jeffp51

This is my first attempt at a build thread. The biplane in the pdf below is my own design drawn on ancient modelcad software. Let me give a little background. About 8 years ago I was part of a National Guard unit that was transitioning to air defense artillery. We inherited a huge stack of target dronesthe Mig-23 floggers and it was decided to get rid of them. I rescued one and gave it a home. Unfortunately, I didn't have space at the time for the airframe, but each plane came with a K&B .61which I saved from a certain death. Ever since, that engine has been waiting for a home. This design is my attempt. I suspect that I have too much wing for it, but I am looking to build a light as I reasonably can. I hope for a flight envelope similar to my Big Stik .40not full 3-D, but relatively gentle landing characteristics.

I have also drawn a full set of templates. My son's junior high shop teacher has a laser cutter at the school. Wood should be here this week, then we will cut as soon as possible after that. The local copy shop was able to print two sheets for a total of 8 dollars.

I am fairly confident that it will fly. This is my second biplane I have designed. The first died due to pilot error, but some of the lessons I learned from it have been incorporated into this one. But this is not a ARF fan's plane. There will be plenty of sanding and carving and careful work required. I really enjoy the building process; for me it is at least half of the joy of the hobby. I have never owned an ARF because seeing an idea develop into a plan and then into a creation that actually flies is so much more rewarding than paying for immediate gratification, that I cannot see the point.

So much for my soapbox. I hope I am among friends here. Tell me what you think.

jeffp51

My wood has arrived. I hope to get it laser cut sometime in the next week. Some of the wood is good quality, some is a little warped (normal, not too bad) but some of it is really heavy. Balsa seems to get more and more like basswood every year. I guess I could have spent extra to have the stock hand-selected, so I can't complain. Since my tail is relatively large, it may be good to have stiff wood.

davidgeorge212

You designed a neat looking biplane. Im right there with you about building your own planes, I think that is the best part also. Do you do your own laser cutting or are you sending your plans out and having someone else cutting your parts? Up until now I have always cut my airplane parts using a scroll saw and bandsaw. I have autocad and Pro-e and minimal experience with a laser cutter. I have been wanting to buy one so I can start cutting parts out on it.


David

jeffp51

My son's shop teacher has a laser cutter at the school. I am going to take my wood to him and he will help me cut it. This will be my first attempt at a laser-cut plane as well, so I am curious to see how it turns out. Normally I use my scroll saw.

By the way, I teach German in my day job, so Falke is German for falcon.

jeffp51

today was cutting day. I almost succeeded in cutting out all the parts on the laser cutter. We had a few issues with the scaling for some reason. Pretty sure I got them solved, but I will have to order a new sheet of 1/16 ply.

These ribs are not for a tapered wing. They are supposed to be all the same size.

Mtthwacrss

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Keep it coming.  I love building also and look foward to seeing how your plane comes out.

jeffp51

I made a fair amount of progress this weekend. But since I spent my time building instead of posting, the pictures will come later. I got the tail feathers cut out, the fuse sides, and attached the fuse doublers to the fuse sides. The laser cutting has already saved a bunch of assembly time, and I think it will help in building a straight plane.

I have the plans in PDF format and I have been trying to decide what to do with them. I don't have great aspirations of making any money by selling them, but am not very excited to just set them loose in the internet. I don't even know if anyone would be interested in them. I kind of like the shareware concept, where people can pay what they think they are worth.

jeffp51

Here are some pictures of my progress on the fuselage sides and doublers. It took an extra two trips to the copy shop for the template, because when I printed one out on my home printer, it came up more than half an inch shorter than the copyshop plans. The first trip back I had my own print sizing mixed up, but the second time was perfect. Luckily the local shop is much cheaper than kinkos. My full sized plans on two 36x48 sheets was only 8 dollars. the templates were $1.60 each.

For the doublers, it is important to put in the alignment lines first, as there is a top and a bottom doubler ( I suppose these could be connected without too much trouble, but I didn't think about that when I was drawing the plans). The bottom doubler aligns with the airfoil cutout, and the top is 1/8" above that. There will be a top and bottom former that are inset flush with the top of the sides, so care has to be taken to keep the spacing even. F-4 and the firewall can be used to help set up the alignment.

I also cut dove-tail notches in the bottom of the F-4 former so that it interlocks with the bottom former. This was something the laser cutter could not do, since the dove tails are at an angle.

jeffp51

Here is the template for the fuse. Both sides are held together with adhesive spray and tape

jeffp51

and some more pictures of the fuse in progress. I had to resort to an old-school exacto knife for some of the cutouts, because the jigsaw could not reach.

jeffp51

Here are the pictures of the stab and fin. Like I said before, the wood is hard and heavy. I have planned to cut lightening holes in them, but I want to wait and see how it balances. I would be interested if any one knows of a website that can calculate balance points. I know how to figure cg, but how do I calculate where it will balance given known weight for motor, fins, wings, etc as a product of the moment arms between themie given a 1 pound motor 9 inches ahead of the cg, will a 7 oz tail balance it with a moment of 39 inches to the tail? does anyone understand what I am trying to ask?

I also wonder if a standard servo will be strong enough for the large control surfaces on this plane. On the plans I have a large and a small elevator drawn. I went with the large here, but I am not sure if standard servos will be overtaxed.

jeffp51

I have three dimensions now. Before assembling the fuse I decided to reinforce the last section of the fuse with some carbon fiber, because the cross grain seemed likely to break out. Originally I had planned to build the sides upside down over the top former. this proved to be impractical for a couple of different reasons, mostly that it was too hard to align everything. Instead, I glued in the central formers where the box is untapered. Then I glued in the top former and brought the trailing edge of the fuse together. the center former can then be slid into place.

jeffp51

The bottom former also has a dovetail tab built into it like the top former. I had to design the top former in two pieces just because it wasn't practical to buy a single piece of plywood that long. (and it would not have fit on the laser cutter). But this also means I had to cut in a dovetail on the bottom of F-4. It is a silly thing, but I like the way this part turned out. the fit is good and it feels like it ties everything together nicely.

The 3/8" triangle stringers on the back of the fuse are there so that I can round off the bottom of the fuse. Unfortunately I didn't account for the ply doublers and so I had a gap under one of the formers that I had to adjust.

With the main parts of the fuse put together I can start adding the mounting blocks. I always like to put in the blind nuts before I glue in the parts. Both the landing gear block and the firewall are reinforced with 3/8" triangle stock. Altogether the laser cutting has allowed the fuse to go together very straightbetter than a number of other planes I have put together in the past. I have a tendency to build bananas that I have to try to straighten out with a sanding block.

flyingagin

Good looking work.
I am looking foward to seeing the progress and finnish.
Keep up the build thread. I have one on this site that is also my first. I really enjoy doing the thread. Started another build thread on another site today.

Ken

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jeffp51

I have spent some time cutting wing pieces. this is where I begin to question the swept wing design. If I had a flat wing, I would need half as many pieces since they could reach full span. Instead, I have to make a left and a right of everything, plus a top wing and a bottom wing. I count 28 span-wise spars and stringers between the top and bottom wings.

I was also able to get the ribs laser cut today. It went much smoother today compared to last time. Everything looks pretty good

jeffp51

Here are pictures of the wing tube and sleeve. The tube is a replacement part for some airplane on tower hobbies, and Extra, I think, but I don't remember now. But it didn't come with a sleeve, so I had to make my own. I am sort of proud of how it turned outsilly, I knowso I am going to explain what I did: I started by wrapping the tube in plastic wrap so that the sleeve would release after it was glued. I cut the paper from a grocery bag by spiraling the cut around and around the bag until I had a two-inch wide strip about ten feet long. This I wrapped diagonally around the wing tube and plastic while soaking the paper in wood glue thinned with water. I wrapped it a second time in the opposite direction. When it dried, the paper tightened up around the tube, but it came out relatively easily. Then I had to sand the outside smooth, and of course it was still too thick for the holes in the ribs by just a little bit, so I wrapped a drill bit in sand paper and put it in the drill press (poor man's drum sander) and sanded it until everything fit just right. Total weight was 7 grams.

jeffp51

More progress on the lower wing. The first thing I had to do was assemble the trailing edge/ ailerons. Because of the shape of the airfoil, any single piece of stock even if I had been able to find one the right shape would have been very heavy, so I chose to design a built-up TE. I was about 10 shy of the little triangle pieces somehow, but they were easy to make without the laser cutter, so it wasn't a big problem. the main spar of the TE has to be tapered on both sides so that it forms an isosceles triangle, which will allow the final angle of the piece to sit straight on the back of the wing. What you don't see in this picture is the solid balsa filler needed for the wing bolts or for the clevis mounting. I got all excited about gluing them together, that I forgot to add them before I put the top on. Instead, I had to cut open each spot and add filler and then sand back smooth.

There is also a 1/16 x 1/2 plywood strip between the top and bottom sheet to harden the TE after everything is sanded to shape. The top sheet should probably be sanded before it was glued on, but it seemed a little easier to glue it on firstwhich caused a big flat spot on the last 1/2 inch. Most of that can be quickly sanded out. If the ply were set back from the TE by 1/16" or so, then the sanding would go even better.

jeffp51

You would think that I could learn to keep my thumbs out of the way. when trimming the filler stock, my knife slipped yet again and I sliced into my thumb. Good thing we humans have evolved thumbnails to stop x-acto blades with. I wish I had a dollar for every knife scar on my left hand.

Assembly of the wing was fairly straight forward. Due to the sweep, each spar slot has to be sanded a little to account for the angle. Each rib is tabbed to keep it at the correct alignment, and I shimmed the LE and the TE to keep them straight across the entire span. Everything else is standard D-tube construction. Sheeted front section top and bottom, vertical sheer webbing butting against both ribs in each bay across the full span and cap strips on top of each rib. I decided to extend the plywood spar that holds the wing pegs out an extra rib bay for added strength, since the sweep makes it hard to reinforce the center joint. I extended the center sheeting for the same reason.

With the wing mostly framed (I still need to add the bottom sheeting) I can start to get an idea for the overall shape of the plane. Things are starting to look a little like an aircraft. The tail still looks big, but I don't think that will be a bad thing.

jeffp51

One more not today on weight So far I am under my weight expectations. I did calculations for wing loadings between 18 and 22 oz/sq. ft. The low end would mean a weight of 7.3 lbs, and the top end would be 8.9 lbs. My fixed weights (wheels, engine, battery, etc.) plus the rough (unsanded) parts built so far come out to 5.5 pounds. If the top wing is about the same weight as the bottom, the frame without covering will be just about 6.3 pounds, which means I just might be able to get this bird in under 7 pounds. Since I don't believe the K&B .65 is going to be a real powerhouse, and since I would like landings to be gentle, the lighter it ends up being, the better.

jeffp51

slow but important progress. I sheeted the center section of the lower wing, glassed it, and added the mounting pegs. I also installed the mounting block to the fuse, and drilled the bolt holes and tapped them for 1/4-20 nylon bolts. Next I sanded the wing tip blocks to shape and hollowed them out to save weight. It looks like I that may save me all of 5 grams. It doesn't hardly seem worth it, but I suppose it all adds up. there is a 1/32 ply insert at the TE to keep it strong. I also did some work on control surfaces. I cut out the ailerons from the TE and tapered their LEs. The elevator halves are joined with 1/8" music wire.

jeffp51

for the next section, I need some advice. I built the N-struts and reinforced them with carbon fiber, But they look a bit tall in real life as opposed to the plans. The goal was to get the top wing one cord-width above the bottom wing, or about 9 inches, to reduce interferance between the two wings. Aesthetically, I don't think I like it that high, and I worry about an upward pitching moment with the top wing so high above the thrustline. I think with some semi-major surgery, I can lower the top wing an inch or so (probably closer to 3/4 of an inch) by cutting off the bottoms of the N-struts, but it would also mean cutting my already laser-cut wing struts in half and taking a chunk out of the middle. Do I make the change, or leave it as-is?

jeffp51

I have a taped together plane now. After I roughed in the top wing, I cut apart the center section. I also cut down the wing tube by 2 inches, since it extends past the ribs, and therefore adds weight, but no strength. I saved 3/4 ounce. Next comes attaching the trailing edge, cutting out ailerons, and more fun with wingtips.

jeffp51

Sanding, sanding, sanding. Not much to show, but a lot of work done over the weekend. Wing tips are hollowed out, balanced, and sanded to their final shape. I hope balsa dust isn't carcinogenic. I wrapped the wing struts with carbon fiber tow and epoxied them with finishing resin. The stuff sands pretty well, without fraying like fiberglass does. I am not sure how much strength it will add, but maybe it will reduce side-to-side flexing. Adding wire cross bracing would keep things perfectly rigid, but also add to set up complexity, which I would like to avoid.

All up weight is approaching 7 pounds.

Views of my thread have now passed 1000. Comments would be appreciated.

dmbaerry

Plan looks cool.. will be following your progress..

jeffp51

some pictures of my progress. I installed the carbon fiber wing joiners, attached the wing tips and sanded them down. After giving the cabane struts an airfoil shape, they were wrapped in carbon fiber tow and impregnated with finishing resin.