Wednesday, May 16, 2007
Chugger is an exercise in low-cost construction, a sturdy little bird built mostly from Box Store lumber, powered by a converted Volkswagen engine spinning a home-made propeller. The landing gear uses go-cart wheels with tires from a baggage cart.
Chugger would normally be called a fabric-covered wooden airplane but the truth is, wooden airplanes use steel fittings at every stress concentration point, such as where the landing gear attaches to the fuselage or the wings to the pylon over the pilot's head. A major cost-savings was to be the use of inexpensive mild steel rather than chrome-moly for fittings, as Erik Clutton has done with his F.R.E.D. This was the standard method in the early days of aviation, prior to the advent of malleable high-strength steels. The only down-side is that when you make a fitting from mild steel it needs to be as much as three times thicker than when made of alloy steel. And that means three times heavier as well, since all steel weighs about the same.
Still, it was obviously do-able and I forged ahead, working out the weight of every single piece of wood and every fitting too, for airplanes are the ultimate Balancing Act. Spread-sheets were a big help here because like most balancing acts you must concern yourself not only with the mass but how far it's located from the point of balance. Mass times distance becomes moment and a relatively small change can have a significant impact when the part is located far from the center of balance.
The hand-maiden of low-cost was ease of fabrication; a flying machine literally anyone could duplicate. But Chugger is also meant to be realistic and today most American's are fat so it had to be big enough to carry a fat person, which caused the wings to grow a bit. Still, it had to be small enough to build in the typical garage. I wanted the landing gear to also serve as a durable trailer, allowing the airplane to be towed on its own wheels. Accordingly, the wings had to fold. Unfortunately, I've since learned that most airport managers will not allow a towable aircraft to be erected and flown from their field unless it first undergoes an inspection by a 'certified' mechanic.
Drag increases exponentially with velocity but I've flown some real tumbleweeds that were not only safe but whose controls had a delightfully positive feel. Since tumbleweeds are never very fast I was more than happy with an expectation of 75 to 80mph for cruise. Accordingly, I paid scant attention to streamlining. But here in the western United States, for an airplane to be useful it must have a range of at least 300 miles, and more would be better. I included tankage for twenty gallons of fuel -- about one hundred and twenty pounds of the stuff. Add that to a 250 pound pilot and I found the wings were acting like Pinochio's nose, which forced me to redesign the fittings.
A ten gallon tank fit nicely in each wing but made it impossible to fold the wings without first draining the fuel. Defueling an aircraft is another thing airport managers get a bit huffy about.
Somewhere along in there it became pretty obvious that in order to fly really well my heavy, draggy, inexpensive, road-towable flying machine was going to need more power than I could get out of an inexpensive engine.
One reason I'd avoided aerodynamic drag was because I'm not very well educated and some of the equations cause my eyes to bleed. Fortunately I found some design texts from the 1930's that addressed low-speed drag in terms I could understand. With the help of a spread sheet I quickly learned that my light-weight, easily built -- but slab-sided -- empennage was generating about six times as much drag as a thicker but more streamlined design. The streamlined design was more difficult to build but the reduction in drag combined with a strict diet put the Chugger back into the realm of Volkswagen-powered aviation.
Like all diets, this one proved expensive, for it meant re-designing all of those lovely, low-cost mild steel fittings for SAE 4130 chrome-moly steel. While this appears to violate a basic tenet of the design, the cost of the 4130 is several thousand dollars less than the cost of replacing the converted Volkswagen with a small Continental. In keeping with the goal of easy fabrication I'm redesigning the fittings to allow them to be made from 4130 steel strips of standard dimensions. Which I should finish any year now :-)
In the meantime the engine is coming along, as is the 68x38 propeller. More detailed drawings and the occasional photo will be posted to the Chuggers Group on Yahoo.