Control-surface hinging

Model Airplane News, May 2003 by Mosher, Vance

SCALE TECHNIQUES

Aside from the ability to get airborne, the most basic requirement for any flying machine is attitude control. With this in mind, let's talk about hingesthe little things that allow the control surfaces to move-and how to make them work properly.

On a scale model, the best hinges are those that work the way full-size ones do. Scale functioning hinges allow the control surfaces to move like the real ones did, and this is an important detail if you want maximum static-competition points. On WW II fighters, for example, the control-surface hinges were inset to help counterbalance the control surfaces so the pilot could handle the loads. In this case, the scale-hinge arrangement adds to the static score while decreasing the servo loads-both good things!

A good hinge is friction-free; the control surface will flop around from its own weight. Stiff Mylar sheet hinges won't allow this, and they aren't recommended for scale models.

* A good hinge is stiff, with no unwanted motion (slop) between the servo and the control surface or between the control surface and the surface it is attached to, even with a reasonable load applied to it.

* A good hinge provides a seal and doesn't allow airflow through the hinge line. Airflow reduces control effectiveness and produces higher drag and higher servo loads. This produces erratic control response, and your model "jumps around" and becomes difficult to trim.

COMMERCIAL HINGES

Though not as effective as scratch-built scale hinging, commercial hinges are suitable for sport scale. To minimize control binding, all the hinges must have their hinge pins in a perfectly straight line. The best way to achieve this is to make an assembly jig to hold the hinges in alignment and with the correct spacing. Using a jig also lets you test the hinges' action before you install them in the model.

THE HINGING JIG

Make the jig out of straight pieces of balsa. The base sheet should be as long as the control surface and about 1x1/4-inch thick. Sheet balsa is preferable to stick balsa because the thin edge can be trimmed to a straight line, and it doesn't readily bend across its width. Thick sheet is more likely to remain flat. Bevel the hinge side about 23 of the width to allow the hinges to move, but try not to alter the straightness of the edge. To keep the jig parts straight, glue them together on top of a flat surface. I use 3M-77 spray adhesive to glue the parts together; to take it apart later, I soak the jig in thinner. This allows me to use it again with different hinge spacing.

Don't get any glue on the hinges; otherwise, you won't be able to remove them from the jig. Place the hinges along the beveled edge, tight against the edge and spaced apart as required. Fill in the spaces between the hinges with balsa sheet that is the same thickness as the hinge. Remove the hinges, and complete the sandwich by adding another hard, straight piece of balsa (as wide as the hinges) on top of the jig. Then bevel the top of the front.

USING THE JIG

Use contact cement to hold the hinges in the jig. Let the cement dry for a few minutes before you place the hinges in the slots. Use the jig as a guide to mark the hinge-slot spacing in both the control and the supporting surface. A long knife blade makes a good marker. Use a straightedge to check the alignment of the slots in both the control surface and the supporting surface, and then cut the slots in the control surface and the supporting surface with a sharp hobby-saw blade. To widen the slot so the hinge can slide in freely, just push it in and twist it back and forth while cutting.

To prevent glue from binding the hinge pin, dip it in melted paraffin or beeswax. Fold the hinge tabs tightly back on themselves, and dip just the barrel into the melted wax. (Don't get any wax on the gluing surface.) Replace the hinges in the jig, and lightly apply slow-setting epoxy or Pacer Hinge Glue in the hinge slot with a knife blade. Smear a bit of glue on the hinge tabs, and glue all the hinges into the control surface (while they are still in the jig). Wipe off any glue that has squeezed out, and allow the adhesive to cure. When the glue has dried, remove the jig from the hinges.

THE REMOVABLE SIDE

Permanently installed control surfaces are hard to work around, so to make it easier to paint and repair the model, I make my control surfaces removable. I use socket-head sheet-metal screws instead of glue to hold the hinges in the slots. Lengthen all the hinge slots in the supporting surface about 1/4 inch at each end. Enlarge the slots to 1/16 inch wider than the hinge tab. Make all the enlargements on the top side of the hinge line. Cut pieces of 1/16-inch birch plywood about 1/2 inch wide by 1 1/8 inches long, or roughly the same length as the slots. Glue these pieces into the tops of each slot, leaving room for the hinges centered along the hinge pin. Hold the control surface in place, and position it so that there is clearance at each end. Press the hinges tightly against the fixed surface's trailing edge, and press the hinge tabs against the underside surface.