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Author Topic: Control horns and clevis or alternatives  (Read 1250 times)
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VinnyTheB
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« on: September 09, 2014, 10:09:42 PM »

I am converting a DHC-2 Beaver (Guillows 305C).  The small scale is throwing me off when it comes to the horns / clevis.  I've done some searching and most builds seem to ignore this phase of design/solutions.  Would anyone have tips and tricks or perhaps know of a thread that I may search that might speak of this in more detail?

Questions I have:

Basswood horns - how to guesstimate the distance of attachment point from hinge line?

Should the horn be keyed in a gusset - or just a butt joint?  I've seen what appears to be a simple butt joint on a build on here.

Carbon fiber rods?  Is .03" too small?  Is a 2-56 tube from sullivan to sloppy of a fit to use as a sleeve through the formers?

What are clevis solutions for small scale?  I can get Dubro's micro stuff - but that bumps up the rod to .047" - seems excessive.

Hopefully my rambling gives a sense of the info I am looking for.

Thanks in advance.

Vince
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Konrad
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« Reply #1 on: September 09, 2014, 10:44:36 PM »

I make mine out of 1/32 ply. Yes bury the hone into the surface. Also think about adding gussets. The more support and added glue area the better, well within reason. The horns take a lot of abuse both while landing and in the hanger!

0.03" to 0.040" carbon is fine. I use heat shrink to fasten the Z bend 0.020" wire to the carbon rod. This  allows for some adjustment. Once the trim is set I usually glue the heat shrink, wire and carbon together with a drop of CA.

I actually like Pull Pull controls for everything other than ailerons.

The control horns on the surface usually wind up being longer than you think (less surface deflection). You want to use as much of the servo movement as possible as these small servos often don't have very good resolution.

I think I've addressed this issue in my build threads
Try look at this one
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=11737.0

All the best,
Konrad
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lincoln
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« Reply #2 on: September 10, 2014, 12:10:14 AM »

If you're going to run the .030" through a tube, it's much bigger than you need. Properly installed, that size can handle a DLG rudder at maybe 80 or more mph! That's a relatively large rudder for a 60 inch model, too. And DLG's tend to have very rigid control systems, out of necessity. Teflon tube is best to run it though.. You can get gluable teflon tube here and there, but it's important to glue it before too much time goes by. Mark Drela's Supergee II uses a .018" steel pushrod in a tube for the rudder. I would guess that you could get away with .010" steel, maybe a used guitar string. I think some of them are that small. Maybe you could get some CA teflon tubing and use that, though you'll have to capture it instead of just gluing it in place. Will save weight compared to other options. Thin pushrods like this, however, should be supported until just short of the point where they would jam something at full deflection.

If you have 90 degrees of servo throw, and want 45 degrees of control surface throw, use a control surface horn something like twice as long as the distance between servo output axis and the hole in the servo horn. Using a lot of throw will make up, to some extent, for tiny servos that may not have the resolution of larger ones.

If you set it up right, 1/64" ply horns ought to be enough. (DLG's use 1/32" sometimes, and it works fine.)

For clevises, I have used a right angle bend in the wire with a separate spring wire glued to the end of the carbon. If you set it up right, you can dispense with the wire and let the stiffness of the pushrod keep the wire in the hole.
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Konrad
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« Reply #3 on: September 10, 2014, 01:11:04 AM »

In my series 300 conversion I use 1mm (0.040") carbon unsupported to actuate the elevator. Thinner would be preferable but I don't have heat shrink that grabs well on smaller rods and wires. I find that unsupported 0.5mm (0.020") carbon too flexible.

I have used the push rod as a retainer with good results but like the classic Z bend better at both ends of the carbon rod. I like Pull Pull the best. But one need to look out for the Ackerman effect (keep the horn centered on the hinge line).

I don't like 1/64 horns as they are not very durable (hanger rash). They are more than adequate for the flight loads on the series 300 models. One needs to look at more than flight loads when making control set ups. Bearing area and adjustability are two other areas of concern.

How about some photos of your built we might have some other suggestion to help.

All the best,
Konrad
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lincoln
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« Reply #4 on: September 10, 2014, 02:52:17 AM »

On a model called a "Moosquito", designed by Ron Fikes, I used pull pull with sewing thread, which was completely adequate and pretty easy to do as well. The original used unsupported carbon pushrods. Also went with bamboo skewers for landing gear, which works quite well with a model around 100 grams. (I think the original was 125 grams.) Unfortunately, the model was quite slow and rather boring to fly, except when flying around obstacles, such as the swingset, barbecue (active), small tree etc. Or at night. 2 grams to add LED's shining on the wings. A stick and tissue model could probably use one LED inside.
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Konrad
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« Reply #5 on: September 10, 2014, 04:38:40 PM »

… Unfortunately, the model was quite slow and rather boring to fly, except when flying around obstacles, such as the swingset, barbecue (active), small tree etc. …
So how did she thermal? Roll Eyes

I saw Ron at the last Ranch San Antonio (RAS) swap meet last week. I need to ask him what he has been working on. I haven't been to RSA for about 6 years nor have I seen Ron in that time period.

All the best,
Konrad
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lincoln
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« Reply #6 on: September 11, 2014, 10:56:35 AM »

I suspect that spending more than a second or two in that particular thermal would have melted the covering on the wings.
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marcin_pl
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« Reply #7 on: September 21, 2014, 07:16:18 AM »

Vince,

Maybe a little late, but I will present my idea of making control horns and pushrods. I make pushrods of round balsa sticks. 3mm diameter is enough for a model with span of around 30 inches or so. Small wire sections are installed at both ends of the pushrod. Their inner tips are bent 90deg, glued into the balsa stick and wrapped with thin thread.
I use tiny metal snap (it's called "barrel") at a servo arm (see picture). It allows me for some trimming. I make control horns of 0,8mm plywood and glue them into the moving part of the stab. Wire section of a pushrod is secured by small parts of wire insulation. After trimming flights I secure it additionally by tiny drops of CA glue.

Marcin
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Konrad
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« Reply #8 on: September 21, 2014, 11:07:38 AM »

Marcin,
Your work looks exquisite!

All the best,
Konrad
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OZPAF
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« Reply #9 on: September 30, 2014, 05:16:15 AM »

Here is another late reply re pushrods. I used 1mm carbon rods on a 135gm - 4.8oz.
The elevator rod ran in a 1.5mm or so paper tube and ailerons were direct driven with short rods.
All horns (2 – 2.5 x inner servo horn hole) were angled sections of 1mm rod with a heat shrink connection between pushrod and horn. The heat shrink provides more than enough flex for the small deflections required and there is no slop at all.
The heat shrink wore well – the model was flown for 3yrs plus and was eventually lost due to an act of stupidity, that had nothing to do with the rods or horns.
I used small wire connections at the servo with keepers from small sections of wire insulation(hot stuffed carefully). Neutrals were set up by sliding the rods into the heat shrink and then when happy - locking the position with CA.
John


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VinnyTheB
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« Reply #10 on: November 21, 2015, 09:49:59 PM »

Well - I might be building this one bones only to get back in the swing of things.  Here is my stab.  Problem - how to join the 2 feathers?  Tips?  Alternative build / design?

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Konrad
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« Reply #11 on: November 21, 2015, 11:29:05 PM »

Maybe "U" shaped carry through, with a channel cutting the aft center section of the stab from the rest of the stab. Glue this aft part back on to the vertical fin or fuselage.

Or "Y" push rod to move each half. This allows for some trimming options.

All the best,
Konrad
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VinnyTheB
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« Reply #12 on: November 22, 2015, 12:04:03 AM »

Thanks for the confirmation Konrad.

I'm going to go the U shaped brace for simplicity.
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Konrad
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« Reply #13 on: November 22, 2015, 11:22:38 AM »

That would be my first choice.
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Bill G
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« Reply #14 on: February 21, 2016, 02:29:17 AM »

Old thread, but figured I'd add this to it.  As of lately, I've been making control horns for small models with clear sheet plastic.  Use different thicknesses depending on model size.  E-Z type clips can also be fabricated from the sheet plastic, where a drop of glue locks the clip portion to the pushrod wire.  On the smallest models like this centrally ducted fan 30mm Comet, the pushrods are made from .015" wire.  The holes are made with pins, carefully enlarging just enough for the wire.
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