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K-18 washout

General discussion on any topic which doesn't have a natural home on any of the other boards.
harry curzon
Posts: 115
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: K-18 washout

Post by harry curzon »

RobbieB wrote: 01 Aug 2019, 10:19 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
Many, many thanks for that Robbie. Unfortunately it doesn't solve the washout issue, in his article he adds all the ribs and then jacks up the washout, which we all agree cannot be done by twisting a box spar! The article doesn't show the part of the wing plan where the washout is stated so we cannot see what the very original plan said.
harry curzon
Posts: 115
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: K-18 washout

Post by harry curzon »

ian davis wrote: 31 Jul 2019, 09:32 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
There are a few of you that have misunderstood the 7 degrees - it is not the angle of the trailing edge jig (which goes from 0mm to 10mm over a long distance) but the change in incidence at the tip rib which is caused by raising its te by 10mm, which is minus 7 degrees compared to the un-washed-out section of the wing.
Actually it is far worse than 7 degrees, since I measured that assuming the section pivoted around the front of the leading edge. It pivots around the spar location making the washout angle considerably steeper than 7 degrees.

I have made the spar cut outs in the port wing ribs based on washout of about 3 degrees, altering the trailing edge jig at the tip from 10mm to 4 mm. If all goes well I will go back to the starboard wing, break it apart and re-jig it.

However it shows that there are still omissions from the plan and the article. The spar location is not at the point of max thickness of the section, but aft of it where the top and bottom surfaces of the ribs are sloping markedly to the trailing edge. With the spar cut-out made in a rib, the 2 cut edges are of a seriously different length. If one matches the depth of the spar, the other will be totally wrong, e.g. if the aft section matches the spar depth, the front section is much deeper and sits way up proud of the spar. The D box sheeting will reach the back of the ribs and find itself 2 or 3 mm above the spar and trying to glue to thin air!!
Now this could be dealt with by facing the top and bottom of the spar with soft balsa and sanding it to an angled profile, but none of this is mentioned on the plan or article.
harry curzon
Posts: 115
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: K-18 washout

Post by harry curzon »

chris williams wrote: 29 Jul 2019, 12:43 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
Chris, I am having a think about doing that. How much difference to its performance do you think an HQ section would make compared to the original?
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RobbieB
Posts: 547
Joined: 07 Mar 2015, 22:22
Location: North West

Re: K-18 washout

Post by RobbieB »

harry curzon wrote: 01 Aug 2019, 17:04
'Many, many thanks for that Robbie. Unfortunately it doesn't solve the washout issue....................................'
I was aware of that Harry, I just thought it might be of interest.

The problem you are experiencing with the washout/spar end is exaggerated as a consequence of the spar end being a bit on the thick side - a common design fault in some spars. Theoretically, and only theoretically, at the tip rib the spar thickness should be zero as there is zero bending moment there. Of course that is not practical for a whole raft of reasons but you frequently see excessive spar thicknesses out at the tip.

You might still want to consider CW's suggestion to refit with a modern HQ section generated in Profili or Compufoil (the offer still stands) but that would necessitate a new spar but one that could be better suited to a more modern way of thinking.
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chris williams
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Joined: 10 Mar 2015, 10:50
Location: Blandford Dorset

Re: K-18 washout

Post by chris williams »

The HQ section will give you a wider speed range, more efficiency and a much simpler construction... Cliff, is that 12 x 6mm spar? I would recommend a 6mm sq spar full length, with a shorter 6mm sub spar let in afterwards: much simpler than physically tapering a flat spar?

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chris williams
Posts: 1549
Joined: 10 Mar 2015, 10:50
Location: Blandford Dorset

Re: K-18 washout

Post by chris williams »

A simple taper down to 6 x 3mm over the last three ribs usually does the job
harry curzon
Posts: 115
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: K-18 washout

Post by harry curzon »

I have received the kit and plan for the updated wing from Cliff at lasercutsailplanes.

Not only are the ribs of the modern HQ section, now they have alignment tabs, the set of parts includes the ply vertical web of an I beam spar and other parts for the leading edge and ailerons, and the plan shows both wings instead of one.

Nice job Cliff, looks like it will be a considerably easier build as well as the hoped for advantages of the HQ section.
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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: K-18 washout

Post by Peter Balcombe »

Harry,
Just sent you a PM.
Peter
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