|Oct 21, 2012, 09:26 PM|
Consider that if you do not cut the cf spar caps to 32" per the plan and leave them at 36", and if you cut yourself some extra ribs and shear webs, you can have an 8" overall span increase for next to no money with almost 80 sqin extra wing area. That is what I did on my Houston eHawk and I must say it is delightful.
Also measure the plan carefully, mine had a nasty case of replicative stretch, so builld to dimension, not on the plan, forewarned! I also noticed the same problem on the FFHstab halves being somewhat different, again, forewarned.
Use 1 hour epoxy with cabosil filler when assembling the spar. Also clean the CF spar caps with alcohol or acetone before assembling the spar.
|Oct 21, 2012, 10:32 PM|
Great ideas and tips. Thanks, Ray.
I used 30 min epoxy thickened with cabosil for the mid panel spars of my Allegro Lite build this summer and that was scary short time to get all those webs and ribs lined up before the clock ran out on those hot evenings. I was able to do it, but not without a lot of anxiety. I moved to 60 minute after those mid panel spars. That was breeze by comparison -- place a rib and a sheer web, have a sandwich, set another, take a stroll around the block, read a book, place another, etc.
With the extra 8" span, is the rudder/fin and stab still sized right, given the tail moment on my HH wood fuselage design (the total length of the fuselage+rudder is right around 50 inches)? I seem to remember reading in one of these threads that the HH stab span could be shortened by one bay on each side. Is that recommended? The stabs on newer planes are so much smaller than they used to be, to my eye.
Of course if I extend the main panels by 8 total inches, my storage problem for this big plane only gets more complicated! Plus that 80 inches would need another roll of covering too. By the way, how many rolls does it take to cover a Houston Hawk? I forgot to budget for that.
Is there a place where I can find target weights for the pieces of the HH so that I can keep track of whether or not I am building light enough?
Sorry to ask so many questions. Soon I will be cutting and gluing and re-cutting and re-gluing.
|Oct 26, 2012, 08:05 PM|
Hey Houston Hawk folks,
I received my fuselage laser-cut parts this week from National Balsa. They are perfect. I strongly recommend the folks at National Balsa. I know it's a cliche but man, isn't laser-cutting the best! Talk about improvements in the hobby. I am starting the build this weekend just as the hurricane is approaching the mid Atlantic where I live. I'll be building by candle light next weekend I bet.
I have decided to build the Houston Hawk wing and the Yardbird fuselage with the wing adapter I have drawn up. I have also ordered parts and wood for the Yardbird pod. I think this will give me the most options for use with either the Houston Hawk or the Yardbird wingset.
One question for the group wisdom: how far does the 1/2" aluminum joiner rod extend into the wing (how long is the aluminum rod altogether)?
>> Answer from Jack: About 5"
|Oct 29, 2012, 04:41 PM|
I used oak blocks that were drilled 1/2" and split for the ends, and a piece of 1" steel rod for the center. I chucked all of this up in a large vice and put the squeeze on it until it was a couple of degrees beyond where I wanted it. It sprang back to perfect...
|Oct 29, 2012, 04:59 PM|
Thanks, Jack. I'm trying to visualize it. Make an oak block. Drill it 1/2". Split it exactly in half along the line of the drilling. Place one split block on the right of the bend, one on the left. Place the 1" rod in the center. Put the whole thing in a big vise and start turning the crank. No heat. Pass the goal by a few degrees and then let off the pressure. 7º bend.
Do I have it right?
if so, all I need is some hard oak and a big vise. I have a steel tube breaker bar (I used to work on car engines for fun) could that work somehow or shall I just go buy a big vise?
|Oct 29, 2012, 05:14 PM|
With the hurricane bearing down and about an hour left of electricity, I decided this weekend to start the fuselage build. I have been taking photos as I go. Should I put them here (I'd kind of be hijacking the thread) or start a new HH-Yardbird wood fuselage build thread? I want to be part of the community but not step on Jack's plans for his own design. Opinions welcome.
ps. As always, the devil is in the details. There are things that my drawings did not anticipate and fits that the laser-cut parts (perfect-to-drawing-but-physical-reality-differs) did not anticipate.
|Oct 29, 2012, 09:26 PM|
hi there from Toledo
Welcome to the world of scratch building and prototypes. This is where partially baked ideas and designs are tried, proven, and refined, when necessary. This is the development side of R&D. This is where I have spent a career in engineering, mostly fun and interesting but sometimes not so much. Oh, well...
|Oct 30, 2012, 04:27 PM|
Power is back after the storm. Pretty rough yesterday but it seems to have calmed down by midnight. We lost power around then and got it back just a bit ago. It's definitely easier to build by light than by flashlight!
So here's the build.
I decided to start with the Yardbird wooden fuselage since with the wing adapter I drew up, it can be used for both the Hawk and Yardbird wing sets.
I ordered laser-cut parts for the Houston Hawk fuselage before I made this decision but they are so similar, modifying it to work for the Yardbird design is not difficult--mainly formers. I also drew up the Houston Hawk cut sheet to include most of the parts for the Yardbird Pod. I ordered the remaining Pod parts at the same time so I have the parts I need to build both the Yardbird fuselage and the Yardbird pod and use either with both wing sets in the future.
All that laser cutting and wood for the parts, balsa sheets and sticks, spruce, and ply cost about $140 including shipping from National Balsa. What they sent seems excellent.
I ordered the short kit for the Houston Hawk wing. I thought I'd start there. I also used the materials list I found here in the build log for the Hawk to order all the other materials, CF, brass tubes and aluminum rods, TOW, fiber glass, etc.
I have attached the materials list with vendors and costs. Currently I have spent about $300 on the project and I think all that's needed is covering now.
|Oct 30, 2012, 04:43 PM|
I began with the wing adapter that should let the Houston Hawk wings mount on top of the Yardbird fuselage. I'm sure I'll probably need to build this at least twice since there is a chicken and egg issue with this part fitting the wing and fuselage perfectly. I could have waited until the fuse and wing were built to make this but I had to do something small to get the project going.
Since I had ordered parts for the Houston Hawk fuselage design, which uses laminated 1/8" lite ply for wing fairings, I decided to use those pieces to cut matching Houston Hawk ribs for the adapter. They're oversize so the ribs could be cut from them pretty easily. I carefully located the holes for the joiner rod and incidence pin and drilled out the set of ribs with a slight offset downward in the central laminates so that the bend joiner rod would fit.
The internal four ribs were laminated with Titebond then wrapped in 1.5 oz. glass wetting with 30-min epoxy before then epoxying the laminate in place. This will serve as the bolt beam for the wing.
I'm using a 1/2" 6061 T6 aluminum rod for the joiner. The adapter is built up to the point where the bent adapter is inserted and potted with cabosil-thickened epoxy. I need to find a way to make the 7º bend and then I can insert it and set it up, finish the sheeting, then glass it.
Attached are the plan for the adapter and the photo of where I am. The rod is just slid in unbent for photo purposes.
|Oct 30, 2012, 04:52 PM|
The fuselage sides for the Houston Hawk design are only slightly different from the Yardbird design. Mainly in the wing mount area and the Hawk's fuse is just a bit taller in that section so the Yardbird shape can be easily cut out from the Hawk side pieces. Knowing this, I continued ahead.
The fuselage sides have several thin ply doublers and a thicker 1/8" lite ply doubler in the wing mount area. I laminated these using Gorilla glue weighing them down with bricks while the glue set.
I chose to use the option of strips of CF and 1/64 ply doublers along the tail of the sides (first laminate the ply. The sand a taper in the front edge of the ply. Then the CF strips on top of those) and top since this is a prototype and I have some concern about the torsional stiffness of the sleek tail. I used 1/4" strips of 0.022 CF for the sides. These run from F5 to the rudder post centered on the inside of the sides and top. I didn't sand a channel in the sides to accomodate the strips so they interfered with the placement of the spruce longerons. I solved this by butting the longerons up against the edges of the CF and letting the spruce exceed the top and bottom edge of the side ply pieces. I then cut off the spruce to match the side contour. The loss of strength from the thinner spruce longerons should be at least partially taken up by the CF so I think it's OK. It's an improvisation at best and I'll redraw the sides to account for the need to address this. I could have cut the CF to a taper but it was more than I wanted to get into with the CF.
I'll describe the fin-stab pylon next. It's shown in the photos below.
|Oct 30, 2012, 05:04 PM|
Since the design of this fuselage is owing to the Sagitta, I have used the Sagitta's fin-stab pylon idea. Basically this is a lamination of hard 3/32" balsa that creates the core of the fin and the hollow section for the bell crank that actuates the full flying stab.
The laser-cut parts worked perfectly for this with the one exception that the bell crank lower arm needed to be longer to ride just above the bottom of the tail. That position would have made the control rod installation simpler. The bell crank is made from two 1/32" ply parts that are sandwiched on CF tow for reinforcement. The resulting thickness just fits smoothly in the cavity of the fin core. Parts of the fin core extend into the tail fuselage sides to strengthen the fin-fuselage structure.
I had to carve up the tail pieces a bit to get smooth runs for the rudder and stab control rods, which are wire running in tube sleeves. It looks pretty ugly but it will all be magically disappeared with sanding, rounding, and other forms of slight of hand.
|Oct 30, 2012, 05:24 PM|
One of the nicest things about laser-cut parts is not having to cut formers. I decided that for the prototype the formers forward of F5 would be two cross-grained 1/8" lite ply laminations. F6-F8 are lite ply laminated to balsa. All of these options are in the part cut sheet making assembly of the formers a pretty nice chore by comparison to days of old.
Since the 1/8" lite ply is not quite 1/8" thick, I laminated a third former layer of balsa then sanded it down in thickness for formers F1-F5 so that the 1/4" thickness could be met.
I began by gluing the formers to the right fuse side being careful to get the angle right so that when the sides are joined, the formers are perpendicular to the center line. Of course, I botched it in a couple of cases. I got too hurried with the epoxy. I used 15 min epoxy thickened with milled glass for these and used the thick epoxy to form fillets front and back of the formers from F1-F5. Formers F4 and F5 carry the wing mount bolts so those took some extra work. F4 and F5 required some adjustments to their width, which I messed up so F4 is bonded to the doubler rather than to the side against the doubler. Live and learn.
Having just built an Allegro Lite, I went with Dr. Drela's approach to the wing mount bolts. Formers F4 and F5 have slots cut into them to tightly hold a metal nut for 10-32 nylon wing bolts. F4 and F5 have ply laminations on their top portion to bolster where the vertical holes are drilled for the bolts. Once the holes were carefully drilled, the nuts were slid into place and glued. Then the former was glassed with 1.5 oz. glass for added peace of mind. The idea is that the wing bolts are carried by the formers and the formers are bonded to the fuselage sides where the added 1/8" side doublers are. F4 is also bonded to a lamination of 1/8" lite ply that forms a beam along the bottom into which the tow hook is screwed. This creates a lite ply box with an added beam along the bottom to transfer tow load through the fuselage to the wing through the bolts. I hope it works for winch launches for this big plane.
|Oct 30, 2012, 05:29 PM|
Design corrections so far
I am keeping tabs of the things that need to be fixed on the drawings. Here they are so far.
F3 from Houston Hawk fuselage is cut too short for Yardbird design
F4 needs dimensional work accounting for 1/8 doubler and tri and for the final dimension of the tow hook beam (1/8" lite ply is not quite 1/8" width). - FIXED IN NEW DRAWING
F5 needs dimensional work accounting for 1/8 doubler and tri - FIXED IN NEW DRAWING
F8 needs dimensional work
Fuselage top pieces under the wing saddle need dimensional work
Stab pivot in the fin-core/stap pylon should be 5/32" brass tubing and not 1/8" The laser cut holes in those parts need to be resized.
Bottom needs to be slightly wider around F4-F5
Fin LE post cannot continue into fuselage due to control rod passage
Control rod holes in formers need to be relocated and lined up straight
Right fuselage side at tail need to have the control exits laser-cut cleanly
Bellcrank bottom arm needs to be longer by about 1/4"
Increase height of nose area under the canopy for added strength back to F3
Fuselage sides doubled with CF need to have that dimension accounted for in spruce stringers
Internal 3/32" balsa fin core front piece is mitered incorrectly to fit with top piece of the fin core.
The tab that joins the last section of the tail bottom needs to be enlarged by 1/64" all the way around to fit tightly (laser cut leaves slot slightly oversized).
Remove the quick inward curve at the bottom of the nose. The doubled ply cannot make that radius easily.
The stab pivot tubing in the fin core should be 5/32" not 1/8". The pivot holes in those parts need to be resized to 5/32.
The fuselage top pieces under the wing saddle need to dimensional revision
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