then lifting that weight with the pencil. It would make a big difference if the weight were tied to one end of the pencil, and you had to lift the weight by holding the pencil at the other end (holding the pencil horizontally). There would be a lot of stress on the pencil lifting the weight that way. But if you tied the weight to the center of the pencil, then lifted the weight by holding the pencil at both ends (as both "ends" of a wing are lifting), there would be much less stress on the body of the pencil.

Do I understand this correctly?

(edited for typos)

[spanner-do]

Which is why......

On the Boeing Twins, the Centre/Auxilliary Tank fuel load (which is located in middle of the wing structure) is used first; to ensure that at higher all up mass the fuel in the Main (wing) tanks assists in the wing bending relief as there is need for greater lift to get the whole thing airborne. Thus preventing the wing tips from being deflected upwards any further than necessary.

[john_tullamarine]

Flight Safety,

With my engineer's hat on I might see a need to clarify a point or two .. but with a pilot's hat on ... I think that you have the idea sufficient for the need ....

The wing lift is distributed along the span rather than acting at the tips. If you like to think of a centroid for the lift (similar to the rule analogy I gave before), rather than a distributed pattern, then the centroid (mean lift position for the half span if you prefer) will be somewhat inboard of quarter span for each wing.

Keep in mind that my description was intended to be a general overview for a pilot's interest ... the engineering detail gets a little more involved but, really, is not necessary for a descriptive overview....

[Dan Winterland]

The VC10 with 4 tail mounted engines, has a wing bending relief system which involes moving the ailerons up about 10 degrees in unison. This system known as aileron upset is switched manually by the crew at certain weights and altitudes.

[MasterGreen]

Dan - Interesting.

Is this to off-load the outer panels (where the ailerons live) ?

[john_tullamarine]

Haven't come across this system before, although there are other active control techniques in the world of aeroplane technology.

Moving the ailerons up together will alter the spanwise lift distribution significantly with the overall effect being to move the semi-span lift centroids inboard with a reduced critical bending moment which is the goal of the exercise.

This is a bit like the lift distribution change you get when trailing edge flaps are extended .. except with the former the overall lift is decreased while, with the latter, the lift increases.

When Dan responds to MasterGreen's query, perhaps he can include some details regarding associated operational limitations and considerations ? I, for one, will be very interested to have my knowledge base extended on this one ....

[411A]

And, let us not forget, the Lockheed TriStar -500 does this automatically. Severe weight (mass for you European folks) penalties if the system is unserviceable.

Wing bending relief is nothing new either. The DC-6B (ten tank model, 5512 US gallons) requires fuel to be used first from the more inboard tanks, same for the 'ole 1649 Connie (9900 gallons).

[bsevenfour]

Some aircraft, such as the 747-400, have a limit imposed on their maximum take off weight when the specific gravity of the fuel is below a certain amount.

With the specific gravity low then the wing bending relief from the fuel in the wing tanks is reduced and hence the lower imposed maximum take off weight.

[MasterGreen]

Yes it is interesting how this is implemented. I have seen some minor (but strange to my uneducated eye) positions on the B777er flaps early in the cruise recently. I rather thought they were artifacts of the system, but come to think of it.... There was reflex on the outers and a slight droop on the inner (flaps) on a Max TOW departure and cruise recently.
As JT says this would neatly move the spanwise lift centre inboard (at the expense of a greater overall AOA) and relieve the wing no end.

I to flew the L1011 and noticed this reflex activity - but then then the L1011 was a law unto itself - a wonderful old lady for sure. Well ahead of it's time I fear... Shame about all the relays... With solid state she would still be in the sky...

MG

[Wander]

Interesting stuff indeed chaps. My lack of knowledge about this was unearthed when I took over an aircraft (A320) and the valve that tells the centre tank to refuel was stuck open. Consequently we had more than the usual amout of fuel in the centre tank and less in the wings. No problem in this instance though as we could use the fuel from the centre tank.

Further investigation in the MEL however says (I think from memory) that if you can't use the centre tank fuel then you are effectively increasing the ZFW and should consequently reduce your traffic load accordingly.

This would also explain why the fuel feeding sequence on the A320 is 1. Centre tank
2. Inner tanks to the low-level sensors
3. Outer tanks (700kg) transfer to the inners.
All designed to keep maximum weight outboard. I believe that having fuel in the outer tanks is also good for turbulence?
:p

[John Farley]

1515Blue
You have not come back to say whether you are now happy, given all the above posts.

If you are, fine. Read no further.

If not consider the following:

1 Jack any aircraft up in the hangar using the fuselage jacking points only.

2 Add weight to the wing (internally as fuel or externally as engines) this will make the wing sag.

3 This sagging is wing bending moment relief in action because when flying wings bend upwards due to fuselage weight. Anything that causes sagging reduces the upward bending and so contributes to wing bending relief.

[fantom]

and why we prefer slim hosties.....

:p

[1515Blue]

thanks to all the gentlemen who responded to my query!

no wonder they mount stuff underneath wings instead of over them... like fighters carryng bombs and extra fuel tanks.

i was observing a 777 taxiing the other day and realized that even on the ground those wingtips go a long ways up!



thanks again

[john_tullamarine]

1515Blue,

Don't think for a moment that you might be R. Crusoe ... self same thing applies to each and every one of us ..... the only problems are those people who try to hide from (or just hide) the fact ...

[Dan Winterland]

Master Green. Apologies for the delay in answering.

The system does just that, reduces the lift at the outer half of the wing and therefore reducing the bending moment.

It's over a year since I last flew the ten, but if my memory is correct, the system operates as follows.

The duplicated system is tested and then armed as part of the pre flight checks. When the flaps are raised after take off and go through 14.5 degrees (take off setting is 20 degrees), the ailerons start to move up taking about 20 seconds to go to the upset of about 10 degrees. (Varies depending on variant of VC10). To put the ailerons back to normal, simply switch of the selected system.

The parameters for use used to be above 131.5 tonnes and below 24,000'. This was changed a few years ago to accommodate a new fatigue management plan, so now graphs are consulted to see if upset is required for the weight, altitude and level of turbulence.

The system is required to be armed for take off as not having armed it will trigger the Take Off Configuration Warning. If it is not required, you have to remember to disarm it before flap retract.

If the system is inoperative, you are restricted to operating within the parameters where it is not required.

When activated, it didn't seem to affect the handling of the aircraft much, but it does increase drag and therefore fuel burn slightly, and also reduces the max flight level for that weight a little.

[PAXboy]

Writing as another who has limited tech knowledge ... Thanks for the explanations which make the process very clear.

In trying to find another way to describe the bending relief given to the structure, would it be correct to say: With regards to wing mounted engine pods - do they act like Shock Absorbers?

[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 ......)