Spoilers - RC Groups
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Jul 17, 2012, 06:39 PM
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Hi, I am a total noob. I am wondering about what materials to use to actuate a spoiler. I have the spoilers figured out per the plan. My question has to do with the strings and tubes used to move them. The plan on my 2 meter Riser said that I should use a 1/8 tube coupled with string to move them. What kind of string? What kind of tube. I think it is such common knowledge for glider guys it is not generally discussed when you read about spoiler instal.

In case you are wondering I am not totally committed to the strings instead of the separate servos in the wings. That said the plans are set up that way and I am leaning toward it as I am trying to stick to the plans and learn a few building skills on this one. If there was a very good reason to use some other method of actuation I would consider changing the plans.

Thank you
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Jul 18, 2012, 12:14 AM
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Strings are ok. If you can find some thin polyethylene or teflon tubing, that would probably be very good. Only has to be large enough to thread the string through. Maybe the smallest Sullivan tube, though that's probably more expensive than you need. The last hobby shop I went to had some suitable plastic tube that was, I think, 1/8" on the outside and 1/16" on the inside. Maybe slightly heavier than necessary, but not by much.

Are you building the Riser from the plans only? The kit should probably have something in it. Radio dial cord used to be a standard material, but good luck finding that these days. And strange things can happen to it when coiled up. Or at least the stuff I got in a kit had the core coming out at each sharp bend.

Braided fishing or kite line is good. Maybe somewhere between 10 and 50 lb test. Some of the lines are very slippery, and that's good, as long as you can actually get a knot to hold. Spider Wire, I think, is one type. And I bet Glide dental floss would work well. Very slippery.

I like the strings myself, but another good way is to put a larger arm on the servo, angled in such a way that it pushes the spoiler up.

No one seems to use strings anymore, so you'll probably get lots of advice to use servos in the wing. In my experience, one is not easier than the other. Strings have an advantage that they cannot be pushed, so you don't overload the servo if it tries to lower the spoiler too much.

I like using a torque rod to retract the spoiler. Somewhere on rc groups I have a post with a picture of this, but I probably can't find it any better than you! From several years ago. Also discussed using stop screws and little magnets for adjusting, and making the final closure more secure.
Jul 18, 2012, 01:00 AM
Registered User
Over the last 40 years, I have actuated spoilers many ways but prefer pull strings. I use spoilers in the wing ONLY when I can't find a good way to route the pull strings. https://www.rcgroups.com/forums/showthread.php?t=731042 shows how I use pull strings to activate spoilers.
Jul 18, 2012, 08:26 AM
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AMBeck's Avatar
McMaster-Carr has some excellent tubing for spoiler cables. The flexible nylon 11 works great and comes in several sizes, from 0.093" id to 0.375" id. For cables, I like nylon coated stainless steel fishing leader cable. Some hobby shops sell it in control cable kits for larger models.
Jul 18, 2012, 09:28 AM
LSF303-AMA Fellow
tkallev's Avatar
A servo in each wing is easy, you can use a tiny servo that doesn't cost or weigh much ... the old string/dial cord worked fine, but there is no need for the Rube Goldberg setup anymore.
Jul 18, 2012, 10:16 AM
Registered User
plus with servos you have a positve hold down factor so spoilers will not pop open on a stall condition. as the string version will do. Like Tom said a small servo in spoiler pocket a piec of balsa with a slit cut in to it long ways and a screw in the servo horn passing thru the slot and bingo positive up and down. And you dont need much of up spoiler 30% Max and 15% is a good band to shoot for. just make sure they travel as close together as possabel
Jul 18, 2012, 01:09 PM
Proud member of LISF and ESL
Strings are great if you have a very simple radio and want to operate both spoilers on one channel. My Sagitta 600 was set up that way.

On my Spirit I put servos in each bay and had them on individual channels on my Futaba 9C so I would trim them more easily from the Radio.

both methods worked well.

Plastic antenna tube works great for routing strings.

if you are going to run servo wires you can use drinking straws as guides as they weigh nothing. Or you can just run the wire through the ribs, but I like the straw design better. neater and the wires don't bang around inside the wing.
Jul 18, 2012, 01:33 PM
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AMBeck's Avatar
The little rubber bands you get from your kid's orthodontist do a great job of holding string actuated spoilers closed.
Jul 18, 2012, 01:59 PM
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craigrrr's Avatar

Keep it simple

I prefer spoilers that are simple, light, and do not ever need adjustment.

I use Hitec HS-55 ($10 or less;weight: 8 grams) in each wing with the long arm attached. I wrap each servo with blue masking tape and lightly epoxy to bottom of wing bay.

There is no linkage. The servo arm makes direct contact with the surface of the spoiler.

Spoiler is spring loaded to retract by itself as the servo arm is lowered.
I use a piece of a small rubber band; one end attached to bottom of wing bay, the other end attached to spoiler. I tie a knot in each end of the rubber band; the knot is held in place by a tiny hook made from a pin (and held in place with CA, two places - bottom of wing bay and bottom surface of spoiler).

By spring loading the spoiler, the spoiler is free of the servo arm in the retracted position; this results in no strain on servo (no buzzing sound) and no adjustment over time. The servo arm does not even contact the spoiler while in retracted position.
Some people use a tiny magnet to keep spoiler closed, but that adds some weight.

I hinge the spoiler to wing top (usually just behind wing spar) using clear Scotch tape. I cover the spoilers with same type of material the wing is covered with, but white color.

Jul 18, 2012, 02:39 PM
Registered User
I have found that, although the rubber band method works well, with some kinds of hinge tape it slowly pulls the spoiler up against one side of the servo bay, thus requiring adjustment. Also, after some period of time, maybe a couple of years, the rubber dries out and needs to be replaced. I prefer torque rods, which are actually faster to set up, in my experience. At least, usually they are.

The strings don't need to be adjusted either, if they are set up right at first, and you can use stop screws so that the spoiler is supported at several locations and is less likely to warp.

I suggest, when using balsa spoilers, make sure they are c grain and cover them with 3/4 ounce glass cloth, with the epoxy blotted, to make them less likely to warp.
Jul 19, 2012, 10:36 AM
"I prefer torque rods, which are actually faster to set up, in my experience."

Would you explain this or show a picture of this?
Jul 19, 2012, 02:45 PM
Just call me crash for short
Quick61's Avatar
Have a look at this pdf. It's something I put together for showing how a spring and string control surface is made. Just needs to be slightly modded for spoiler install, like one leg is bent out and the other down, then once installed on the wing, the leg going down would need to be pulled around to the the spoiler and a strip of wood glued and clamped to hold the spring leg. Anyway, take a look and you'll get the idea.


EDIT : I should of also mentioned that the pdf also explains the benefits of the "T" type of torsion spring over the Z type of setup. I'm planing to use this type of setup in an upcoming Windfree build as there is little room in the wings for a clean servo install
Last edited by Quick61; Jul 19, 2012 at 05:09 PM.
Jul 19, 2012, 04:18 PM
Registered User
I've been using strings for about 40 years ,never a problem or failure.The only add on I did was small magnets to hold them in while on the winch.
Jul 20, 2012, 06:20 PM
Registered User

Originally Posted by jdveirin
"I prefer torque rods, which are actually faster to set up, in my experience."

Would you explain this or show a picture of this?
Jul 20, 2012, 06:21 PM
Registered User
BTW, my picture shows the wire oversize. I'm thinking something in the range of .020" to 1/32" dia is adequate, maybe less if the torque rod is shorter.

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