ISON Tandem AirBike Builder's Log
January - March, 2000
The plans, builder's manual and flight manual arrived today. Very impressive stuff. The plans are excellent and even include a full-size mylar plot of the wing rib. That's a far cry from the last airplane I built which had several distorted photocopies which had to be spliced together to form the rib template!
The only modifications I'm planning for my AirBike involve reducing the power-on stall speed so that it can be used to tow hang gliders and ultralight sailplanes. Wayne Ison (the designer) has been very accommodating about this. Not only did he make minor revisions in the plans to help me increase the wing span by one rib bay per side, but he also is having the pre-welded tail surfaces built extra large to enhance control authority in low-speed flight! I could get used to great service like this. With these mods I am hoping to see power-on stall speeds (with flaps) in the low 20's.
I started out wanting to put a Jabiru 2200 (80 hp) engine on this plane, but I doubt that I'll be able to afford it when it comes time to buy the engine. I talked to the one builder who installed a Jabiru on the Tandem, and he felt it was an outstanding engine for the plane. Not surprisingly, he said that it is capable of climbs that make you feel like you're going straight up. Having this enhanced rate-of-climb would be nice for towing gliders, too, but Wayne feels the Rotax 503 would still have all the power I need. I have a feeling he's right. With the Jabiru being more than twice the money and probably more than twice the complexity, I'll probably be leaning in favor of the 503 when it comes down to it.
I've only ordered the pre-welded kit (fuselage, tail feathers, etc.), the wing kit, the wing-folding kit and the wing tips at this time. I expect to receive these sub-kits in early to mid February. Until then I'll be preparing my workshop, studying the plans and starting the rib jig...
The pre-welded kit and wing kit arrived on Feb. 8. We're having fun now!
The welded assemblies are really nice, featuring what appear to be expertly executed MIG welds on the 4130 tubing. A little cleanup is necessary to remove very minor welding spatter around the joints, along with some minor grinding of the weld beads here and there. I know from my experience with MIG welding that this was done by someone with a lot of experience at it!
It would have been nice, however, if the fabricator had reamed the fuselage members after welding which are fitted with other tubing members by the builder. All of the tubing sockets (except for the landing gear leg sockets) were mildly distorted by the welding and wouldn't accept the mating parts without some reworking. I was able to straighten up the ID of the rear fuselage assembly socket with a 3/4" reamer -- before that I could only get the rear fuselage assembly in about halfway. In the absence of 5/8" reamers (which actually would need to be .627" or larger to accommodate the mating parts) I made a rotary sanding tool to ream out the 5/8" ID sockets. I took a length of 3/8" steel dowel and cut a 1" deep slot in one end with a hacksaw. The slot accepts a length of 1" wide sandpaper (from a used sanding belt), and then the sandpaper is wrapped around the dowel until it will just barely fit into the tubing. Chuck the tool into a hand drill, insert it into the tubing and grind away -- a trick I learned in my machine shop days which really comes in handy.
Above is a photo of my procedure for truing-up the landing gear legs. I made a fixture of two straight pieces of wood nailed together to make a 90 degree corner 60" long. This fixture was set on bricks and leveled, then the gear axles were clamped to the fixture with blocks that ensured the axles were level and coplanar from a top view. Then the fuselage was leveled as much as possible using a plumb-bob and double-checking with a level at numerous points around the fuselage frame. At this point two holes are drilled through the landing gear sockets and legs for the bolts which hold the gear on and aligned. Be sure to step-drill holes like this, folks. There is a lot of steel to go through here, and this is never what I would call fun. I used four drill bits from 3/32" through 1/4", wet with oil. With the rudder pedals installed, one can jump "on" the seat and start making engine noises after only a few hours of work.
I wish I had some wheels and tires now!
I spent the weekend making wing rib gussets. For the full report, visit my new Perfectionist's Guide to Great Gussets -- Fast. (OK, I'm going into detail and I vowed not to. So sue me :)
I have six wing ribs made now, the fuselage is up on its big funky tundra tires and tailwheel, and the rudder pedals are installed on custom teflon bushings. I added a brief addendum to the gusset page showing one way to cut rib sticks to length.
I noticed that when I was taking the ribs out of the assembly jig (above) the trailing edge wasn't quite secure enough with its one staple holding it together, which allowed it to deflect upward slightly and inconsistently. So I made a simple jig (below) which holds the bottom edge of the rib straight while the glue cures. One of the cam clamps holds the trailing edge joint together, and I don't put glue in this joint until the rib is ready to go into this jig for the night. This system works quite well and the ribs are coming out perfectly straight.
This is my young friend Grady, who has taken me up on an offer to help with the Tandem project as often as he likes and then help fly it when it's completed. We got off to a slow start on his first visit, completing only one rib, but my completion package just arrived from ISON and now projects abound for us to work on next time...
Luckily, with the passenger seat all the way forward Grady can just reach the rudder pedals! Notice the new tundra tires on wheels from ASAP. Manly, yes. Low-drag, no. Thanks to Robert for the idea anyway. Since this plane has no suspension to speak of, I like the idea of bouncing on all of that air (hey, I never bounce on landing, but Grady might ;)
My new Tandem-building buddy Robert discovers the joys of using a 6" edge sander while touching-up some resawn compression members for his spars. This is yet another too-small-to-build-an-airplane-in workshop, and you have to crawl through the Tandem's cockpit to get to the sander!