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!

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.

Cliff Evans wrote: ↑29 Jul 2019, 11:05 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

RobbieB wrote: ↑29 Jul 2019, 11:14 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!

How far on are you with the wing construction?

RobbieB wrote: ↑29 Jul 2019, 11:22 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

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

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

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...

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

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:

You can use the original section at the root and transition to HQ35/12 at the tip. I did this with the Rhonadler (Go 535 root) and it worked out very well, even with a root section so thick I could hardly get my hands around it...

Cliff, note that the brakes etc. will need to be adjusted to suit the section thickness (probably thinner as you progress to the HQ section along the panel).

................and will have to match the one spar that has already been built - root to tip. Happy days.

I suspect that the spar will definitely have to change/be rebuilt to suit the new set of ribs as the thickness profile will almost certainly change along the panel span due to the different airfoil section.

However, it sounds as if the spar assembly may have needed to be rebuilt anyway in order to get it pliable enough to move ribs to the correct position to provide the intended washout, else cut/position the ribs as they are inserted, setting up the TE to give the required washout as mentioned previously.
Note that the rib doesn’t necessarily have to be a perfect fit to the spar as it’s main purposes are to define the external airfoil section and help hold the various parts of the structure in correct relationship. I suggest that once a cut rib has been correctly positioned, any small gaps between the cut rib end and the spar assembly can be bridged by small pieces of 1/16” balsa sheet placed each side of the rib for a short distance. The rib capping will generally hide these pieces & only the builder will know that they are there

If a new wing design is used which has the same root rib & all other fuselage interfaces are retained, any rework is confined to the wing build.

Personally, I have never built a complete spar assembly as Cliff Charlesworth recommended (although I know a number of builders who like this method). I always lay up ribs on the bottom spar, with the rib TE resting on a straight edge(s) at the correct level. Then the joiner/top spar is added followed by spar webbing to complete the spar assembly.
My Oly 2B was built this way, using the plan for rib positioning.
At the end of the day, it’s whatever method works for you.

Thanks for all the replies.

I have Cliff's book and see how he does it but fundamental problems remain which he doesn't describe. The top and bottom surface of the box spar is horizontal, the washout ribs come at it at an angle, giving the section a stretched Z shape. That could be fixed by putting a layer of soft balsa on the top and bottom of the spar and chamfering it to match the angle of the ribs, BUT, the ribs are cut to the same depth as the spar and so the thick end of the chamfer will be way proud of the rib.
Also, if the front section of the tip ribs are angled downwards, the spar tip must be raised off the board to allow for it. What amount is not shown on the plan, and any amount will affect the washout angle, given a fixed vale of 10mm at the tip te. Also, raising the spar tip will raise all ribs, requiring a tapered jig along the entire le and te.

In other words, I am finding it impossible to build what is shown and how it is described in Cliff's book. And so far, no-one else has been able to explain it either! Yet, people have built this model so what did they do? No washout? Far less washout? Conventional spars and webs instead of box spar? Some other cunning method?

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 that is easier to build and doesn't need any washout at all...

That's an interesting comment Chris, someone else separate to this forum suggested to me to use an HQ because it will not tip stall. I have a good understanding of stalling, and of geometric and aerodynamic washout, can you point me to an explanation of why the same HQ section from root to tip does not tip stall?

Yes, and at the same time I'll solve the Brexit conundrum... I know of no theoretical explanation of this phenomenon, but I have 40 years of practical data to support it. It's not that HQ35 can't be made to tip stall, a higher aspect ratio wing coupled with a high wing loading will do it, but when this happens it's all about minimising the height loss during recovery. In my view this is the critical component that the HQ section brings to the party...

Yes, you can get anything to tip stall if you abuse it sufficiently but as Chris said, it is as much to do about how that stall occurs - how early/abrupt even how violent it can be and the time taken to un-stall.

I know this all goes against even current aerodynamic theory for stall control but practical experience over a few years now with our teeny models clearly indicates Dr Quabeck seems to have produced a keeper here. Like I said, I am really tempted (only tempted mind) to try it out at a wing tip without washout.

Don't want to bend this thread too much but Brexit was definitely stalled until Team Boris changed the angle of the hot airflow around it ! However, pilots will be well aware that too rapid a stall recovery can induce a flick entry into a spin. Watch and wait.

However, pilots will be well aware that too rapid a stall recovery can induce a flick entry into a spin.

So you're saying that any party that includes HQ35/12 in their manifesto will never make the country crash & burn?

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.

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.

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

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

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.

RobbieB wrote: ↑31 Jul 2019, 23:27 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!

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

Hi Robbie

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

Ian

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.

Hi Robbie

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

Best regards

Ian

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.

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.

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?

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.

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?

Similar to this...

A simple taper down to 6 x 3mm over the last three ribs usually does the job

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.

Harry,
Just sent you a PM.
Peter