- Oct 18, 2003
- Saline Michigan
Hmm. Let's just lay out a 2D expansion of this case:Now I see I made an "error in logic" in the above referenced description.
The above statement: "The connection of the lift strut at the fuselage will also create a compression force upward between the strut attach to the fuse and the wing root fitting" is not correct. This upward acting force on the fuselage bracket where the lift strut attaches does NOT create a compression load between this bracket and the wing spar root to cabane bracket above it. Instead, this upward force on the bracket is reacted by GRAVITY. Duhhh.... This is (in part) where the fuselage is lifted from. This must have been "cringe worthy" to the readers who recognized this error.
If the airplane is flying at some steady speed, g, etc, the fuselage weight times the load factor is carried to the wings. The fuselage pushes down on whatever fittings it has to connect to the wings, and the wings lift up. In the vertical direction, the sum of these forces is zero;
Then the airplane is flying straight ahead and not rotating in the axes, net loads in the lateral direction is zero. This means the loads are symmetric, left and right, and that the detail loads at each wing mount and strut mount are also symmetric. That does NOT define lateral loads;
What do we know? If we sum moments about the wing root mount from lift, we get a moment about that spot. Since we prevent the wing from rotating about the root fittings with a strut, we can compute loads where the strut hangs on the wing. Take the moment we just calculated above and divide by the straight line distance between root fitting and strut connection to the wing, and we have the force perpendicular to the line between root and strut connection. If you have dihedral you may have to divide by the cosine of the dihedral angle to get the true vertical force there;
Since the strut is at some known angle from vertical and the strut is generally flexible enough that it is modelled as pin jointed at both ends, we can assume that the strut carries all load along its long axis. The load vertical and the load horizontal are in proportion to the angle of the strut. Tangent of the strut angle from vertical gives us the proportion;
At the strut mount on the wing, we have a given downward force (that keeps the wing from rotating upward) and a horizontal force that is usually bigger than the vertical pulling the wing towards the root. At the other end of the strut we have forces of same magnitude but opposite direction;
The horizontal force from having the angled strut is resisted at the root fitting, and the spar between strut and root is thus in compression;
Last up is that unless the wing lift and weight balance on the strut mount of the wing, there will be net rotating moments about the strut mount on the wing. This moment can be calculated by summing moments from lift and from distributed weight of the wing times load factor about the strut mount. Understand that much but not all will cancel. Then divide by the distance from strut mount to root fitting, and that is vertical reaction at the root fitting.
You can check your work by checking that the sum of vertical forces at the root fittings is equal to lift created by the wing. You can also check that the moment about the root fitting from lift and from horizontal forces at root fitting and fuselage strut mount cancel.
Once you have the 2D loads straight and working and know the strut tension for a case, you can displace the strut fitting forward or aft, view it all from above, compute the angle from lateral, divide the strut load by the cosine of that angle and get the linear force in the strut. By viewing it all from above, you can also use trig to get the forces in the fore-aft direction at both ends of the strut and into the wing and fuselage.
I shall look at the rest of the post during my next break from making parts. I have a piece of acrylic that has been preheated to 240F for 22 hours to drive off moisture. Now I get to warm that piece of acrylic to 360F and see if it will vacuum stretch form over a buck. Wish me luck.