# Is the wing attach fitting ever loaded with shear force?

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#### David Teahay

##### Well-Known Member
Do wing attach fittings of semi-cantilevered plane(eg a wire braced ultralight) ever get subjected to shear force?(for example during roll,does the centrifugal force of the wings load the wing attach fittings with shear?).

#### Dana

##### Super Moderator
Staff member
Wing attach fittings are generally designed so that the bolts are loaded primarily in shear at all times.

#### wsimpso1

##### Super Moderator
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Listen to Dana.

Then, let's look at the bigger topic. First you need to know your way around the beam theory portion of a Mechanics of Materials textbook. This knowledge will let you run the numbers on shear load and bending moment at anypoint on any wing.

Typically, a pair of bolts or pins, parallel to the chordline attach the root end of the wing to the fittings at the fuselage. No moment can transmitted at the pinned joint, but a substantial amount of the lift is applied vertically through that joint. And when you go through things carefully, reactions from the bracing also result in pretty substantial lateral loads through those joints. There are a lot of ways to design these joints, but they all can end up with shear loads in some the pieces making the joint.

Please find and study a chapter or two on beam theory. Your questions will get a lot bigger and more rewarding for both the folks answering the questions and for the guy asking them too.

Billski

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#### Turd Ferguson

##### Well-Known Member
Wing attach fittings are generally designed so that the bolts are loaded primarily in shear at all times.

lol, with Beechcraft excluded from the way planes are generally designed.

#### Bill-Higdon

##### Well-Known Member
lol, with Beechcraft excluded from the way planes are generally designed.
Don't forget the DC-2 & 3,:ban: I don't know the number bolts the DC-2 took but because of the DC2 1/2 I suspect it's the same number per wing for both aircraft something like 328 per side.

Bill

#### wsimpso1

##### Super Moderator
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Don't forget the DC-2 & 3,:ban: I don't know the number bolts the DC-2 took but because of the DC2 1/2 I suspect it's the same number per wing for both aircraft something like 328 per side.

Bill
Not wire braced...

#### wsimpso1

##### Super Moderator
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lol, with Beechcraft excluded from the way planes are generally designed.
After the Staggerwing, Beechcraft were not wire braced...

#### TFF

##### Well-Known Member
Wing bolts on bonanzas are in tension. Big, high strength cap screw things. Usually need special tools to get out. I want to say 250 ft/lb each. Four a side.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Do wing attach fittings of semi-cantilevered plane(eg a wire braced ultralight) ever get subjected to shear force?(for example during roll,does the centrifugal force of the wings load the wing attach fittings with shear?).
Centrifugal force?

#### wsimpso1

##### Super Moderator
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The OP asked about wire braced wings. Yeah, he called them semi-cantilever, but that does not mean any of those fully cantilever birds you guys keep mentioning.

Even in the Bonanza/Baron style mount, the bolts themselves are in tension, but shear loads are seen in the buckets. Then you can read up on Mohr's circle and von Mises yield criteria - that is where you find out that if you look at any loading case except triaxial compression there is always shear. Usually, shear is how deformation occurs, even in pure tension.

Billski

#### TFF

##### Well-Known Member
I think the real answer is you want to try and put bolts in double shear instead of single shear.

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#### ddsrph

##### Well-Known Member
A & P friend of mine was doing an annual on a certified strut braced high wing plane the year after a total rebuild. When he took off one wings root fairing he was surprised to see two screwdrivers holding the wing on instead of bolts.

#### pictsidhe

##### Well-Known Member
A & P friend of mine was doing an annual on a certified strut braced high wing plane the year after a total rebuild. When he took off one wings root fairing he was surprised to see two screwdrivers holding the wing on instead of bolts.
Not harbor freight ones, then! Decent screwdrivers are made of alloy steel, should take a lot of shear!

#### proppastie

##### Well-Known Member
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"Red-neck total rebuild" new paint?

#### gtae07

##### Well-Known Member
Sounds like temporary pins that got left in by mistake, akin to finding clecos still attached in a flying production aircraft. But more serious.

#### TFF

##### Well-Known Member
Yea, but can I have them back.

I knew someone who test flew a helicopter after an annual, and as he looked up, when getting out, he noticed he did not put the nuts on the bolts that held the blades on.

#### Pops

##### Well-Known Member
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Must been another redneck

#### Chilton

##### Well-Known Member
I remember about 30 years back an Aeronca L3? From a total rebuild where the root fittings were not even fitted with the bolts, only friction holding them, plane flew a circuit and the join failed in the flare, rolled the plane in a ball, luckily minor injuries only. My first job in school holidays was the start of rebuilding the replacement aircraft which had to be done free by the company, I remember it being VERY carefully checked before flight!

#### Riggerrob

##### Well-Known Member
Most aircraft bolts are designed to be loaded in shear - ideally double shear - because shear loads lock in bolts in place, creating so much friction that bolts cannot fall out in flight.

Tension is the worst load on bolts, because normal
flight loads might pull components apart.

Similarly, most bolts are installed with bolt heads on top, so gravity naturally keeps them in place.
The other “safer” option is installing bolt heads facing into the wind in hopes that wind will hold bolts in place until you land.
All these “best practices” are based on the question “What if the nut falls off in flight?
Conscientious aircraft mechanics always install lock nuts, lock washers, cotter pins or lock wire to reduce the risk of loose nuts or bolts.