Washout at the tip or the whole wing?

[Cliff Harvey]

Hi Guys, the plan on my 3m glider build suggests adding washout to just the ailerons, there's a solid balsa tip that finishes the wing off but that would mean either shaping the tip in a way to accommodate the upward sweep of the aileron or leaving it scale and the ailerons would stick up awkwardly!

My idea is to build the wing on a twisted building board so the whole wing has washout.

My question is would the second option be feasible?

Interested to hear what you guys think?

[wookman]

Do you mean flying with ailerons set slightly up at neutral?

[mikef]

It would be useful to know a bit more about the planform of the wing.  I would certainly plan to have a continuous twist, without incidence “steps”,  applied to the tips alone - but a twist from root to tip seems strange.  I assume it would be  applied to both wings symmetrically…

[Cliff Harvey]

14 hours ago, wookman said:

Do you mean flying with ailerons set slightly up at neutral?

Hi Wookman, no, the plan shows the half-length aileron neutral at the inboard end and twisted up a little at the tip, this would add washout but would look ugly.

[oipigface]

19 minutes ago, Cliff Harvey said:

Hi Wookman, no, the plan shows the half-length aileron neutral at the inboard end and twisted up a little at the tip, this would add washout but would look ugly.

It would also create more drag at the tip. Whether this extra drag would be enough to negate the anti-stall intention of the washout is unclear. Your instinct is right: Don’t do it.

A popular way of building washout into a tip is to place a tapered jig underneath the trailing edge at the tip. Any wingtip block that you might want to fit should also be on top of the jig. This is a simple method by which the angle of incidence can be reduced progressively towards the tip.

I can’t immediately see any advantage to extending washout across the whole span. After all, tip-stalling occurs at the tip.

If you want to do anything fancier than what is suggested above, probably the easiest thing to do is learn how to use a CAD package. (Aka ‘entering a world of pain’.) These make it comparatively easy to produce not only wing components, but also jigs to produce wings with very complicated shapes. Take a look at Angel Wings’ Prandtl:  https://cdn.shopify.com/s/files/1/0300/4864/9309/files/FVT3v3_Build_instructions.pdf?v=1708266941

[wookman]

As OPF says tip the whole trailing edge up over the the last third to a quarter of the span. 

[Cliff Harvey]

Hi guys, here's the port wing of the LF-107 Lunak, from rib R8 to the tip 6mm washout is built into the aileron. If I do it this way then I would make the tip sweep up on the trailing edge to match the aileron. I was suggesting making the whole wing with a slight twist to allieviate the wing tip 'sweep'. By twisting the building board the wing and aileron could be built 'flat'.

Perhaps I should just go for option A and jig the aileron?

[mikef]

The designer has reduced the airfoil camber at the tip by twisting the aileron and carving the tip block to match the aileron's tip angle.  You could make each aileron on its own twisted board but the basic wing is flat as designed.  If you twist the whole wing, you will lose the airfoil transition to the less cambered tip section.  Don't forget to steam a suitable twist into the aileron leading and trailing edges before you assemble the ailerons, otherwise they will spring flat when you take them off the board.  The 0.4 mm ply wrapped around the aileron leading edge will be fun to do.  Be careful not to alter the twist putting the ply on.

[oipigface]

17 hours ago, Cliff Harvey said:

Hi guys, here's the port wing of the LF-107 Lunak, from rib R8 to the tip 6mm washout is built into the aileron. If I do it this way then I would make the tip sweep up on the trailing edge to match the aileron. I was suggesting making the whole wing with a slight twist to allieviate the wing tip 'sweep'. By twisting the building board the wing and aileron could be built 'flat'.

Perhaps I should just go for option A and jig the aileron?

The plan certainly makes clear the designer’s intentions. Is this a scale feature? (I’ve done a bit of checking and apparently it is. This makes it worth replicating.) It’s not a form of washout I’ve ever seen before, but there’s no reason why it shouldn’t do the job of reducing tip-stalling. 
 

To build it, I would build the aileron separately from the rest of the wing, both on a flat board. Trailing edge of aileron should be supported from Rib 8 outwards, by a tapered piece of something (balsa’s good) that’s been treated with release agent. Making this slightly wider than the trailing edge would enable it to support the ribs as well. I wouldn’t bother trying to twist the TE. Instead make sure each rib is installed at an appropriate angle to the board. Painstaking? Yes.

Let us know how you get on.

[oipigface]

More thoughts. Seeing the plan makes this trickier than I thought!

i) It looks as if there is no LE to twist as suggested by mikef. Instead a piece of 1/64” ply is formed around the semicircular front end of the ribs. I would think seriously about preforming this before assembling the aileron. If there’s no taper in the LE I would try doing this with a rod of the right diameter, steam and tape. 
[One of my current projects features a similar idea, but I’m using a halved piece of 17mm CF tube, and there’s no washout]
ii) Getting the tip right looks more difficult than I would expect too. I think I would try to preform that as much as possible, but it would also be helpful if it could be glued to the tip during assembly. This would mean shaping it so that the front part could lie flat on the building board, with the rear part tapered from the hinge point to sit on the same tapered jig as the aileron. The top surface could then be shaped with the whole wing assembled on the board. The effect you are looking for is shown well from about 30secs intp this video of a model a bit larger than yours: https://www.youtube.com/watch?v=5h9_BhHnqII

 

 

 

[mikef]

 

I like the preforming idea for the ply.  The shaped 6mm balsa under it is clear but the extent of the ply isn't.  I would make it wide enough to meet the  straight lines in the aileron surfaces.

[Cliff Harvey]

Thanks for the video link oipigface, I did actually watch that video a month ago and forgot about it, the tip looks pretty good in that model, it appears again at 2:00 for a good look. It's very subtle but quite doable I think.

MikeF, I'm thinking of doing away with the .4mm ply facing on the aileron, I've ditched it on the elevator and it seems pretty strong, one thing less to contend with.

I've also had a thought that I might make the wing an open structure with just the leading edge to main spar sheeted, I like open structures and the iron-on film would go on nicer as well ... just at thought, there's a way to go.