Building and Flying an Airbike
Ian Henehan - April, 2002
I first ran across the Airbike in an ultralight magazine about seven years ago. I had been looking into ultralights, hoping to find something that would expedite my desire to start flying. The magazine had an article about the new Airbike prototype that TEAM (now ISON Aircraft) brought to Sun ‘N Fun. There was a picture of this cute little tail dragger with a very open cockpit. It had a wood wing and chromoly tube fuselage. A very conventional little airplane compared to the rest of the ultralight field at the time. The prototype was powered by a 22hp two-stroke and weighed in at 225 empty. Fuel capacity was only 2.5 gallons! Just enough fuel to fly around the patch a few times.
A few years ago I finished up my private glider rating and started working on my power ticket. I decided it was time to get serious about building a plane. I had a couple simple criteria. First, I wanted something relatively simple. If I could complete a smaller project, I might be willing to take on something more involved later. Second, I was looking for a plane that had had a fair amount of successful builders. A proven design should make the project more likely to succeed. I was also interested in keeping the costs under control. In hindsight, I think two out of three isn’t too bad.
First I bought some plans for TEAM’s V-Max. Similar to the Mini-Max, but VW powered. While researching VW power, I found the Sonerai aircraft by John Monnet. Soon, I also had a nice set of plans for the Sonerai IILS. Notice the trend developing? Fortunately, I remembered my original intent before it was too late. In April of 2000, I ordered a wing kit for the Airbike.
I remember the first model kit my dad gave me as a kid. It was a stick and tissue J3 Cub from Guillows. On the box was a great picture of the little yellow airplane contained inside. The collection of sticks and paper inside the box bore little resemblance to the picture. In fact, except for the red propeller, nothing else looked much like airplane parts. Opening up the wing kit for the Airbike was exactly the same. I had just bought a crate of very nice looking sticks and plywood. None of which were recognizable airplane parts. The plans, however, had many detailed pictures of airplane parts. All of which would be made from the big box of sticks. I was starting to wonder what I had gotten myself into.
I had built a 16’x4’ table in the basement. This left about four feet on each end and a few feet across to maneuver. It was a little tight, but workable. With tools and plans laid out, it was time to commence building.
Working with Wood
While I had built plenty of things out of wood, this was the first time I worked with aircraft grade lumber. The wood in the kit was beautiful stuff. The wings are built from spruce, pine, and a couple different types of plywood. The ribs are of a truss type construction, using ¼" spruce sticks and 1/16" birch plywood gussets. There are 26 ribs, with 15 gussets per side and 14 spruce sticks. It took about two evenings, just to cut all the parts for the ribs. All ribs are laid up in the same jig. It took about 100 hours to complete the ribs. This was about the time I realized I might not finish up in a few months.
It was a little intimidating to tackle the spars. It actually took more time to get the jig setup, than build the spars. Once the jig was straight, the spars were pretty easy. There are two spars per wing and they all came out straight and dimensioned correctly. I even avoided making two sets for the same wing. There is a point where it is easy to screw up and not make a right and left set. The hardest part of spar fabrication was cutting the bevel in the top caps. This allows the spars to match up with the curve on the top of the wing. The recommended procedure is to rip the 10-degree angle cut using a table saw. It’s not easy to stuff your brand new spars into a screaming table saw. This is one of those times that a little paranoia really helps out. I spent a good couple hours making sure the saw was setup and jigged correctly. It only took a few minutes to make the actual cuts.
Wing assembly went fast with the spars finished. Keeping things straight and square was not too difficult. This is the first time you get see something take shape that looks like it belongs on an airplane. Sheeting the leading edge with plywood was a little like trying to catch a greased pig, but it worked out in the end. A few coats of varnish and the wings were ready for fabric.
The fuselage is 4130 tubing and all welding is done. Most of the work was rigging the empennage and installing the engine and instruments. After cleaning up the welds, I used AFS steel conversion coating to protect from rust. This stuff wipes on with a shop rag and both converts the existing rust and leaves a coat of clear acrylic. Even with all the welding done, there was more detail work than I expected. Completing the fuselage probably took ¼ to 1/3 of the total build time.
Installing the landing gear was the first order of business. Followed directly by a trial installation of the seat. Any builder that says they didn’t climb in and make engine noises once or twice is probably lying. Sitting in the seat for the first time was a high point in the project.
Installation of the Hirth 40hp engine was simple. Wiring and plumbing are minimal and except for finding the right spot for the fuel pump, there was not much to it. I installed the minimum day VFR instrumentation and a handheld radio for communications. Panel space is severely limited on my ‘Bike, so I opted for a Grand Rapids EIS system for the engine. It took up less space than the analog gauges, is more accurate and cost about the same. It was not a hard decision to make and paid off later during the first few flights.
Covering and Paint
Covering with fabric was a lot of fun. The job goes quickly and you see the airplane really taking shape. The kit supplies a Poly Fiber glider weight cloth. I had decided on the AFS waterborne paint system to allow me to continue working at the house. Rib stitching was not strictly required, as the Vne is 80mph. I opted to go ahead and stitch the wings and tail anyway. Stitching didn’t take as long as I thought. I was able to stitch a complete wing in a little less than four hours. Finishing tapes and finish ironing took longer in the end.
The Kansas City Dawn Patrol has been using the AFS system for a few years now. More recently they’ve been experimenting with a different method of application. Instead of spraying the paint, it is rolled on with a foam roller and then tipped with a brush. The brush is drug across the paint, just barely touching the bubbles formed by the roller. Very little paint gets on the brush if done correctly. I decided to give this a try. I figured It would give me more control and less mess since I was still working in the basement. I started on the tail and had mixed success. The paint self levels very well, so it wasn’t too hard to get the "wet" polyurethane look. It was difficult to get color to cover evenly. The blue on the stabilizer and elevator show this the worst. For the wing, I went a head and borrowed an HVLP sprayer. This worked out much better, even though I’m not very good at shooting paint. By the time I really was getting good, I was done. Oh well, save the new skills for the next plane. Overall, the paint came out better than I was willing to except. Even a bad paint job rarely makes an airplane un-airworthy.
Paperwork and Inspection
I decided early on, that I would register the ‘Bike as amateur built experimental. Only a few builders managed to keep their ‘Bikes ultralight legal with 40hp engines. I also would be able to add more fuel capacity and log the tail wheel time. I picked up the homebuilder packet from the FSDO and just followed the recommendations. It was a lot easier than I expected. I had an N number and registration months before I was finished. I prepared the paperwork expected for the inspection and called up Frank Sneed.
The inspection only took a couple hours. The Airbike is so open and simple the inspection really went quickly. It took almost as long to finish up the paperwork and make sure everything was complete. Airbike N40613 received an airworthiness certificate on April 14, 2002, almost exactly two years after buying the wing kit. About 850 hours went into construction.
I had two limiting conditions that had to be satisfied before the first flight. The first was finishing up my tail wheel endorsement. I had been signed off to solo a Taylorcraft L2 last summer and had about 15 hours of flying time. I needed the endorsement to be legal to fly the ‘Bike. The other problem was weather. For three days after the inspection, the weather was horrible.
Finally on Thursday, I was able to get some time in a J3 and finish up my tail wheel work in the L2. Around 5:30pm I had both the sign-off in my logbook and perfect weather for a first flight. By the time I got back to Twin Oaks, folks were starting to show up for the monthly EAA meeting. This worked out well, since I needed a couple volunteers anyway.
I pulled the plane out did the second thorough pre-flight of the day. After strapping on the parachute and climbing on the ‘Bike, I realized a forgot something crucial to the operation… the key! In all the excitement and anticipation, I had left it in the toolbox. Wally retrieved the key and I was back in business. A couple pulls on the handle and the engine fired up. After letting it warm up for a few minutes I headed for the runway 20. After the engine run-up checked out, there was little left to do but go fly. The windsock still looked like it was nailed to the pole, so I pulled out and brought up the power.
There were a couple things that happened pretty quickly. About three seconds after applying full power, I was airborne. The plane got off in about 200 feet or so. Highest on my list of concerns, were engine temps. The CHTs were coming up pretty quick and EGTs were on the high side. By the end of the runway I was coming up on 1000 feet. CHTs looked like they might make a run for the limits, so I had to back off the power slightly. CHTs stabilized and the engine was still running. So far, so good. At around 1500 feet, I had a chance to look around a bit. This is when I first noticed the Airbike cockpit is really OPEN. While that was something I liked about the design, the realization in flight was a bit startling. Good time to check the harness and maybe snug it up a bit. I continued up to about 2500 feet.
The goal of the flight was to try to get a feel for the engine and also to do some slow flight close to stall. The engine was staying within limits, though on the high side. After about 10 minutes of tunnel vision on the engine monitor, I decided I’d better check out slow flight behavior. The plane slows down nicely with all the drag from the open fuselage and round wing struts. Controls started to get soft around 40mph indicated. I was able to fly down to 20 mph indicated and not get a clean break. The descent rate increased, but no real break to the stall. At this attitude, I’m sure the airspeed indicated included plenty of error. After getting back up to speed, I did a few more shallow turns and then headed back to the airport.
The high-speed taxi tests revealed that the tail wheel was a little sensitive. The tail wheel on the Airbike is connected directly to the rudder control horn with a pushrod and ball joints. It doesn’t caster and has no dampening. Because of this, I decided a wheel landing might work better for the first landing. I came across the threshold of runway 2 at 55mph and continued to fly the plane right down to the runway. I kept some power in as I leveled off and then carefully set it down by reducing power. Just the slightest amount of forward pressure kept the tail in the air as the wheels squeaked onto the pavement. I held the tail up as long as possible and when it finally came down I pinned it with the elevator. There were a couple little wobbles on the tail wheel at that speed, but nothing too bad. By the time the plane slowed to walking speed, I was only about halfway to the first turn off!
With the success of the first flight nearly in the bag, I just had to taxi back to the hangar without running into anything. Not as easy as it sounds after so much excitement. By the time I got back to the north end of the field, even more folks had shown up to watch the proceedings. It took a few minutes to climb out after shutting down. What a fun day, seeing two years of work come together for a first flight and safe return to the ground.
I’ve got a few more hours on the plane now. A couple of engine adjustments settled down the temperature problems. The elevator trim spring was tightened up to give a 65mph cruise. Other than some very thorough inspections, not much else has been required. Just need to finish up the flight-testing and fly off the 40 hours. My biggest concern now is deciding what to start building next.