USING TWO AILERON SERVOS
by
Ed Moorman


Let's talk about using two servos for your ailerons. Most trainers and small sport planes still use a single servo with torque rods or belcranks for aileron control, but more and more we are seeing the use of dual servos, one for each aileron. There are always some questions as to how you install them and hook them up.
 
 
WHY USE TWO SERVOS?
In the early days of RC, even into the 70's, RC was a very expensive hobby. It was not uncommon for servos to cost $35-$40 each. This may not be costly now, but think of the inflation and value of the dollar in the last 35-40 years. Maybe a banker or accountant can give us the exact figure, but I would guess it is at least 3 to 5 times. That means a $35 servo cost the equivalent of slightly over $100, maybe even more. Today's sport servos, which cost under $20, are a huge bargain by comparison, so there is really no cost impediment to using two servos for ailerons.

 

Using two aileron servos is also better from an aerodynamic standpoint, especially if your plane was designed for belcranks. I have always hated belcranks! It seems impossible to get all the slop out of the system since there are so many connections. I tend to fly fast, powerful planes and belcrank controlled ailerons tend to be more prone to flutter because of the slop in the controls.
 

If you use two aileron servos, you never have to worry about interference between the servos in the fuselage and your aileron servo when you attach the wing. I had this happen on a new plane once and had to go back and remount the servos.
 

Another thing you might not think about is reliability. Two servos means if one fails, the other can control the plane. I have landed more than a few times using the rudder for steering, and believe me, steering a high performance plane with rudder is not like steering a three channel trainer. Lots of grass is a desirable addition to skill and cunning. I can testify, landing with one aileron is much easier than with no ailerons.
 

MOUNTING THE SERVOS: All right, how do you mount and hook up two servos to ailerons. If your plane has a built up wing, run two 3 ply or hardwood servo rails between two ribs so the servo is about centered on the aileron. Use some thin plywood (1/32 or 1/16 regular ply or 1/8 lite ply) as a doubler to reinforce the rib where you notched for the servo rails.
 

The height of the servo is your choice. I used to hide them under a cover plate until I got lazy. Then I mounted then so only the servo arm was showing. This is pretty easy. You install the rails so only the splined shaft for the servo arm shows above the surface of the wing. Then you Monokote over it, but before you do the final shrinking, you take a sharp X-acto knife and cut a little hole for the shaft. Clean and neat. You do have to cut the Monokote if you have to replace or repair the servo. Finally, I started putting the rails flush with the wing surface so the whole top of the servo hung out in the breeze. This is the easiest and, besides, they're on the bottom where you can't see them. I haven't been able to tell any performance difference so ease and expediency works for me.
 

To mount servos in a foam wing, the easiest method is to cut the hole in the foam core-I use a hacksaw blade-then line it with balsa or thin plywood. Epoxy the plywood or hardwood servo rails to the lining material, recessing them the correct amount for the servo height you want and screw the servo in place.
 

To really hide the servo, you'll need to mount it laying on its side recessed in the wing and covered with a plywood or balsa plate. The way you think to do this is to mount the servo through a rib on a plywood doubler. This makes the screws, both the mounting screws and the servo arm screw, hard to get to. A much nicer way is to mount the servo on the plywood cover plate, then screw the plate to mounts in the wing. Let's say you are using 1/16 ply for the plate. Attach a side servo mount to the plate or make a mount from two pieces of hardwood. Epoxy them to the ply so the servo fits between them in the correct position. You may want to install small screws through the plywood into the hardwood mount to make sure the epoxy doesn't crack loose. Cut a slot in the cover plate for the push rod or the servo arm, if you're using a long one. The plywood plate mounts to rails in the wing. Very clean and convenient.
 

This method also works well in foam wings. Cut a hole in the foam core where you will be mounting the servo and line it with balsa or thin ply. Epoxy your rails in place, recessing them the thickness of the cover plate. Mount the servo to the cover plate and screw it to the rails in the wing.
 

'If you are using inexpensive servos and consider them "throw aways," here's a method I sometimes use. Find a convenient corner in your wing, say between a rib and the shear webs, and glue the servo in with silicone rubber. It sticks like mad and it also gives some vibration insulation. When I build foam wings for small (.40 size) planes, I normally sheet the leading and trailing edges, cap strip and then cut out the foam between the cap strips, making foam ribs. You do need to measure the cap strip locations so the tops and bottoms line up. Now I just squirt some silicone on the servo and shove it in place. You can cut the servo out, if necessary, to replace or work on it, but this has been rare for me.
 

SERVO LEADS: You will have to cut holes in the inboard ribs to allow passage of the servo lead. Many people install a rolled paper tube as a conduit for the lead. I never bother with this. My normal method if I have to remove a servo is to tie a string on the servo lead and drag it through the wing. When I reinstall the servo, I use the string to pull the lead back through. If you forgot and pulled the servo out without the string. You can tie a small weight-I normally use a nut- to the end of a string, stand the wing on end and shake the weight through the holes in the ribs.
 

If you are using foam wings, you should plan ahead and melt a hole in the wing before you join the two halves. Figure out where you will be mounting the servo and where you want the lead to come out. Hold a piece of 1/8 music wire with vice grips and heat it with a torch, then melt a hole through each wing half for the servo lead. Notch the side of the core where the lead will come out of the wing.
 

HOOKING UP DUAL SERVOS TO YOUR RECEIVER: To hook up dual aileron servos, you use a Y connector plugged into the aileron channel. Plug the Y connector's single end into your receiver. This gives you two aileron extensions instead of one. Plug the aileron leads into either the Y extensions-it doesn't matter which side you use, both are electrically the same.
 

All the radio manufacturers and several aftermarket manufacturers make long extensions for inside your wing and the Y's. When I use an extension inside the wing of a giant, I always use heat shrink tubing or tape on the connection of the servo lead and the extension. It might just get pulled loose up inside the wing.
 

A final option is available to those using two aileron servos and have a computer radio. With a computer radio you can turn the flaperon option on and plug one aileron servo into the aileron channel and the other into the flap channel. The flaperon set up causes the ailerons to droop like flaps. With this arrangement, you can also mix flaps and throttle for up aileron type spoilers. See a previous column on Spoilers for more information.