Sailplane radios

Model Airplane News, Aug 1998 by Edberg, Don

This time, I'm going to talk about modern sailplanes and describe those features that can only be found on sailplane computer radiosand why you might need them. In case you're wondering, my last column was in the June '98 issue and covered helicopter functions.

WHY DO SAILPLANES NEED SPECIAL RADIOS?

If you haven't seen a sailplane lately, you might be wondering why they need special radios. After all, they fly slowly and only need two or three channels. That's hardly a challenge for even a regular radio, right? Well ... no; it isn't so anymore! Modem sailplanes, molded of high-tech composites such as carbon fiber and Kevlar, routinely fly at over 150mph (one holds the absolute R/C speed record; they're faster than powered models!) and have flown more than 100 miles cross-country. They aren't as simple as they used to be. To understand where glider radios are today, you need to hear a bit about glider evolution.

At first, sailplanes used only rudder and elevator (even today, many beginners' sailplanes still use only rudder and elevator). To make spot landings easier, modelers started adding spoilers to kill the lift on the wings. Some folks were concerned that we were cutting slots for spoilers and putting bumps in the most critical portion of the wing, where the wing needed to be strong and where the airfoil shape was critical. To compensate for the lost lift caused by spoilers, modelers added flaps. Whether you use spoilers with or without flaps, you usually have to compensate for trim changes by holding up-elevator as well.

Soon, with gliders flying speed and distance events in international competitions, maneuverability became important, and ailerons were added. Some bright pilots figured out that if the flaps could be used together with the ailerons, the "turning power" of the model could be improved; if used in opposition, landings could be made steeper and the spoilers could be eliminated, making the wing "cleaner" with less drag. To keep track of all these interactions (and others that you'll read about below), the need for a computer radio for competition sailplanes became very apparent. A good sailplane radio will contain a set of sailplane-specific programming functions to handle everything shown in Figure 1.

That being said, I want to emphasize the following: sailplane beginners certainly don't need a computer radio. In fact, you can get by with just a 2- or 3-channel system, which is going to cost less than half as much as a computer radio (just don't buy one of the really cheap 2-channel, 2-stick units!). If you think you're going to be seriously involved in sailplanes, however, you should strongly consider getting a computer radio. You'll grow into it as you become more proficient, and you'll be glad to have the extra capabilities when you need them. Later, I'll give some specific information on which manufacturers supply sailplane radios.

SAILPLANE FUNCTION DESCRIPTION

Let's take a look at the typical highperformance sailplane as shown in Figure 2. Seven servos are used: right and left ailerons, right and left flaps, elevator, rudder and spoilers. If the model happens to have a vee-tail, all the functions are the same except for the response of the two tail controls, which both move for elevator and rudder commands. Scale ships may have spoilers, which operate for glide-path control in conjunction with the flaps and ailerons. (Calling the wing controls "flaps" and "ailerons" is really misleading, since all four wing controls work with one another both as flaps and as ailerons. Even so, we'll still describe the outer controls as ailerons and the inner ones as flaps.) Aileron->flap mixing is used to get the wing's inner controls ("flaps") to work with the outer ones ("ailerons"). This makes the sailplane roll faster and reduces drag as well. You can select the amount of mixing, from 0 to 100 percent. About 50 percent or less is recommended.

Aileron differential is another function intended to improve the sailplane's efficiency. You set up the function so the up-moving aileron moves up more than the down-moving aileron moves down, often in a ratio of 2:1 or higher (down motion is 50 percent-or less-of up motion). If ailerons are coupled into flaps, the flaps also obey this relationship; this reduces drag and helps to make coordinated turns, where the fuselage is aligned with the relative wind.

Aileron->rudder mixing is used along with differential to make the sailplane fly coordinated turns, for highest efficiency. Quite often, the amount of mixing can be set to vary depending on whether you are launch ing, cruising, or landing. It doesn't take much, perhaps 15 to 25 percent mixing value.