Flush Mounting Aileron Servos

 Younger modelers may not know that once upon a time servos were big, heavy and expensive.  So many older model plans show ailerons driven by one central servo, with elaborate mechanisms invoving "bellcranks" (whatever those were).  These had to be built very carefully to avoid excessive friction, slop, and springiness which could result in "aileron flutter" (which is really, really bad).

Sometimes these linkages bordered on bizarre.  Here's what the plans show for Jim R's PZL P-11c.  The aileron servo was supposed to be mounted below the center section, with pushrods crossing over and making a sharp bend at just the right place.  Then they had to be led through the gull wing to the bellcranks.  What could possibly go wrong?

Fortunately today servos are much smaller, lighter, and more powerful than the "good old days".  (And cheaper, allowing for inflation.)  So most models use one servo per aileron.

What's the best way to set this up?  Everyone has their favorite, but here is a simple method that has worked well on several models for Jim.

First, box in around two ribs, flush with the lower surface of the wing.  The old bellcrank location is usually a good place.  Material here is leftover 1/8" lite-ply from laser or die cut part sheets.  If you are very weight conscious balsa is fine, just reinforce with ply where the mounting screws come through.  (The longer ply strip and doublers are for the wing strut mounting.)

Cut a 1/32" plywood plate, large enough to cover the boxed in area.  This is thin enough not to require inletting into the wing, but thick enough to support the servo.  Locate and drill holes to mount the plate with #2 screws.  Find the line of action of the aileron pushrod, which should be square to the hinge line (unless you use ball links).  Cut a slot the length of the servo arm and wide enough for the clevis link.

Some older guys remember when you could buy plastic servo trays, including one for mounting servos on their sides.  The inside of the plywood plate becomes your tray.  Servo is mounted using scrap bits of lite-ply for rails, reinforced with ply or balsa wedges.

It helps to measure the servo and locate the slot so the mounting doesn't interfere with the framework.  Jim will try to do better next time, meanwhile we won't tell anyone.

This method has proven to be strong enough for mini servos in models weighing up to about 8lb.  For his quarter scale Cub, Jim used 1/16" plywood, six mounting screws, and thicker stock for servo rails.  Also assembled the tray with epoxy instead of CA.

Do you have a building or finishing tip to share?  Write it up and send it in!