Fitting Wings to Fuselage

January 20, 2012

MVP = Major visual Progress.  When you first fit your wings up to the fuselage, it is one of those MVP moments in your project.  Great care must be exercised to assure that you get your wings set up to be accurate.  Doing this part correctly, will assure that your airplane flies correctly.

Several objectives must be met:

1. The wings should be parallel with each other, (neither swept back nor forward). A string stretched along the leading edge of the wing from the left wingtip to the right wingtip should reflect that the wings are neither swept back nor swept forward.

2. Take a measurement from each wing tip to a fixed point on the tailpost.  The two wings should be within 1" of each other.

3. (Probably the most important of all) The angle of incidence for both wings must be identical with each other. Use a digital level if you have one.

4. The dihedral (angle from wing root to wingtip) must be 1 degree with the fuselage level.

The holes in the fuselage are predrilled undersized to 1/4".  The holes in the wing spars are predrilled undersized to 3/16"  This will give you a little bit of adjustment room so you can position your wings to achieve the above criteria.

The first step is to get your wings up in position and the brace them up with a couple of "T'" braces made from 2 x 4 lumber.  Here are my wings ready to be lifted into position by a few helpers and a sheetrock lift.:

Note that I have a few "helpers" from my local EAA chapter. Left to right is Burl Nelson, Mickey Whittenburg and my father-in-law Lonnie Gibbons.

The fuselage was positioned to be level fore/aft as well as side-to-side.  Note the sheetrock lift on the right side of the photo below:

The ladder is positioned so I can quickly slip 3/16" bolts in the front and rear spar attachments.

The sheetrock lift was rented for the weekend for $25 and well worth every penny.  We placed a piece of plywood along with some Styrofoam sheet on the lift to protect the wing.  The wing was lifted up and placed on top of the sheetrock lift.  Then with one helper at the wingtip, balancing the wing and one helper cranking the sheetrock lift handle, we lifted the wing into position and slid in some 3/16" bolts at the front and rear spar attachment points.

A "T" brace made from some 2 x 4 lumber was then positioned to support the wing out near the wingtip.  The sheetrock lift was lowered and the whole process was repeated for the other wing.  Here we are with the wings in position and braced up:



Next we will go ahead and position the wings to meet criteria 1, 2 and 3 listed above.  Remember, you don't want to end up with an oblong hole in the spar attach fittings so watch the amount of hole showing in your wing attachments at the fuselage.  You have more adjustment capabilities at the forward wing attachment than at the rear because you will be opening the hole up to 3/8" and that gives you a little more wiggle room. The hole can be off slightly and still clean up when you drill it out to full size.  On the aft attachment, it only gets opened up to 5/16"  so it can't be off very much still end up with a round hole.

Once we got the wings where we wanted them,  we opened the holes to their final size (3/8" for the main spar, 5/16" for the rear spar), using a couple of drill jigs that Mark sent me.

One end has a spacer in it to fit a 1/4" drill bit.  You position the jig in place on the spar attachment and insert a 1/4' drill bit from the inside and into the fuselage spar attachment.  This keeps the jig centered where it belongs.  The drill side of the jig is sized to accept a 5/16" bit.  With that, you open the hole 5/16".  Once both the front and rear attachments are opened to 5/16, there is another jig like the above pictured, but this one is shimmed to accept 5/16" bit on one end and 3/8" bit on the front end. Repeat the process for the front spar attachment to accept a 3/8" bolt.  The rear stays 5/16"

Here is Mickey drilling out the front spar attach hole with an extension drill:

With the spar attachment holes drilled to final size and bolts inserted, we can move on to the struts.  First, the dihedral must be properly set to 1 degree.  Make sure the fuselage is level side-to-side (we had to let the air out of one of the tires).  Then position the wings so they have 1 degree of positive dihedral.

This can measured several ways.  You can use a digital level and get the wings to be 1 degree higher at the tips than at the root. Or you can use a long clear tube filled with water and set the wings to be 2 7/8" higher at the wing tips that at the root.. Or you can use a bubble level with a spacer attached to one end.  The formula is .017" of rise for each inch of length. So for instance if you take a 48" long level and make a mark on it exactly 45" from one end and put a spacer at that mark.  The spacer should be 45" x .017" = .765" thick. You then simply place the level with spacer along the spar line and adjust the wing up our down at the tip until the bubble is centered. 

To calculate the strut length, take a measurement from the center of the hole at the strut attachment on the wing to the center of the hole at the strut attachment on the fuselage (near the front landing gear attach point.  Once you have that measurement, your actual strut material needs to be exactly 3" shorter than that measurement.  This will allow for the 1 1/2' that the strut attachment fittings must stick out from the strut at each end as shown in the drawings.

To cut the struts to length, we used a horizontal band saw:

The ends were then filed smooth and deburred.  

Next, you will need to smash the ends of the struts down to properly fit the strut end fittings.  To accomplish this, we used an arbor press with a couple of blocks of wood to protect the strut material:

You could do the same thing with a bench vise.

The end fitting now slides in with a snug fit:

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