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Installing servos in confined space

Micro servos in tailplane

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Stephen Belshaw13/02/2019 07:11:17
73 forum posts
14 photos

I'm building a scale glider from a Mike Trew plan which is a V tail "ruddervator" model, originally designed to be activated by pushrods. I'm fitting micro servos in each tailplane instead, I've sourced the thinnest servos I can find at 9mm thick to fit within the fully sheeted tailplane halves which are 13mm at their deepest point.

They will go in but it leaves very little room for mounting options, if I bond them directly to the skins is this likeley to lead to flex of the 1/16' balsa sheeting? Once fitted they will be inaccesible so I want to be sure they are secure and set up properly.

Any suggestions?

Stearman6513/02/2019 08:04:51
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769 forum posts
893 photos

Mount the servos on 1 or 2mm ply, which is the usual method in this situation. The ply becomes the top skin of the tail. slot the ply for the servo arm to protrude. Add an inner hardwood frame to the balsa skin to take the screws to hold in place the new ply skin/mount.

Frank Skilbeck13/02/2019 08:42:46
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4360 forum posts
101 photos

Is this the Fauvette? (I've got the plan and it's on my build list yes)

What servos are you planning on using, if you are using the thin wing servos, e.g. Corona CS-239, it maybe possible to fit in a ply mounting plate.

Stephen Belshaw13/02/2019 09:25:41
73 forum posts
14 photos
Posted by Frank Skilbeck on 13/02/2019 08:42:46:

Is this the Fauvette? (I've got the plan and it's on my build list yes)

What servos are you planning on using, if you are using the thin wing servos, e.g. Corona CS-239, it maybe possible to fit in a ply mounting plate.

Yes Frank it is, here is a picture of the servo and tailplane:

img_1748.jpg

Stephen Belshaw13/02/2019 09:27:32
73 forum posts
14 photos
Posted by Stearman65 on 13/02/2019 08:04:51:

Mount the servos on 1 or 2mm ply, which is the usual method in this situation. The ply becomes the top skin of the tail. slot the ply for the servo arm to protrude. Add an inner hardwood frame to the balsa skin to take the screws to hold in place the new ply skin/mount.

That's a method I might choose to use on a wing but in a tailplane that's tight for space it's overly complicated and heavy. I'm looking for a simpler lighter solution.

Edited By Stephen Belshaw on 13/02/2019 09:28:38

Andy Meade13/02/2019 10:09:27
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2549 forum posts
655 photos

I designed and print my own low-profile servo mounts in "carbon fibre infused" PLA plastic. Work a treat!

20190104_125409.jpg

Robert Welford13/02/2019 10:25:40
152 forum posts
4 photos
Posted by Stephen Belshaw on 13/02/2019 09:27:32:
Posted by Stearman65 on 13/02/2019 08:04:51:

Mount the servos on 1 or 2mm ply, which is the usual method in this situation. The ply becomes the top skin of the tail. slot the ply for the servo arm to protrude. Add an inner hardwood frame to the balsa skin to take the screws to hold in place the new ply skin/mount.

That's a method I might choose to use on a wing but in a tailplane that's tight for space it's overly complicated and heavy. I'm looking for a simpler lighter solution.

Edited By Stephen Belshaw on 13/02/2019 09:28:38

If ply it too thick and/or too heavy then reinforce the inside of the wing skin with glass (0.8 oz) and epoxy.Otherwise scarf in ply in the area of the servo replacing the balsa skin in that area.

Your options are finite ...

Dickw13/02/2019 13:13:11
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423 forum posts
71 photos
Posted by Stephen Belshaw on 13/02/2019 07:11:17:

..........

They will go in but it leaves very little room for mounting options, if I bond them directly to the skins is this likeley to lead to flex of the 1/16' balsa sheeting? Once fitted they will be inaccesible so I want to be sure they are secure and set up properly.

Any suggestions?

The balsa might flex if the servo is only fixed to one skin, but should be OK if it is glued to both skins.

When installing servos in a thin wing or tail it is fairly common to just wrap the servo in masking tape or heat-shrink and then glue (e.g. with a dab of epoxy) between the tape and both wing skins. I even cut the servo mounting lugs off in tight installations.

Robert Welford’s suggestion of thin glass and epoxy reinforcing could be included in the glueing bit. You can also add a few scraps of balsa either side of the servo if you want to take up any excessive side thrusts/knocks. A bit of thin polythene or similar is useful to stop the glue getting in the wrong places on the servo!

If you don’t have access to the servo you don’t need to make it removeable by screwing it to a mount, but if you ever need to remove a glued in servo, cutting the tape/ heat-shrink makes it easy given the major surgery to get to it in the first place.

Dick

Simon Chaddock13/02/2019 15:08:15
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5371 forum posts
2818 photos

With my admittedly lightweight planes (although some are EDFs) is to cut off the servo lugs and simply glue them in. If necessary a thin wall can be built around them carefully sanded to match the wing section.

The aileron servo of my 61" (1550 mm) Bombardier Q400 twin glued to the spar shear web and the bottom skin..

Aileron servo

In this case a carefully shaped packing piece was glued to the upper servo surface so it would be "sandwiched" between the skins when the top one was glued on.

I work on the principle that if the servo fails it will be cut out and a new one added with the appropriate inserts to the skin.

Works for me!..

Edited By Simon Chaddock on 13/02/2019 15:09:33

Stephen Belshaw13/02/2019 18:00:44
73 forum posts
14 photos

Thanks for all the suggestions, here's what I did - probably a combination of all your ideas:

tp servo mounted.jpg

I mounted the servo on 1/16" ply, strengthened around the area with 1/16" balsa and a couple of spruce bits to prevent it twisting, I hot glued the servo in place after wrapping in insulation tape, put a piece of 1/32" ply on top of the servo which made a tight sandwich when I skinned it.

Trevor13/02/2019 20:44:44
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349 forum posts
51 photos

My preferred method is to mount the servo through a rib as illustrated here:

**LINK**

Using hex head servo screws (supplied by ModelFixings) makes it easier.

Trevor

Simon Chaddock13/02/2019 20:47:26
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5371 forum posts
2818 photos

Looks good.

Just make sure your servo arm is exactly the right place on the splines. wink 2

Stephen Belshaw13/02/2019 21:28:30
73 forum posts
14 photos

Thanks Simon, yes something I was concerned with getting right before sealing it up.

I programmed my Taranis with V tail set-up and made sure all was centred and in good working order prior to adding the second skin. As a get out for this though the spline fixing screw is accesible through a tiny hole in the root rib and I can prise off the servo arm and re-position if neeeds must, but it would of course be easier to make any adjustments through the Tx.

Trevor, that's a very neat way of doing it and eminently suiyed to aileron/flap/brakes in a wing but, within the confined space of the tailplane I'm working on,  just not enouh room.

Edited By Stephen Belshaw on 13/02/2019 21:33:23

Ronaldo13/02/2019 23:23:44
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226 forum posts
17 photos

In my hotliner gliders where there is a firm surface of glass fibre or ply, but only just room to take a servo without a mount, I wrap the servo case in masking tape and epoxy it in. It can then be removed if needed by carefully cutting the masking tape and easing the servo out without damaging the case.

John Duncker14/02/2019 15:28:06
77 forum posts
7 photos
Posted by Ronaldo on 13/02/2019 23:23:44:

In my hotliner gliders where there is a firm surface of glass fibre or ply, but only just room to take a servo without a mount, I wrap the servo case in masking tape and epoxy it in. It can then be removed if needed by carefully cutting the masking tape and easing the servo out without damaging the case.

+ 1 on the tape though I used a hot glue gun.

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