rear spar bending load

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Rob de Bie

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Huh? 'Clamped beam' or 'clamped boundary condition' is a standard English word used in structural analysis. Actually I don't even know of another word to describe it. Therefore I used 'clamped rear spar' to describe a continuous rear spar at the root, or one that has such a connection to the fuselage that bending moments can be transfered. 'Simply supported rear spar' would be the equivalent description of a pin-jointed rear spar.

Rob
 
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PiperCruisin

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Huh? 'Clamped beam' or 'clambed boundary condition' is a standard English word used in structural analysis. Actually I don't even know of another word to describe it. Therefore I used 'clamped rear spar' to describe a continuous rear spar at the root, or one that has such a connection to the fuselage that bending moments can be transfered. 'Simply supported rear spar' would be the equivalent description of a pin-jointed rear spar.

Rob
I'm guessing 'clamped beam' is cantilevered. Or refer to the boundary condition as fixed in translation and rotation. Pinned is only fixed in translation. Some wing spars are cantilevered, but actually pinned in two locations so they are rarely "clamped" although that might be a way to test a beam that is cantilevered.

There are many ways to setup boundary conditions and they matter.
 
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wsimpso1

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ME undergraduate study in the late 1970's, grad work in mid 1980's, and a whole career in ME spanning nine separate engineering organizations. "Clamped" has not been in my nomenclature on beams. I doubt that simply calling something 'clamped" and having others know what you mean is universal. When someone says clamped without other definition, I am left to imagine what is meant. Clamping is compression load between members that also creates friction. The joint can slip, but it is up to the designer to either put capacity to resist slip outside of operating envelope, or - when slipping within envelope - to specify how much slip will happen before constraints change.

Joints generally have 6 degrees of freedom possible, three in translation and three in rotation. Each will have stiffness and strength or capacity. We can design for anything from zero capacity (swiveling and sliding) to allowing the beams to reach operating strength in any or all of these degrees of freedom. Being as aircraft use min FOS of 1.5 to 2.0 in many parts, getting the boundary conditions right is important to having safe structures come out of our calcs. Permutations are many...

The joint I have discussed as a preferred drag spar connection is pinned. One pin oriented with its long axis along the direction of travel and perpendicular to the shear webs being connected. Significant rotation about the pin or bolt is allowed and expected, as well as small but significant rotations in the other two axes. This has very little torque capacity to resist rotation in the axis of the pin. If the pin is changed to a substantial bolt that is torqued significantly, the joint is now clamped - significant stiffness and strength can be designed into two rotation axes and all three translation axes, leaving the joint with rather low capacity before it will slip in rotation about the pin's long axis. This could could be considered clamped because we tightened the nut and safety wired it. Is that what we want folks to think we are talking about?

Therefore I used 'clamped rear spar' to describe a continuous rear spar at the root, or one that has such a connection to the fuselage that bending moments can be transmitted. 'Simply supported rear spar' would be the equivalent description of a pin-jointed rear spar.

A clamped joint and a continuous beam are not quite the same thing. A continuous beam is used and we already discussed that - loads are distributed per the relative stiffnesses of the components in each axis of load and movement... This extends to a beam system with one or more joints, although the joints generally soften the system locally. In airplanes, we generally have fairly soft structures that move visibly under flight and landing loads.

The joint could fully restrain all six degrees of freedom to load levels outside of the operating range. I am used to calling this "cantilevered in all three rotation axes". We have to know something about the degree of constraint - stiffness and strength become really important in cantilevered structures. I can visualize two widely spaced bolts in bushings that are quite stiff to lift and drag loads on the outer panel, but would be rather soft to pitching moments. These types of joints allow rotation - curvature of the the two beams being coupled may be quite different. I can also imagine a lot of other joint schemes.

Boundary conditions in each axis should be thought through and the total stiffnesses, including the joints, used in figuring distribution of loads and then deflections - bound or crimped flight controls are not good things. FEA is great for much of this, but there are classic closed form methods too. Roark's is a great book, and usually allows us several ways to upper and lower bound our estimates, so at least we can put a box around the loads and deflections and expect the real world to be inside the box. That is a whole lot better than guessing...

A simply supported joint is a pinned joint in three axes when radial clearance is sufficient to allow system rotations and small relative translational movement. When the rotations exceed bushing clearance, the boundary condition for the axes making contact goes from free to restrained with a rising spring rate at loads further increase. Contact stresses become significant actors in the pins and bushings. Pins are already in shear, but bolts get shear loads added to their already present tensile loads.

Let's define our terms or get awful surprises.

Billski
 
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