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( 2.8 / 405 )Based on design calculations for the strut and root attach fittings, I had two choices - use five 1/4" bolts in bare wood, or use three 1/4" bolts in 1/2" phenolic bushings. Since drilling holes in the spar makes me nervous, I went with the option that required less drilling. The problem was that I did not have 1/2" phenolic rod - only 3/4". A few minutes with a lathe solve that issue, and I am now ready to install the bushings into the spar.
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( 2.9 / 333 )I got a shipment of GL2 birch ply that should keep me busy for quite a while. I have been trying to find an inexpensive source for the plywood, and I think I've done the best I can do - Plywood and Door Manufactures Corporation in Chicago, IL. They sold me the 1/16" 50"x50" sheets at $23 each, plus shipping. With the freight, the cost was still well below that charged by the usual suspects. I had the plywood one day after I ordered it.
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( 2.9 / 440 )I glued in the leading edge stringer as well as an additional lamination on the wing tip bow. The reason for the extra lamination was that after shaping the nose area of the tip bow, there wasn't enough area for the leading edge skin to attach. The extra 6 inches of lamination along the underside of the leading edge gave 1/4-inch of glue area for the skin after shaping. That should be sufficient for the low stress joint.
I am trying a new (actually old) adhesive - Weldwood Plastic Resin. I really like T-88, but it isn't available locally. I can get it in St. Paul, MN at Rockler, but they only carry the very expensive 8-oz. kit. I can currently get WPR at the local Ace Hardware store, and it is MUCH cheaper. I think the plastic resin works fine, though I can see where it would be unforgiving of gaps.
Speaking of locally (well, kind of local) available products, I stopped at McCormick's Lumber in Madison, WI this weekend to get some 1/16" birch plywood. They were out of the plywood, but they did have a very nice selection of Sitka Spruce. The quarter-sawn rough-cut boards were 1"x9" in lengths from 14' to 18'. I didn't have to go into the stack more than a few boards to find one that will make some nice longerons.
Yes, the spruce is more expensive than the lumber at the local lumber yard (we do still have a lumber yard in Eau Claire, in addition to Menards) - but I haven't had much luck finding wood that I would make a spar or longeron out of. The structural grade douglas fir at Lyman Lumber was one exception - I found a few 2x10s that I could have used. Each timber could have yielded a couple one-piece longerons or caps, maybe double that with scarfing and laminating. That douglas fir was better than half the cost of the nearly perfect spruce - not worth my time to save so little.
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( 2.9 / 220 )I sprayed the leading edge ply with near-boiling hot water to soften it, then used a dozen cargo straps to pull it into position around the leading edge ribs on the wing.
The radius is pretty sharp for the 1/16" bias-ply birch. It took a fair amount of coaxing to get the skin tight to the ribs, especially from 2" to 6" back from the nose on the top. In the image above you can see where I needed to use some oak boards under the straps for extra pressure in that area.
On the strut-braced wing with two struts per wing, the leading edge is not normally load carrying - the chordwise moment is directly carried by the struts. So why use the expensive 45-degree plywood? The primary reason is because the bias-ply holds shape better between the ribs with less tendency for warpage. The other reason is that it does offer some degree of structural redundancy. Even though the skin cannot resist the moment from all the design flight loads, there is a good chance that it would be sufficient to resist 1G loads in the event of a rear strut failure.
1 hr.
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( 3 / 229 )I cut and fit the nose stringer. The 12-foot stock I had was just a few inches short of extending from the root rib out to the wing tip bow, so I scarfed on a short extension.
I also ripped the 45-degree birch ply into wide strips for the leading edge. These strips were scarfed end-to-end to make one large piece for the leading edge skin. The plans call for two butt joints in the leading edge skin over heavier nose ribs with cap strips. Aside from the PITA scarf joints, I feel that the one piece leading edge skin will be easier to build and will form a smoother leading edge.
It is interesting to note that the 45-degree birch ply has a very pronounced sheen that the 90-degree ply does not. The 45-degree ply is mil-spec, whereas the 90-degree stuff is GL-2. I'd wager that the mil-spec material is more sensitive to needing the surface scuffed. Testing that I did with the GL-2 ply and epoxy didn't show much difference in bond between scuffed and un-scuffed.
2 hr
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( 3 / 209 )Cut and fit the lower aileron diagonals today.
1 hr
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( 2.9 / 249 )In my earlier post, I derived the performance envelope for the aircraft for which we now need to find the airloads. Let's go in alphabetical order and start with estimating the airloads in Condition A. We will determine the loads on the front and rear spars by summing moments about the rear and front spars, respectively. This is done on a spanwise per-unit-length basis using the local coefficient of lift in each of about ten wing segments.
Condition A represents Va, the design maneuvering speed. From the previous posts, the load factor, n, is +3.8, and the airspeed is 85 kts. The aircraft weight is 1200 lbs. One new piece of information we need today is that the wing weight is estimated at 220 lbs. This acts at the wing CG which I've estimated at 26.6% of the chord length back from the leading edge. This weight includes 90 lbs of fuel (7.25 gallons per side), the tanks and fittings and half of the strut weight in addition to the basic airframe and covering.
To get the loads on the front spar, we add all the twisting forces acting around the rear spar, and divide by the distance between the front and rear spars to find the force required by the front spar to keep things in equilibrium. There are four forces/moments at work in our simplified (CAM 04 Appx IV) analysis:
1) Force of lift acting at 25% of the chord
2) Force of drag acting at 25% of the chord
3) Pitching moment of the airfoil
4) Weight of the wing acting at the wing CG
;)
The lift force is defined as follows:
...where Cl_local is the local coefficient of lift, A is the wing planform area, and q is the dynamic pressure:
...which works out to 24 psf for this speed at sea level. FAR 23, Appendix A in paragraph A23.7(e)(1) states that we need to increase the positive loads by 5%. We also need to account for the angle of attack, as the diagram above illustrates. Multiplying the resulting lift force by the moment arm (distance from the rear spar to the aerodynamic center) gives us the moment about the rear spar due to lift. Using the chord length instead of the area in the equation above, and dividing the result by the moment arm of the front spar gives the force per unit length in the front spar. The equation looks like this, where rac and rfrontspar are the moment arms for the aerodynamic center and the front spar, respectively, about the rear spar:
Drag is made up two components, form drag and induced drag. The equation for the combined drag coefficient for our example looks like this:
AR is the aspect ratio of the wing, Cd0 is the zero-lift drag coefficient from the airfoil data (0.011), and Cl is the wing coefficient of lift. Coefficient of lift can be calculated using the following equation:
S is the gross wing area (135 sf), Wgross is the max gross weight of the aircraft (1200 lbf). This Cl computes to 1.4 which should, and does, come out pretty close to the max wing coefficient of lift of 1.38. Substituting that result into our drag coefficient equation gives a Cd of 0.105.
The equation to find the required reaction in the front spar due to the contributing component of the drag force is as follows:
To be continued...
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( 2.9 / 207 )I attached five of the noseribs today. I also glued in a few more of the aileron diagonals. All the top diagonals are finished. When the bottom diagonals are all in, it will be time to cut the ailerons free.
2 hours
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( 3.1 / 226 )I cut out the eleven nose ribs today. They were roughed out from 1/4" birch plywood with a bandsaw then matched to my noserib template using a laminate trimmer router bit that has the bearing. These were all clamped together and stack sanded. While they were clamped, I also cut the notch in the nose for the leading edge moulding.
2 hours
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