ISON Tandem AirBike Builder's Log

August, 2000 -- Not much visible progress


I don't have a lot of airplane building to show for the month of August. After getting back from Oshkosh, I had to catch up with lots of work and general "life" stuff, and then I had to dust off the ol' resume and go get a job. Now I'm on my way to being able to purchase my engine and parachute!

Speaking of the engine, I did spend much of the month of August researching various engines for my Tandem. I sought and received advice from dozens of people -- aviation newsgroup and mailing list lurkers, engine dealers, AirBike owners, tug plane pilots, aircraft designers/builders, prominent engine repair shop proprietors, propellor manufacturers... It's been a long, twisted and frustrating search, and I'm here to say that I've come full circle and have settled on the Rotax 503 for my Tandem's powerplant. Yes, that is the engine that the Tandem was designed for -- go figure.


At one time I had decided to use the 4-stroke, 80hp, air-cooled Jabiru 2200 (above). Some factors that swayed me away from it are: high prop speed, inability to swing a large prop, heavier weight, increased installation complexity and high cost.

Then I decided on the 2-stroke, 65hp, water-cooled Rotax 582 (above). Factors that swayed me away from it are: reliability and maintenance concerns, installation complexity issues, and weight. I still consider the 2200 and 582 to be good second choices, though.

The MZ engine line (above) remains very appealing to me. These engines are being brought to market very carefully and deliberately, and I very much look forward to seeing how they perform on test vehicles and in the marketplace.

The 4-stroke, air-cooled, 60hp HKS 700-E deserves honorable mention here, as well. It's a very attractive engine as well, and right now I can't think of any factors that swayed me away from it except for the higher price (almost as much as the Jabiru), lingering doubts about its true power/thrust output, and the current lack of availability. But I'm going to be watching Robert's experiences with his soon-to-be-manifested HKS with great interest.

Someday I'm gonna work up some charts and graphs that list the significant aircraft engines in the 50 - 80 hp range so we can compare their performance specs at a glance. Watch this space, but don't expect it any too soon (I have an airplane to build first).

So here's the winner -- the good ol' 2-stroke, air-cooled, 52hp Rotax 503 (above). I hear only good things about this engine, and several people have expressed to me their belief that this is the best 2-stroke aircraft powerplant there is right now. It won't have quite the power of the 582, but the reduced weight will help make up for that a little. I understand that Flightstar uses the 503 on their tug model, so that is reassuring for the limited tug use my Tandem will see. Factors that swayed me toward the 503 are: light weight, good power/weight ratio, low cost (relatively), low installation complexity, a healthy, geared torque that allows it to swing a big, efficient prop, and an awesome reputation for reliability.

Of course the fan assembly has to go, to be replaced by a Flightstar Supercowl (see the one on Tariq's Tandem if you haven't already). The way I understand it, the fan's intake is on the appropriate end of the engine for pusher installations, but not for a tractor configuration. Tariq had cooling problems with his tractor-mounted 503 until he removed the fan and installed the Supercowl, so I'm benefitting from his experience in this matter.

I received several reports about the Powerfin being the best prop for my use, so I called Stewart Gort of Powerfin and had a chat with him. He's a busy guy, but was generous with his propeller knowledge between interruptions. Even though I could swing a 74" prop because of the big tundra tires, he thought my best combo would be a 3-blade 70" "F" prop, mounted on Rotax's 3.47:1 gearbox. A 74" prop would suggest the use of the 4:1 gearbox, and Stewart recommends avoiding the 4:1 and 3:1 gearboxes because of an "increased propensity to resonate" (vibrate). The prop/gearbox combo he suggests will provide some rather ideal numbers for prop pitch and tip speeds, so I'll be happy to take his advice and use those components. A highly-efficient prop/gearbox combo will help me squeeze the highest amount of performance out of the 503.

Bottom line regarding the engine choice: I'm happy to have settled on a relatively low-cost powerplant, and also to have a nearly drop-in engine installation. I just received my 503 installation kit from TEAM, and it looks like the "height" of simplicity.


I was able to get in one day of work on actual airplane building this month, which was enough time to get the trailing edge (TE) installed on the left wing. I ended up jigging this procedure just like I did on the Ragwing Ultra-Piet: As shown above I blocked the trailing edge off the table (with the front spar held against the table's straightedge) and clamped some unused rib members to the bottoms of the ribs to align the TE stock with. It seems to me that getting the TE stock in a straight line with the rib bottoms is the most important parameter here, and this handily accomplishes that. With a layer of waxed paper between the parts, the TE is simply clamped to the auxiliary stick on the bottom of each rib until the glue cures. Then I come back later and attach the ply gussetts to these joints.

The procedure in the builder's manual has you nailing the TE down to the table along the straightedge and then butting the wing assembly up to it. This sounds like an attempt to produce a nice straight TE, but I feel that a straight TE is accomplished by consistent ribs and a true rib/spar assembly, and that there is little you can do after those major components are assembled that will affect the straightness of the TE.

Why does any of this matter? It's just one of my hang-ups. Whenever I'm ogling someone's aeronautical handiwork, one of the first things I do is sight down the trailing edge to see how good of a craftsman he is ;-)

Hopefully this horrible heat wave will pass soon and I will have more to show in September! Working in a 100° workshop isn't very conducive to airplane building...