F1D TailBoom / Fin / Stabilizer Construction
An article by Steve Brown from Indoor News and Views (INAV)
Completing the fuselage of an F1d involves the same tradeoffs between weight and strength as does motorstick construction. A well built model will maintain its adjustments under power, accept the stress of ground handling and still meet a weight target. The comments below refer to an F1d with a suspended, unbraced parabolic stabilizer. Many of the techniques also apply to braced stabilizers. I favor unbraced stabilizers because, in my experience, they recover from tailslides better than braced stabilizer models and they are easier to build to weight.
Boom wood selection - The factors previously discussed for motorstick wood selection apply to the wood for tail-booms: uniform thickness, straight, consistent "C" grain, and sheets that lie relatively fiat when placed on a flat surface. I say "relatively" because the majority of wood, when cut to .007" or .008", displays some tendency to curl or twist. More latitude in sheet weight exists when selecting boom wood. Booms can be built with or-without boron reinforcement. Look for .007" thick wood that weighs .0060-.0070 oz, but good booms can be built of heavier wood, up to .008 oz, without boron.
Rolling the boom - I use a 22" long former tapering from .235" to .125". This gives an i.d. of .160" at the small end of a 13" boom. The idea for using a Iow-taper former originated with Stan Chilton. It is not only stiffer, but it has less tendency to split at the small end if a model tailslides. You can obtain low taper formers by looking though the fiberglass rod components at a fishing equipment shop.
I usually use the traditional method of cutting an exact size tapered blank. Assuming that you are using a .250" i.d. motorstick, locate the .235" o.d. point on the boom form. Wrap a piece of .125" wide masking tape around the .235" diameter. Measure 13" down the form and wrap a second .125" piece of tape around the form at that point. Slice through the overlapping tape and remove both pieces from the former. Mark two points 13" apart on the sheet of wood and trim one edge of the wood straight. Using magnification, place the .125" strips of tape on the wood perpendicular to the straight edge. Position a straightedge adjacent to the ends of the tape. Allow about .015" extra width to compensate for shrinkage. Cut the second side of the blank. I usually make the blank about .250" longer on both ends than my target length, to allow for an exact fit with the motorstick i.d.
Soak the wood for 15 minutes in cool water. Cut a piece of 00 Silkspan !o a wedge shape about 2" wide at the wide end, 1.25" wide at the narrow end, and 14" long. Place the Silkspan on a flat work surface and soak it with water applied with a 1" wide brush. Brush out all the wrinkles. Position the boom former along the bottom edge of the Silkspan and attach the Silkspan to the former. Roll the former about 2/3 turn. Place the wood blank adjacent to the former and roll. Bake at 150 degrees F for 20 minutes.
Carefully remove the blank from the former 00 Silkspan is easier to separate from the wood than is Japanese tissue. The tube will almost always have a slight curve. If the tube is very curved or "doglegged" discard it. It is impossible to straighten a boom that is excessively curved.
Gluinq the seam - Spray the former with aerosol Teflon and allow to dry. Place the former in a simple jig like that shown in the sketch. Weigh the rolled balsa tube and then place it on the
former. Using the wire loop pull the small end of the former down about .180". Don't overdo it. The bend helps counteract the tendency of the glue seam to warp the boom. Position the wood with its curvature opposed by the bent former.
Recently I have been using Stan Chilton's method of applying glue to seams with a brush. Use Ambroid thinned 50/50 with a mixture of 1/2 acetone, 1/2 lacquer thinner to retard drying, and 4 drops of TOF to the ounce of thinned mixture. The disadvantage of the brush technique is that so little glue is applied that it tends to dry before the joint can be made. If more is applied it is easy to glue the wood to the former. By slowing the drying time and spraying the former with a non-stick coating these problems can be minimized.
Begin gluing in the middle of the tube, aligning the edges to produce a straight seam. When complete, leave the tube on the former for 6-8 hours to allow the glue to dry completely Weigh the glued tube, subtract the weight of the bare rolled tube and you should have a seam weight of .0002-.00025 oz.
Applying boron - Remove the tube from the former. Now is the time for a "judgment call." If the tube is straight and seems very stiff, omit boron. Tubes that don't require boron weigh about .0030-.0033 oz. for a 13" length. If the tube is still curved or is very light (.0027-.0030 oz) boron is usually necessary.
Using the same jig as described in my motorstick construction article I apply .003" o.d. boron (from Model Research Labs) to the top and bottom of the tube. Apply the boron to the side away from the curve first, using thinned Ambroid with TOF in a 26 gauge hypodermic needle. Two boron strips will add about .00035 oz.
Buildinq the suspended, unbraced stabilizer - Obtain .028 - .032" thick, 24" long wood from Indoor Model Supply. Grade the wood to find a sheet of about 5.5 lb. density that is stiff and springy. Place the wood on a flat surface, mark the center with an indelible marker and begin sanding from the center to the ends with a 1.125" wide sanding block Only experience will tell you how much wood can be removed. I usually taper the sheet from .028" thick on the ends to .022" at the center.
Once the sheet has had the taper sanded into it tape it on a cutting board and true the edge. Mark each end to .050" tall by dragging the sharp edge of a razor blade through a felt-tip marker or ink pad and making a tiny cut at the .050" mark. Repeat the process to mark .030" at the center. Position a straightedge against these marks and make two cuts. You will have a double-tapered spar.
Soak the spars for 30 minutes in cool water. I bend the spars, two at a time, around a parabolic form made from 1/8" balsa Secure the large ends to the form using tiny rubber bands made by cutting 3/32" wide pieces of a toy balloon These balloons are
available in bags and are about 1/4" wide and 5" long. Bake the form for 20 minutes at 150 degrees F.
I build and cover on the same smooth, matte-finished painted particle board surface. Assemble the stabilizer over a pencil outline. The building board has 1/6" thick wood sub-ribs under all three ribs. This assures that the ribs will stay at the proper height during covering. Be sure that the covering board has one or two vent holes to allow air under the microfilm to escape. Positioning the two halves of the stab outline over the pencil outline on the building board. Hold the pieces in place with small weights. Tack glue the large ends of the outlines together at the two points where they overlap. Remove the outline from the form, cut scarf joints and glue.
Reposition the complete outline over pencil outline. If the wood does not lay fiat on the building board weight the entire perimeter of the wood down to the board with lead weights and brush on water. Allow to dry overnight and check for flatness. Repeat if needed. Soaking the wood helps relieve any warps or stresses.
Cut three ribs from 5.2 LB "A" grain balsa. Make the two outer ribs of .025" thick wood .030" tall. Make the center rib .025" thick and .045" tall. It is important to make the center rib strong. With experience you may be able to reduce the height of the center rib to about .040". Glue all three ribs using Ambroid with no plasticizer. I always "double glue" rib/spar joints by lightly coating both surfaces with tiny amounts of glue, waiting about 10 seconds, and then applying a second coat of glue to one of the surfaces Hold in position until dry.
The stabilizer should weigh .0028 - .0032 oz. before covering. If it is less I would suspect it won't be stiff enough to work unbraced. If more, it is too heavy.
Covering the stabilizer - Select a sheet of straw or gold microfilm that has been aged at least 60 days. Slacken the film by placing 1/4" wide strips of masking tape about 1/8" inside the perimeter of the film frame. Cut the sheet loose so that it is suspended by the corners. Place small pieces of tape at the mid-points of the long sides to reduce the billow of the film. I generally make the sheet quite slack, since stabilizers are easily warped by taut film.
Position the outline on the building board and adhere the balsa to the board with water. Do not get water on the inside of the outline. Using a 3/16 round sable brush adhere the ribs to the balsa sub-ribs with water. This will keep the ribs straight. Make one pass around the outside of the stab leaving a light layer of water on the board outside of the balsa outline.
Lower the film frame over the building board. Blow lightly on the film the be sure that it touches the tops of all the ribs and outline surfaces. Let dry overnight. The outline will shrink as it dries which will add more slack.
Fin construction - Make a circle of .004" boron 4.25" in diameter Glue the overlapping joint and cut off the excess boron. Cut a 3.75" tall vertical balsa upright from a sheet of stiff .028" 6.0 lb. "A" grain balsa. I usually taper this piece from .055" wide in the center to .040" wide at the ends. Position the boron circle so that the overlapping joint is behind the vertical upright and glue the two points where the circle contacts the tips of the wood. Let the glue dry well since considerable stress is transferred through these joints when the boron is broken off. Using smooth-jawed needle-nose pliers or hemostats, gently squeeze the boron/wood joint and break off the unwanted part of the boron circle, leaving a "D" shaped structure.
Cover the fin by outlining a 5" X 5" area of violet or blue slack film with 1/4' tape. Using a #1 sable brush apply water or saliva to both sides of the wood and to one side of the boron. Immediately drop the fin onto the film and allow to dry. After 30 minutes or so blow lightly on the film to check for un-attached areas of the boron. If you find any work a tiny amount of moisture under the outer edge of the boron and allow to dry. Cut both the stab and fin loose with a hot wire or battery powered soldering iron.
Assembly - Use a jig to position the stabilizer with the stab tilted at an angle parallel with the angle of the rear wing spar. The boom should be supported so that the correct incidence angle is formed with the stabilizer. Tack glue the stab to the jig. Butt-joint the boom to the stab and then align the large end of the boom with the centerline marked on the jig. Tack glue the large end of the boom to the jig.
Position the fin at the boom/stab juncture and glue the rear joint. Make sure the upright is vertical both from the side and the front by visually comparing it to a square. Cut a piece of balsa .020" X .020" X .750" long and butt glue it to the side of the boom so that it touches the front edge of the boron circle of the fin. Glue the boron to the balsa to provide .125" of rudder offset. Clip off the excess balsa with sharp scissors. Glue a .020" square peg of balsa on the opposite side of the boom near the front of the fin. I brace stabilizers with .0003" tungsten wire from Ray Harlan or Indoor Model Supply. I prefer it to thicker wire because it bends around curves more readily.
Weight one end of the wire with a weight of about .002 oz attached with a 1/16" wide piece of masking tape. Measure about 22" of wire and attach a second weight. Cut the wire behind the second weight and position the wire beginning at the front at the .020" square peg, over the top of the upright, around the back of the stab and under to return to the peg. Examine the wire in a cross-light to look for kinks or areas that are not properly tensioned. The wire should be taut. Glue all points, beginning at the rear of the stab and finishing at the peg on the boom. When dry, cut off the weights. Remove the complete assembly from the jig by softening the glue joint at the front and rotating the dowels gently under the stab spars. Weigh the complete assembly. With a boom length of 13" I look for a weight of about .0075 oz.
Stabilizer incidence can be adjusted in the field by softening the glue joint with acetone and lightly repositioning the stabilizer. I find that I only have to do this once or twice with a new fuselage and the adjustment is generally good until the boom/stick joint is altered, usually following damage
Please contact me if you have any questions or comments: 297 Hartman Ct., San Dimas, CA 91773-2152. (909) 394-9685.
