I have a young pilot in my family who just started flying control line airplanes past Summer. We began with a 1/2A Proto Speed model powered by a Cox reed valve engine (0.8 cubic centimeter). The small model is fun but it is time to move to something more serious. So we agreed to biuld a simple trainer airplane with a 2.5 cc (.15 cubic inch) motor, which will be:
- Easy to build (and repair).
- Easy to handle for a novice pilot and not too fast.
- Takes off the ground but can be converted for operation with a dolly.
- Completely FAI legal and can be entered into a competition (this means proper wing area/loading, use of a piped engine and fuel shutoff).
These requirements can be satisfied by a symmetrical airplane with flat fuselage. For the motor it was decided to use a Russian replica of the Rossi 15 - the ЦСТКАМ 2.5 КР. If we need to gain more speed we can upgrade to a real Rossi or an Irvine 15R.
With winter at our door, the construction work began. I did not make a drawing - the plan is to improvise and produce one in the end. On the photos below one can see the materials I picked up at a local hobby shop: some 1/8" balsa and basswood sheet for the wing and stabilizer, 1/4" basswood for fuselage, 1/32" plywood, 1/8 x 1/8" carbon rod for the wing spar, 4-40 threaded inserts for the motor mount. I also found a tank from one of my old F2A airplanes and a titanium bellcrank.
Below the construction process is detailed with some photos.
A.Valishev, January 10, 2011
The stabilizer was made of a 1/8" (3.2 mm) 50x270 mm balsa sheet reinforced with a piece of pinewood along the leading edge. The pine slat is 10 mm wide at the root and tapers off to zero at the tips. The stabilizer was sanded with a 220-grit sanding block to create a symmetrical airfoil. Finally, the stabilizer was covered with one layer of glass cloth (epoxy thinned with alcohol was used as bonding compound, during curing the stab was sandwithed beween two sheets of mylar film for a smoother finish).
The elevator has the dimensions of 18x75 mm and was cut out of the finished stabilizer. Two hinges were made of 1.2 mm music wire and 0.3 mm tin sheet. One of the hinges is simultaneously the control horn. Don't forget to drill a few holes in the tin part of the hinges (hinges in the photo are not drilled yet), this makes the hinge lighter and also makes the bond stronger. Before glueing the elevator in place cover its edges with thinned epoxy or clearcoat to protect from fuel. Weight of the assembled stabilizer is 13 g.
Is 1/8" thick and was made of three main parts: 1"-wide basswood sheet along the leading edge, a 1/8" square carbon fiber spar, and a balsa sheet along the trailing edge. 10mm-wide pinewood tips were added to add some torsional rigidity. The wing is 70 mm wide at the root, 50 mm at the tips, and the span is 627 mm. Similar to the stabilizer, the wing was sanded with a sanding block to create an airfoil (semi-symmetrical in this case). Finally, the wing was covered with one layer of glass cloth on thinned epoxy. Weight of the finished wing is 44 g.
Fuselage is basically a 1/4" basswood plank reinforced with plywood in the nose. The side facing outboard (engine) is 1.5 mm plywood, the inboard side is 1 mm plywood. It also has a window behind the engine that will accommodate the fuel tank. Four 4-40 (2.84x0.63 mm) threaded inserts are permanently glued to the motor mount, and two similar inserts are glued near the tank bay. Weight of the fuselage is 38 g.
The fuel tank was reused from an old sidewinder airplane. It is of the conventional 'coffin' design with the dimensions shown in the figure below. The volume is 32 cc. The tank is comprised of two parts bent from 0.008" (0.2 mm) tin sheet and soldered with silver solder. I'm using acid flux, just don't forget to rinse the tank thorougly when finished. For mounting on the trained, two L-shaped brass strips were soldered to the sides of the tank against the threaded inserts in the fuselage. Weight of the tank is 20 g.
For now the nalding gear is a very concervative two-weel design, which should help the pilot learning and improve the model robustness. However, it is easily detachable and can be replaced with a simple skid for dolly operation. The gear is bent of two pieces of 1.6 mm piano wire and silver-soldered. The wheels are 1-1/8" diameter, made of polyurethane. The gear is attached to the bottom of the fuselage with the use of a brass bracket, which on one side is held by the motor lug.
As it was mentioned in the Stabilizer section, the control horn is an integral part of the elevator, and is made of 1.2 mm piano wire. The horn is 13 mm high. The pushrod is also 1.2 mm piano wire, for stiffness it is placed inside a polyethilene coctail straw that is glued to the fuselage. Thin cotton thread was wrapped around the straw to make the bond more reliable. The bellcrank (1 mm titanium) is attached to the wing via an aluminium axis. Finally, the fuel shut-off is actuated by a wire that runs on a brass roller (see photo below).
To make sure the airplane is straight, I mount the engine on the fuselage and put the crankshaft in the lathe chuck when making markings for final assembly (refer to photo below). The CG is located 12 mm from the wing leading edge. The wing line lead-out is positioned so that the engine axis is 90 degrees to the lines when the airplane is hanged by the wing tip.
Finally, the airplane was covered with clearcoat.