Back to Basics S400 (oz14843)

About this Plan

Back to Basics S400. Radio control electric-powered sailplane model.

Quote: "Forget the winch or hi-start, build this Speed-400-powered sailplane that's designed to soar with the best. Back to Basics, by Leon Kincaid.

When I learned, in May 1999, that there was a competition class restricted to small Speed-400-sized motors and a seven-cell battery, I was impressed. I subsequently joined the National Electric Aircraft Council and subscribed to S&E Modeler.

Then I immediately scaled my 2-meter Back-To-Basics sailplane design down to a 60-inch wingspan, resulting in 410 square inches of wing area. It's funny, 60 years ago I started flying free-flight airplanes with gas engines for power, then 28 years ago I took the engines off and started flying R/C sailplanes. Now, I'm putting a motor back on, an electric motor!

Analysis: For this Back-To-Basics Speed-400 model, I built five fuselages, four wings and five stabs and rudders, keeping records of all part weights. The first wing had shear webs all the way to the tips. It was overkill. The next wing had webs in the center panels only. This was lighter and more sensible. The next two had webs only in the first four rib bays. It was sufficient and about 10 grams lighter.

The first two fuselages had 1/8 sheet balsa sides. The next three had 3/32 sheeting and reduced the weight by S to 9 grams, and they were still rugged enough.

I constructed five stabs and rudders. Two had 1/8-inch-square diagonal ribs. One had 1/8-inch-square ribs in a parallel configuration 2 inches apart. Another had parallel ribs with 1/16 x 1/8 diagonals between the ribs. One was all sheet, 3/32 thick. The all-sheet tail was the heaviest. After covering the open structured surfaces, and using seven coats of dope (cut 50/50) on the sheet tail, the results were as follows: parallel ribs 17 grams, sheet tail 18 grams, and diagonals 20 grams. It doesn't make a lot of difference but the diagonals provide a more rigid structure.

Construction: Empennage Construct the plane as shown on the plans. Use the hardest 1/8-inch by 1/4-inch strip stock down the center of the stab and for the rudder fin post. Be sure to bevel the leading edges of the elevator and rudder about 30 degrees for proper movement.

Wings: Cut out all the ribs. Then cut spars, leading and trailing edges to length. The trailing edge stock is too thick on its back edge, so plane and sand it to a sharp edge; then notch it for the ribs. Pin down the leading and trailing edges, lay the bottom spar in place, and add all the ribs except W1l, W2, and W9. Next, add the top spar. When dry, sand the end of the panels to the correct angle. Then spot-glue the tip panel to the center panels and glue the center panels together. Now you can add all of the dihedral braces.

Lay the left center panel back down on the building board. Add the center bottom 1/32 sheeting and W2. Flop the right panel down flat and add its bottom sheeting and the other W2 and W1. Now you can install the two 1/16 by 3/16 turbulator spars in place. Next install the top center sheeting, gussets, and wing tips. Contour the top of the tip blocks to the airfoil. Now you can plane the leading edge to shape and complete the overall sanding.

Fuselage: The fuselage is shown with its bottom down flat, the way you build it. The top of the fuselage is its flying position and provides the reference line that everything is measured from. From this line, the stab is set at 00 incident. The wing is then set at 2.5 degrees positive. Since the motor is set parallel with the bottom, it has about 4 degrees down-thrust. The main motor support bulkhead F3 has a 1-3/32 hole in it, about 1/32-inch to the left..."

From S&E Modeler, September 2000.

Direct submission to Outerzone.

Update 4/4/2025: Replaced this plan with a clearer copy, essentially redrawn from scratch, thanks to TonyP.

Supplementary file notes

Article pages, thanks to RFJ.
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