I'm just getting started on a Sterling Yak-9 and already have a problem with the wing. All of the old kit plywood parts have de-bonded so I'm using fresh ply to re-make them, including the rectangular braces behind the angled LE joint and the center joint of the two spar sticks. My birch ply is much stiffer than the old kit wood. The plans say nothing about the spar being angled at the center joint, but when the ribs are slid onto the spar and fitted into the slots in the LE, the spar has to be pulled tightly into a bowed shape to fit. You can see in this picture of the dry-fit parts, I used rubber bands to pull the outer ends of the spar into place so the outer ribs will reach their slots. None of them have been glued.

This is just waiting to spring loose and gut every rib the first time I bounce off its nose. I've been known to do that. The plans show the spar assembled straight. If that's right, then the spar holes in the ribs are placed incorrectly. I could cut the spar back in two and glue the center together at the correct angle. Or I could just leave it out. Or open up the holes on the ribs to let it fit straight. I know a lot of Yaks have been built here at Stunthangar, so I'm open to any thoughts on which option is best. Has anyone run into this in a Sterling Yak before? I could search out no mention of it. What do y'all think of leaving the spar out completely?
Thanks,
Rusty

Rusty,

I would just open up the rib holes a little and then glue a small rib doubler behind the spar to hold the spar tight in the rib.  I would not leave the spar out.  It looks like the rib holes are cut incorrectly.  Odd... I've never seen that before.

Paul

Thanks for the replies.
I hope to make some time to spend on it this evening. If I get anything accomplished I'll post back with whatever I work out.

Rusty,
The Sterling kits that I built had the worst wood and fit of any manufacturers product I assembled.   I throw better wood away then they used.   I had to replace a lot of pieces when building their kits.  I am rebuilding a Ringmaster that my son built 25 years ago and the profile fuselage has saw marks on the sides.  The wood was never planed.  Sterling used nice designs but terrible materials.

Joe

Please, don't go into a "lightening hole" frenzy. It will be easier to just cut a new fuselage. If you can't find any 1/2", two 1/4" pieces can be epoxyied together to make a very strong, and straight (if done correctly) piece.

Thanks Ty. My skills improve with every build. I'm finally at the point where I have a pretty dependable little fleet because either I fly them better or build them to fly better. Or a bit of both. At least I don't spend so much time repairing any more.

I went out to the shop this afternoon and cut the spar back in half. I'm not sure what kind of wood it is, but it's harder than balsa. Anyway, I threaded one side of the ribs back onto it and fitted them into the LE, and the TE fits right up. So since it all fits, I can sand that angle where the two spar sticks butt, glue them back together, and build it that way.

I'd guess that you joined your leading edge with inadequate sweep, forcing the spar to sweep forward to accommodate.  That you think your trailing edge join angles are off reinforces that suspicion.  How confident are you of your leading edge join angle?

There is a ply biscuit that presses into a slot cut in one end of each LE stick. I test fit it and walked it into place pressing with my thumbs and once it was fully seated, the butting ends were perfectly flush. Had you not asked, I would have said 100% confident, based on how smooth the transition is across the joint. There's barely a visible seam. Consider that there is about an inch of bow from the spar center to each end. That would create a helluva gap in the front of the LE if it were adjusted to follow those lines.

I tried every combination of butting the TE sticks together, and there's only one way that comes close, but it is off by a couple of degrees. You could be right though. I assembled the LE as it was cut, but maybe it was cut at the wrong angle rather than the TE.
One or the other is wrong. If I wasn't going to use the flaps, I'd just say, heck with it, let's glue. But I want to make sure the TE is no more forward swept than designed, or I'm going to have trouble with binding controls.

Here's a picture of the TE joint with the sticks roughly aligned with the rib ends. It supports your thoughts. I'm going to take a protractor and measure the angles on the tiny pictures on the instruction sheet and see which angle is off.

Rusty,

This evening I'll dig out my plans for the Sterling Yak-9 that I recently got from Barry Baxter and measure the sweep for you.  As I recall, it's not trivial.  I can't view your picture here at work but I will be able to from home.  I'll let you know.

Gary, Jim and TY,
Looks like about 5 degrees on my glued LE, and closer to 8 degrees on the picture. It fit so well, it just had to be right. I wasn't going to glue anything else. To add insult to injury, it's epoxied because my ply brace(not the biscuit) was so stiff I wanted to make sure id couldn't pop back off. I usually only use epoxy sparingly, but this was an unfortunate exception.

Not a good first flapped plane... yes I have to agree, Jim. But I'm sure I've left you shaking your head at my folly, or stubbornness, or both, so here we go again. I have to learn the hard way. Besides, maybe I'll learn how to make Lucky Boxes.

Funny, Ty, maybe a good last plane.

I do, however, have a plan. You purists ain't gonna like it, but it'll fly just fine. I will leave the LE like it is, and make a new TE with a trapezoidal shape that returns the forward sweep to its original angle. Or maybe I'll get wild and reduce it, allowing for easier flap hinging.

I appreciate all of your help. It's going to have to wait till after my vacation now. It's time for my second hobby, surf fishing.
Rusty

Rusty,

I measured my plans by placing a yardstick with the ends just touching the back side of the leading edge and the 18" mark on the centerline.  The leading edge joint was 1-7/16 forward of the edge of the yardstick.  This calculates to about 4 1/2 degrees sweepback on each side.  I also measured the trailing edge sweep the same way.  The trailing edge joint was 1-3/32 aft of the edge of yardstick.  This calculates to about 3-1/2 degrees forward sweep on each side.  The spar is straight.

Thanks, Gary. You nailed the culprit. My Lead edge sweep is too shallow.

When it starts looking like a plane, I'll post back and show what I've got.
I appreciate you and everyone else taking the time to help me out.
Rusty