K-18 washout

[RobbieB]

Harry,

Cliff has sent me a full copy of Cliff C's K18 wing plan for me to have a look at.

As per the comment above, that is absolutely ridiculous.

The whole idea of a pre-built box spar is that they are very rigid in torsion - just like full size. Don't try and twist any washout into it (never been a fan of twisted-in washout anyway - sometimes has a habit of twisting out over time).

Don't do anything for the moment and let me have a good look at the drawings and have a think.

[harry curzon]

That just does not look right!

Exactly! What am I doing wrong? The only way to preserve the section is as Trevor suggests, but I don't believe that the box spar will allow the 7 degree twist that is required

[harry curzon]

Harry,

Don't do anything for the moment and let me have a good look at the drawings and have a think.

Thanks Robbie, I am leaving well alone until this is sorted!

[RobbieB]

How far on are you with the wing construction?

[harry curzon]

How far on are you with the wing construction?

Both box spars built.
On right wing only, ribs attached to box spar but can be broken off and re attached. No sheeting etc added until this issue is solved

[BrettW]

I've also got a 1/4 scale Charlesworth K18 from Cliff slowly (oh, so slowly!) taking shape. Fuz only at this stage whilst I contemplated the wing construction on the back burner. I've no doubt about the strength and general utility of the wing/spar arrangement, but I do have serious doubts about my own ability to cut all the different ribs at the right places and to the right angle to give the desired washout. My personal super-power is screwing up jobs that become too complicated!

Has anyone tried out alternative methods with this model, such as using a tapered woven circular CF spar (aka fishing rod blank)? And would rotating the angle of the outer family of ribs provide an acceptable way of building in washout? The question in my mind (as a newcomer to largish-scale model building) is whether the resulting dip in the LE would be deleterious or not. However, this is probably not too different from the situation where a square/rectangular is used to provide the datum.

Thoughts and alternatives all gladly received - otherwise this beautiful model may never be completed!

Brett

[Mike F]

Hi, I think that Cliff intended that the spar, training edge and leading edge would be packed as necessary to achieve the washout described on the plan. In his book (Scale Model Gliders) he refers to this on page 75 for a D box section, the 'over the edge' method and on page 141 in the construction of the Oly wing.

Harry, I can lend you the book if you do not have one.

Rgds
Mike

[chris williams]

My advice would be to get Cliff to draw you a more standard wing in Profili; make use of the widely-used HQ35/12 section. Then you would have a more efficient wing that is easier to build and doesn't need any washout at all...

[Ian Davis]

I find it difficult to understand most of the learned word so far on this subject. I have and regularly fly my K18. The model build was overseen by Cliff himself. As I remember, I didn't use anything like 7 degrees more like 2 . After 30 years the washout is still there and the model will not tip stall even when provoked and will always stall straight ahead.

The method of building in the washout was easy. First,Build the spars and ribs as per plan(floppy state as someone described it). Next , arrange a straight edge of the same length as the aileron at the required angle to give the required washout on the vertical edge of board. Align the wing so that the aileron TE part sits on the vertical straight edge. Now complete all spar webbing ,top D box sheeting and top TE. making sure that the wing inboard of the aileron is weighted down during the process. Next. Finish lower TE and Glue lower D box in place using masking tape to hold it . Reposition as before on the washout jig while the glue dries.

Ian

[RobbieB]

From Cliff C's original drawings the sections he has chosen for both root and tip ribs show no reduction in thickness at the tip. Not a really big deal but not ideal.

CW's suggestion to replace with a modern Quabeck section seems an eminently sensible thing to do and given its ability to cope with the tips even better (might even try one at the tip without washout myself one day).

Now we have the problem with the already built box spar. Even if there is no reduction in rib % thickness at the tip, as a consequence of the compound taper of the wing, the taper on the spar will change out towards the tip from the start of the wing taper. If the section thins as well then this will be significant i.e. the bottom of the spar will not be in contact with the building board for the whole of its length - some packing up will be necessary.

What I would take issue with is this jacking up of the trailing edge at some value or other if you are not sure what that translates to in terms of angle. In the case of our models, two to three degrees is usually more than enough if you want to do it that way.

I have the full size drawings of the AS K8 ribs and they bear a remarkable similarity to those that Cliff has used on the K18 and the tip rib on that is washed out by three degrees (also thinned to 12% from 16% at the root).

Also, on Cliff's K8 model drawings, he shows the washout there as 3/16" at the tip which roughly translates as 4.5mm - give or take.

Building wings like this tends to deal with most of these issues but it's not for everyone: