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Peter Russell 302/08/2015 17:30:48
8 forum posts

2 bits that confuse me.

The C of G is shown as in the middle of the wing - how do you do this? the nearest I can achieve is to set up a pivot point on the under side of the wing.

Are we just looking for balance prop to tail or are we looking for balance port to Starboard as well.?

The Elevator - Im using a snake- which is probably adding to my problem. Has any one got a sketch or picture of the Horn fitted to the elevator and the push rod coming out of the fuselage.

Solve these and Im ready to get someone to teach me to fly.

Geoff Sleath02/08/2015 18:07:51
3486 forum posts
319 photos

The CoG (which is probably nearer the front of the wing - ie the leding edge) is the fore and aft balance point. It's checked as you imply by setting up a pivot point and seeing if the model balances there. At its crudest it can be your fingers if the model is light enough. It's very important to get right and is adjusted, if necessary, by adding weight at either the nose or tail. In an electric model the eaiest, and best way is to move the power pack because you don't add dead weight.

Lateral balnce is much less important and in any case is unlikely to be wildly out. I do check it but a small imbalance isn't very critical unless you're trying to improve aerobatic precision.

I don't use snakes all that often but they're usually supplied with short lengths of 2mm threaded rod which you screw into the snake and attach a clevis to the other end which clips onto the control surface horn. Snake outers must be rigidly held so the whole thing doesn't flex and the full movement of the servo is transmitted to the control surface.


kc03/08/2015 13:36:11
6079 forum posts
169 photos

For a high wing model using rubber bands then a lolly stick or similar piece of wood is used. Drill a hole in the stick,thread a piece of string thro the hole and tuck it under the rubber bands at Cg position. Suspend the model from the string and see if it balances. see if you can rearrange battery etc rather than add weight.

Note 4 or 6 proper wing rubber bands are needed for flight, they are normally used diagonally.

The M2 threaded parts are from SLEC like those here

They are difficult to insert but the trick is to use a metal clevis with  the arms bent out and another M2 bolt used to jam the insert from the other end.  Wind the insert into snake a safe distance ( many threads) then release the bolt and remove the clevis.  Insert stays in snake, while clevis is used only for this purpose.  Fit a new clevis.   metal type are dearer but easier to adjust.  Metal type needs a M2 nut as locknut.

Edited By kc on 03/08/2015 13:50:49

kc03/08/2015 14:11:20
6079 forum posts
169 photos

Thi is a quick photo of a different model showing clevis, M2 brass insert and snake. Also shown lying on the tailplane is the inserting device - a scrap clevis opened out with a 'bolt' for jamming ( bolt is actually a bit of thread with soldered on grip )


Notice adequate clearance for movement and that snake outer is epoxied into fuselage. If the part with the insert actually goes into the snake outer it tends to jam a little.  However too much unsupported inner is equally bad.   Missing is the M2 locknut!  Note also that there needs to be suffucient thread into both the clevis and snake.  My model has a little too much bare thread - but it is only the rudder!

Edited By kc on 03/08/2015 14:15:36

Edited By kc on 03/08/2015 14:19:28

Peter Russell 304/08/2015 08:10:31
8 forum posts

kc thanks for that.

Its basically what i am doing - however the elevator is angled away from the fuselage to clear the throw of the rudder which makes the angle and distance the snake comes out of the fuselage a bit awkward.

It seems to work ok as I have it so fingers crossed.

Next bit is elastic bands - how tight should they be? the dowels are 11 inches apart and 12 inches diagonal with the wing 11/4 inches thick - ive got 8 inch but think its a bit tight - dont want to crush the wing!


Mowerman04/08/2015 10:23:30
1542 forum posts
105 photos

8 inch bands are fine, thats what I use.

Mowerman04/08/2015 10:24:07
1542 forum posts
105 photos

8 inch bands are fine, thats what I use.

kc04/08/2015 10:55:12
6079 forum posts
169 photos

Rubber bands need to be tight enough so they don't really sag much if you pick the model up by one band. = pretty tight= right size bands.   A piece of very thin ply - 1/64 inch - can be used to reinforce where the bands touch wing.  Just a 1 inch square with the edges sanded to paper thin  (before gluing! ) will not show under the covering.

An elevator snake which extends out from the fuselage can be supported by a triangle infill of 1/4 balsa ( grooved to fit snake ) Cover the infill before gluing or paint to match. remove covering from glue area. Avoid weight build up at tail as far as possible.

When planning snake installation ( probably too late now! ) it helps to get a straight run by having elev servo on ( say ) right if exit is on left ( or vice versa ) Inserting a straight length of 10SWG piano wire into the empty snake outer helps get a dead straight run. Snakes are crossed but there is always a slight difference in height possible by adjusting rudder horn height.  Glue does not always grip snake outers but a couple of turns of masking tape will grip snake and can then be glued to fuselage.

Much easier if snakes are installed before fuselage is boxed in at top or bottom. Drawing the line of snakes onto plan helps place horns & servos exactly in the right position, but remember to allow for servo arm height etc and allow 1 hole up and one down on the horn for possible adjustment of throw later.. A well drawn plan will already have a dotted line for elev and also rudder snakes or pushrod line to horn - it's a matter of looking carefully or drawing your own.

Edited By kc on 04/08/2015 10:58:01

Edited By kc on 04/08/2015 11:02:01

Martin Harris07/08/2015 12:26:36
8951 forum posts
221 photos

Looking at your posts here and another thread suggests that you may not have fellow flyers to call on for advice? Are you in a club, or do you have one nearby? It might pay you to pop in for a chat - most modellers are enthusiasts and I'm sure they would be happy to share their experiences with you even if you aren't ready to join their club yet. Such things as correct wing band tension and linkage geometry are much easier to demonstrate than explain...

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