Balancing your Sailplane: Tips and Technique
by Len Buffinton
Whether you build a small micro flyer or a 12 meter scale giant, they both have something in common, they need to be balanced before they will fly correctly. For some, this process is a painstaking endeavor, but it doesn’t have to be, remember, its a good thing, it means you are about ready to go flying.
Balancing a sailplane can be a challenge because of the long wings, long fuse, and sometimes delicate nature of the airplane itself. With a little patients and a good plan, I think we can help you develop a process to take the pain out of balancing your ships.
The first issue we need to address is the balance point. When getting a new plane from a manufacturer, the balance point will be given with the kit. But, when purchasing the plane second hand, you will hardly ever be “given” the balance point. It’s a GREAT idea to permanently mark the balance point on the fuselage root when you get it figured out. That way, if you sell the plane or make an equipment change somewhere in the future, you are not stuck searching through the mess of drawers trying to find the original instructions. You can use a permanent Sharpie, vinyl tape or even a paint dot to mark the point.
If you have no Idea where the CG is, then it gets a bit more complicated. But don’t worry, we’ll help. We have put together an article to help you with that, click here. We’ll wait.
Assuming you know the balance point, we can proceed.
When balancing a plane, be sure everything is included in the plane, the batteries are in, the switches are mounted, the PILOT figure is in place, etc. Also fully assemble the aircraft. With sailplanes, that means bolting on the stabilizer with BOTH bolts. Sailplanes are far more CG sensitive than power planes. With a power plane, two fingers somewhere around the CG is close enough if the nose drops when you pick it up. Not so with sailplanes. Proper CG will greatly enhance the flight characteristics and make the ship able to detect lift much better.
For this tutorial, I’ll discuss balancing an average size sailplane, 2-6 meters, later I’ll talk about some options for balancing large sailplanes.
The balance point on your sailplane is typically measured from the LE back toward the TE, right at the fuselage to wing connection.(wing root) Mark the spot on the fuselage with a piece of thin colored tape, cut in a triangle, pointing to the CG point. Most builders will have a balancing device in their shop, so I will not get into building one or which one is best, we’ll cover that in another article. For now we will assume you have a way to hold the plane on a CG machine of some sorts. Set the appropriate CG on the balancer.
Before going any further, we need to get the weight ready. I use lead shot for balancing my sailplanes. Its easy to use, conforms to the nose of the plane and you can adjust the weight fractions at a time. The best way to prepare the plane for balance it attach a cup to the nose of the glide..
Taping a cup to the nose
WHAT?
Yes, attach a cup to the nose of the glider. Take a paper cup, tape it carefully to the nose of the plane, use a good tape for this, you do NOT want it to let go when you’re in the middle of filling it with lead. Tape from front to back and side to side as in the picture.
Once secure, have your lead ready to go, put it on the bench next to you, and get a couple rolls of paper towels too. No, not for the mess, to act as a bumper guard for your wings and tail. Place the sailplane on the balancer, with the CG lined up properly.
Homemade balancer for large models
The plane will be tail heavy 10 out of 10 times. If you have room to move any battery backs or other equipment up front, now is the time. Actually, earlier when you were setting it up was the time, but we all know how that goes.
Start pouring in lead shot into the cup, or placing other lead pieces into the cup. Start slow, sometimes it doesn’t take much, other times, it takes 3-5 lbs. It all depends on the plane of course. Hold the ship level and gently let go of the nose, does it still drop? Add some more lead and repeat the process. When the glider finally stays level, your close. You can now make small adjustments as needed. Test it a few times. Hold the plane level, test it. Hold the nose slightly down, test it. Hold the nose slightly high, test it.
Once you are happy with the balance point, carefully take the plane off the balancer and set it aside. The cup can be taken off the nose, but be careful not to spill all the shot. Now we need to weight the shot, cup and tape. If you don’t have a small scale, now is a great time to get one. They are cheap and come in very handy for this application. Weigh everything you took off the nose and right down that measurement. Place another cup on the scale and ZERO it out. Pour the shot into the cup, add more to make up for the tape and cup you had on the nose. When you get the exact measurement you need, take out a couple ounces, more or less depending on how much you are adding. The idea is to have a little room to “final trim” the amount of weight you need. If you glue it all in now, you will not be able to remove some later if needed.
How are we going to secure the weight in the nose? Can we get it really close to the tip? These are questions you have to answer. The best way to secure the weight is to mix the shot with epoxy resin, and pour it into the nose of the sailplane. When doing this, there are some precautions that need to be taken to insure everything works out in the end.
1) You need to protect the tow release. You dont want to pour the mixture into the nose, only to have it glue the tow release shut. One very good way to protect it is to use a paper tube, larger than the entire mechanism, including the arm you attach the servo rod. Push the tube down tight to the nose, sealing off any epoxy from entering as it flows around. Just make certain you will be able to operate the release once the mixture is dry.
When you mix up the epoxy, which should be at least 30 minute epoxy, weight it once its mixed up. Then, take away that much lead from your cup. Again, we are trying to be pretty exact with out initial setup.
Pour the epoxy into the cup of lead and mix it up good. A wet mix is way better than a dry mix. You dont want any little “Shot” pieces rolling around in the plane because we didn’t get any glue on them. Holding the fuse nose down, carefully pour the mixture in. Check to be sure you did not get any mix where you didn’t want it. Place the fuselage up against something to hold it upright while you go get a towel soaked in cold water. This is for fiberglass fuselages, you need to wrap the nose with this wet towel, so the heat from the epoxy curing will not affect the finish on the outside of the plane. Epoxy is a chemical, it will produce a LOT of heat when it “kicks off”.
That’s about it. Clean up any mess you made with rubbing alcohol while it is still wet. Once dry, you’ll need a chisel and 36 grit sandpaper to clean it up, so you might want to clean it before it dries, its up to you.
When you take the plane to field for the first flight, bring along some extra weight for trimming. This weight is best if it has sticky foam attached. That way, you can add it to the plane at the field, then take it off at home and replace it with glued in lead latter. To determine the exact CG for flying, you should read this article.
