Wing Fold & Locking Mechanism
 

Looking at a typical aircraft with folding wings such as the Skyraider.

When the wings are unfolded, how is the stress from airloads on the outer wing panel transmitted through the join in the spar. Logic would suggest that some locking pin is used, but the pin is smaller than the spar, so how does the pin not that is much smaller than the spar carry the loads. Is it as simple as a bloody strong pin?

I can count 16 pins on this one:
How folding wings work.

I would suggest going to the next glider field and checking out how the wings are secured there, after all they are built to be taken apart easily (folding the wings, often at several points along the wingspan), also to carry the loads once everything is back in place. And they have design at least +5.3g/-2.65g in normal operation (yes, every glider is according to EASA at least an utility category plane). From my experience, and that is a bit out of date, it is usually one or two bolts for the central point, and quite small pins for mid to outer span points.

On the Buccaneer it pretty much was a bloody great pin.


On this picture, about 1/3 of the way from the left hand edge of the wing there is a pale grey ‘spade’ shaped thing with a hole in. When the wing was unfolded, that would engage with the sloped bit on the inboard section of the wing and then the big pin would sequence down through the hole.

HtH

@ the original poster:
One can also have a steel pin in steel fittings, attached to aluminum spars.
Google something like 'Corsair wing fold mechanism'. I'm not sure but there seems to be just a main spar pin to take mainly bending forces (rotating around a top one, pinned at the bottom), and some mechanism near the leading edge to take torque forces.

Bigger faster aircraft may use a bunch of pins. (Newer ones will also be certified under 'fail safe' rules requiring more damage tolerance, designs where even for the wing in general, there isn't just 1 critical spar, but a bunch of spar webs in the wing.)

No question about the "bloody great pin" a third of the way down from the left-hand side of the pic, in fact the top of that pin is visible. It works vertically. However, the red locking fork on the right is surely a transit securing piece that's removed prior to flight? It too has a pin that operates horizontally and it is more clearly visible.

I think.

Correct on both counts.

One thing that needs to be considered, is that inflight, the lower part of spar is in tension, while upper is in compression

IMHO it's very clever engineering, not just the hinge and locking mechanism, but also pivoting fuel and hydraulic lines, control runs, electrical feeds, and in the case of the Buccaneer, ducting for the 'blown lift'.

Re the bloody great pin that some steel alloys are rated at 100 tons per square inch in tension. So not too many pins are needed to support the air loads. Probably more for redundancy in multiple load paths

Gentlemen that reminds me... A question off on a tangent if I may which has come to mind watching that big ole bird start up.
Does a piston engine have a choke for a cold start? Or is there another arrangement like the fifth injector on my car?

In the Skyraiders case, No and no. Engine is started on primer (a button you press, located right side of cockpit next to the starter switch) with mixture lever (left side of cockpit) in idle cut off, once running on primer mixture is moved to rich and primer button released.

As I understand it, the weight and forces from underslung gas turbine engines, on today's passenger jets, are transmitted to the wing with just two pins. They are big pins though.......

Usually, turbofans are attached via a combination of bolts (which taken mainly vertical and static loads) and shear pins (which take thrust loads, and are designed to release the engine in an impact).

The 737NG, for example, uses eight bolts and three shear pins:


https://cimg3.ibsrv.net/gimg/pprune....0665b40e92.jpg

DaveReidUK,

Methinks Uplinker may be referring to the engine strut fuse pins.

Rgds McHale.

Thank you both, I stand corrected, (and am slightly relieved, to be honest) :ok:

Don't forget that wing fold sheer bolts are only loaded in sheer. Simple to calculate and easy to design.

Big G they just slide a bit o timber across so it wont let the wing fold up mate! Treated Hardwood mind you! NOT Wet Pine.....( Wet pine dries and warps )