Why does the upper wing skin fail in buckling and lower wing skin does not?

I am aware of the fact that the buckling is mainly caused in the regions of the specimen which are in compression. But the wing skin doesn't take any axial loads and is used to resist shear, atleast that is what is taught at the Bachelor's level of education in schools. The buckling in the upper skin, according to what I think, happens because of shear. But at the same time, the lower skin also is undergoing shear but the lower skin is not usually seen to experience any buckling failure. Why is that?

• How is a wing loaded? Mar 10, 2021 at 12:49
• Simple Aerodynamic forces coming in at at angle of attack, for example 10 deg, with no sideslip wind. Mar 10, 2021 at 13:01
• So check out how a Lancaster's wing bends when loaded with a Tallboy. And think about why. Mar 10, 2021 at 13:06

In a wing the normal situation is that the aerodynamic force is upwards (resisting gravity).

You are right that there is some shear and a fair amount of torsion, but the result is that:

• the top side of the wing is in compression, while
• the bottom is in tension.

So in essence what happens is the opposite from the picture below (the force is applied upwards, therefore there is tension below, and compression above).

Buckling only occurs when there are sufficient compression forces (even shear is a combination of tension and compression), therefore the top side is favorable for the onset of buckling.

UPDATE:

The wings panel are designed not to buckle, because that would be catastrophic for the aerodynamics. So the main structure (spar, ribs and the stringers) receive most of the load. Depending on the implementation, the structure might looks like the following image:

or

Also it is important that the skin panels are fastened onto the ribs and panels with different methods (see an example below). So movement of the stringer/spar will affect the loading of the panel

The wing skins are there to receive and distribute the aerodynamic pressure to the underlying load bearing structure.

In general the distribution of structural forces due to aerodynamic loading will look like:

However, as the underlying structure bends and flexes (see picture below of an example of a similar structure under bending), with it the skin panels are subjected to compression and tension (top and bottom respectively).

As I understand it (correct me if I am wrong) the shear is mainly due to the drag forces (unless you are talking about the vertical shear forces due to lift, which are resisted by e.g. the spar), and more or less they are the same for top and bottom

As as a result you have the top skin which subjected to complex loading (compression due to lift and shear due to drag), and the bottom skin that is also subjected to complex loading ( tension from the lift and shear due to drag). Additionally there is the aerodynamic pressure which during flight creates positive pressure always puts the panels in tension (and unless there is flow detachment and a negative pressure which can have unpredictable effects).

Again the skin structures do not carry much loading, although they are the outer most fibers on the beam, this is due to their relatively small thickness. They almost behave as membranes. So the resistance to local buckling is also small.

In a sense is like a bed sheet, that if you shear it, it has higher chance to resist buckling if you apply tension at the edges.

• I completely agree with what you have wrote and what is depicted in the pictures which you have attached. I am also aware of the fact that the shear itself is a combination of tension and compression. But I am still not able to comprehend that why doesn't the lower skin experience any kind of buckling then? I mean it also has shear, meaning a combination of tension and compression also is present there, but the upper skin is buckling and lower is not. Why does the overall effect of the shear forces acting on the top skin is causing it to compress, while causing the bottom skin to stretch? Mar 10, 2021 at 20:00
• If it was not a skin, which kind of behaves like a web, then I can agree on this fact that it is taking any axial forces, like what you have already shown in the second picture. But that is not what is taught to us. What is taught to us at the Bachelor's level is that the flanges of the spar only carries the axial loads while the skin and webs are used to resist merely shear. How does the overall effect of the aerodynamic forces cause the upper skin to come in compression and lower skin in tension? If that is so, then why do we even need shear, if shear can be broken down to the axial forces? Mar 10, 2021 at 20:03
• As kamran said, the wings panel are designed not to buckle, because that would be catastrophic for the aerodynamics. So the main structure (spar, ribs and the stringers) receive most of the load. The wing skins are there to receive and distribute the aerodynamic pressure to the underlying structure. However, the underlying structure bends and flexes and, with it the skin panels are subjected to compression and tension (top and bottom respectively). As I understand it (correct me if I am wrong) the shear is mainly due to the drag forces, and more or less they are the same for top and bottom. Mar 10, 2021 at 20:24
• As as a result you have a top skin structure which has a complex loading (compression due to lift and shear due to drag), and a bottom that has also a complex loading ( tension from the lift and shear due to drag). Again the skin structures do not carry much loading, although they are the outer most fibers on the beam, this is due to their relatively small thickness. They almost behave as membranes. So the resistance to local buckling is also small. In a sense is like a bed sheet, that if you shear it, it has higher chance to resist buckling if you apply tension at the edges. Mar 10, 2021 at 20:32
• "As I understand it the shear is mainly due to the drag forces [...]" -> Not necessarily. The shear stresses on the skin are caused by both torsion and lateral forces, including the lift force. Depending on the load state, it is not unsual for the lift-induced shear stress acting on the horizontal part of the skin to be larger than the drag-induced Mar 11, 2021 at 11:28

Wings are designed as a complex structure of spars and ribs, clad with aluminum, titanium, or new composite sheathing, or in the fabric, in the early planes. The Frame has been designed to support all the loads, bending moment of the wings, torsion, shear, compression, and tension.

It has been designed to never let the surface buckle because it would be catastrophic

The sheathing or skin collects mostly two types of forces normal forces in terms of surface pressure and drags tangential forces in limited boxed up cells with the structure of the wing supporting it. It also has in-plane shear and some stresses due to the rivets or connections, but all stresses are designed to never reach critical levels. Any bending compression, torsion, or other stresses are resisted by the frame of the wing, not by the skin. (except on monocoque planes, but that too follows the same concept of spars and ribs)

As we may have witnessed in a fight on a jetliner in choppy air, the wings are designed to vibrate large amplitudes, sometimes 5-6 feet up and down. But they can handle the shock and vibration.

This is a figure of a part of the wing structure.

.

Construction of wing box of a commercial aircraft. -1 – center spar,

-2 – front spar,

-3 – rear spar,

-4 – rib,

-5 – skin panel,

-6 – underside of skin panel,

-7 – stringers [10].

• Some wings will bend far more than your measly 5 feet - some are tested to destruction and bend more than 20 feet but also they are large wings... Mar 10, 2021 at 19:53
• @SolarMike, yes i know. I have seen 10-12 feet flapping in a 777 off Frankfort airport, but that alarmed even the crew who ran to their seats and buckled. Mar 10, 2021 at 20:08

Buckling occurs because of bending moments which place the upper skin in compression and the lower skin in tension.