CONTROL LINE AEROBATICS
Curt Contrata, 6783 Nightwind Cir., Orlando FL 32818; E-mail: [email protected]
Introduction
Books could be written describing the many methods developed in an attempt to construct the perfect flap for a competitive Stunt model. The quest for a control surface that resists twisting under loads while being as light as possible never seems to end. Consistent with the adage that simplicity is often the best solution, it is tough to beat a plain, old sheet-balsa flap.
This article focuses on detail-sanding flaps using the "rod method" and reviews a few other details of control-surface construction.
Preparing the flap
- Begin by cutting the flaps just a pencil-line larger than the plans show.
- Sand the leading edge (LE) true using a long T-bar sander.
- Test the hinge-line fit to the trailing edge (TE) of the wing and work slowly until they mate perfectly.
Some find it easier to true an edge by applying a long strip of self-adhesive sandpaper to the surface of a bench, then dragging the part across it. With that method, use a square to ensure the part is sanded perpendicular to the bench.
For flap and elevator horn clips, basswood works well: it is lighter than plywood, easier to cut, and much easier to blend into the balsa flap. To cap the opening on the clip, use 1/64" plywood with the grain running vertical. If the flaps are not full span, add a cap of basswood or plywood on the other end of the flaps to keep them from getting rounded off during the finishing process.
Once true, mark a centerline on the LE of the flap and mark all hinge locations.
Slotting and installing hinges
Take extra time as you slot the flaps and wing for the hinges. Do not rush this step, and make sure the hinges are in the center of both the wing and the flap. If you must fill a slot and restart, do it. Even if you have to move all the slots over a bit to get a fresh start, it can be worth starting over.
To obtain a perfect hinge line, recess the hinge barrels into a small notch on the LE of the flap. Make a maple block the width of a hinge with 120-grit sandpaper glued to one side. Use this block to cut a notch in the LE of the flaps at each hinge location, then use finer papers for finishing. With all hinges in place and the flaps temporarily mounted to the wing, there should be no gaps at the hinge line. If you run your fingers along the hinge line, the top and bottom of the flap should align with the top and bottom of the wing TE. If something is not right, fix it before moving on.
With everything in alignment, the flaps are ready for shaping.
Rod method for shaping flaps and elevators
If you have never tried the rod method, you will find it very effective for accurately shaping flaps and elevators from sheet balsa. With careful handling around fragile ribs, you can even use it for sanding to shape built-up control surfaces that will be sheeted.
The basic idea:
- Lay the part flat on a work surface.
- Place one metal rod against the LE and a thinner metal rod at the TE. The rod diameters are chosen to produce the desired final edge thicknesses.
- Use a flat sanding block to sand the part flush with the two rods, shaping the part between them.
Key points:
- Completely sand one side before flipping to shape the other side; this prevents warps.
- Use a flat sanding block so the surface comes out smooth and uniform.
- A smooth, clean work surface is essential — glass works extremely well. Make sure there is no glue or debris that can dent or scratch the part.
Example process for a 3/8" sheet flap with a 3/8" LE and 1/8" TE:
- LE rod: 3/8" diameter.
- TE rod for the first side: calculate as half the wood thickness plus half the desired TE thickness. For 3/8" wood: half = 3/16"; half of final TE (1/8") = 1/16"; sum = 1/4". So use a 1/4" rod at the TE for the first side.
- Sand using an 80-grit block, going lightly. Let the paper do the work; excessive pressure causes permanent warps.
- Before reaching the metal rods with coarse paper, switch to 110-grit, then 220-, 320-, and finish with 400-grit as you approach the rods.
- When the first side is finished, clean away dust, flip the part, place the 3/8" rod at the LE, and use a 1/8" rod at the TE for the second side (to achieve the final 1/8" TE thickness).
- Repeat the sanding progression on the second side.
If the part has a twist or an inconsistent TE after sanding, that usually means too much pressure was used while sanding.
This technique works best with control surfaces that do not have elliptical or highly curved TEs. If the flap outline curves slightly on the plans, leave it squared off for the rod-sanding step and do the final outline shaping after you finish with the rods and begin rounding the TE.
Rod sizes (typical)
- For a 3/8" flap: 3/8", 1/4", and 1/8" rods.
- For a 1/2" flap: 1/2", 5/16", and 1/8" rods.
The LE rod size equals the sheet-balsa thickness. TE rod sizes require the simple half-thickness math shown above.
Reinforcing and finishing with carbon veil
To add rigidity, carbon veil applied with dope is a very effective method. Carbon veil adds stiffness quickly and fills fast, making the finishing process easier. Carbon veil looks like black silkspar and is applied dry, with dope, over any solid surface.
Common issues and how to address them:
- Difficulty conforming the veil around curves: this depends on the veil type. Softer veil conforms to corners; harder veil is fine on flat surfaces. If you have the hard variety, apply a strip of silkspar around edges first, then use the veil on the flat surface.
- Pinholes showing up during finishing: carbon veil is fragile with only one coat of dope and becomes very hard after three or more coats. Sanding must be timed carefully. If you sand too early, you'll sand the veil off; if you wait too long, it may be too hard and pinhole when sanded. Experience teaches the right timing and amount to sand.
- Pinholes most commonly appear where more than one layer of veil has been applied. If applying multiple layers, wait until the bottom layer is smooth and sand it smooth before adding the next layer.
- It helps to iron the veil flat before using it.
Final tips
- Work slowly and precisely, especially when slotting for hinges and when sanding to shape.
- Use progressively finer grit papers as you approach the metal rods.
- Keep pressure light while sanding to avoid warps and deep scratches.
- Use a clean, smooth work surface (glass recommended) to avoid damage while shaping.
Transcribed from original scans by AI. Minor OCR errors may remain.
