Prepare the struts with these steps:
Install the Wings
You can’t make the struts until you’ve finished this step.
With the wings in place, you’ll be able to measure for strut length. If you are building a Model B 4-Place, you won’t be trimming the struts at all. For an original 4-Place, you might end up needing to trim to around 79.25 inches, but measure first (twice) and be sure to allow for 1.5” between the attach bolt holes and the end of the strut as depicted on the plans.
If you do end up trimming the strut, make the cut square so your subsequent measurements are accurate. Whatever method you use (chop saw, horizontal hacksaw, hand hacksaw) make sure you are working to a line you marked all the way around the strut. Use a fine point felt marker. After trimming the struts go back and smooth the cut surface, first with files, then with sand paper or Scotchbrite. Make sure any burrs left from cutting are gone. Sharp edges and burrs are all stress risers that could potentially become fatigue cracks.
Mark Centerlines
Mark a centerline on the outside of both sides of the strut at both ends. This line should be lined up with the center of the flat spots on the inside of the struts.
Squeeze the Struts Snug to Fittings
The internal dimension of the strut material is slightly wider than the fittings so the last 8 inches at the ends of the struts must be squeezed down so the fittings will slide in with a snug fit. This is normal with struts on many aircraft. The squeezing will be done in a vice with a piece of hardwood on both sides of the strut to keep from leaving marks and to keep the pressure even.
Make certain the strut is square in the vice, as indicated by the flat spots on the inside of the strut, before squeezing.
Make up a spacer that’s about 3/16” thinner than the fittings and insert inside the strut while squeezing. This is to make sure the flat surfaces inside the strut remain parallel. Just tighten the vice until the opening is a little too small to let the fitting slide in. Release the pressure on the vice and see how much the strut springs back by sliding the fitting in. Keep repeating that process until, with no pressure on the vice, the fittings slide in with a snug fit. We don’t want them so tight we have to hammer them in as that could leave marks and distort the aluminum fittings.
Sequence of Events to Come:
To determine where the fittings will be positioned in the struts and to get ready to drill the holes, we’re going to bolt the fittings to the fuselage and wing attach points and clamp the strut to the fittings with the fittings laying on the top surface of the strut. The fittings will be outside of the tube, alongside the tube. This works as long as both fittings are outside of the tube. In other words, after drilling the bottom fitting, don’t insert it into the tube before drilling the top fitting, since that will alter the geometry.
The holes in the fuselage end fitting (bottom of the strut) are to be parallel to the centerline of the strut, but the top one has a slight angle relative to the centerline. We’ll first determine the position of the bottom fitting, drill and bolt it in place on the outside of the strut and then finalize the position of the top fitting before final drilling it. This operation will involve a few iterations of assembly and disassembly. Use plain or castelated nuts for these steps, and use fresh nylon lock nuts for the final assembly.
With the struts prepared, it is time to locate them precisely.
Put the spacer blocks back inside each end of the strut so sufficient clamping force can be exerted without crushing the strut.
Install the fittings to the fuselage and wings using their single large bolts and clamp the struts to the bottom of them. We’ll position and drill the bottom fitting first. With the strut clamped to the fittings, move it lengthwise until it is centered between the bolts. That should leave 1.5” between each strut end and the respective bolt. Using a very fine point Sharpie, mark a line across each fitting where the strut crosses it. This line will serve as a reference for how deep the fitting goes into the strut, and also the angle relative to the strut (top fitting). Make sure each clamp is tight and remove the strut.
If using a drill press make sure everything is level and square.
Drill Preliminary Bottom Fitting Holes
Center the bottom fitting with the strut centerline going through the center of the holes. Make certain the lengthwise position is correct as indicated by the line previously marked on the fitting. Clamp tightly and recheck alignment. Double check that the face of the fitting is parallel to the flats inside the strut. Be careful: it is very easy to clamp the fitting at a slight angle on the strut because of the curved surface. Before drilling any holes, check and double check that the fitting is parallel to the flats. Otherwise, your first holes will be at a slight angle, which will result in a twisted strut that can’t be installed on the airplane.
Use two clamps to hold it in place, leaving access to the hole closest to the outboard end of the fitting for drilling. Drill the first hole through one surface only, not through the entire strut, and put a bolt through the one side and reach in side to put a nut on it. Snug it down. Recheck it for alignment, re-clamp and drill the last hole inboard. Put a bolt in it and snug it down. The bottom fitting is now secured to one side of the strut with two bolts and we can do the top fitting.
Drill Preliminary Top Fitting Holes
As a last minute check to make sure nothing moved, put the strut back up on the airplane with the bottom fitting bolted to the outside of the strut and the top fitting still clamped to the outside. Re-check dihedral measurement. This is the last chance you’ll have to correct anything. Re-check the line you drew across the top fitting. If it’s not right, correct it.
Remove the strut. Verify that a central hole is located over the centerline that you marked lengthwise on the strut, and ensure that the fitting hasn’t changed position lengthwise, as indicated by the line across the fitting. Drill one hole through one side of the strut as we did before, and install and tighten a nut inside the strut. Drill a second hole and install a second nut, and put the strut back up on the airplane. If everything is right, you now have a strut that has both fittings bolted alongside it with two bolts. Repeat this process on both sides of the airplane continually checking and rechecking the dihedral.
Drill One Side
Once both struts have the fittings secured with two bolts, go ahead and drill the rest of the holes through one side of the struts only. Before taking everything apart, make identifying marks on both the fittings and the struts so you can easily determine which fitting goes with which strut and what direction they were facing when they were drilled.
Drill the Rest of the Holes
Take everything apart. Reach inside the struts with 320 grit paper and knock down any burrs from the drilling operation.
Slide the fittings inside the struts and index them in position with a couple of bolts (the bolts can’t go all the way through the strut because there are no holes on the other side yet). Firmly clamp down on the middle of the fitting between the two outboard holes. Make the clamps are tight enough to pull the strut together forcefully so that the flats inside the strut lay flat on the fittings. Drill completely through the strut in the the open holes, put bolts in those holes and through the struts. Put nuts on them and tighten them up, then drill the other holes.
It is not necessary to use a drill press to drill the holes all the way through the strut. In fact, it is highly advisable that a clamp be placed between the two outboard holes and tightened as much as possible. With locating bolts in the outboard holes, this guarantees that the fitting squares up with the flats inside. Then, when the two outside holes are drilled and bolted, they hold the fitting square so the balance of the holes can be through-drilled.
Once all the holes are drilled, disassemble again and clean out the inside with 320 and Scotchbrite. Outside of the strut, lightly camber the outside edges of the drilled holes by twirling a large diameter bit in the holes with your fingers. You want to just barely break the edges.
You’re ready to bolt them together permanently. Congratulations on a job well done! Your wings are now ready to be attached for good, any time you choose.
***MY AIRCRAFT IS NOT FINISHED AND FLYING YET – BE WARNED***
Most of the write-ups on this bearhawk tips page are spot-on, but this one left me pretty confused, even after I had finished the struts!
Here’s my 2-cents worth, to be read in conjunction with the main info above:
1) I used a water levels to set the dihedral. After mucking around for a whole morning, I discovered it’s much easier if you have BOTH ends fixed. If you clamp the outboard end of the tube to the wing tip, the “belly” of the loop moves up and down with the wing, and you chase your tail. Obvious, I know! Use a 5ml syringe to add one of two drops of water, that’s all you need to get the water level into “the zone”.
2) Cutting to size – when you get you struts, check that the ends have been cut 90 degrees. If they have, you’re set. Designate and label the two inboard (bottom) ends. Using a 12″ square, run centre lines up the strut, both faces, both ends, both struts. It has to be about 10″ long, because you’ll be chopping a lot of it off ONE END (4-place only).
3) Using a fine sharpie, carefully mark the inboard strut fittings with a centreline that wraps right around the fitting, and goes through the exact middle of the all the holes. Designate and label the fitting faces with the strut faces.
4) Buy 4x Irwin hand snap clamps, there’s a photo on the forums. These are great, but they are fractionally shallow, and tend to pull your fittings forward toward the leading edge. Be vigilant.
5) Make up blocks of wood about the size and shape of the fittings, but thicker so that you can only slide them inside the strut when you compress it chord-wise with the clamps. Slide one inside the strut, and line up the inboard strut fittings on the upper surface so that all the reference lines line up. Remember your 1.5″ (38mm) distance between the lug hole and the end of the strut, this allows for edge distance for your AN4 bolts. Now, re-read all of the bearhawk tips info above, so that you can see where this is all going.
6) When they talk about “twisted fittings”, the fitting has to be drilled off while aligned to the flat surfaces INSIDE the strut. Again, the Irwin clamps pull you around.
7) Clamp hard, pray, and drill off. Use a 1/2″ pneumatic drill, slow speed, spray the hole with CRC, and use the wood insert to reduce breakout.
8) Now, with the 4-place strut, it’ll still be about 5 inches too long. The info above talks about cutting each end to 90 degrees, but because the lower end is your reference, just hack enough off the upper end so it clears the wing lower surface. Note the centre line was extended beyond the cut off section now we’ve lost the reference end. My wing spar fittings were so tight, I couldn’t slide them on or off with the fitting clamped to the top of the strut. So I had to install the fittings onto the wing, and just hold everything up there. A loop of rope around the top of the wing will help hold the strut up while you’re attaching the bottom end, or use a helper.
9) There’s not a lot of room to clamp, but get one on there, and check the forward-aft alignment of the strut relative to the centreline. This one will be skewed on an angle, so go for the best compromise either side of your reference line. Trace around the fitting, so you can reposition on the bench. Drill off with the block of wood as above.
10) I made 0.080″ shims, rather than using a vise to squeeze the strut. You’ll need two per fitting, 8 total, and while I don’t know if I’ll need it down the track, it seems shimming the strut up or down might help with aileron cable alignment (check the forums). Clamp the fitting to a sheet of 80thou, and use it as a drilling guide. I also primed everything before final assembly. With the shims, the fittings are a nice fit, you have to compress the strut chord to fit everything in. Place the fittings and shims inside the strut, clamp the wooden block to the bottom of the strut to stop break-out, and drill right through.
11) The RC-2000 deburring tool does a great job here, and is fun to use.
12) Don’t forget your 1.5″ edge distance, and trim the upper end of the strut with an angle grinder. Polish everything with 3m scotchbrite discs and a die grinder.
12) If you use shims, you may need longer bolts to clamp up the fittings than those supplied in the kit.
A fun job!