About this Plan
The 73.40. Control line speed model.
Quote: "The 73.40 by John Youngs. Here's a snappy 1/2A speed buggy that you 'yo-yo' fans will find fits right down your racing alley.
The 73.40 is a scaled-down version of previous speed planes in the .065 and .074 range of displacements, to meet AMA rules for a 1/2A job. This plane was recently clocked at 73.40 miles per hour, and a little work on props and fuel should bring speeds of eighty and more.
Construction is conventional in most respects. A few evenings at the work bench can add another class for the speed fan or offer an inexpensive and easy way for the novice to get started in speed.
Fuselage: Select two pieces of 8-1/4 by 1-1/4 by 5/8 sugar pine. Tack-cement these two pieces and shape as shown on the plans (Pine properly hollowed is preferred by the author because it affords increased strength with no addition of weight). Be sure to leave not more than 3/32 wall thickness when hollowing the fuselage.
The radial engine mount is shaped from 1/4 in oak or plywood. Cement the mount in place, then pre-drill and set small wood screws as shown. Be very careful at this point to assure perfect alignment. Shape the bellcrank plate and cement it in place. Then, fit the top shell by cutting out for the cylinder and needle valve.
Wing and Tail Surfaces: Start the wing by cementing together pieces of 9/32 by 3/16 by 8 in pine and 1-1/4 by 3/16 by 8 in medium balsa. While this is drying, shape the tail surfaces and install the elevator hinges. The control horn is fabricated from tin-can stock. Cut the wing to outline shape and sand carefully to airfoil shape. Next, install the wing line-guide. Lines are fastened directly to the bellcrank to eliminate drag - extremely important in models of this size.
ENGINE COWLING: Two cowling sides are cut as shown in the top view. To these cement the cowling top. Fit the fuselage halves together and install the cowling snugly behind the engine cylinder. When the proper fit has been obtained, cement cowl in place. This type of cowling was used in order not to restrict the 3600 exhaust of the Wasp .049. Restricting the exhaust on this type of engine appreciably reduces power output.
Final Asembly: Cement stabilizer, fin and wing in place and carve the top half to produce a snug fit between top and bottom shells. Cut the top half diagonally along the line shown and permanently cement the rear half in place. Install the bellcrank and push-rod, allowing 30° up and 15° down motion in the elevator. Then, fit the forward part of the shell with small wood screws, which go through to the fire wall. The fuel tank is made of .008- brass shim stock. Hold the tank in place with two 4-40 machine screws.
Finishing: Remove the engine, bell-crank and tank and sand the entire model with No 400 sandpaper. Apply two coats of dope and talc, sanding between each coat. Now apply two light coats of synthetic enamel. Allow the paint to dry thoroughly before installing the engine for a test run.
Flying: It is not necessary to wait for a cairn day to fly this model. The original model is very stable, even in a stiff breeze. Be sure, however, to hand-launch downwind - this is a must!
Any 5 or 6 in pitch propeller is good for a start. To increase speed, reduce area and increase pitch as you become accustomed to the model. You won't get much line pull, but you may be surprised to see just how much speed you can get out of this model!"
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