Fuselage construction with sandbags, clamps and masking tape (above left) holding the formers and fuselage sides in alignment while the epoxy cures. Nose
a vectored raster format in order to import it into CAD. Once I had the outline in CAD, I could scale it to the size I wanted. I set the wingspan at 72 inches without the wing tips, simply so that each of the wing panel sections could use a standard 24-inch length of wood. I then moved on with the design, drawing up some other items in CAD such as the tur- bine, fuel tank, retracts, wheels and servos and placed them in the drawing where I thought they should go. During this process I adjusted the outline accordingly to fit the components while still retaining the general outline of the A-10. In the end, I shortened the tail and added about six inches to the nose to compensate for the weight of the en- gine being behind the center of gravity. I then modeled the basic A-10 design in 3D CAD to visualize it from different angles and confirm that it looked proportionally correct. Another major advantage of CAD is that it lets you plan out your control rod routing from the servo to the control surface. You know exactly where to drill holes for the control rods in the fuselage formers and where they will exit the fuselage. In addi- tion, the CAD files can be sent out for accu- rate laser cutting.
Once I had the outline drawn to size, it was time to draw up the internal structure such as formers, landing gear mounts, spars, ribs, etc. I was targeting an all-up
and canopy (above right) are made from rigid pink foam carved to shape and covered with fiberglass cloth and epoxy resin.
weight around 13 to 15 pounds dry, without fuel. This would line up nicely with the 13.5 pounds of thrust the P60 engine delivers. The fuselage is a basic box structure con- structed simply from lite-ply sides with a few aircraft plywood formers and balsa sheeted top/bottom decks. The wing is a foam core sheeted with balsa, incorporating a plywood rib/spar crutch structure to sup- port the flight loads and main landing gear plates. The tail surfaces are flat balsa sheeted surfaces with a balsa internal frame. Some- what unique are the functional twin rud- ders, driven by an internal linkage that runs inside the horizontal stabilizer. A working rudder is an AMA requirement for turbine jets. The curved parts, such as the nose, canopy, wing tips, engine and wheel na- celles, are made from rigid foam and covered with fiberglass cloth and epoxy to keep the plane light.
One of the most challenging parts to design was the turbine exhaust pipe. There is not a lot of detailed information available on ex- haust pipe design so, as a start, I utilized the radial dimensions of the exhaust pipe from another jet powered by a P60 engine. The ex- haust pipe is relatively short and to make construction easier I designed a straight pipe with a constant diameter versus a tapered pipe with a smaller diameter outlet.
The inner tube of the double wall exhaust
pipe is constructed out of stainless steel shim stock while I used aluminum roof flashing for the outer tube. Both are held to- gether with stainless steel 2-56 bolts, lock washers and nuts, alleviating the need for a spot welder.
The center of gravity is the most critical calculation and I used multiple methods to verify the answer. The first method was a graphical method in CAD in which the root chord length is added to the tip and tip chord length added to the root to find the mean aerodynamic chord (MAC). I then conserva- tively located the CG at 25 percent of the MAC.
I also found a number of Microsoft Excel spreadsheets on line, each with different methods to calculate the CG. To use these spreadsheets, you enter the dimensions of the wing, fuselage, tail, distance from wing to tail, etc. and it calculates the CG. All of these computations yielded approximately the same range for the CG. I found using the 25-percent MAC location for the CG to be conservative for the maiden flight and proved very successful. It is always better to err on the side of nose heavy than tail heavy. Construction of the A-10 should go fairly quickly. There are not a lot of parts to cut out. All of the parts for the prototype were cut out using only a band saw, jig saw and
Wing ribs, sub-ribs, spars and landing gear plates (at left) cut out from 5-ply aircraft plywood and ready for assembly. Wing center section (above) foam core marked and cut out for ply ribs and spars.
FLYING MODELS 27
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