ISON Tandem AirBike Builder's Log

November, 2000
Towlines, new friends and HEAVY ailerons


I had been planning to set up a V-bridle arrangement for the tow line like the Bailey-Moyes Dragonfly uses, but then it occured to me that the Dragonfly is a pusher and the Tandem is a tractor. Will this be a factor?

After some rough sketching, I got concerned enough to draw the towline system in AutoCAD. This confirmed my concerns but still wasn't convincing enough, so I made a little 2D model of the fuselage and attached a fishing line V-bridle to the rear end. Sure enough, this is what I found (click on the image for an enlargement):

With this arrangement the towline is above the normal thrust line, and it appears that the glider would pull the tail of the tug downward, forcing the tug into a high angle of attack. Probably not a good idea. It's clear to me now that the V-bridle arrangement was intended to center the towline with the pusher engine's thrustline on the Dragonfly and is not as relevant to my application.

So I called my tug-pilot friend Les to ask how she has her tractor Flightstar II set up for towing. Turns out she has a single-point attach for her towline, located four or so inches below the Flightstar's thrust line, and it works just fine. What a coincidence -- the tow hook I'm planning to install just above the tailwheel happens to be about that distance from the thrust line of the Tandem...

So I'm happy to find simplicity winning in this matter. The drawing above shows how I'm planning to set up the towline now. An aluminum tube will mount between the fuselage and the tailwheel, and will hold the tow hook far enough aft to keep the towline away from the rudder. This seems like a good time for one of those pertinent quotes:

In anything at all, perfection is finally attained
not when there is no longer anything to add
but when there is no longer anything to take away.
Antoine de Saint Exupery


Southern airplane tour, Part 2

Over the Thanksgiving break I was very fortunate to meet three very fine Tandem AirBike fellows in Alabama, and I'm pleased that they are my new friends. Above, John Vincent of Huntsville shows me the beginnings of his Tandem fuselage on its wooden jig. Photos of John's wings and tail group are available here on the ISON site.

John shows me the small Harris aviation torch he's using on his tail feathers and fuselage. He sure got the hang of welding quickly -- his finished joints are nothing short of beautiful.

John also brought up a concern about the full-span RS-17 1/4 x 1 1/4 stringers that are located just ahead of the ailerons. One builder had already warned me that his had come loose in several places on his finished AirBike, and John had heard reports of this also. More on this subject below.


The next stop was with Bill Bailey of Pelham, AL. Bill is enjoying the heck out of his beautiful single-place AirBike, and has a good start on a stretched-height Tandem powered by a Rotax 582. He has already been generous to me with his airplane-building know-how, and I'm very pleased to have him to exchange ideas with as we both begin the task of installing the 582. Bill does beautiful work, and seems to be a very hard and efficient worker -- he gets a lot of airplane built in a short period of time!

Here Bill holds one of his Tandem ribs, which he assembles with 1/4" brass aircraft nails instead of staples. He leaves these cement-coated nails in the ribs, instead of removing them like we do with the staples. He also cuts his gussets to shape with lightly serrated hand shears. I had tried this early on with some shears, but mine made such a mess of the plywood that I discounted it as a bad idea. Bill gets a beautifully clean and crisp edge with his shears, which resemble the ones you may have seen advertised on late-night TV cutting pennies in half.

Bill talked me out of buying an airbrush to paint my fuselage with. He has one and doesn't think it would be up to the job. Instead he showed me a small touch-up spray gun he has, and suggested one like that for painting the fuselage tubes instead. Sounds like good advice to me. I bought a cute little $20 sand-blasting gun that I'm going to prepare the fuselage with -- more on both of these tools when I begin painting the steel parts.

Bill also told me that although Wayne hasn't personally seen a 582 on a Tandem, he thinks a heavier engine boom tube would be a good idea to compensate for the increased torque. After seeing Richard's 503 shaking all over the place (below) I second that emotion. The stock engine tube is 2 x 2 x .125 square aluminum tube. Bill is planning on getting some 2 x 2 x .25 tube from Wayne, but that sounds like overkill (too much weight) to me. Jorgensen Steel has 2 x 2 x .188 6061 tube, and that is what I'm planning to use. Bill says he's gonna use a B or C gearbox and move the engine forward slightly to accommodate a side-mounted starter on its back side.

"Has Richard told you about his heavy ailerons?" Bill asked. "Check out the spades I made for him while you're down there."


Richard Stumpf of Ozark, AL, didn't build this Tandem, but he is a wealth of information and ideas about making it better. I helped him find this Tandem when it was for sale, and he thanked me by taking me up for my first Tandem ride. More on that below.

A few of the mechanical tips I got from Richard are:

  • Install a plywood gusset on the jury strut blocks, opposite the plywood spar webs. He had one or more of the blocks pop loose from the lower spar cap, and of course you have to cut fabric to get in there and fix them.
  • Use bolts instead of pins on the lift strut attachments. He saw the aluminum lift strut brackets splaying outwards in flight with just the pins.
  • Install removable covers over the fuel tanks instead of just covering them with fabric. If (when) a fuel tank starts leaking, how ya gonna fix it otherwise?
  • Buy a product called Nev-R Dull at an auto parts store for keeping aluminum parts polished (like lift struts). I guess it's great on brass and silver also. Sure makes those aluminum tubes shiny and purty.

One reason I wanted to visit a completed Tandem at this time was to investigate alternate locations for instruments and gauges. Sitting in the back seat behind Richard's broad shoulders confirmed the inability of the back seat driver to see the instrument panel, and since I'm setting mine up for back-seat piloting (as intended) I'll need to make some adjustments.

I was interested in putting some streamlined bumps on the bottom of the wing which would house instruments and gauges, but that idea didn't pan out. To keep them clear of the forward occupant's helmet arc, they would need to be mounted a foot or more outboard of the wing root, and further forward than I would have guessed. Their proximity to the leading edge of the wing would preclude any effective streamlining. And the forward occupant (passenger) would be unable to see them in this location.

So I backed up a little and realized that all the passenger needs is a prominent ASI and maybe a slip indicator (more on this below). Everything else can be visible only to the PIC in the back seat. So I investigated my other wild idea -- mounting an Engine Information System (EIS) on the back of the passenger backrest, facing up. This appeared to be a very workable solution, and should only be a concern if I have a very tall passenger and I'm bundled up with a thick winter coat. Otherwise, there's plenty of room between me and the backrest for an EIS.

This EIS by Grand Rapids Technologies is only 2 1/2" tall (or deep in my case). In this very small package I can have right below my nose:

  • Tachometer
  • Dual EGT
  • Dual CHT
  • Coolant Temperature
  • Altimeter/VSI
  • Battery voltage
  • Fuel level
  • Flight time
  • Engine hours
  • ...and most importantly: OUTSIDE AIR TEMPERATURE! ;-)

(If there are other EIS brands out there that I should consider, I'd appreciate you telling me about them.)

As far as the ASI goes, I'm thinking of a nice 3" unit up front that I can see when flying a small passenger or solo, and the ISON-supplied Hall tube indicator on a lift strut. The Hall will make a nice accurate low-speed indicator when towing a glider, and could also be used when a passenger blocks my view of the dash-mounted ASI.

What does that leave? Switches, choke, radio, GPS. I hope to mount the switches overhead in the wing gap. The electronics, well, perhaps a clever design will present itself that will locate these items in the front seat when I'm solo and in the front panel when I'm not. Just dreaming out loud here...

Richard solos his Tandem from the front seat (with 13 lbs. of lead in the tail, I should add), and just ties the rear seat belt harnesses around the rear seat to keep them from flapping around. He said I would have to find a creative way to stow the front harness away when I'm flying alone. We decided that maybe I should build a small compartment into the bottom of each wing panel that the forward/upper straps can be stashed in.

My first Tandem ride
Subtitle: Oh my, those heavy ailerons!

We left my sister's house at Lake Martin at 6:30am, trying to get to Richard's hangar in Enterprise by 8:00 and hopefully before the wind and rain. The drive turned out to be twice as long as expected, and the rain beat us there. Amazingly, the rain, which was supposed to be severe and prolonged, let up upon our arrival and gave us a small window in which we could commit aviation. Richard did a short hop by himself and reported "It's OK if you don't mind flying sideways," so I bundled up for the 48 degree ride and up we went.

He let me fly it around for a while, and complained a little that I was flying with one wing low. Heck, I was getting used to lot of new things at once -- in turbulent conditions -- and it's kinda hard to see what's going on from that back seat anyway! I'm sure that once I'm acclimated to this airplane (interesting choice of words for this particular flight) the reduced visibility with a large passenger up front will be no big deal.

Richard also complained about my pitch management, or should I say lack of pitch management! With him right in front of me I couldn't really tell what our attitude was, so when he told me "that's where you want it" I lined up the bottom of his helmet with the horizon and used that as my pitch reference thenceforth. No more pitch complaints.

After I got the pitch under control I attempted some coordinated turns. After a few awkward moments where I sorely missed having a visible slip indicator, I noticed that you don't really need one in this plane! All you have to do is pay attention to the wind on your face. Wind on left cheek = "you're slipping left" and vice versa. Light, equal wind on both cheeks = "you're doing OK." Once you're aware of these cues it's hard to ignore them.

Richard told me I was going about 60 mph, and this is when I suddenly noticed the stick pressure required for roll inputs at cruise speed. Oh my... he said they were stiff but I would never have guessed they were this stiff. At times I was genuinely afraid that I would bend something in the aileron control system if I really told the airplane where to go instead of just gingerly guiding it around like I had been, or if I had to correct hard for a wind gust. During level flight I looked out at the left aileron and watched it as I gave left aileron pressure. The plane banked, but I could not detect any motion in the aileron itself. After getting back home and lifting some 8 and 10 lb. dumbbells, I estimated the aileron stick force to be somewhere between those two weights. That's heavy stick pressure.

At this time I noticed that I was still toasty-warm in my long underwear, jeans, three shirts, vest, and down coat. Amazing. I get cold very easily, so this is a good sign. But my host was getting cold in the front seat, and began saying things like "We can fly as long as you want to, but I'm freezing up here!" (hint, hint.) So we headed back to the airport for more valuable "hangar flying."

Richard's miniMAX-flying, hangar-next-door buddy Tom is the only other person Richard has taken up so far, and Tom had warned me that riding in the Tandem gives you a feeling similar to trying to balance your butt on top of a skinny fence. He said "you'll feel like you're gonna slip off the seat but you won't." Let me tell you, he wasn't kidding. I'm not terribly squeamish about this kind of thing, but it wasn't exactly a secure feeling. I felt like I would be OK as long as a side gust didn't come along and blow the airplane out from under me. Seriously. If Richard's Tandem has the ISON Aircraft-supplied harness, I recommend some kind of addition to it that will make it feel more secure, especially for passengers not inclined toward aviation in the first place. Although my shoulder straps were plenty tight, as I moved around in flight one of them fell off my shoulder. Something that ties the two shoulder straps together across your chest would go a long way toward enhancing one's sense of security. Also on this topic, Richard accidentally brushed against his seat belt buckle and popped it open twice as we taxied. This is NOT an airplane where you can afford to become unexpectedly strapless, so perhaps a better harness is in order on all counts.


The gaps between the ailerons and the wings on this plane are larger than shown in the plans, and you can kinda see it in this photo (notice the wind sock also). I'd like to find out that this is contributing to the heavy aileron forces, but I really doubt it. Richard is preparing to install the large set of aileron spades that Bill made for him in hopes that this "power steering" will lighten the stick forces.

I failed to photograph the spades (which Bill did an excellent job on), so I'll try a brief explanation for those unfamiliar with spades. Below each aileron will hang one "spade" or flat plate, roughly 7" x 9" in size. The spades will be aligned so that they point into the relative wind and create virtually no drag. When an aileron is deflected upward, its spade is deflected downward. The spade then scoops air like a shovel, which increases its propensity for going downward. This energy is transmitted into the aileron as upward motion, thereby reducing the stick force required to deflect the aileron upward.

I sincerely hope the spades work, Richard and Bill, but I fear they won't. The spades require motion in the ailerons before they "kick in," and the lack of aileron motion I observed at cruise speed leads me to fear that they'll never have a chance to become effective. I hope I'm wrong. BTW, Wayne has apparently given his blessings on this little experiment.

What bugs me is that I haven't yet heard a good explanation for these high aileron forces, except for the Tandem's increased wingspan. The stock Tandem's wing is about 5' longer than the single-place AirBike, and about 6' longer than the miniMAX. Both the AirBike and the miniMAX are reported to have very light ailerons and "can be flown with two fingers." Should an extra 2 1/2 - 3 feet on each wing panel really make this much difference? If yes, then my extended-wing Tandem might be unflyable without some modifications to its ailerons. Right now I'm considering some surgery to change the full-span flaperons into separate ailerons at the tip and flaps in the middle. Perhaps that is the best thing to do aerodynamically, but it will have the unfortunate consequence of complicating what is now a wonderfully simple flaperon control system. (2/23/01 note: I've talked to three other Tandem drivers since my flight with Richard, and they all report heavy aileron stick pressures.)

To be fair, the only real complaint I had about the flying qualities of Richard's Tandem was the heavy ailerons. It's a very solid-feeling plane in the air, and I know I'll love the wide-open visibility it provides... when I'm wearing goggles, that is. In the flight with Richard I was afraid to turn my head too far left or right for fear that I would lose my glasses to the slipstream! I had meant to bring my light skydiving goggles along, but failed to pack them for the trip.

Oh yeah -- back to those RS-17 stringers just ahead of the ailerons. The stories I had heard about these coming loose were confirmed for me by another miniMAX in Tom's hangar. The tension of the upper wing fabric was pulling these stringers away from the ribs in a severe manner. (Also, I think the oversized aileron gaps in Richard's wing might be largely caused by the fabric bowing the stringers out between the ribs.) I haven't gotten to this point in the instructions yet, but I'm told that ISON is careful to specify that the final heat-shrink operation on the fabric must not be done around the edges of the wing, including the area just ahead of these stringers. Probably when it's done right there is no problem, as with most of ISON Aircraft's instructions, but I'm going to consider some strategic strengthening of these stringers just to be safe. When covering my Ragwing Aircamper wing I was being very careful to not apply too much heat, but at the last minute things got way too tight and warped some ribs and wing tip parts out of alignment. This overtensioning really sneaked up on me, and I would rather not go through that again.

Whew. Thanks for hanging in there through another long update! And thanks, John, Bill and Richard for everything you so generously shared during my visits.