Front and Rear Spar Alignment!!!!!!

[DesertBearhawk]

Guys, I made a post about framing up the wing and got some interesting replies! There is some chatter about how to line up the front and rear spars. Can you guys who have built your wings please elaborate on this. Are the bolt holes lined up or are they off set? what can you tell me? I am in the process of lining up my wings now!

Looking forward to your replies!

[BTAZ]

For my LSA, I set the front/rear spar holes in coaxial alignment as that would match both the wing attach fittings as drawn on the fuselage plans and the "Use a 1/4" rod to maintain spar mount alignment when welding" in the book.

My intention is to set the wing on my assembly table(once finished), run a string or rod between the spar attach holes, shim between the wing and table for the required angle, measure the resulting angle of the string/rod, and then use this angle when positioning the spar brackets on the fuselage.

I expect the critical item is to have both wings match more so then an absolute angle(within reason).

[mswain]

I offset my spars as described in the link I shared in your other post, and my centers ribs fit perfectly square to the spars. I know 2 builders locally who built their wings with the holes aligned and experienced no issues. If you're trimming your ribs to fit I don't think you'll have an issue either way, it's a very slight difference.

I had some correspondence with KYBearhawk via email right after he began assembling his wing. I think he lined up the holes on his spar and found the ribs to be very slightly out of square with the spar. Maybe he could elaborate.

[Enewton57]

I aligned the holes with plumb bob for two reason. The first is that is the way Bob Barrows showed it in his photo book. The second is because when you build your fuselage, the front and rear wing mounts are aligned with each other using a steel rod through the aft wing mounts and running all the way through the front wing mounts. For me it stands to reason that if the wing mounts on the fuselage are aligned with each other, so then should the wing spars that attach to those fuselage mounts. If in doubt, contact Bob.

[KyBearhawk]

Eric has built plenty of planes now and if aligning with a plumb bob was good enough for him it was definitely good enough for me. I did notice however that the center ribs did not sit perfectly flush with the spars, like they were at a little of an angle. When I say little I mean less than an 1/8 inch, Take in mind this is my first construction and I suck at this, so it could be that I cut them at an angle and not the spar alignment issue. I actually first heard of the alignment thing in the yahoo group after I had already started wing assembly. If I had it to do over I would still do it the same way but I would have added an extra 1/2 inch to the length of the center ribs so I could trim to fit instead of mass producing. I am finding that as the wing assembly is coming to an end the perfection of not trimming to fit has went out the window. Example I just discovered that my false ribs have the hole for the flap tube in the wrong spot by an 1/8 of an inch. So now I have to oblong my perfect round holes on all 4 of them.

[DesertBearhawk]

Guys, doing some initial assembly/lining-up I am finding that when my ribs are flush and square on BOTH spars the bolt holes are lining up within a 32nd of an inch. I can't be 100% sure if I'm stright but its looking good. I'm with Eric, if thats the way Bob did it AND the mounting lugs on the airframe are in line with each other I am going with the line up the holes method….Hopefully Im not back here in a few days telling you otherwise!

[bergy]

I am about to weld the wing attach fittings to my Patrol longerons. When the fuselage is level, my top longeron slope about 2" in the 32" between front and rear spar-mounts, which is approximately a 3.5 degree slope. The side-view of the fuselage on the first page of the plans shows the center-line of the airfoil inclined at +2 degrees. It also appears in the drawings for wing-attach fittings that the holes in the spar-ends are approximately parallel to the top longeron. In my mind there is a slight conflict there. I gather that there will need to be a slight difference between the slope of a 1/4" rod used to line up the wing mounts and the slope of the upper longeron. That is, all wing-mounts are to have a little kink in them to give the proper 2.0 degree slope to the the wing airfoil.

Anyone care to explain?

[mswain]

Anyone care to explain?

The builders I know here put the fittings on the wing, put the wing in position and tacked the fitting.....while more labor intensive it's a sure fire way to get it right.

[bergy]

Sounds like a good way to get it correct ... problem for me however, I don't have me wings done yet

[Chris In Milwaukee]

I'm pretty sure that the 2 degrees is built into the wing attach fittings on the wing root, not the fuselage. It seems to me that the top longeron between the spars is level to the ground reference when the tail is up.

[bergy]

According to Patrol plans page 7 the bolt-hole thru the front spar is 1.5" above the spar centerlline
The bolt-hole thru the rear spar is 1.125" above the rear-spar center line.

If one draws the centerline of the cord from Bob's Mylar airfoil profile and measures bolt-hole positions, the bolt-hole in the front spar is ~ 3.125 above the airfoil centerline. The bolt-hole in the rear spar is ~2.625 above the airfoil centerline.

Also ... according to the drawings it appears that the faces of the front spar and rear spar are parallel. That is, the spars are oriented perpendicular to the centerline of the airfoil.

From this the only conclusion one can make is that the attachment holes in the ends of both the front and rear spars are not drilled perpendicular (squarely) through the spar-ends if one uses the 1/4- inch rod technique for lining things up.

The difference of offsets from the airfoil centerline is 3.125 - 2.625 = approx 0.5" which amounts to approx 0.9 degrees

If my fuselage tail is raised until the cockpit floor is level the top longerons slope rearward approx 2" in the 32" from front spar position to rear spar position. That amounts to approx 3.2 degrees.

Subtracting the angular offset of bolt holes between the front and rear spars of 0.9 degrees gives me an angle of attack for the Patrol's airfoil of + 2.3 degrees. Close enough to the 2 degree spec on the Patrol's plans , I guess.

Measure with a micrometer, mark it with a crayon.

Bergy

[Bdflies]

I can't imagine myself welding the attach fittings onto the fuselage without having the wing to align them to tack. I'm pretty sure I'd end up creating new cuss words at some point of that process.

Bill

[whee]

I can't imagine myself welding the attach fittings onto the fuselage without having the wing to align them to tack. I'm pretty sure I'd end up creating new cuss words at some point of that process.

Bill

Agreed. That's the way I did it. I'm sure more than one person has ended up cutting their fittings off their finished fuselage because they didn't match their wings.

[Mark Goldberg]

I think that if both wings are the same angle of incidence (most important) and you are close to the two degrees spec'ed you will be fine. MG

[BTAZ]

For my LSA, I welded the fittings without mating the wings to the fuselage.

But I put a lot of thought and effort into jigging it.

I built my wings with the spar holes colinear per a plumb bob as described. If one doesn't, then the front/rear spar attach holes will NOT be colinear so you can't simply use a round bar as described below.. Note I didn't trust a simple piece of all thread to be straight or rigid enough for the accuracy required.

I raided the remnant pile at my local metal supplier for some 3/4" solid rod and some 1"x .5"(might have been 1"x 3/8") rectangular bar. Maybe $25 worth of steel.

I cut the round bar to the appropriate length that would fit between the spars(made sure to account for the mounting ear thickness etc.). I dressed the ends square in my lathe and then center drilled/drilled/tapped for 10-32 to match my mount brackets.You can bolt your mounting tabs to the bar and then "sanity check" against the wings.

I cut the rectangular bar into two matching pieces with 1/4" through holes drilled at the appropriate width for the tabs.

I cut two more bars and drilled for enough drop(IIRC, 3") to put the front cross bar above the fuselage top. These were bolted to the cross bars and set square.

I cut two more bars and drilled holes the same drop as for the front plus the additional drop shown in the plans(I.E. 3" plus the drop shown in the plans between the front and rear mounting ears).

This gave me a pretty rigid setup that was fairly easy to align and clamp to the fuselage with a good chance everything would stay in place after welding.

I used the same round bar approach to weld my landing gear brackets on.

And it did work out as I found out a year and a half later when I actually mated the wings.......