F1D Motorstick Construction
An article by Steve Brown from Indoor News and Views (INAV)
The motorstick may be the most important single component of an F1d. You can change the wing, even prop, but it seems that motorstick determines the way the model flies. The stick is also the single heaviest part of the model and is a logical candidate for weight savings
Wood Selection: Wood that is suitable is rare and stringent selection is required. Examine each sheet by laying it on a fiat surface. If the sheet isn't perfectly flat remove it from consideration. The grain should be parallel to the edges. Hold the sheet up to a light and look for density variations or heavy streaks. Compare all the sheets to each other to find the stiffest sheets
The most significant variable in the weight of a completed stick is the weight of the raw wood that forms the tube. I eliminate all sheets that weigh more than .0098 oz for a .013' x 1 1/8' x 18" sheet. The density of a sheet of wood can be misleading since it tells nothing of the uniformity of the grain and resistance to bending. The "density" of the sheet is actually an average of the variations (hopefully few) in density along the length of the sheet, it is rare to find wool less than 3.8 lb. density that is useful.
Use a dial thickness gauge and measure the thickness of the wood. I usually check 7-8 spots at random along the sheet. Take care not to compress the wood as you check it. Look for thin spots in the middle. Boron will not prevent uneven bending if there are thin spots in the middle of the tube.
Rolling the Tube: I use a .250" o.d. rod to form the motorstick. I've tried rod diameters as small as .210" for F1d and, while the weight savings can be significant, I have never had any success with smaller diameter. The resistance of the wood to the twisting force of the rubber torque vanes with the density and character of the wood and the diameter of the tube. Small diameter tubes allow tail tilt and wing wash adjustments to change excessively under high torque.
Aluminum arrow shafts (available from archery suppliers) make good forming rods as they are available in 1/64" size increments, have thick walls and are light and easy to handle. Hobby shop tubing, or steel drill rod will also work.
Don't cut the balsa sheet to size before rolling. Trim one edge of the sheet perfectly straight. Position the sheet edge along the edge of your work surface and sand a bevel into the straight edge along the entire 18" length. The bevel should extend about .10" onto the sheet and feather at the edge. Use a waterproof pen (such as a Sharpie) to identify which edge of the sheet has been beveled. Mark both ends.
Soak the sheet in cool water for 15 - 20 minutes. While the wood is soaking cut a 3" X 19" piece of white Japanese tissue. Lay the tissue on the working surface (I use plate glass) and wet the tissue with a soft 1ú wide brush. Use the brush to smooth the wrinkles. Place the forming rod along the edge of the tissue and attach it to the rod. Roll the rod about 2/3 revolution so that the paper is evenly attached.
Place the wood on the tissue adjacent to, and almost touching the rod with the waterproof ink marks facing down. Do not attempt to force the sheet into a perfectly parallel position against the rod. The wood will do whatever its internal stresses dictate when it is baked and it isn't possible to force it to be straight. Roll the tube and bake at 200 degrees F for 30 minutes. Remove the rod from the oven after baking and allow to cool to room temperature. Do not unwrap the rod at this time.
Cutting the Joint: Secure both ends of the rod to the work surface with masking tape to prevent roiling. The ink mark at the overlap should be up. Position a wide metal straightedge as shown. Tape the rear edge of the straightedge down to the work surface. Smoothly cut a clean joint using a new, sharp razor blade edge, while applying light pressure to the straightedge with the other hand. Make 2 or 3 passes with the blade to be sure that all the layers of paper and wood have been cut through. About the only thing that can go wrong with this method is failing to cut through all the layers.
TAPE STRAIGHTEDGE TO WORK SURFACE
Carefully unwrap the outer layers of tissue until the wood is exposed. Using a very fine felt-tipped marker (Sakura Pigma .005 or similar) make 4 or 5 small marks across the seam .dong the length of the tube. These marks can later be aligned and will assist in gluing a straight seam. Remove the wood and the rest of the paper from the rod. Weigh and record the weight of the tube before putting it back on the rod.
Gluing the Seam: I use Ambroid glue thinned 50/50 with acetone for all construction. I plasticize the glue to be used for stick and boom seams with 3-4 drops of TOF plasticizer per ounce of thinned glue. Apply the glue using a 26 gauge needle with the sharp point removed and smoothed, on a plastic syringe.
It is most important that glue be applied only the edges of the wood. Use eye magnification. Non prescription magnifying eyeglasses work well. The glue seam can vary in weight as much as 100% depending on the thickness and amount of glue applied.
Clamp the end of the metal rod in a vise sc that both hands are free. Beginning in the middle of the tube, with the small ink marks aligned, lightly preglue about 1/2" of both edges of the wood at a time. After about 10 seconds apply a second light coat to one side and press the joint together. Minimize pressure from your finger's to the wood, since it i? easy to skew the seam or warp the wood from the moisture on your hands. Allow the glue to dry completely, usually 1-2 hours depending on the temperature and humidity, before removing the tube from the rod. Weigh the glued tube and record the weight. Subtract the weight of the unglued tube from the weight of the glued tube and you will know the glue seam weight. Look for a seam weight of about .00035 oz. for an 18" length.
Thrust Bearing, Webs, Rear Hook: I use a Ray Harlan F ld thrust bearing modified to remove excess metal. The stock bearing weighs .00077 oz. Remove metal with a file from the sides and notch the edges of the top of the bearing until it weighs about .0006 oz. Roughen the top of the bearing where it ',viii contact the motorstick. Be careful not to remove too much metal or break off the pigtails. The aluminum cannot be bent more than once without reducing strength.
I use 4.5 lb. C-grain wood, .018-.020" thick for webbing. Orient the grain vertically. A .013" music wire hook will handle torque up to .50 in./oz, without deformation. Whatever hook shape you choose, remember that it is most important that the rubber motor O-dng be easily attached and removed. I reinforce the joint between the rear hook wire and the wood web with one layer of Japanese tissue, but I don't use any CyA because of its weight.
Front End: The glued tube will probably have a slight curve. The location of the seam doesn't matter, just look at the actual curvature. The tube should be oriented to 'arc "down", that is, to pull against the bracing wire. Mark the top and bottom of the tube 180 degrees apart. Place the tube back on the forming rod and tape both down to your work surface with one of the marks "up". Cut .015" X .750" slots on the top and bottom of the tube, about .25" from the front end. This will help keep the tube round as you work on it. Cut the slot narrower than the thickness of the web and widen it to an exact fit by lightly sanding it with a small piece of 600 grit sandpaper. Install the front web and glue in place. When dry, slice the front of the tube off and install a .013" cap.
Attaching Boron Filament: Boron filament should be held in a jig that tensions the filament and frees both hands. It is critical that the Filaments be glued along their complete length. I use the same plasticized Ambroid glue as for the stick seam, applied using a 26 gauge needle and plastic syringe. Mark the locations for the boron filaments with small dots of ink along the entire length of the tube. Placement of the boron at 12, 3, 6 and 9 o'clock produces the straightest sticks. Unfortunately, locating it at 12 and 6 o'clock causes interference with the stick bracing post. I feel the strength and improved straightness of the
tube is worth the extra work required to install the post. Cut the wood tube to length and place the tube on the forming rod into the jig. Glue the boron filaments, alternating the sides of the tube to equalize stress. Four .004" boron fibers glued on a 14.5" stick will add .0011 to .0012 oz.
Assembly: Once the boron fibers have been installed lay the tube, on the rod, against a metal straightedge and mark the locations at the rear of the tube that correspond to the top and bottom of the web at the front. Cut, a slots about .015" X .060". Remove the tube from the rod. Using a scrap piece of .015" wood inserted in the slots, adjust the slots using 600 grit sandpaper so that both the front and rear webs ,are in alignment. Cut and insert the previously completed web and hook assembly and glue in place. When the glue is dry slice off the excess wood. Cut the excess .013' wire off so that only about .040" extends above the top of the tube.
Install the thrust bearing with no downthrust ,and 2 degrees of left thrust. I use Ambroid with no plasticizer for this joint. Coat the bottom of the bearing and the location on the wood tube with thin coats of glue and allow to dry for 10-15 seconds. Apply a second coat of glue to one of the surfaces and place the bearing on the wood. Set the thrustline by placing a straight piece of .013" wire about 2.5" long in the bearing using it to adjust the angle of the bearing. This must be done quickly or the joint will be weakened. Once the glue dries apply a second coat in a small "fillet" along the edges of the bearing where it joins the wood.
I use a single 1.75" tall bracing post on all my models. In conjunction with 4 boron filaments it is the strongest bracing method I've tried. Make the center bracing post of 6.5-7.0 lb. "A" grain wood, cut and drilled as shown below. Mark the appropriate locations on the top and bottom of the wood tube and make small holes on either side of the boron with a sharp pin. The holes should be slightly smaller that the bottom diameter of the bracing post. install the post by lightly "worrying" it into position. The boron filaments will locate themselves in the drilled holes. Be sure that the wood tube stays perfectly round. Bracing: A bracing jig assures repeatability when tensioning the stick bracing Wire. I use .001' tungsten Wire from indoor Model Supply and haven't encountered any breakage. 1 don't see a need for heavier Wire or double strands. Don't use wire that will stretch, such as nichrome.
CENTER SADDLE .040" LOWER THAN FRONT AND REAR SADDLES SECURE TUBE TO CENTER AND REAR SADDLES WITH MASKING TAPE
Place the motorstick in the jig and use small pieces of masking tape with tissue pads to secure the tube at the rear and to pull the tube down to touch the center saddle. Tie the tungsten wire around the stub of the rear hook that protrudes through the top of the stick and glue. Weight the wire with 2 Quarter coins. It is better to have a little too much bow braced into the stick than too little. You can always adjust the tension of the wire at the flying site by lightly sanding the top of the bracing post. The goal is to obtain a bracing tension that allows the stick to ice straight (no up- or downthrust) at full winds. Don't glue the wire to the top of the post, it is helpful to be able to remove it later.
I use rectangular wing tubes formed by 3 turns of Japanese tissue around a .035' X .064' brass former. I install them by placing the motorstick in a jig that makes round pilot holes at the appropriate angle. The pilot holes are then enlarged with a rectangular toothpick that has been sanded smooth with the comers rounded and the proper width marked.
It isn't possible to cover ail the details in an article this short. If I can answer any questions write me at 297 Hartman Ct., San Dimas, CA 91773, or call (909) 394-9685 evenings or weekends.
