Arvin and Greg Larson: Kitfox IV and Number 11
By Chad Jensen, EAA Homebuilders Community Manager, EAA 755575
Arvin Larson in his shop with "Number 11"
Capitol Drive Airport in Pewaukee, Wisconsin, is one of those airports that I hope never goes away. It's a small airport with a north/south paved runway and an east/west grass strip. The row of hangars can't be seen from the road (which is why I missed my turn initially during my visit), and it has a great small town airport feel, even though it sits on the outskirts of Milwaukee. I made the hour-long drive to this airport to see one specific modification on one specific airplane - a Kitfox IV. Built by father and son team Arvin and Greg Larson, this little plane has a unique external throttle handle designed for use while hand-propping the engine.
Capitol Drive Airport as seen from above in Google Earth
Arvin and Greg have been building airplanes together for a long, long time. Aviation nuts these guys are. They've started 11 airplane builds, but not all were completed. At some point along the way, on several of their projects, goals and objectives changed and the airframes being built were cut up for use of their parts on the next project. The airplane that I made the trip specifically to see is the Kitfox IV. It has several modifications to it that make it unique. The two biggest: the external throttle handle and the fact that they are running an A-65 up front, with its own set of mods.
The external throttle. Pretty straightforward, I'd say!
Let's start with the external throttle because they are awfully proud of this modification. Every year we hear stories around the country of airplanes that "got away" while being hand-propped to start the engine. It's an art form that is slowly becoming a thing of the past, although there are still plenty of airplanes that don't have electric starters. The once-common knowledge of how to properly do the trick isn't being passed along today as it has been in the past. I've personally never hand-propped an airplane. I learned to fly tailwheel airplanes in a J-3 Cub without a starter, but it wasn't an airplane that I would ever rent or borrow from the owner to fly solo, and he chose to start the engine each time we would fly. I would sit in the airplane and stay on the brakes. It made the start of each flight lesson particularly safe, though I missed out on learning the procedure other than what I was able to glean from watching my instructor in action. I've flown many tailwheel airplanes since then, and all of them have had starters. So it hasn't been an issue for me - but I'd still like to learn. This external throttle idea is certainly something I would want to incorporate into an airplane like (for example) a Baby Ace, if one were to be in the works at my shop.
|The location of the external throttle and magneto switch is evident in this view. Easy to control both while standing in one spot.||A view from under the panel shows the torque tube connected to the inside throttle, the linkage through the firewall, and to the outside throttle lever.|
This idea is so simple and easy that it's almost a no-brainer to install on airplanes like this. So, how does it work? It's a lever on the outside of the fuselage, near the passenger entry door where the magneto switch or switches are located that can be handled at the same time the prop is flung. The real beauty of the external lever is that it has a hard mechanical stop built into it so when pulled to idle it can be used to hold on to the airplane to keep it from rolling anywhere.
It's welded directly to the same piece of tubing that the inside throttle is connected to. That makes a lot of sense. With a lever outside in the slipstream, you'd think it would tend to "walk" back toward lower power settings while in flight. Not so with this design because the throttle has a thread lock on it, as every throttle should, and that keeps the external lever in place. They are directly connected to one another via a torque tube so once the power is set it stays set until the threads are loosened. The external throttle is simply for use on the ground when starting the airplane. Brilliant. The Larsons have also incorporated this concept on their next airplane, "Number 11," but I'll get to that in a minute.
|The Kitfox IV sitting in its hangar. Notice the Zenith 701 wing hanging from the rafters.||Under the hood: the modified Continental A-65|
The other rather unique mod on this particular Kitfox IV is the use of an A-65 engine. While most are powered by Rotax engines, the seemingly ubiquitous little Continental (although these days, they do seem to be harder to come by) makes for a great engine on this airframe. It didn't get installed without its share of problems to figure out, though, and the engine itself has been modified to become something of a C-65/85/O-200 conglomerate. It has an A-65 case with O-200 cylinders, and C-85 pistons, as well as an O-200 carburetor with an accelerator pump. The use of the A-65 had put the spark plugs and wires way too close to the cowl, in fact, rubbing the original cowl. To fix that problem, they added a nice, classic-looking cowl bump. Does it sound like these guys enjoy experimental aviation? The propeller driving the Kitfox is a two-blade, carbon-fiber, ground-adjustable Warp Drive.
|The vinyl tubing plug over the gas cap vent to keep out the bugs.||Same thing on the pitot tube.|
|A Van's tailwheel assembly-becoming more and more the standard for conventional homebuilt airplanes.||A cowl bump had to be fashioned to give clearance to the spark plugs and wires on the A-65. Nice classic touch.|
Other mods on the airplane include the engine mount being lowered by 5 inches to accommodate the Continental over a Rotax, a Zenith 701 windshield (which came from the 701 sitting in the back of the hangar that houses the Kitfox), and the aforementioned Van's tailwheel assembly.
The Kitfox is kept in an open hangar, so precautions are taken by the Larsons to prevent critters and insects from intruding in and on the airframe. One little idea that I really liked is a cover to prevent mud daubers from taking up residence in the pitot tube and fuel vents. They used clear vinyl tubing with a plug glued in place. That tubing slides over the openings in the lines, but a tiny little hole is drilled in the vinyl to allow equalization of pressures for expansion and contraction to occur with no damage and no insect intrusion.
|The cockpit of the Kitfox IV looks like a fun place to spend some time in the air!||Clear doors provide excellent downward visibility and a much roomier feeling inside the cockpit.|
I mentioned earlier that the father and son team has started 11 airplanes. Not all made it, but 11 airplane starts sort of sounds like an addiction - the good kind, though. There have been Zeniths, Tailwinds, and the Kitfox in the past, and now Number 11. So what's Number 11?
Number 11 as it sat on the day of my visit. The fuselage is finish-welded, ready for wing installation.
It's a design of their own with specific financial and build-time goals set from the start. It began as a wooden mock-up to see if the design was visually appealing and of sound design philosophy.
The origin of Number 11, a wooden mock-up.
Feeling good about the mock-up results, they began to work through the details. It would (obviously) be a tube-and-fabric fuselage but with metal wings, using a Riblett airfoil.
Arvin showing me pictures from the initial wing mounting, which happened the week before I arrived. They also have already performed a weight and balance check to be sure things were on track.
With the wooden proof-of-concept mock-up as proof enough that it could be done, the goals were set, and with enough ambition to make it happen, construction commenced. They wanted to build the airframe (firewall aft) in 400 hours for $4,000. Power was intended to be Volkswagen or another Continental A-65. The plans on which the father and son team are loosely basing their design is from LuceAir's replica of the Wittman Buttercup. So far, they are on track to meet the present goals with each wing taking only 50 hours, and the balance of the airframe is still expected to come in right at 300 hours.
The red tail is all Buttercup except for the rib channels. Those were changed to tubing.
The tail of the airplane is built straight from the Buttercup plans. The rest of the airplane is built from their own design, but using the Buttercup plans for suggested tubing sizes and basic dimensions. It is a single-seat airplane now, and though it began as a tandem two-seater, it became clear that in order to meet their goals, a single-seater was the way to go.
The external throttle is in place for the A-65 that they may use.
The external throttle idea is used on Number 11 as well. With a now-planned A-65 up front, the setup will be virtually identical to the Kitfox they built. During my visit, they were in the process of hunting down another A-65 for this project. With an expected empty weight of 575 pounds, this little single-seat fun machine will really get going on 65 hp - and fly on the cheap!
That's about as far as I can go into detail on Number 11, other than letting the pictures speak for themselves. Arvin and Greg are working diligently on this design, and they are very passionate about it. I think they have a neat little winner of a design here, and it's exactly what a lot of people want to build these days.
Watch the video above to see the hand-propped Kitfox and external throttle design in action. Don't be alarmed by the appearance of the prop bending. That's just an illusion caused by the type of shutter in the camera being used.