# Shear flow in wing box and single strut

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#### rapidshot

##### Well-Known Member
Hi all,

I have been thinking on the subject for a while but couldn’t make an approach for hand calculations,

Any recommendations are welcome, how does a single strut attached to front spar affect the shear flow in the box due to torsion,

Do you think that shifting the elastic axis linearly from wing tip to strut attachment point is a good approach?

Thank you,

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
The strut is best thought of as a pure linear element- only compression or only tension- and attached to the main spar. The wing outboard is a pure cantilever.

The root connections to main and drag spars reacts linear loads only. Pitching moment is reacted by opposite sense vertical loads and is superimposed on the compression and lift forces there.

As to shear flow in the box from torsion, torsion accumulates continuously from tip toward root, and the strut really won't carry any torsion off as they are pretty soft torsionally.

#### rapidshot

##### Well-Known Member
Thanks for your response,

I think my question was misunderstood, i agree since strut has no bending stiffness and pin connected it shouldnt have any effect on the torsional stiffness of the box, may be a small amonuts due to rib at that point, but it certainly affects the rotations, the elastic center location etc,

So with the existence of the strut how much the rotations change when compared to a cantilever wing, sorry for the wrongly expressed question,

Thanks a lot,

#### BBerson

##### Well-Known Member
HBA Supporter
The torsion elastic axis is spanwise. So shifting is chord wise. So shifting the strut attach forward on the box would cause a negative pitching moment. And aft of the box center of torsion a positive twist. Probably best to put it on or near the box torsion center but most go to the main spar.

#### BJC

##### Well-Known Member
HBA Supporter
The torsion elastic axis is spanwise. So shifting is chord wise. So shifting the strut attach forward on the box would cause a negative pitching moment. And aft of the box center of torsion a positive twist. Probably best to put it on or near the box torsion center but most go to the main spar.
The Sportsman's strut is unusual in two ways: It attaches to a structure in the wing between the main and rear spars, and it attaches to the fuselage vertically below the rear spar attach. That arrangement allows the wing to fold aft by pivoting on the rear spar attach and the strut-to-fuselage attach. No disconnecting of wiring, fuel lines, flap controls, aileron controls or pitot tubing is required. It also makes access to the front seats easy.

BJC

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#### BBerson

##### Well-Known Member
HBA Supporter
I think the wing folding requires some compromises from the theoretical ideal strut location.
There is always compromises. And ideal is rare.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Thanks for your response,

I think my question was misunderstood, i agree since strut has no bending stiffness and pin connected it shouldnt have any effect on the torsional stiffness of the box, may be a small amonuts due to rib at that point, but it certainly affects the rotations, the elastic center location etc,

So with the existence of the strut how much the rotations change when compared to a cantilever wing, sorry for the wrongly expressed question,

Thanks a lot,
Oh, this is just statics. You can resolve this by examination or by cross products. This was Sophomore year when I was in engineering school.

I would compute the vertical load generated at the strut, then the lateral and any fore-aft components, and superimpose those on the wing structure at the attach point. Yes, if the strut attaches aft of the 1/4 c point, the downward force will superimpose a nose up moment, any fore-aft component will drive a moment about the vertical axis, and the inward force will be split among the spars by lever rules.

Billski

#### rapidshot

##### Well-Known Member
This is a simple box beam model to clarify my question, only torsional moments are applied,

The one on the left is supported at its torsional center while the other one is supported at left spar, those green points around mid points of the box are the restraints, both have 2 hinges at the far end,

There is huge difference between the tip deflections, almost doubled in the rhs model which brings me the conclusion that if the single strut is attached to front spar only shear center shifts towards the spar, i think this will be a rough but safe approximation for preliminary hand calculations,

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#### BBerson

##### Well-Known Member
HBA Supporter
It isn't a box beam without skin. Put skin on it and it won't twist much.

#### rapidshot

##### Well-Known Member
This box is 600 mm x 200 mm , sides are 3 mm, top and bottom 1 mm, ribs 1 mm thick,

It doesnt look like an actual wing box but a quick model, torsional moments have arbitrary value uniformly distributed along span just to capture the behaviour,

#### BBerson

##### Well-Known Member
HBA Supporter
Aircraft structures has a page for two spar design. Briefly, the front spar moment of inertia should be 60% and rear spar 40% and the strut at the elastic center

Thank you,