K-18 washout

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harry curzon
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K-18 washout

Post by harry curzon » 14 Jun 2019, 07:11

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?

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Cliff Evans
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Re: K-18 washout

Post by Cliff Evans » 14 Jun 2019, 08:09

Harry. That is what it states on the plan, I have seen many of these built to this spec, all fly very well. That washout starts at rib 28 (Inboard aileron position to tip) Which actually calculates to 0.75 degrees. Not 7 degrees!

John Vella
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Re: K-18 washout

Post by John Vella » 14 Jun 2019, 08:54

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.

BrettW
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Re: K-18 washout

Post by BrettW » 14 Jun 2019, 12:14

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

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Cliff Evans
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Re: K-18 washout

Post by Cliff Evans » 14 Jun 2019, 12:18

Most modellers have their own prefered methods! Not one way can be deemed as being better than the other. All I am sure will have been tried at some point by someone over the years! Whatever floats your sailplane!

harry curzon
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Re: K-18 washout

Post by harry curzon » 28 Jul 2019, 20:28

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!!

Keith
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Re: K-18 washout

Post by Keith » 28 Jul 2019, 20:42

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

harry curzon
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Re: K-18 washout

Post by harry curzon » 28 Jul 2019, 21:33

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?

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RobbieB
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Re: K-18 washout

Post by RobbieB » 28 Jul 2019, 23:21

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?

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Trevor
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Re: K-18 washout

Post by Trevor » 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.

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