K-18 washout

[chris williams]

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...

[Peter Balcombe]

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).

[RobbieB]

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

[Peter Balcombe]

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.

[harry curzon]

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?

[harry curzon]

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?

[chris williams]

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...

[RobbieB]

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.

[spike spencer]

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.

[chris williams]

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?