Monday, 1 January 2001

Mounting wing servos

As moulded models are becoming more and more commonplace so are the associated questions they pose. One of the most frequently heard is "what is the best way to mount the wing servos?" The technique used by myself and many others is very simple but a little explanation of the way it evolved is probably warranted.

Since servos first shrank enough to fit in wings builders have experimented with different installation techniques. There are many and varied methods used today, some naturally being better than others.

One factor affecting the way your servos are mounted is the servo gear material used. The best plastic gears in the world when put in our less than friendly wing environment are liable to resent the abuse once in a while. On the ground this can be a desirable weak link and save the rest of the airframe from further damage.

For this reason I would advocate the use of screws and blocks in a manner appropriate to the make of servo you are using. In using plastic geared servos you are accepting that the gears are probably going to require replacing at some point, whether in sport flying or in competition the easier you can make it for yourself the better. However, as anyone who has ever seen a wing servo come adrift in flight will tell you, although ease of access is nice, security is most definitely paramount.

Metal geared servos pose different problems. If you have purchased wisely the likelihood is that there will be no requirement to deliberately remove them from their mountings, as they should outlive the airframe. Of course life isn’t as simple as that and problems can arise both mechanically and through some ham fisted stick twiddler not tucking their crows away on cue (if anyone tells you they’ve never done it don’t believe them). This means that an amount of serviceability has to be built into the system, but hopefully not as much as with plastic gears.

I am full of admiration for builders who craft clever, well-made screw mounts, however I wouldn’t recommend it for metal-geared servos. When your protruding flap gets a whack something will have to give, if it isn’t going to be your gears the chances are it’ll be that clever mounting system you laboured over for many an evening. Someone will probably have come up with a clever system with a weak link but is it going to be worthwhile timewise when compared with the simple method that follows?

Firstly, yet often overlooked, is the need in most cases to strengthen the wing skin to which the servos will be attached. This means that if a servo is knocked off it won’t take your lovely wing skin with it, secondly it will reduce any skin flexing that can be a cause of flutter. A square of medium weight glass, carbon or Kevlar cloth as large as you can get to adhere properly applied with wing skinning epoxy to the area where the servo will be glued will do the trick nicely. Whilst on the subject it is often prudent to spend a few minutes shaping vertical grained balsa ribs to bridge the upper and lower skins of a moulded wing thus spreading the mounting loads further. These can be worked in through the servo access holes prior to the servo fitting, but after the aforementioned skin reinforcement, and secured in place with some five minute epoxy or cyano. Some would regard this skin bridging as overkill and they might be right but for the sake of half an hour’s work why take the chance?

What you have now is a strong and sturdy base to which the servo can be glued. If you feel the need to protect your servo cases simply wrap a few layers of tape around it. Masking tape works well but be sure to sand the gluing surface to remove any release agent. Personally I don't use any tape.

Mix up a paste of top quality five-minute epoxy and micro balloons and apply to the appropriate surface of the servo. What we want is for the excess glue to be pushed away from the servo arm and creep out at the other three sides. This is simply achieved by holding the servo by the servo arm and applying that end to the wing skin first, as the rest of the servo is pushed down the excess glue will be pushed out away from the servo arm. A wise precaution is to have a few cotton wool buds on standby to get any unwanted glue away from the moving parts, you shouldn’t need them but if you fail to plan, you plan to fail!

What you are left with is firmly mounted servo that won’t let go unless really clouted. When it does let go all that will break is the glue joint leaving you with a pre-formed epoxy/microballoons "cup" to receive the servo again with a light smear of epoxy and no loss of trim.

Of course when mounting servos in this or any other manner you wouldn’t forget the old golden rules of double checking your linkage and servo arm geometry, would you? Another cause of premature swearing is forgetting to centre your trims, sub trims, brake settings and other gadgetry before screwing the servo arm in place and securing the servo. Most of us have done this at least once and experienced the less than blissful spectacle of plugging the servo into the RX and watching in horror as the servo arm despatches itself to centre in a position that you hadn’t catered for. I have seen someone fiddling with a Christmas cracker screwdriver who’s head had been bent at ninety degrees to adjust the servo arm in a wing mounted servo. Whilst it didn’t look the most elegant of procedures and seemed to require a lot of verbal encouragement it did eventually work and was better than ripping the servo out, worth bearing in mind in an emergency.

I know a few people who use silicon sealant to mount servos and there can be no doubt that it does work but be warned. If you need to replace gears or the servo gets knocked off nothing is going to stick it back again except more silicon sealant. Not practical for a slopeside job.