Calculating Centre Of Gravity

Hi everyone. In one of my earlier posts, I talked about calculating flight speed and how a larger diameter of propeller offers more thrust and more pitch will increase the pitch speed because the propeller is taking a larger bite of air. One thing I want to add is that at the point where the pitch speed of the propeller is equal to the speed that the plane is moving through the air, there is no thrust.

CENTRE OF GRAVITY 

My Dad and I are trying to design an RC plane right now that will have a flight speed of about 90 km/h and a thrust to weight ratio of at least 1.2 for vertical climbs. We decided that since we know what our possibilities are for motors and batteries etc. we decided to estimate the centre of gravity to see if our design was any good in terms of balance. Here is a drawing I made of a design we made on the computer:

Of course, the vertical stabilizer is too small and the plane will be much more streamlined, but this is the best drawing that I could do. The big line going through the airfoil is where we want the centre of gravity to be. We estimated the c of g on a spreadsheet but I will show you as best as I can what we did. The arm is the distance of these different main parts from the centre of gravity and the motion comes from multiplying the weight by the arm. So, if the motion is roughly the same for behind the centre of gravity and in front of it, we will know that the plane is fairly well balanced. 

                          Front Of Plane                                    Back Of Plane 

              Weight     Arm  Motion                          Motion     Weight      Arm 

      Battery: 65 grams 7 cm  455                          1350 Motor: 54 grams 25cm

      Gear:     80 grams 20cm  1600                       315  ESC:   35 grams 9cm 

                                                                             354   Gear:  118 grams 3cm 

         Total Front Of Plane Motion:               Total For Back Of Plane:

                         2055                                                         2019

I should probably say that I don't remember exactly what the different arms where from the actual spreadsheet for this plane, but I think that most of the weights of the different parts are accurate, and these motions were very close to the ones that we got from the spreadsheet. On the actual spreadsheet, the difference between the front C of G and back C of G was only around100 which as far as I know is not very significant considering that the motions were around 2500.