Originally by Bob Aberle
I created a new Radio Control (RC) trainer design that I call the "Scratch-One." The idea was to utilize the RC system and the electric power system from the Aero Craft Pogo Almost-Ready-to-Fly (ARF) model. The Scratch-One RC trainer design is slightly larger and slightly heavier than the Pogo. Specifically, the wing area is 247 square inches, the wingspan is 45 inches and the all-up weight is 16.9 ounces (approximately an ounce heavier than the Pogo).
The Scratch-One design can be classified as a RC electric-powered trainer sailplane. It has proven to be one of the most forgiving designs to fly, and perfect for the RC beginner, but keep in mind that the thrust of this article is to get you to build your first model from scratch. The pieces won't be fabricated or preassembled; it is all going to be strictly up to you. As part of the process, you will also cover all or part of your model.
Obtaining the necessary balsa, plywood and spruce pieces to construct this model took at least three visits to local hobby shops; not every hobby dealer will have every stick you need. I thought this might be an initial point of frustration, so I came up with an idea.
Craig Wagner, who owns Aero Craft, agreed to make a box of wood for the Scratch-One. It will have all of the wood material you need, and in the correct sizes. The "kit" of materials is $19.95 plus $6 shipping and handling. This kit will come packed in a 4 x 4 x 36-inch mailer box. The only things you will have to purchase on your own are the cements, covering material and hardware, such as control rods, control horns and control-surface hinges. You can reach Craig at Aero Craft Ltd., P.O. Box 1474, Riverhead, New York 11901; Tel.: (631) 369-9319; Website: http://www.aerocraftrc.com/.
About the Design
Before discussing the construction, I want to point out some of the Scratch-One's design features. Cutting out wing ribs can be a tedious job, especially for a beginner. So in this design, all wing ribs and substituted balsa sticks were eliminated.
The bottom stick is 1/8 x 1/4 balsa. Next come three different-size wing spars, then 1/16 x 1/4 balsa sticks are bent over the spars to provide the necessary airfoil shape. The leading edge is commonly a 3/16-inch-diameter hardwood dowel. You won't likely experience much damage on rough landings with this kind of construction.
I also made the wing's center-section flat, so that no center joiner or brace is necessary. Both tips are raised 41/2 inches for what we call "polyhedral." These raised tips provide overall stability in flight.
Many models have "internal" battery compartments; to access the battery pack for charging purposes, you must remove the wing. Removing the wing can be time-consuming, so in this design, the battery compartment is on the bottom of the fuselage where it can be accessed directly without touching the wing.
Probably the most difficult task for the beginner is to mount the servos and hook up the control rods that operate the rudder and elevator. To keep it simple, I placed the two servos on top of the fuselage, just aft of the wing trailing edge. It's kind of like "letting it all hang out"!
The control rods are run externally from the servo output arms back to the control horns on the rudder and elevator. This makes for easy control throw adjustments and easy centering of the controls. These few ideas made the Scratch-One extremely simple to build and fly!
It is helpful to make a "kit" of parts before starting the assembly. On this model, you must make two fuselage sides and all of the tail pieces from 3/32 balsa. You also have to cut out four fuselage formers from 1/16 plywood and a fifth former from 3/32 balsa. The last items are two 1/16 plywood wing-panel joiners—one for each tip panel.
To cut out these parts, take the plans to a copying store, such as Kinko's, and have photocopies made of the parts to be cut out. Then, paste these copies to manila-folder stock (file folders opened up) using rubber cement and 3M Magic Tape.
After the cement dries, cut out the pieces, which make handy templates. Then it is just a matter of transcribing the outline of the parts onto the balsa and plywood sheet material with a ballpoint pen, which is shown in the second image below. Since you made copies of the full-size plans, the parts you cut out should fit perfectly, such as the parts displayed in the last image below.
You will be using the same cement that you used to assemble the Pogo. I purchase all of mine from Balsa Products Inc. in Iselin, New Jersey, but there are many supply sources. You will need thin and thick cyanoacrylate glue (CyA) with an accelerator (in a spray bottle) and five-minute epoxy cement for the high-stress areas such as the firewall, wing-panel joints and stabilizer/vertical fin attachment to the fuselage.
Because the wing requires no ribs, all you need is a lot of balsa sticks. The bottom of the airfoil is made from 1/8 balsa sticks, which are shown in the first image below. Add the three spars, and the last step is to bend 1/16-inch-thick strips over the spars, forming the airfoil shape of the wing, as shown in the second image below. You may find that briefly soaking these strips in water will make them bend easier.
When assembling the wing, take note that the middle spar in the center panel is 1/8 x 3/8 spruce (a hardwood); that is for extra strength. On the tip panels, to maintain a gradually thinning airfoil shape, it is necessary to taper the spars from the panel joint out to the tip. For that reason, all of the tip-panel spars are made from balsa, so that they can be easily tapered (cut). Each wingtip is simply capped off with 3/32 balsa and sanded to blend in.
1. Trial-mount your Speed 400 motor to the plywood firewall F1. You will need to drill a clearance hole in the center along with two screw holes. The screws are 2.6 millimeter, which you can obtain from Kirk Massey at New Creations R/C. Leave the motor off until after F1 is cemented to the fuselage sides.
2. Add 1/16 x 1/4 spruce stiffeners to the wing-mount and battery-compartment areas. This provides extra strength and should not be omitted. When you cement the stiffeners in place, remember that you are making one left fuselage side and one right side.
3. Now, you can cement the formers in place, first to one side using a square or triangle to make sure that they are aligned properly. You can use thin CyA to spot the formers in place, and then follow up with thick CyA, which adds more strength.
4. Attach the firewall (F1) with five-minute epoxy cement. Once formers F1 through F4 are in place, add the other fuselage side. Try to eyeball-align the two sides. The last thing you do is join the two fuselage sides at the rear. Hold them temporarily with a clothespin and cement them in place (with thick CyA).
5. Add the two 1/8 x 1/4 spruce servo-mounting rails, which are located on top of the fuselage, just aft of the wing trailing edge. The Hitec HS-81 servos are mounted to these rails using the supplied wood screws (two for each servo). Run the cables coming from these servos through the hole in former F4 and into the receiver compartment, just below the wing.
6. Mount the Speed 400 motor in place with the two screws, and install the Jeti JES 110 Electronic Speed Control (ESC). The motor cables run through the hole in former F2. Then the ESC servo cable passes through the hole in former F3. Using double-stick tape to hold the ESC to the fuselage side is recommended. Install the ESC switch on the left side of the fuselage. Before adding the sheeting, install the battery-compartment floor, which is made from 3/32 balsa.
7. Lastly, with this done, you can apply the 1/16 sheet top and bottom to most of the model. For example, connect the fuselage top from the servos to the leading edge of the stabilizer, then sheet the entire bottom of the fuselage. With all of this sheeting, the grain runs across or at right angles to the fuselage sides. The only sheeting you leave open at this point is the top from the firewall to the wing leading edge.
For your convenience, the plans and specifications are included with this article. With this information, you can purchase the balsa, cut out the parts and do the assembly work. Thank you for your patience.
Manufacturers recommended by the author:
Du-Bro Products (control rods, control horns, keepers)
P.O. Box 815
Wauconda, IL 60084
Phone: (800) 848-9411
Fax: (847) 526-1604
Hitec RCD (Neon RC system)
12115 Paine St.
Poway, CA 92064
Phone: (858) 748-6948
Fax: (858) 748-1767
SR Batteries Inc. (Gapless Hinge Tape, R/C Techniques on covering application)
1335 Broadway Ave.
Holbrook, NY 11741
Phone: (631) 286-0079
Balsa and plywood raw-material suppliers (in addition to Aero Craft Inc.):
Lone Star Models
13511 Greywood Dr.
Sugarland, TX 77498
National Balsa Co.
60 East St., Suite C
Ware, MA 01082
Phone: (413) 277-9500
Fax: (413) 277-9502