K-18 washout

[harry curzon]

I have just spoken to a friend who built two of these decades ago and he remembers having a lot of trouble with the washout, and building the second one to a different method, perhaps separate spars rather than box but it was so long ago...

I think I have worked out how it can be done, except for the Z shape problem -
1. Cut out the spar section from the ribs as per the book, which is pretty much what I did. (page 141 of Cliff Charlesworth's book)
2. Glue the front and rear sections of the ribs to the spar, from the root to where the washout starts. Do not attach ribs affected by washout.
3. Raise the whole structure off the board by some amount, say using 3mm balsa under the entire length of the spar, and under the te of the ribs added so far. This keeps the whole thing level but raised. This gives clearance above the building board for the remaining nose ribs to be added with their downwash angle.
4. The trailing edge downwash jig now just needs to be amended to account for whatever amount the structure was raised by so instead of the jig taper being 0mm to 10mm, in this example it will be 3mm to 13mm.
5. The problem remains of the stretched Z shape formed by the level top and bottom of the box spar versus the extreme angle of the tip rib. My friend who had built 2 says that any attempt to go faster than cruise resulted in the wing tips bending downwards, and that it was extremely well behaved as regards stalling. My gut feeling then is that the washout is more than is required (perhaps a typing error in the original publication of the plan way back in 1979?) and there is much room for it to be reduced to half or less what is shown on the plan. That would reduce the Z shape and a careful sanding of the sheeting could take out most of what remains.

[Peter Balcombe]

For info., the Charlesworth Olympia 2b has 5mm washout starting from approx. 40% from the inboard end of the aileron (roughly 12” into a 32” aileron length) & this has a fairly benign stall characteristic - but maybe a different airfoil section from the K18.

[Ian Davis]

Hi Harry

Twisting the ready built box spar to provide the washout is never going to work as you point out. The building method described on page 141 of the great mans book is not the way I built the wings of my K18. Just as your friend recalls, separate spars and infill shear webbing make it an easy process. The ribs are not cut into two pieces but remain complete and are attached to the bottom spar over the plan and then the top spar added. The jigging you described can then be used to complete the wing.

It's tough that you've spent so much time building the box spars but my advice is to discard them.

Incidentally, I've just measured the washout on one of the wings of my K18 and it's 2 degrees. There have been so many iterations of Cliffs original drawings that errors such a the 7 degrees will always happen. Given that my original drawing shows the washout starting at the aileron and continuing to the tip a 7 degree angle would raise the tip at the rear by 4.29" (Tan7x35) making the tip more like an airbrake.

Ian

[ChrisGordon]

Hi Harry.
I have had a look at a set of ASK18 plans that I acquired recently. It comprises of the usual 2 plans.
They are issued by R. C. M & E. and numbered RC/1372. They are undated.
The stb wing is drawn and bears the note 'Build in washout over this length 3/8 " at tip.
The 'length' starts at the inbound end of the aileron and finishes at the tip. I measure this distance as 34 5/8 inches in old money.
Presumably the rest of the wing is built flat to the modelling board.
I do not have a set of instructions but do have a completed, and flown, model. I shall put it together and try to measure the actual washout but, as Ian seems to suggest, it should not be very much.
Hope the helps.
Kind regards all
Chris

[RobbieB]

If anyone would like a copy of Cliff's original article from RCM&E let me know and I'll scan and pdf it on the forum here.

[harry curzon]

If anyone would like a copy of Cliff's original article from RCM&E let me know and I'll scan and pdf it on the forum here.

Yes please!

[RobbieB]

Okee dokee, just getting steam up on the scanner....................and sending the search party out for the mag. Fear not, it is here somewhere.

[Ian Davis]

Hi Robbie

I've just been reading my copy. Let me know if you can't find it

Ian

[RobbieB]

Thanks Ian, we did find it.

That said, after a few re-homings since August 1979 it has become a little dog eared and faded so if yours is in better condition, yes, by all means.

Had to scan at quite high res hence the chunky file size.

K18_scan.pdf

(5.67 MiB) Downloaded 96 times

[Ian Davis]

Hi Robbie

That came out well. I don't think mine would be any better.

Best regards

Ian