On second thought, the dictionary definition of "hinge" is not too far off for miniature aircraft control surfaces - "A device allowing one part to turn upon another." The word "allowing" is technically correct but we would like to approach it in much stronger terms such as "assuring." Then we would also want our hinging to be pleasing to the eye.

Ten years use of a particular hinge, in five aircraft involving about 400 hours of flying, without having any control surface problems is quite "assuring." The Robart 3/16 inch diameter hinge has proven to be very reliable. Admittedly, it is not the easiest hinge to install but the nice appearance of the hidden hinge line makes up for the slightly more involved installation. In fact, a good friend urged me to write this article because the installation looks so good. He thought some information would encourage those who were intimidated by the seemingly difficult job.

I have used three 3/16 inch Robart hinges per surface on all my airplanes except for POGO IV, a 9 1/4 pound airplane with a Zenoah 23 engine. On this airplane I used three 1/8 inch hinges per surface. The 3/16 inch hinges have proven satisfactory for the larger 18-19 pound airplanes. All installations were like, or similar to, the Tiger II elevator shown in the figure.

The concave part of the hinging area seems to be the largest concern - so lets start there. Two tools are needed that are not usually available. I use a half round professional woodworking tool with a cutting radius of 4.75 mm or .14 in. This is small enough but not as large as required for some control surfaces. However, it can be used where larger radii are needed by rough cutting to the approximate radius. This tool is relatively expensive so a substitute tool may be in order. The brass tubing tool shown in the figure is made from model airplane store brass tubing of the diameter for your job. The wall thickness is exaggerated to show the cutting edge better. I prefer to cut the balsa somewhat undersize, then sand the concave surface to the finished size. The sanding is done with a wooden dowel (s) of appropriate diameter and about 4 inches 1ong. Use an aliphatic glue to glue sandpaper (about #80) around the dowel. The brass tubing can be sharpened by grinding or filing a 45 degree cut on the outer diameter and using a propeller reamer on the inner diameter as shown in the figure. Use several cuts to approach the final shape of the concave surface and use the sanding dowel (s) for the final finished surface. A tapered wing or surface is obtained easily with two or three sanding dowels of the appropriate diameter.

The convex leading edge of the control surface is made very simply. I use a razor plane to get the approximate shape while checking for fit with the concave surface. A sanding block with #80 paper is used to get the final shape and fit.

The hinging involves putting the elevator surface into the concave surface of the horizontal stabilizer (for example) and aligning per the plan view in order to mark the hinge locations. Mark hinge locations with a straight edge and a soft pencil with a soft touch to prevent denting the balsa wood. Separate the pieces then mark the end view center lines on the horizontal stabilizer and elevator by using a small square to maintain plan view alignment with the previous pencil lines. I use two steps in drilling the 3/16 inch holes for the hinges and I always finish the control surface then go to the main surface. I drill all holes by hand using pin vises. An undersize hole in a wooden dowel will provide a good grip on the drill for hand drilling if you don't have pin vises. The first step uses a 1/16 inch drill for the pilot hole for the final 3/16 inch drill. The 1/16 inch hole is drilled slowly and carefully while checking for plan and end view alignment as the drilling progresses to proper depth. The 3/16 inch drilling in the pilot hole is done similarly. This drill should be very sharp. The control surface is notched to allow about 15 - 20 degrees of hinge deflection. Hinges are temporarily put in the control surface and deflected while aligning with the main surface to check for hinge alignment with marks on the main surface. After any slight corrections are made, if necessary, the outermost hinge hole is drilled in the main surface using the same procedure used on the control surface.

The outermost hinge is installed in the control surface and this is installed on the main surface to check for alignment. Misalignment up to about 1/16 inch can be corrected by using a small rat tail file and balsa slivers to shim a misaligned hole. This procedure is used as each hinge is temporarily installed. The next step is to check the hinge line by proper insertion depth of the hinges and to check for any interference with control surface deflection. Keep in mind that the hinge line should be in the center of the control surface leading edge radius. About 1/32 inch clearance is necessary to allow for the concave and convex surface covering. Hinges are not glued in until the airplane is completed and covered. The concave surface of the horizontal stabilizer is covered using the Top Flite Trim Tool. The deepest part of the concave surface is ironed down first for the full length, then the ironing progresses outward to assure good conformity of the covering to the concave shape. A hot soldering iron tip is used to remove the covering over the hinge holes.

I prefer to glue the hinges in two sequences. The first sequence is to glue the hinges only to the elevator surface. I use an aliphatic glue (Pica Gluit for example) for this job. Use a toothpick to work glue into the hinge holes in the elevator. This should be done to get a fairly thin uniform coating. Use fingers to get a thin uniform coating on the barbs of one half of each hinge. Insert these barbs into the elevator to the depth of the hinge line. Then insert the dry barbs into the dry hinge holes of the main surface while aligning and checking for proper hinge line location and elevator deflection. Allow glue to set before removing the elevator. The last sequence is to remove the elevator then use the procedure above to glue the main surface half of the hinges in place. The hinge line gap should only be enough to prevent parts from rubbing.

This extra work and care is only a small percentage of the overall time invested in your Giant Scale Model and in addition to the appearance provides some other small advantages. This hinging moves the hinge line to the rear giving a very small amount of aero and mass balance. It also can give a little better aerodynamic seal. Incidentally, here are a few things to keep in mind relative to control surfaces. A shock mounted engine will reduce control surface hinge wear. Short stiff push rod connection from servo to control surface will reduce possibility of flutter. A lighter and less massive control surface to the rear of the hinge line (consistent with required strength and stiffness) will also reduce the possibility of flutter.