# Wing skin deformations

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#### rapidshot

##### Well-Known Member
When designing the skin metal plates of a light aircraft under pressure we get relatively high deformations, apart from aerodynamic considerations and shear buckling due to shear flow can we just use one of the design philosophy approximations to reduce deflections and avoid narrow rib spacing, stringers

1. higher load factors are transient , so compute the deflections under 1g load condition
2. may be considering fatique and depending on the stiffness of the perimeter supports (ribs, front and rear spars ) use max load factor and large deformation theory to reduce deflections allowing in plane forces in the plate,

before having several lower back surgeries i had a light aircraft, as an amateur flying engineer i hated to see those wrinkles on the upper skin during gusty conditions,

Any source and/or advice all are welcome

thank you

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#### pictsidhe

##### Well-Known Member
If it's a true monocoque, the wrinkles need to be kept in check. If the skin is not taking all the bending, then it's more of an aesthetic problem. Thin skins would need frequent bracing. Thick skins weigh too much...

#### rapidshot

##### Well-Known Member
Thanks Pictsidhe, the skin doesnt take any bending, only the spar flanges,

#### TFF

##### Well-Known Member
Some of the design tradeoff is minimum weight or robustness. Aesthetic, ground handling, mission requirements. Adding thoughtful beef can check any questions of is there really enough there. So you might have to gain 20 lbs to get rid of wrinkles for whatever the choice is. It just depends on the design and where it has to be at weight. Super minimalist design, oil can city. Live with it because it’s part of your mission. Medium duty IFR plane, it better look like it is carved out of billet. It’s all designed choices.

#### BBerson

##### Well-Known Member
HBA Supporter
The skin provides torsional "stiffness" (stiff in deflection, not ultimate strength) needed for flutter prevention.
Look up "FAA Simplified Flutter Criteria".

Good point

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
Took this snapshot this morning, noting skin deflection. Can't stop it any more than you can prevent power lines sagging between towers...

##### Well-Known Member
Limiting factor for skin gauges are often driven by nonstructral reasons i.e. handling damage, ability of actaully get the loads out of skin, fuel tank issues, etc.

There are plenty of GA have .032 skins and no stringers. GAF Nomads with their super thin skins (.012 trim tabs and the like) did not always age well).

I know of at least one GA aircraft where the skin was omitted from the structural bending test of the wing.

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#### wktaylor

##### Well-Known Member
One element of all sheet metal that cannot be avoided... except at PREMIUM cost...

All sheet metal from the factory has allowable deviation/tolerances for waviness and flatness. When 'perfectly flat sheet' is specified... in reality very-low allowed waviness/flatness tolerance-deviations... then the mills demand premium s and high quantities.

IF/when sheet metal is formed, bent, draped, etc... then 'stock sheet metal' is useful and practical. However with large unsupported flat sheet is applied to common rib/stiffener the apparent waviness/flatness deviations inherent in the metal may be pronounced.

Aluminum + honeycomb-core adhesive bonded structure is created, then the close spacing of the core-cells and the very rigid/contour controlled tooling can make parts with low contour deviation... and exceptional stiffness and buckling resistance.

NOTE.
Sheet-metal skins-fastened appropriately to substructure... such as frames/ribs, stiffeners, fittings, etc... with fastener sizes/spacing to avoid/minimize inter-rivet buckling... do more than contribute to shear/torsional stiffness/strength... there is an effective width of skin material that can legitimately added to the primary adjacent/attaching flange for compression/shear/bending loading.