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How Do I Balance My Airplane?
Lots of you have asked about balancing your airplane. So let's start by answering a few frequent questions, and then we'll show you how it's done.
FAQ's
Q. Why is it so important?
A. An airplane that is not properly balanced will fly poorly, or may not fly at all.
Q. Why?
A. If an airplane is nose heavy, it will be sluggish in pitch maneuvers, tend to dive in turns, and make for some pretty fast landings. If it is tail heavy, it will be extremely sensitive to pitch controls, and could snap at a moments notice. The old saying is: "A nose-heavy plane won't fly well, and a tail-heavy plane won't fly long."
Q. Why won't a tail heavy plane fly long?
A. Think of an arrow (It is said that the man who made the first arrow was the worlds first aeronautical engineer). An arrow has a weight on one end of the shaft, and feathers on the other. If you held an arrow horizontally and dropped it from a tall building, it would automatically point nose down because there is a weight at one end with little wind resistance, while the other end is light with a large wind resistance. If you added a large weight to the tail end of the arrow, it would not drop nose down. An airplane is similar. Too much weight in the tail, and the tail wants to lead the way. Needless to say, if you're flying along and the tail of your plane suddenly spins around to the front, it makes for a very interesting flight.
Q. Ok, so where should the plane balance?
A. Check your plans and/or instructions. They will either tell you where it should balance, or show you the location of the proper CG (Center of Gravity).
Datum Mar
k Showing The CG Range
Now What!
Q. My instructions say the plane should balance 3 1/2 inches back from the leading edge of the wing. Now what do I do?
A. First, locate the center of the datum mark from the plans, or measure the distance the instructions told you; then, put a mark on the plane in some manner. You can draw it with a felt tip pen, or mark it with a piece of tape, or whatever you can dream up. You will find that the CG (or balancing point) will usually be on or near the main wing spar. Now by lifting the model at this point, you'll be able to check whether it balances properly.
To do this, you have a choice: you can buy or make a jig to rest the plane on, or you can get someone to help you hold it while you check the balance. I don't have a jig, but there is usually someone around the house whose finger I can borrow for a few minutes. Then, starting with the plane level and the fuel tank empty, place a finger under each wingtip at the CG mark, and lift the plane off the workbench. It should balance either level, or very slightly nose down. If it does not, reposition your battery to try to compensate. If you have moved everything that you can and it still doesn't balance, you'll have to add weights to the nose or tail, but, don't fret, many planes have a chunk or two of lead hidden somewhere (even Full-Scale planes do it).
Q. If the fuel tank is empty, won't it make the plane nose heavy when it's full?
A. Yes, it will, slightly. But remember, nose heavy is better than tail heavy. You don't want to use up your fuel, and then find that you have to land a tail-heavy airplane.
Q. I heard that you should balance a low wing plane upside down. Is this true?
A. Yes. Since a low wing plane is top heavy, the top will sometimes try to flip over to the bottom. The amount of dihedral is a factor in this, but the bottom line is, you will usually get a more accurate measurement if the plane is at rest and not trying to flip itself over. (However lifting at the wing tips will sometimes cancel out this effect if there is enough dihedral).
Q. Ok, I moved the battery as far back as I can, but it still needs tail weight. What can I use for weight?
A. You can use anything that's heavy. There are stick-on weights available, or you can use fishing weights, solder, washers, nuts, bolts, etc.. The main thing is to make sure they are secure, especially in the case of nose weights. You can use screws, epoxy, CA, or whatever you like to secure them, but make sure that they are SECURE! The vibrations from the engine can loosen them if they are not held down properly! (You should also avoid doing something like putting the loose weights in a case and securing the case to the plane, as the case could open and spill out all of the weight.) You can attach them to the firewall, put them in the tank compartment, or cut a small hole in the rear fuse bottom and glue them in there. They can be attached to the outside, or inside, but, you want to put them as far away from the CG as possible to avoid having to use too much extra weight.
Q. My plane is a little sluggish and lands fast, but I have it balanced PERFECTLY. What can I do?
A. You can try moving the CG back a little, i.e., removing nose weight or adding tail weight.
Q. But it balances EXACTLY where the manufacturer suggests.
A. All I can say is: all airplanes are different. Depending on how your plane was built, and how you want it to fly, the CG will change. The manufacturer only gives you a reference point for starting out. The only way to know for sure where your plane should balance is to fly it - which is a good reason to have an experienced pilot do the maiden flight if you are not one yourself. Add small amounts of weight, enough to move the CG in 1/8" increments, and fly it again. Keep repeating this until the plane behaves as YOU think it should.
Lateral Balancing
Q. What about Lateral Balance?
A. Lateral balance (side to side) is not as critical as forward to back, (in fact, I flew in the 60's and 70's without ever balancing a plane laterally) but it really is a good idea to check and adjust (and it's a MUST if you're flying aerobatics).
Q. So how do you check it?
A. There are as many ways as there are pilots. I have heard of suspending the plane from strings attached to the prop and to the tail wheel (OK if you have a tail wheel), but here is what I do: First, the nose must be held. You can ask someone to hold the prop for you, or if you're alone, you can set a block, or a paint can, or something similar under the spinner (or under the engine's thrust plate). Then stand at the rear of the plane and put one finger under the fuse all the way at the back and lift it a few inches off the table. Notice which way it tips. Repeat this several times. If it falls to the same side each time, add weights to the lighter wing tip. To determine the amount of weight needed, BB's or other small weights can be placed in a sandwich bag and set on the wing tip (Since I am also a fisherman, I often use those little fishing weights known as "Split Shots"). Keep repeating this until either: (a.) balance is achieved, or (b.) the model tips 50-50 to either side.
Next, put a small hole in the bottom of the wing tip (or outboard rib bay if you have solid tips), coat the weights with epoxy (Any kind of epoxy will do, i.e., 5 min. - 15 min. - 30 min., etc.), and drop them into the wing. Then stand the wing up weight side down until the epoxy cures. Finally, cover the hole with a patch of covering or plastic tape (Of course, this is assuming that you wing is covered with an iron-on or other plastic covering).
Q. That's all there is to it?
A. Well, that's all you need to know to get you flying. Of course there are some finer details that can be too complicated to get into here, but you now have enough info to keep your plane from falling out of the sky due to balance problems.
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