Hobby Shop EZB, Part 4
An article by Larry Coslick
Illustrated by Steve Gardner,
published in Indoor News and Views (INAV)
WING
| Projected Weight.----------------------------------------------------------- | .15 to .16 gr. | |||
| LE ------------------------- | .030" X .067" X 10.5"------------------------ | 5.5# ---------------- | .028 gr. | |
| LE ----------------------------------------- | Deflection--------------------- | 5/16" with .340 gr., paper clip at 5" | ||
| Trailing edge --------------- | .027" X .067" X 16.5"------------------------ | 5.5# ---------------- | .031 gr. | |
| Trailing edge ------------------------------- | Deflection ------------------- | 1- 1/16" with .20 gr. clip at 8" | ||
| Tips ------------------------ | .025" X .058" | -------- .025" X .035" ------- | 4# ------------------- | (2) .022 gr. |
| Ribs ------------------------ | .020" X .055" X 3"---------------------------- | 4.5# ----------------- | (3) .010 gr. | |
| Posts ----------------------- | .035 X .062 X 1.25" ------------------------- | 6# ------------------- | (2) .009 gr. | |
| Paper tubes ---------------- | 3 wraps of Condenser Paper, or light Japanese tissue------------------ | (2) .003 gr. | ||
The leading and training edge spars are cut from selected sheets of A grain stock as described in the wood selection article. Use a Jim Jones or Harlan stripper to cut the spars to shape. Test each spar for weight and stiffness using the deflection gauge. Select the L/E and trailing edge spar that comes closest to the spec sheet. The front spar is the most important component of the wing. It must be stiffer then the rear spar for the wing to resist unwanted flexing. To save weight the wing tips can be cut from very light wood. If you can find 3.5# use it.
Leading edge spar - This spar is 10.5" long and is not tapered except for the last 3/4" on each end. Hand sand or cut this taper from .067" to .058".
Trailing edge spar - This spar is 16.5" long and the last 4" of the top of each end tapers from .067" to .035". Scribe a line to show the taper and sand or cut along the line. Mark the top of this spar with a felt marker to prevent turning the spar up side down.
Tips - The tip wood needs to be sanded from 1/32" stock to around .025", not less than .022". Use 4# wood or less. Use a Harlan stripper, if you have one, or a straight edge to taper the 8" tips from .058" to .035".
Template - Mat board of the kind used to mount pictures or photos makes very good template material It is available at all art stores and most picture framers. Balsa sheet 1/16" thick is also good. Make sure that allow for the width of the spars and another .050" when you make the template to stay under the 3" chord limit for EZBs. Apply CA glue around the entire template edge and sand smooth when dry. This will prevent the template from swelling when you use water to make the bend in the tips. Pin the template to your building board with poster pins. These are 3/8" long pins with plastic heads. Push the pin all the way down to the heads so that they are not in the way of construction.
Construction - The first step is to soak the tip wood in water to allow them to be bent around the template. Gene Joshu suggested a good way to soak the tip and stab outlines. Lay the wood on a Formica counter, top or table and use a watercolor paintbrush to run a bead of water along both sides of the wood. Let the water soak for about a minute, then place the tip with the .035" end at the rear splice marked on the plans. Trap this end of the tip in place with a balsa block and a pin and wrap the wood around the template while holding a very light tension. The other end of the tip will extend past the front splice. This will be trimmed off later when it will be matched to the leading edge spar. Once the tips are dry ( about an hour) lay the rear spar in place with the top side marking up, and cut the scarf joints in the spar and the tip. Pre-glue and attach each tip to the rear spar. Place the leading edge on the template. The wood will extend beyond the rib. Make a scarf joint 1/8" beyond the rib and attach both tips to the leading edge spar. Be careful when making the last joint, its easy to cut either the tip or the spar too short.
Ribs - Sand a small sheet of 4.5# A grain balsa to .020". Strip 5 straight ribs .020" X .055" X 3.25", two of these are spares. Soak the ribs and then stack them on the rib form to dry. (See illustration) The ribs are placed with the front end against the leading edge spar, then they are carefully trimmed to length at the trailing edge spar. Check to be certain that the rib is not too long, forcing the spar apart or adding bend to the rib. Pre-glue the ends of the rib and the spot on the spars where the rib will be glued. Wait about ten seconds and place glue on one end of the rib and attach it to the spar in the proper place, then glue the other end of the rib to the spar. Make sure that the rib is vertical before this glue dries. After the ribs are placed its best to leave the wing on the template for one day. Make sure that the center rib is installed perpendicular to the wing spars to properly locate the wing posts. The wing post jig centers each post on the rib location. This jig is illustrated in the final assembly section.
Covering - This subject is not covered in this issue. I did a covering article which appeared in INAV issue 65,66,67 Jan93. If you need a copy, send a self addressed stamped envelope to INAV.
Placing Dihedral - After the wing is covered turn the wing over on a clean flat surface. Take a sharp double edged blade and cut scarf joints on the tip side next to each rib. Don't cut all the way through the spars. Lift the center section of the wing 2" above the table and break each joint where the cut was made. The tips will touch the table. Now support the center section with balsa blocks. Place a small amount of thinned carpenters glue in each joint. After 2 minutes re-glue the joint. Carefully turn the wing over and block up each tip 1.7". Make sure the wing is not over 18" long from tip to tip. Place a small weight on top of the spar at each tip rib. After about one hour lift the wing and inspect each dihedral break. If there is a gap, close it with a sliver of balsa.
Wing Posts - Strip the posts 1/32" X 1/16" X 1" , 6# wood. Wing post installation is described in the final assembly section.
Paper tubes - Cut another piece of 1/32" X 1/16" balsa to use as a form for the tubes. Cut the tissue or condenser paper .into 3/8"X1" pieces. Apply a bit of ambroid glue to one end of the form and place the tissue so that it is ready to wrap. The tissue should extend off the end of the form by about a 1/16" so that you will have an end to grab when you pull the tube off of the form. The glue will help you start the wrapping by holding the end of the tissue. After the first turn, when the tissue is starting its second layer, put a fairly large blob of glue on the tissue right at the form Now as you continue to wrap the tissue around the form this glue will spread out and coat each wrap in the whole length of the tube. Once you have three or four turns wrapped around the form immediately grasp the end of the tube extruding past the end of the form with your finger and pull the tube off the form. Set aside to dry an hour, then place back on the form and recoat the outside of the tube. Once the glue is on the tube pull the tube off again and let dry completely. Do not put the tubes on the wing posts too soon, or they will stick. A good idea from Steve Gardner.
Prop
| Projected Weight----------------------------- | ---------------------------------- | .170 gr. | ||
| Prop Spar------- | 12.5" X .047" X .075"----- | 025" X .025"--------- | 5.5# ----- | .035 gr. |
| Prop Spar------- | B grain--------------------- | Deflection 3/8" each side with a .20 gr. paper clip | ||
| Prop Spar Wire------------------------------ | .010 music wire + spar----------- | .044 gr. | ||
| Prop Blades----- | 5.0 sq. in each blade------- | 4.0# .008"----------------------- | (2)----.120 gr. | |
| Prop------------------------------------------ | 14" X 25" Pitch | |||
Prop Spar - The spar is double tapered from 1/16" B grain, 5.5#. Look for clear uniform grain and cut several 1" X 7". Sand a taper from 050" to .025" using a 220 grit sanding block. The spars are double tapered by cutting the second taper into them when they are cut from the sheet. Use a Harlan stripper or a straight edge to make this cut. Make several spar sets from each sheet. Test each spar for deflection as you did the boom. Both prop spare should match each other closely in deflection. Record the deflection of each set of spars. Pick the lightest stiffest set of spars to use for the prop. When your final selection is made, cut a long scarf joint on the big end of each spar. (see drawing) Pre-glue the ends of the spars and join the two with ambroid. Pick up the spar after several minutes of drying time and realign if necessary.
Prop Shaft - I have used several styles of prop hooks and the S hook works best for me. It centers the 0 ring and does not creep up the hook. Sharpen one end of .009" wire and punch a hole through the narrow portion of the spar.(see drawing) Hone the end of the .010" prop shaft and push it through this hole in the spar. Leave just enough wire to accept 1 thrust washer and clear the end of the bearing by 1/16". Place needle nose pliers at the front of the prop spar and push the prop spar back towards the hook. Bend a 90 degree angle in the wire. Leave .2" of wire to glue to the prop spar. CA the wire to the spar using a straight pin to apply the glue. It just takes a small amount of CA so do not overdo it.. Check the spar on the dummy motor stick for trueness. The .2" of wire on top of the prop spar allows for easy handling when truing up the prop spar.
Blades - If at all possible, order .008" C grain from Indoor Model Supply. It's difficult to find good C grain at a hobby shop. If you want to use hobby shop wood for the prop you must choose the lightest piece of C grain 1/32" balsa that you can find. You can't use 5# wood and expect the prop to weigh .17 grams. The EZB will fly OK with a heavier prop, but the performance will fall off quickly with every bit of extra weight.
Blade Construction - The blades are assembled on a 4"x10" piece of the green cutting mat from the fabric or stationery department of Wal Mart. My prop blade template is cut from thin aluminum flashing material (available at any hardware store). Diagonal lines are drawn on the template to indicate the overlap. Place the lip of the template over one end of the balsa sheet. Outline the tip with a series of dots 1/8" away from the template. Move the template tip down the sheet and outline the tip again. Do each section two at a time. The reason for placing the two sections together is in hopes that the grain will match as closely as possible. After the pieces are cut out the first tip (A) goes with the first center section (A) and so forth. The sections are glued together so that the diagonal joints face the hub and toward the front of the spar. The tip will overlap the center section, and on down the line. Each overlap is about .025". Use very thin ambroid and lay a thin line of glue along each face to be glued. When dry, lay the tip over the center section .025". Hold the two sections together on the mat and run a small brush loaded with acetone across half the joint. After 10 seconds, slightly rotate the two sections so they won't stick to the mat. Now do the other half. Do not use any more glue or acetone. Repeat this process on the remaining sections.
Place the glued prop blades in a heavy book and press overnight The next day, lay the blades, stacked on top of each other, on the green mat. Make sure that the diagonal lines match up. Lay the metal prop template over the wood. Use a new razor blade and cut both sides of the template. As you come toward the tip make small straight cuts instead of trying to get the blade to follow the sharp curve of the tip. Work around the tip and rotate the mat as you go. If the cuts are small enough you will have a perfect curve and no sanding be needed. Weigh and record the weight of both blades. Draw a spar line on the back side of each blade where the spar will be placed. This can easily be done by stacking the blades together and pricking the wood with a straight pin. Place a straight edge along the two small holes, and draw the line with a very thin tipped marker. Do not use a sharp pencil or an ink pen as this will damage the thin balsa.
