Q-141: I currently have a CL model that is quite nose heavy. Obviously I need added weight in the tail of my model to move back the CG location. My particular model has a semiprofile fuselage which does not leave me much room to hang any lead weights on the tail (rear) of the aircraft. What would you do in a case like this?
A-141: I don't want to appear to be a know-it-all, but I generally try to avoid out-of-balance aircraft while I’m still in the building process. Often on original designs, I have made a profile mockup of my fuselage and then placed simulated size/weight tail surfaces on the rear end and either the actual engine/motor or representative weight at the nose. Then I shift the location of the nose and tail weights until I can obtain a reasonably good balance point. By going to this extra work, I don’t end up with an impossibly out-of-balance situation when the model is completed.
However, there will be times when you might want to add or remove weight from your model aircraft. I tried this question on our Aeromodeling Editor, Bob Hunt, who is a World CL Stunt Champion. Bob told me of an idea he uses on his championship CL models. He actually builds a tube into the center of the vertical fin on his aircraft. The tube can vary from 3/8 to 1/2 inch in diameter, depending on the thickness of the actual fin. Then Bob can add small lead fishing sinkers or lead shot into this vertical tube to add weight. The reverse is also possible by removing the weights. Each time the weight is changed he places a piece of foam rubber into the tube to prevent the ballast from moving in flight. Then he plugs the top of the tube with a small cap or bottle stopper.
I had never heard of this idea before, but it does sound practical and it will
allow you to achieve precise CG changes in your model in the least amount of
time. It will also allow you to tailor your CG for exact weather conditions. If
the idea sounds good, try building a hollow vertical tube into the fin of your
next model.