July, 2000 -- Wing Assembly
Here are my favorite tools for mixing the T-88 structural adhesive. I usually use a graduated cup like the one at upper left, available from Wicks Aircraft Supply. For mixing smaller batches, I've always used any old scrap of whatever I could find, like cardboard or plywood. But recently someone told me that epoxy should not be mixed on porous surfaces like this, as certain elements of the glue might be disproportionately leached into the material. I'm highly skeptical of this occuring to a degree that could cause a problem, but I am avoiding porous mixing surfaces anyway.
I've come across a slick (pun unintended, but I'll let it be) and permanent mixing surface -- 1/4" thick teflon sheet. In the photo above you see the 5" x 5" square I use. Each corner of the square is used for one batch, and I let all of the residue harden in place. Then it is a simple matter to take a chisel and slip off all of the residue, quickly making the board ready for four more batches. Next time you're at a plastics supply house, see if they have a scrap of Teflon for you. (I think this is where someone comes back and says that Teflon will leach solvents into my glue mix, making it unsafe? ;)
I also advocate the use of a round dowel for mixing the glue. I usually use 1/8" dowel with rounded ends (also in photo). During the stirring it is very easy to twirl all of the unmixed glue off the stick onto the board, and you can squeegee the unmixed glue off the bottom of the teflon with the side of the stick. Using a lot of twirling and scraping movements, you don't have to worry about the unmixed glue that tends to remain after mixing with things like popsicle sticks or square sticks. Wipe off the dowel with a dead paper towel in the trash can after each use, and you'll be able to use it hundreds of times.
While we're on the topic of misc. tips, I photographed some sandpaper tips that might be of interest to you. This shows how to make my favorite sanding block, and also shows a great way to store sandpaper.
After the hinge blocks were in place on my tip ribs and the 1/4" hinge holes drilled, I clamped ribs 5 & 9 to the underside of a tip rib and transferred the hinge holes through these ribs with the drill press. If the rest of the wing panel is built perfectly (in theory, right?) all four hinge holes will now line up in a perfectly straight line.
The ribs with aileron hinges are completed and ready to install onto the spars. Tip rib is on top, middle rib is the #5 or #9 rib. The root rib at bottom has the long block where the aileron pushrods will attach to the hole on the right.
FINALLY, all of the ribs for the first wing panel are varnished. A stack of varnished ribs is really something to admire.
The router table with the laminating bit makes quick work of trimming the 1/8" ply faces to the nose ribs and cap strips on the root and tip ribs.
And finally, out of the rubble emerges an airplane wing. Or at least a very good start toward an airplane wing... Still LOTS of work to go before this first wing panel can be declared done. It's difficult to appreciate from this angle, but this wing panel is HUGE. It's over 16 1/2 feet long, and this is less than half of the finished wing span. The center gap and the drooped wing tips will bring the total wing span to nearly 34 feet. Stock Tandem AirBikes have a 31'-4" wingspan. This extended wing will have about 8% more wing area than stock Tandems.
I've heard a lot of inquiries about what to do on the root ribs where the wing attach brackets protrude through the 1/4" x 1/4" verticals. Indeed, if the cut-outs are made as the instructions seem to suggest, the verticals will be cut clear through and the ribs will spring open. I don't know what ISON Aircraft's answer is, but the two photos above show how I dealt with the problem. I simply added extra 1/4" x 1/4" sticks to the areas where the fore and aft cut-outs need to be made, which preserved the integrity of the ribs around the cut-outs.
At Oshkosh "Airventure 2000" I saw this tug made from a Kolb Slingshot with an Ultratec 80hp (I assume) engine. I'm trying to contact the owner to find out how successful it has been as a tug. The tail feathers look small to me (tugs need a lot of pitch and yaw authority to overcome gliders that aren't following properly) and with the heavy engine and small wing I'm really curious how low of a stall speed this rig is able to muster. Hang gliders need to be towed at around 32mph, so a power-on stall speed over 26mph would seem to make it pretty marginal...
Also at Oshkosh I visited the ASAP booth and checked out their MZ line of engines. This is the MZ202 engine, which is now my #2 engine choice (Jabiru 2200 being #1). The 202 is 64hp with carbs and 67hp with fuel injection, weighing 88lbs with free air and 92lbs with the nice fan cooling shown here. I was told that the fan cooling works very well in either pusher or tractor configuration. Very simple and light engine. It produces more torque than a Rotax 582, and weighs less than a 503!
The front view shows the centrifugal clutch that connects the crank to the gearbox (not shown). The engine starts and idles with the prop still, and the clutch engages at 2000rpm. This protects the engine from the sometimes damaging pulse mismatches between the prop and crank at low rpms. It would also be a bonus on a tug plane, as the drag of the free-wheeling prop would make steeper descents possible.
Also note the elegant and compact starter at left. This is definitely a powerplant line to watch!
After Oshkosh I spent a few days at Raven Sky Sports in Whitewater, WI for some more hang gliding instruction. I made some good progress and am approaching checkout status for solo aerotow (in the glider, not the tug). In the background you can see the NorthWing T2 glider in which I enjoyed three tandem instructional flights. Raven uses the best tandem harness I have seen yet -- the instructor is suspended above and behind the student. This way the student is centered on the control bar and isn't even aware of the instructor unless he is talking to you. Very sweet setup which helps accelerate learning. The red Dragonfly tug is warming up for our first morning flight.
I got to pick the brains of some expert tug pilots here and learned a lot about aerotowing. I was happy to meet Les Taff, who is planning to use her FlightStar II ultralight as a tug. When she solos the FlightStar she gets a stall speed of 26mph, which she says is slow enough to be useful as a tug. This is good news for me, as I am confident I can get the Tandem AirBike down to at least a 26mph stall.
Notice the slotted ailerons on the Dragonfly -- these keep the ailerons flying and effective even at high angles of attack and slow airspeeds. I've become interested in building slots into the Tandem's wings, right in front of the flaperons in the outer 3 or 4 rib bays. These slots should help reattach the boundary layer over the outer flaperon surfaces when the flaps are deployed and at high angles of attack.
Not a lot of progress to report on the Tandem at this point... The diagonal braces have been installed between the left wing spars, and here the fuel tank is set into place just for fun. The slots for the wing attach brackets were cut into the plywood root rib before it was glued into place.