wing bending relief

[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.