what control horn

[Andrew Cousins]

Hello all. i am after a little advice. I am starting my first 1/4 scale build a L19 bird dog. its going well but i have hit a little snag and i wounder if you all could help.

OK the aileron is the issue. Now on smaller models i have always made sure that the control horn position is such that when the clevis is attached to the horn this joint sits directly over the hinge point. The point the wing trailing edge and aileron leading edge pivot. hope i described that correctly.

Ok on this 1/4 scale the aileron leading edge is 3/8 deep balsa then i have a 9.5mm thick by 15mm deep ply plate to attach the control horn to. This is to the plan. my problem is the largest horn i have, only half of it sits on the ply if i try to get positioning of the pivot point as i described above.

question is this position really that critical and any control horn recommendations.

Many thanks in advance

[Peter Miller]

If the connection is not in line with the hinge you will get some differential movement of the control, in this case more up than down which is normally quite acceptable.

This can be compenstated for by putting the connection to the servo output arm off centre if you need to.

[Mike T]

You need a horn that is angled forward of its mounting plate, but I'm not aware of any commercial types being available.

As I usually top hinge ailerons, I frequently have to mount a horn well behind the pivot point axis, so I make my own. Cut a suitable horn from 2-3mm paxolin or fibreglass board and slot the ply plate to glue this into. I usually size the 'glued-in' bit so it ends up flush with the top surface of the aileron, to give maximum gluing area.

The lever part of the horn is angled forward so that a clevis hole drilled in its end lines up with the pivot axis. (If you use copper-clad board, like me, you can solder a small washer on to pack-out the area that the clevis pin goes through.)

PS - what Peter says is right, of course, but my solution addresses your point about keeping the clevis pivot point and the hinge axis in line...

 

Edited By Mike T on 16/06/2020 14:00:15

[Nigel R]

As Pete says, given what aircraft it is, I'd have the horn a little aft of the hinge line and see what differential you end up with. If it is too much then rotate the servo horn rearward.

[Andrew Cousins]

Thanks guys very helpful so far. I have just read on the plan ( a different sheet to the wing build) the plan states that the aileron throws are 1 1/2 inch up and 1 inch down. So as i under stand it offsetting the pivot point back (towards the T/E of the aileron) might actually help my situation?

[John Lee]

Nexus have a good variety of larger horns, including raked forward variants. Link

Edited By John Lee on 16/06/2020 14:28:34

[Mike T]

Posted by Andrew Cousins on 16/06/2020 14:21:17:

Thanks guys very helpful so far. I have just read on the plan ( a different sheet to the wing build) the plan states that the aileron throws are 1 1/2 inch up and 1 inch down. So as i under stand it offsetting the pivot point back (towards the T/E of the aileron) might actually help my situation?

Nope! With your servo arm zeroed (equal arc either way) you will get more down than up which is a BAD THING. If you do as others suggest and offset the servo arm (towards the wing L/E), you will compensate for this BUT to get the actual differential you want you will have to move the arm further forward again, which is going to get messy, ugly and mechanically inefficient. Get the geometry right in the first place and you'll be fine.

I'm glad there are swept forward horns available, but ask yourself - do you want placky tacky horns on your nice 1/4-scaler?

[Former Member]

[This posting has been removed]

[Nigel R]

"So as i under stand it offsetting the pivot point back (towards the T/E of the aileron) might actually help my situation?"

The pivot, as in where the aileron hinge point is, would be fixed, no?

I would assume from this you mean the connection between aileron horn and control link?

If so, then yes, you will find a little more up than down, in this scenario.

[Engine Doctor]

Why not make your own control horns ? That way you can ensure that the pivot points in line with the hinge line. 3 mm Paxolin is ideal and when epoxied a slot cut into aileron is immensely strong, plus no ugly screws showing. Nexus used t sell Paxolin sheet.

Edited By Engine Doctor on 17/06/2020 14:16:06

[Andy Gates]

I concur with the making of your own horn.

I use glass fibre board which you can get from your local model shop or buy on line.

If you make the horn tilt forwards so that the point that the hole which the push rod connects to is directly below or above the hinge line then you should not get differential movement. If the connection point is behind the hinge line - then you will get more down than up movement which is not what you would want as Mike T said.

As it is a build up aileron (from your description), I would not use the ply horn mount or if you have already fitted it I would cut a slot in it to take your home made plate horn, then add 2 ribs into your aileron for your horn to fit between to increase the gluing surface area.