Found the criteria... OK, I looked over the document. The page you extracted the math we were discussing cites two methods for determining design loads for the wing mounts, with one being the equation we were discussing and the other considering the angular inertia of the glider. Angular inertia and angular momentum are two very different things...

Angular inertia and mass moment of inertia are the same thing - it is mass resisting rotation. I would run an estimate of forces at wing mounts and other spots as they are induced by wing tip touch down and drag for several reasons. The biggest is that your airframe masses are quite a bit lower than those assumed in the Criteria. The next biggest is that the distribution of inertia in your airframe may be quite different from those assumed in the criteria.

I will take this as far as I can remember off the top of my head, and get the rest later. You will need an estimate of the rearward force and the mass moment of inertia of your airplane so you can compute an estimate of the rotational acceleration from the wing tip being grabbed by the ground. From that we can compute the loads that generates at any place in the airframe...

I would do a piecewise estimate of masses every foot of the span and half foot of the fuselage, then you can calculate MMOI of the whole plane about its CG. If you already have the mass distribution in other units that are pretty small compared to the span, go for it. I would also get some sort of idea what the biggest drag load on your wingtip would be.

Mass mount of inertia - On a plan view of your airplane, establish the x and y position of every increment of your span and fuselage, and establish what the weight of that chunk of the airplane weighs. We need everything, wing structures, fuselage structures, landing gear, instruments, pilot, engine. A seated human's CG is about their belly button.

Here is a hint: even experienced degreed ME's can get tangled up in their undershorts trying to do inertia in Brit units. I use SI units and avoid a lot of trouble.

- Set up a column in Excel for X and Y dimensions in inches and weights in pounds. Put in the positions and weight of everything. Do yourself a favor - start at the wingtip you are putting the braking load on, and work your way down the table by first going in the wing that is hitting, then the fusealge, then the other wing;
- Set up columns for X and Y in meters and mass in kg. Divide weights in pounds by 2.2 to get kg. Divide distances in inches by 39.37 to get distance in meters;
- Next two columns are first moments - X*m and Y*m. At the bottom sum the mass, X*m, and Y*m columns;
- Divide Sum X*m and Sum Y*m by Sum m and those are your CG. I put those on the summation line under the X's and Y's;
- You need a new set of X and Y columns for how far the X or Y is from the CG.
- Now on a last column, you put in m*(X^2 + Y^2) for each line, using the m in kg and X and Y in meters from the CG. That is your mass moment of inertia.

Next step is to compute moment trying to rotate your bird about its CG. That force you think is conservative for rearward force at the wing tip (might be 150 pounds, might be less) times the lateral distance to the CG is the moment. But we gotta put it in SI units. Force in pounds time 4.45 gives force in Newtons. Distance in inches divided by 39.37 is distance in meters. Multiply your Newtons times your meters and you have a moment in N*m.

F = m*a and T = MMOI*alpha. Solving for alpha = T/MMOI. Units are radians/s^2.

Next we want to look at the wing root, so we need distance from the tip load to the wing root (call that L). alpha*L is the acceleration at the wing root.

This is where I have research to do to make sure we will compute the right values. Get started and I will get back to this in a day or two...

Billski