1. The problem statement, all variables and given/known data Okay, so this isn't so much a problem as a design issue while following an example given in a book. (Relevant material is here: http://www.aero.us.es/adesign/Slide... 12. Desig of Control Surfaces (Elevator).pdf ) Now, the calculations require a mass moment of inertia about the main gear to be calculated in order to be factored into equations. Sure enough this is already provided in the example they give, but in order to make sure that while applying it to the design I'm making I decided to check my formula against the info given in the PDF... The details of the airplane in the PDF are a total weight of 20 tons, Iyy 150,000 kg m^2, and this drawing of the plane: 2. Relevant equations What formula must I use for such an object. For a slightly different design (instead of using a tubular body, instead using a flying wing-style cabin) is there a general formula that can be used - such as the one for a rod rotated at its end, etc. 3. The attempt at a solution The formula I used was that for a rod being rotated at its end (ML^2)/3 ... However that didn't come out right, no matter how I tried it. As a distance I tried using 1.1, 1.7, 0.6, 2.02 (hypotenuse that links CG and the main gear wheel), and several values between 8 and 12 (since the main gear seems to be right about the center, figured the distance to the nose would be the same)... Nothing comes out right - the only value that would fit is about 4m if my memory serves me right. I then proceeded to try several formulas. The one that finally got me in that ballpark of 150 tons per square meter was the one for a rectangular plane rotating about an axis at its end ( m/12 * (4h^2 * w^2)) where I fed it 1.1 as height and 2 as width as that is the rough radius of a private jet like this. (to be noted, the design I'm personally working on does NOT use a round fuselage, it is more of a flying wing cabin design for two pilots). I thought that this was finally the proper solution, but it didn't feel right... Also attempting this on other problems from this book, it still didn't fit quite right.