The following is my experience as I learned the process for attaching the wing (root) to the fuselage. Both main and rear spars come predrilled with 3/16” holes at the root attach point. The fuselage attachment fittings are predrilled with 1/4” holes. The end result is to attach the wing root to the fuselage by match drilling the main spar to the front attachment fitting to 3/8” (AN-6), and the rear spar to the rear attachment fitting to 5/16” (AN-5), both left and right. So we are enlarging existing smaller holes in both the spars and the fuselage to match the aforementioned sizes.
But first, wings on all airplanes have a set Angle of Incidence. And the Angle of Incidence for both wings must match each other when installed to the fuselage. The Bearhawk factory does amazing work building their fuselages and wings. Their pre-established attachment points are extremely accurate. However, the wings Angle of Incidence is a fixed design feature that MUST be checked by the builder and can require some fine tuning.
What is the Angle of Incidence? On fixed-wing aircraft, the angle of incidence (sometimes referred to as the mounting angle) is the angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis along the fuselage (often the direction of minimum drag, or where applicable, the longitudinal axis). The angle of incidence is fixed in the design of the aircraft, and with rare exceptions, cannot be varied in flight.
MATCHING the Angle of Incidence – MATCH is the key word here. I found the instructions to suggest that we are SETTING the Angle of Incidence, which is a little more daunting. That part confused me, and it wasn’t until I read further that I understood what we are doing here.
From the manual….
We want the same angle of incidence for both wings. The goal is to have no more than .2 degree cumulative difference, left to right, as measured by a digital level. So one wing can be .1 degree up and the other the same amount down or any combination as long as they don’t total more than .2 degree. This represents a dimensional difference in height of .100” accumulated difference front to rear spar (.050 per side or any combination that totals less than .100”).
Leveling the Fuselage - Leveling right and left shouldn’t be done with a digital level because it’s always possible that there’s a welding induced difference in part of the structure so it shouldn’t be used for measuring over such a short distance. The important points to be leveled right and left are the holes in the front wing fittings. This is best done with a water level, which is not only easily made but will also prove useful in setting the dihedral.
Wing Drilling Steps for N6408C
(These are my steps as interpreted from all of the Bearhawk manuals and BearhawkTips website)
Set the Angle of Incidence; Mounting the Wings to the Fuselage Attachment Fittings:
1. Level the fuselage.
· 2-25-2020 - I have test leveled the fuselage with a water level
2. Re-check the attachment fittings on the fuselage (upper longeron) using a ¼” steel rod to see if the angles match.
· 2-25-2020 - I have matched the angles for both side attachment fittings. (13.2 degrees in that environment). Slightly opened the right-side holes to match the left.
3. Measure the bolt holes in the wing spars to see if there is a difference.
4. Support the wings move them into place on the attachment fittings using a 3/16” rod through the spars to loosely pin them in place. The fuselage attachment fittings are pre-drilled to 1/4", allowing room for adjustment.
· The rod will bridge between the front and rear fuselage attachment fittings.
· Use a digital level to MATCH the angle of incidence on both `wings.
· Lock the angle of incidence on both wings with a large Vise Grip squeezing the bottom of the attachment fittings.
· Use shims between the spars and the attachment fittings
· The wings should be locked and will not move
5. Confirm fuselage is still level
6. THEN, using the drill guides and bushing, drill both the attachment fittings and wings incrementally starting with three 12” drill bits; 1/4”, 5/16”, and finally for the FRONT ONLY, 3/8”. Final ream if necessary.
7. The end result will be 3/8” front (AN-6), and 5/16” rear (AN-5). Tighten these enough that the wings can still move up and down in order to set the dihedral.
During the wing drilling exercise from above, we did attempt to set the dihedral. However, time was running short and my helpers had other commitments. We temporarily attached the structs to the attachment fittings as shown in the pictures below, but we could not establish accuracy. We determined to not proceed and would take up this task again upon final assembly and when we had more time to ponder. I should have a hangar during that time and we will have the luxury of working at our own pace where we can come and go at will. These will be the steps that we will follow...
Set the Dihedral; Attaching the Wing Struts:
Set the Dihedral; Attaching the Wing Struts:
1. Confirm the fuselage is level.
2. Set a water level with one end at the lower skin of the wing at the root. Set the other end of the water level at the outer end, lower skin at the tip. The plans call for 1 degree of dihedral, which is 2.9” (2 29/32”) difference from the root to the tip.
3. Check fuselage level again... Set the dihedral.
4. Wing Struts – 4-Place B Model struts do not need trimming and should be 79.25” in length. Other models may need trimming. There will be 1.5” from the end of the strut to the attach fitting bolt holes.
5. Mark a centerline on the outside of both strut ends. Centered on the inside flats.
6. Squeeze the last 4” of the strut end in a bench vice using hardwood to cover the jaws and maintain even pressure. The strut should be square in the vice. Use a spacer that is 3/16” thinner than the fittings to prevent over squeezing. Squeeze until the fitting is snug. Do not tap or hammer the fitting in.
7. With the wings in place, bolt the fittings themselves to the fuselage and wing attach points.
8. Clamp each fitting to the outside bottom of the fitting, outside top of the strut.
· NOTE: After drilling, the fittings will be inserted all together. Do not drill one and insert it before the other is drilled. Otherwise the geometry is affected.
· NOTE: The top fitting has a slight angle relative to the centerline. The bottom fitting position must be established first, drilled, then bolted to the OUTSIDE of the strut before finalizing the position of the top fitting to the OUTSIDE of the strut.
9. With the strut clamped to the attached fittings, move it lengthwise until it is centered between the bolts. Leaving 1.5” between the strut ends and the respective bolt.
10. Using a fine point Sharpie, mark a line across each fitting where the strut crosses it. This will serve as a reference for how deep the fitting goes into the strut. And also the angle relative to the top strut.
11. DRILLING - If using a drill press make sure everything is level and square.
12. Center the bottom fitting with the strut centerline going through the center of the holes.
13. Make certain the lengthwise position is correct by the line previously marked on the fitting.
14. Tightly clamp and re-check the alignment. Re-check the face of the fitting is parallel to the flats inside the strut.
· NOTE: It is very easy to clamp the fitting at a slight angle on the strut because of the curved surface. Double check that the fitting is parallel to the flats. Otherwise, the first holes will be at an angle which will result in a twisted strut that cannot be used on the airplane.
· NOTE: Use two clamps to hold the fitting in place understanding that the first hole to be drilled will be the OUTBOARD END of the fitting (furthest from the strut end).
15. Drill the first hole through one surface only, not through the entire strut. Place a bolt through that hole and reach inside to put a nut on it. Snug it down.
16. Recheck for alignment, re-clamp and drill the last hole inboard (closest to the strut end)
17. Put a bolt in it and snug it down. Move to the top fitting.
18. Place the strut back on the airplane with the bottom fitting bolted to the outside of the strut and the top fitting still clamped to the outside of the strut. Re-check the dihedral.
· NOTE: This is the last chance to get it right! Re-check the line across the top fitting. Correct if necessary.
19. Remove the strut. Verify that the holes are located over the lengthwise centerline. Ensure that the fitting has not changed position lengthwise.
20. Drill the same pattern as before and snug down two bolts on the outboard and inboard holes of the fitting.
21. Take back to the airplane and re-check everything. If it is correct, repeat steps 11 through 21 for the other strut.
22. Once both struts have the fittings secured with two bolts and are accurate, drill the rest of the holes through just the one side of the struts only. Before taking everything apart, mark the fittings and the strut ends to determine which fitting goes with which strut and what direction they were facing when drilled.
23. Take the fittings apart from the struts. Use 320 sandpaper to remove any drilling burrs inside the struts.
24. Slide the fittings inside the struts and index them in position with bolts.
25. Firmly clamp down on the middle of the fitting between the two holes. Clamp tightly enough to pull the flats against the fittings.
26. Drill completely through the strut in the open holes. Put bolts in those holes and tighten with nuts on a washer.
27. Drill the rest of the holes.
28. Clean the inside of the struts again with sandpaper
29. Lightly camber the outside edges of the holes. Just barely break the edges.
Done!
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