I have a set of Hitec digital wing servos I planned to use in my Aegea Mantis. However even if I cut the mounting tabs of the Hitecs and glue them in they are still 3/8” deeper (measuring top to bottom) and 1/8” wider than a Hitec 81 or 85. This would entail cutting what seems like a lot of carbon from the bottom of the wing and getting close to the spar. Will this be OK or should I bite the bullet and get a set of Hitec 81 sized (probably JR 321s) servos. I would actually like to use the hitecs without cutting the mounting tabs off as I don’t like gluing sevos in but this them makes them even wider.
I have read about reinforcing the servo holes so you don’t need to rewrite about this.
The cost of a new set of wing servos is a huge consideration for me and might not be possible this year.
I am not familiar with the Hitec servos specifically so I will have to assume that the servos are adequate in terms of torque and power for the application. My answer will address only the issue of enlarging the servo hole cutouts in the wing.
Enlarging the servo holes by the dimensions that you mention should not be a problem. You absolutely should trim off the mounting tabs from the servos. A distinction should be made between the flap servos and the aileron servos.
Aileron servos/servo holes; You do not need to reinforce these servo holes. The aileron servos are mounted in an area of the wing that is overbuilt in terms of strength. The wing tip spar tube runs past the aileron servo hole and provides plenty of strength for that part of the wing. Just router out the foam to a depth to match your servo thickness. Widen the hole in either direction as needed to just match the height and width of the servo. The simplest installation would be to just insert the servo in the hole, wedge it in tight with pieces of thin balsa or plywood and then secure with a few drops of 5 minute epoxy and a piece of tape over the servo (packing tape works well). I make "shim sticks" for the purpose of wedging the servos. They are about 1" wide by maybe a foot long. I just slide whatever thickness is needed between the servo and the servo bay to tightly wedge the servo in place then score the shim stick with a knife and snap it off. More elegant methods can be used but this is fast and simple. The servo can work loose after some time so a few drops of 5 minute epoxy can be used to help lock the servo in place and this epoxy pops off easily with a knife if you want to remove the servo later.
Flap servos/servo holes; The flap servo holes must be reinforced as described in other posts in this forum. It is a guarantee that the wing will crease over the flap servo hole if you don't use some method to stop the wing skin from buckling over the servo cutout. Making the hole larger in the bottom skin by the amount Bernie mentioned is not a problem. The top skin is carrying a very large compression load during a winch launch and wants to relieve this stress by buckling. It will very happily do this over an unreinforced flap servo hole. The bottom skin is in tension during a launch. Carbon handles tension loads very well and the servo hole itself does not present any problems for the bottom skin since buckling is not an issue for the bottom skin. Cutting a hole in the bottom skin does, of course, weaken the bottom skin but the bottom skin is no where near the point of actually breaking in tension on even the hardest launch. The first failure mode of almost any sailplane wing will be bucking of the top skin. You would have to remove a lot of carbon from the bottom skin before the carbon would actually fail in tension.
It is not a problem if the servo holes get close to the spar tube as long as you don't actually cut into the tube.
The servo holes as they are when you get the wing are sized for JR DS368 servos. When I built my wing I had to widen and lengthen the flap servo holes by 1/8" each way to make room for the 1/16" plywood box that I used.
Wing Servos
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Posted - 04/01/2004 : 9:08:27 PM
