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Wing rib holes

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Allan Knights13/03/2020 13:53:06
1 forum posts

Gents, how to I calculate where to place the holes in wing ribs to except a wing tube as the dihedral changes.

Long story short. The model I wish to build has a built up centre section and foam outer panel wings. I wish to convert the wing to a whole built up one with detachable wings. (ie, three part wing)

My problem is, I don't know how to calculate where the wing rib holes need to be in order to take a wing tube as the dihedral changes.

Any advice welcomed

kc13/03/2020 16:54:27
6587 forum posts
173 photos

I suggest drawing a full size sectional drawing of the dihedral section ot the wing would enable measuring the hole location in each rib.

Don Fry13/03/2020 17:35:11
4557 forum posts
54 photos

If you are happy working with sines, cosines, can be done. But I would still do it as KC suggests.

How big is the model, smaller jobs can use plywood load carries, the the box the carrier goes into is parallel to the wing panel.

Edited By Don Fry on 13/03/2020 17:38:14

kc13/03/2020 18:11:02
6587 forum posts
173 photos

Welcome to the forum Allan.

Wing joiners are often used on gliders ( not my thing) and I seem to remember a drawing showing piano wire joiners going parallel into wing but the tubes going into the centre section at the angle and being soldered together at an angle in the centre section. ( ie brass tubes slightly crossed to achieve dihedral and holes in ribs being offset by the thickness of the tubes ) Probably simpler?

If you state the model then someone may have done this before and give benefit of their experience.

Denis Watkins13/03/2020 18:16:13
4544 forum posts
123 photos

Without Mathematics, you can resort to the goop box method.

The Wing tips are set with fixed parallel/ straight tube outer and inner

But on the centre section, you build a lightweight box as high and as low as the outer wing ribs

Running with the main spar direction

Spaced apart to the wing tube dimension

The Wing tube can move tightly diagonally up and down in this box

This is where you smear the sides with goop, ie, microballoons and 30 minute epoxy

Then offer up the wing tip wing tube and daddy it around to the required dihedral

This tube then sets at the correct dihedral angle for each removable wing tip.


Edited By Denis Watkins on 13/03/2020 18:36:48

kc13/03/2020 18:44:33
6587 forum posts
173 photos

The method I had in mind is shown in Peter Holland's now out of print book Model Aeroplane Building Sketch by Sketch ( still available secondhand from Amazon- sometimes as low as a couple of pounds but about 9 pounds now) He shows the flat steel blade and flat tubes instead of piano wire but it's the same method really.


Bob Cotsford13/03/2020 18:59:57
8646 forum posts
483 photos

When I did my King Altair split wing I worked it out by measuring the rib spacing and the dihedral angle then using these to calculate how much the hole drops between successive ribs:

drop in inches = spacing (inches) * dihedral (degrees) * 0.017.

Alternately, if for example there are 10 rib bays (ie 11 ribs including root and tip) over a half span for 2.5" dihedral a side then the tube drops by 0.25" for each bay - 2.5/10!

I cheat by making ply washers that are a close fit on the tube and cutting the rib holes a shade oversize. Then I slide the tube in adding a washer for each rib, set the dihedral and finally glue the washers to the ribs.

Don Fry13/03/2020 19:01:25
4557 forum posts
54 photos

Depending on the angle of the dihedral, I built one with a carbon tube. The holes in the ribs were oversize. But the tube had balsa washers. On assembly, the washers were glued to the ribs.

Mike T14/03/2020 14:34:04
531 forum posts
45 photos
Posted by Bob Cotsford on 13/03/2020 18:59:57:

....I cheat by making ply washers that are a close fit on the tube and cutting the rib holes a shade oversize. Then I slide the tube in adding a washer for each rib, set the dihedral and finally glue the washers to the ribs.

Yup! I do the same as you (and Don). Also use this method for accurately positioning wing locating dowels in bulkheads

Edited By Mike T on 14/03/2020 14:35:10

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