Wing re-build, washout question

[cbcfly]

I'm in the middle of rebuilding the right wing on my 46 BC-12D. I'm finally in the re-assembly stage. My question regards washout. I've found the rigging instructions on here that describe leveling the airplane and measuring 1 5/16 under the front spar - which I believe I understand. But is there any guidance for when the wing is actually being built and covered?

I trammeled the wing flat on saw horses. I've begun nailing ribs. Should I be doing this with washout already set? Or just continue with a flat wing? What about when I get to the fabric stage? My initial thoughts had been to build and cover it flat and then set the wash out with the rear strut when finished and mounted, but now thinking maybe I should have the twist already set during assembly/covering. If so, is there any guidance on how to set the washout when the wing isn't on mounted on the airplane? i.e. flat at root, raise rear spar x amount at last full rib?

[Guest]

Re: Wing re-build, washout question

I have never given that a thought and have successfully repaired a number of wings.

The wing is pretty flexible with regard to twisting it.

Dave R

[cbcfly]

Re: Wing re-build, washout question

Thanks Dave. I'll press on with how I've started. I live on a little airpark and one of my neighbors stopped by yesterday. Not a Taylorcraft guy, but he was saying to make sure and build the washout into the wing. It made me question myself a little. If it was an aluminum wing and I was riveting a skin on, I could see that being being a concern. Not so much with the way the Taylorcraft is built.

Another quick question: The aileron bay skins (long S-shaped pieces with lots of lightning holes) originally had fabric on both sides of the aluminum. Is it necessary? If so, why on the inside? Does higher pressure inside the wing try to pull the fabric away from the skin?

[3Dreaming]

Re: Wing re-build, washout question

Thanks Dave. I'll press on with how I've started. I live on a little airpark and one of my neighbors stopped by yesterday. Not a Taylorcraft guy, but he was saying to make sure and build the washout into the wing. It made me question myself a little. If it was an aluminum wing and I was riveting a skin on, I could see that being being a concern. Not so much with the way the Taylorcraft is built.

Another quick question: The aileron bay skins (long S-shaped pieces with lots of lightning holes) originally had fabric on both sides of the aluminum. Is it necessary? If so, why on the inside? Does higher pressure inside the wing try to pull the fabric away from the skin?

With the older dope and fabric processes the fabric as it shrunk would not stay attached to the inside curve of the aluminum. With the holes and fabric on the inside it would stay stuck down.

Oh, build the wing flat and let the struts twist the wing for the wash out.

[N96337]

Re: Wing re-build, washout question

I've only been building these since the mid 80's and always throw about a degree and a half in the wing from the beginning of construction to the end. Makes your wing parts have less stress on them when it's finished. At least that's the way the old timers that taught me did it, and the way I still do. Think about it...if you're twisting everything to get there, everything is in a bind when it's done...I'd rather have it with the least amount of stress on it when at rest.
John

[PA1195]

Re: Wing re-build, washout question

What size blocking or end to end offset would give you that 1.5* of twist?

Gary

[Edwin Otha]

Re: Wing re-build, washout question

The fabric on the inside of the aileron well is used to glue the Grade A to itself. The old lacquer cements didn't stick too well to aluminum, so the holes allowed the wing fabric to be glued to fabric. Just what the cement was designed for. The new stuff works pretty good on everything.
EO

[N96337]

Re: Wing re-build, washout question

What size blocking or end to end offset would give you that 1.5* of twist?

Gary

Gary,
I just use a digital level, so it doesn't matter where it starts, you just rig in the washout. I start at the butt, then move out to the furthest spot the spars are full sized. If you're using a regular level, measure to the middle of the two spars and figure the twist from there. I did the math years ago and probably have that written down somewhere, but I don't remember the exact block size. Technology has dumbed me down! One of the beauties of the Cub wing is that it's pretty much flat on the bottom, so it's easier to level from there.
John

[PA1195]

Re: Wing re-build, washout question

Thanks John for the reply. That's what I ended to doing to rig mine...digital level at the same rear location of the inboard and outboard ribs to get close then fly to fine tune. I had a local builder say use a 3/4" thick 1x4 under the rear of the wing outboard when assembling and covering. Close enough.

Gary

[skip]

Re: Wing re-build, washout question

When I rebuilt my wings I assembled ribs, spars, etc on a level set of supports with the correct washout dialed in by small blocks under the spars where needed to set the proper twist. I don't remember the size of the blocks but the calculation was fairly simple. it turned out that until the drag/anti-drag wires are tightened as the wing is trammeled it is quite floppy. The wires tighten thing up considerably. When I assembled to the fuselage before covering there was little need to adjust much. The washout was properly set. The wings with the drag wires tight were stiff enough to ride the rotating wing stands so that covering the wings didn't change the structure very much and the final result has been a plane that flys straight and hands off.

I am glad that I built it as I did with everything set as right as possible from the start. Having said that, I probably overthought and overdid the job a bit, I believe that there is enough flex in the wing that just assembling on a fairly level table and then following the book for final rigging would lead to just as good a result.

[waltermrich]

Re: Wing re-build, washout question

What book ?

[skip]

Re: Wing re-build, washout question

What book ?

Sorry, You're right. my reference wasn't clear.

I meant the standard washout rigging as described in the Taylorcraft service manual

http://www.taylorcraft.org/docs/taylorcraft%20service%20manual%20BB12.pdf

Page 6 for the diagram.

[Guest]

Re: Wing re-build, washout question

When a wing is "trammeled" the drag and antidrag wires in a bay are supposed to be in the same geometric plane.

That means that the spars are parallel (horz. and vert.).

By adding twist to the wing before you trammel violate that principle.

So you end up having to violate trammeling with wires in the same plane (which results in the bays being not exactly 90 degree corners, rectangles).

If you trammel with no blocks then cover then twist the wing you stress the fabric and its attachments by some (small) amount.

If you add the blocks and twist after you trammel and before you cover then you reduce the stress on the fabric compared to above.

Does any of it really matter?

I am not sure it does.

Dave R

[Robert Lees]

Re: Wing re-build, washout question

When a wing is "trammeled" the drag and antidrag wires in a bay are supposed to be in the same geometric plane.
That means that the spars are parallel (horz. and vert.).......

Dave R

I respectfully disagree. Tramelling is ensuring that the compression struts are normal to the spars at each bay, and that the spars are straight and parallel across all bays (and hence the whole span). So tramelling is in two dimensions only. Washout (spar "twist" hence wing "twist") is a separate matter, as already dicussed herein, and is so miniscule over the 16-foot wing length as to be insignificant.

Rob

[Guest]

Re: Wing re-build, washout question

I respectfully disagree. Tramelling is ensuring that the compression struts are normal to the spars at each bay, and that the spars are straight and parallel across all bays (and hence the whole span). So tramelling is in two dimensions only. Washout (spar "twist" hence wing "twist") is a separate matter, as already dicussed herein, and is so miniscule over the 16-foot wing length as to be insignificant.

Rob

Hi Rob,

I agree with your assertion to achieve it we equalize the diagonal lengths, correct?

Those diagonal lengths need to be co planar with the triangles that they form and equal magnitude to assure true diagonal equivalence and therefore to assure the square corners or as you described "compression struts are normal to the spars at each bay, and that the spars are straight and parallel across all bays".

Imagine if I had put a 30 degree twist in and my point becomes more obvious, I think.

Dave R

p.s. I don't think that I stated very well in my first post. Being in the same plane is only a goal insofar is it allows our method to be geometrically correct.