Replacing a Wing Servo in a Glass Slipper Wing

Tired of hassling with servo frames in ridiculously tight spaces?

I have an old, beaten up RCRCM E-Typhoon, now in its ninth year of use and abuse. She’s been repaired several times, the wings are not as stiff as they used to be, and she looks like she’s been through several battles: some won, some lost. But she still flies great; it’s like putting on your favourite running shoes to go for a run. Comfortable, familiar, fun.

Last flying session I had a bit of a hard arrival. Couldn’t call it a landing really. At best a one-pointer: right on the nose. I had to replace the spinner and prop, no further damage. While at it, I also replaced the receiver (Rx), as I am refitting my fleet to Futaba. So when all was connected, I did some table flying — without the prop — to program my transmitter (Tx). One flap servo heated up and stopped working. I’ve had similar problems with this brand and type, so I yanked it out and looked in my spares drawer for a replacement.

Here’s how I, and most members of my club, do it. I’m not reinventing the wheel here. It works for us, as long as you have a regular pushrod setup — not a linear drive system or other fancy stuff — and don’t do DS (dynamic soaring). We use it for all our F5J and F3F gliders.

To start, first prepare the servo. Trial fit, and remove any or all of the lugs to make it fit in the servo well. Center the servo arm by either connecting the servo to your Rx/Tx, or a servo tester. If it is a flap servo, centre, and then position the servo arm 15 to 20° off centre to get maximum flap throw.

Get some heat shrink tubing that fits the servo. You can get all kinds on the internet, just make sure it is not the rubbery one. Glue doesn’t adhere to it so well — I can write you another article on how I found out!

As noted above, remove any or all tabs from the servo, so it fits easy in the servo well, screw in the servo arm set screw, and tape the servo wire to the servo housing, so it’ll come out of the heat shrink tubing where you want it.

Fit the heat shrink on the servo keeping in mind where you want the servo wiring to exit, and zap it with a heat gun. Hold the servo with a pair of long needle nose pliers while you do that. Or burn your fingers, whatever you prefer.

When shrunk, let it cool for a bit, and then cut off any excess heat shrink and making sure not to slice through the servo wire! (Yeah, I got the t-shirt for that one as well).

Now to prepare the servo well in the wing. The previous servo was glued in as well, so I needed to clear any glue remnants. I do that with a flat grinding stone attachment to my Dremel. This must be done very slowly and carefully — you don’t want to Dremel a hole in the wing! Make sure all wiring and the pushrod are well out of the way, and keep them there with masking tape. To keep the pushrod out of harm’s way I put a screwdriver through the clevis. You can use any stick of wood, toothpick or similar and tape it in place as well.

Use a well lit, well ventilated area and wear a face mask. If you can, do it outside. Now grind away all glue residue, until you have a more or less clear, flat area. I find it best to put the wing on a table, weigh it down with some magazines so it won’t wander while I’m working on it, and hold the Dremel with two hands. The name of the game is go ssssssslllllllooow and steady. I use only the flat bottom of the grinding stone. Don’t get tempted to use the sharp edge. You’ll be through the wing skin in a matter of milliseconds — and yes, I got the t-shirt to prove it.

Once done, trial fit the servo, and make sure it doesn’t wobble due to an uneven servo well. Measure where the servo should be, and mark it on the masking tape you have put around the servo well. Give both the servo and the servo well a rough sanding, so the glue has something to hold on to, and clean all areas with some alcohol. I use the stuff my wife and daughters use for their nails.

Move all wiring and the pushrod out of the way. Use masking tape if needed. Have some handy weights ready to put on the servo, to have some pressure while the glue cures. If the servo is below the wing surface, you will need something to put on the servo — like another servo or a piece of balsa — and put the weight on that. You don’t need a ton of bricks. I use some fishing weights, totalling just under 100g.

So all is sanded, cleaned, and taped out of the way. I use thick CA for the next step. I’ve tried different brands and types of epoxy, I’ve tried UHU, I’ve tried Gorilla Contact Glue. They all failed after a very short while. Either on the servo side or the wing side. Only thick CA has lasted years with me.

So, a splodge of thick CA on the servo — not too much as you don’t want it to ooze out on all sides — press it into place, making sure the servo arm is in the right place and make sure there is no glue anywhere near the servo arm. Yes, I have glued in a servo — and after curing found out the servo arm couldn’t move because I had glued it in place with the servo.

Once you’re sure it is all where it should be, put some weights on the servo. Do this carefully without moving the servo. Thick CA does not cure as quick as thin or medium CA. Just leave it alone for an hour or so. There’s really no need to heap on huge weights. All you want is to put some pressure on the servo so it settles nicely on top of the CA. In the picture below you can see I placed a box of X-acto knife replacements and some fishing weights, totalling just over 100g on top. Personally, I like to leave it overnight this way. You could use kicker, but as the glue is under the servo, not all CA may be affected by the kicker, and then you’ll get uneven curing which may — or may not ?— affect the long term use.

After curing, make sure the servo will not budge if you try to move it. Now re-attach the pushrod. Because I use a different brand/type servo, I needed to adjust the pushrod length by turning the clevis. After a bit of trial-and-error I got it right. If you ground a bit of the clevis down, so it won’t bind with the servo arm’s root, make sure the clevis is the right way up.

Before going any further, plug in a servo tester or your Rx/Tx, and make sure it all works. You might want to remove any masking tape you put over the flap or aileron first.

If all’s well, the servo works, you get the throws you need, and the servo doesn’t show any flex or binding, we can do the last bit. To make sure the servo has some help to stay seated, I like to add some fillets on at least two sides of the servo. Here I like to use medium CA mixed with micro balloons. The medium CA thickens nicely with the balloons, and is relatively easy to push in place with a lollipop stick or even a toothpick. I did one side at the time, and you’ll see why. I made the CA/balloons mixture about 50/50, and with a lollipop stick pushed it right next to the servo. Then I tilted the wing about 45° to keep the CA mixture from lying flat, and make a nice fillet between the servo well floor and the servo. I held it like that for a few minutes, keeping an eye on it, to make sure it didn’t leak where it shouldn’t go. Then I hit it with some kicker. Before doing the second side, I gave it 20 minutes or so for the kicker to evaporate. Then I repeated the process on the other side.

All that is left now, is to plug the servo into the wing wiring — don’t forget to tape up the connectors! — make sure all wiring is in place and can’t run foul with the servo arm and pushrod, and then tape the servo cover in place DONE!

Why Do It This Way?

There are lots of different ways to mount servos, so what’s to recommend this way of doing it?

• No hassle with servo frames
• No problem inserting a differently shaped servo from the original one
• Large glue area will keep the servo in place better
• To remove, simply cut the heat shrink and pop out the servo
• KISS (that is, “keep it simple…silly”)

Thanks for reading, good luck with your own re-servoing project and see you on the flight line or slope!

Flying on the east shore of the Lake of Tiberias. (credit: Eyal Radomsky)

©2023 Rene Wallage

Resources

• E-Typhoon — “ outstanding performance and excellent thermal flight qualities. The flight of the Typhoon is highly agile and maneuverable, it will go where you like in a dream. It is able to fly in virtually all conditions…”

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