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My tips for building gliders

by Steve Smith

April 24, 2000

I flew model airplanes a lot as a kid, and I still fly hand-launch gliders for fun (16 inch span).

I think that for everyone in the class to have a satisfying experience, there is one most important thing, and that is the process of getting the glider to fly nicely. This is far less dependent on the details of the design, and much more dependent on the trim adjustments that are made during flight testing. I can make even a poorly built glider of a bad design glide fairly well.

1st thing that many beginners have trouble with is a nice smooth, level throw. For the first flights, you don't want to be throwing it hard so that it loops or zooms, the goal is to get a steady, established glide. So it takes some practice, (and a bit of discipline) to develop a smooth, level launch and a gentle release.

2nd thing is the trim. There are actually two things going on during a steady glide - stability, and trim. They are interrelated, but it is helpful to think of them separately. Both depend on the location of the center of gravity, and the angle of the horizontal tail. The center of gravity is adjusted by adding or removing modeling clay on the nose. The angle of the tail is adjusted by bending or warping the tail.

So, for the students to go through this process of adjusting the center of gravity and the trim angle of the horizontal tail, can be frustrating and discouraging. Once a person gets the hang of it, it can be done very quickly. I would think that a class discussion about what the goal is, and what steps to get there, would go a long way toward getting them to be successful. Here's maybe a discussion outline:

1) goal of flight tests - to adjust center of gravity and tail angle to get a smooth, shallow glide.


2) effect of adding clay to the nose for trim, ( and maybe a word about stability, depending on the class)


3) effect of bending the tail for trim. Tips on how to bend it. Maybe have everyone practice on some scraps, so they get some feel for it before they do it to their gliders and break them. You could have everyone try to get the most bend, to see how much they can get without splitting the wood. (by this I mean permanent bend, not just flexing the wood and holding it)


4) How to do the launch to get smooth level flight.


5) trial and error process of testing, making adjustments. Maybe just change one thing at a time. I get the center of gravity where I think it should be, then focus on trim with the tail.. If I'm having trouble, then I move the c.g. and go back to tail adjustments.

Then, if there is time and interest, you might want to go on to advanced launching techniques, where you throw the glider up high, with a very hard throw, and special adjustments make it transition to a smooth glide at the top of the throw, giving the longest endurance. By the way, on a totally calm day, with no thermal activity at all, I get gliding endurance of about 45 seconds. With just the smallest amount of thermal bubbles, I get flights well over a minute, and if I launch in the afternoon on a school yard, I will pretty often get 2-5 minute flights, and occasionally loose one in a thermal.

Below are some tips for you, and see how best to guide the kids once you have the idea. (I'm assuming you are not a champion hand-launch glider builder that would already know this. I apologize if you already have all this)

It takes a little technique to bend the balsa wood so it will stay bent, but gently enough so it doesn't break. I do this by bending the trailing edge of the tail in the desired direction (usually up) and then using my thumb to compress the wood fibers on the concave side, and spread the fibers a little on the convex side. If you don't do this, the wood won't stay bent. But the amount of gentle compression and stretching you can do determines how much bend remains when you release it. a bit of moisture or warm breath helps too - because it allows the wood fibers to stretch easier on the convex side.

OK, anyway, for any particular location of the center of gravity, it is theoretically possible to get the tail angle set so that a smooth, trimmed glide with a shallow glide slope will result. If the trailing edge is up too much, you will get a scalloping flight path with successive pitch-ups, stalls, dives, pullouts. If the trailing edge is down too much, you will get a nose-dive. As you get close to the proper trim, you will get steady glides. A slightly nose-up glide will be on the verge of wing stall and will glide very slowly, looking kind of mushy. You might see some lateral wing rocking at the same time. A slightly nose-down glide will just be rather fast and not as shallow a glide slope as you would like. Perfect trim will give a nice smooth, slow glide, but with no signs of mushing. (pretty qualitative and subjective, I admit, but after a bit of experimenting, the kids will see what I mean).

Now, what happens if the center of gravity is too far back is that the slight changes in tail angle that you get from warping/bending will have a BIG effect, and you will probably alternate from the extreme scalloping flight to violent pitch-down flight, or just get scalloping flight no matter what. If the center of gravity is too far forward, then it will be very difficult to get enough tail angle to prevent a steep dive. When the center of gravity is about right, then minor adjustments to the tail angle will make it possible to get the nice smooth, level glide that you want.

The center of gravity location is the primary variable for stability. It may be possible to get the trim right on an unstable glider, but a little turbulence would upset it. Most likely, its just too hard to get trimmed. When the glider is stable, then it can be trimmed easily, and once trimmed, will tend to stay that way. Its way beyond a middle school exercise to try to calculate the correct c.g. point for stability. A good rule of thumb is that for most hand launch glider designs, the center of gravity should be between 35% and 50% of the wing's chord. That is, slightly ahead of the middle of the wing. If you have any drawings of plans for gliders, they usually put the desired center of gravity on the plans. If you have a glider with a rather small tail, then the center of gravity may have to be further forward, say up to the 25% chord point. If you have one with an unusually large tail, then the center of gravity may need to be slightly behind the middle of the wing.

The advanced adjustments and launch techniques are a little bit involved.

Good Luck Just giving kids a chance to cut out some balsa wood, sand a approximate airfoil shape on the wings, glue everything together - is a great exposure. Something I took for granted growing up, but then realized most kids never get to build much of anything, don't learn to use tools, etc.

 
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