Scale Soaring UK

I am building the Charlesworth 1/4 K-18 from the lasercutsailplanes kit and plan. Plan says that the washout is 10mm at tip t/e. My calculation says that is about 7 degrees which seems far too much. Can anyone confirm what it should be for this model?

Harry, that seems about right, and knowing several Charlesworth models he does not get that sort of thing wrong. The MU 13 I am building goes for 6mm washout . Regards John.

Talking of wings for Cliff Charlesworth's 1/4 scale K18 has just reminded me I need to ask a question regarding the (very slow) build of my own LCS kit

Screwing up jobs once they get too complicated is my amazing superpower, so I'd rather not cut all the ribs to attach to a built up box spar. Has anyone tried a different aproach, such as using a tapered CF wrapped tube I wonder? All suggestions welcome - especially if they've actually been tried out and found to work!

Brett

Well, I have run into major problems trying to build the wing, perhaps there is some info that Cliff Charlesworth left off the plan or it assumes something that I do not know. I have read Cliff's book about scale gliders and am none the wiser. I am getting something very wrong or am missing some vital piece of knowledge. Anyway, I need help!

The problem is with the washout. Plan says it starts at rib 28, finishing with 10mm washout at the tip. It was easy enough to make a tapered jig from 0mm to 10mm to go under the trailing edges from rib 28 to tip. But:

1. Tip rib is only about 90mm long and raising its te by 10mm gives a washout angle near 7 degrees which is huge, it means the tips will be negative when the main part of the wing is still only at a modest angle. It does seem an awful lot of washout.

2. The wing has a box spar, by the tip rib it is pretty much square in section. Since the box spar is not twisted along its axis, the top and bottom edges of ribs near the root match the flat top and bottom of the box spar, but the top and bottom edges of ribs near the tip which are angled so heavily downward with washout are now at a very different angle to the top and bottom of the spar. It gives the ribs a Z shape section.

3. With the ribs being split into 2 because of the box spar, how do I add a washout angle to the leading edge part of the rib when all I know is how much the te is raised?

I had assumed that I build the wing with the spar on the board, all rib trailing edges on the board except where raised by the washout jig, and as per Cliff's book I set the root and tip front part of the ribs, pulled a tight thread between their leading edge centers and aligned all the other front rib parts to that thread. Perhaps there should be other jigs under the spar and trailing edges that aren't mentioned on the plan or are assumed to be known about?

What am I doing wrong?

thanks for any help!!

Hi Harry, I built my K18 as per plan, I only raised the trailing edge and left the front part of the ribs flat on the plan,, my model flew without fault, you dont need extra jigs, I can only tell you what I did others will have different advice.

Keith

Thanks Keith.
With the rear part of the tip rib raised so much, what did you do about the overall shape of the tip section where the spar and front rib are level but the rear rib is angled steeply upward? There is a sharp change in angle at the joint between rear rib and spar.
Did you build the wing with spar on the board and all trailing edges on the board, apart from at the washout jig?

Harry, is it possible to measure the percentage thickness of the wing section up to the start of the washout together with the percentage thickness of the tip section?

Also, while you're at it, is the taper on the spar (depth) consistent root to tip or does it change at any point along its length?

I haven't built this model but my normal practice would be to first attach the ribs to the spars, false leading edge and trailing edge/aileron spar then, when you have a complete but floppy framework, insert the washout jig at the trailing edge before sheeting in the leading edge D box which locks the washout in place. It may well be that when you insert the washout jig the spar lifts from the board slightly towards the tip.

Trevor wrote: ↑29 Jul 2019, 08:28 I haven't built this model but my normal practice would be to first attach the ribs to the spars, false leading edge and trailing edge/aileron spar then, when you have a complete but floppy framework, insert the washout jig at the trailing edge before sheeting in the leading edge D box which locks the washout in place. It may well be that when you insert the washout jig the spar lifts from the board slightly towards the tip.

A nice idea but, it would work with individual top and bottom spars however the box spar is very strong and resistant to that sort of bending or twisting. It can be bent and twisted but it is so strong that even if all the sheeting was glued in place, I think that as soon as the jigs were removed the box spar would reassert itself and pull it all flat again.
The force required to twist the box spar is so high that I think the ribs would break off from it

Here's a photo to show what I mean. Keeping the front part of the rib and the spar level, but raising the te of the tip rib results in a flattened V shape section. This is a false washout since the important front part of the section is not angled downward, and all the washout angle occurs aft of the spar which means the rear part of the rib is raised 10mm on a 47mm part-chord which is an angle of 12 degrees.
If the front part of the rib is angled downwards, as it should be, the box spar will give it a flattened Z shape tip section!
The only way for it to work is for the spar to twist along its length, which it is extremely resistant to doing!