What It Takes to Make a Good Indoor Glider

Originally by Kurt Krempetz for AMA Glider

The indoor glider events are simple yet hard to master. When one thinks about the events, it quickly boils down to three requirements. These are:

1) To get the glider as high as you can or to the very top of the ceiling
2) To get the glider to transition from the high speed launch to nominal
glider speed
3) To get the best sink rate possible

Now, to excel at these requirements three major issues become important.
1) Weight
2) Strength
3) Aerodynamics

Weight and aerodynamics (airfoil) are the two important factors in determining the sink rate. The lighter the glider is, the better the sink rate. It is very simple. Also, an airfoil with a larger lift coefficient translates into a better sink rate. Weight is the most important of these two factors. One must remember this and build the glider as light as possible. However, strength is the most important factor for getting the glider high. One must never forget that the easiest way to turn good times is to get higher. At most contests, the person who gets the highest typically gets the best times, so one must build the glider strong enough to get to the top of the building. The easiest way to get strength is to build the glider heavier. The higher density balsa woods typically are stiffer and stronger. Also, if one makes the wing thicker, strength is gained. Glider height is also influenced by aerodynamics and weight. One needs a low drag airfoil because drag is the second most important factor in obtaining good height. It also turns out that the heavier the glider is, the higher it will go. However, the weight of the glider does not strongly influence the height. Also, don't forget weight is very bad for sink rate. One can easily see that the factors which influence one important requirement will also influence one of the other important requirements in the opposite direction. That is what makes gliders so challenging.

Now, let's not forget about transition. Transition is influenced mainly by aerodynamics. In this article, aerodynamics doesn't mean airfoils.  Topics like dihedral in the wing, nose and tail lengths or moments, rudder and elevator sizes and positions is what I am referring to when I discuss aerodynamics in this article. A good design has usually worked out all of these issues. The designer has typically arrived at a design by building it, flying it and then building another glider with some small improvements. There are a few things that are important to note. First, how strong you are really doesn't give you a great big advantage, especially in catapult gliders. Yes, it is easy to put more rubber on your launcher and pull back harder, and the glider might go up a little higher, but you will be surprised how much more force it takes to get just a little higher. Even in hand launch gliders, I am sure I could get my glider higher than the best major league pitcher, if his glider was a flexible, high drag and light plane. Second, don't worry about getting the lightest wood available on this planet. I have 3.5 lbs. /ft³ of balsa, which is useless for gliders because it has no strength. It is really a ratio of strength vs. weight that you are interested in, so if you can't find the lightest wood, don't worry about it. Try your best to get light wood, but heavy wood that is used appropriately has and can work. Yes, maybe the wing thickness needs to be thinner, but remember thinner wings equal less drag.

Unfortunately, not every plane is successful!

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