Hobby Shop EZB, Part 3
An article by Larry Coslick
Illustrated by Steve Gardner,
published in Indoor News and Views (INAV)
Wire Bearing and Rear Hook
The wire bearing, called a "thrust bearing", is made from .010 music wire. To make the bearing the wire is tightly wrapped around a piece of forming wire that is .001" larger than the bearing wire, or about .011". All the "music wire" mentioned in these instructions can be purchased very inexpensively at the local music store in the form of Guitar strings. A very good pair of needle nose pliers are a very nice thing to have when making thrust bearings, if you are going to fly indoor, get some! See the illustration on bending the bearing. Note that the bearing supports the prop shaft at two points. There is the front of the bearing, and there is the "pig tail", so called because that is its shape. After the bearing is formed, it will usually require some adjustment. The pig tail might be out of align with the from of the bearing, or vice-versa. Insert the forming wire in either the pig tail or front of the bearing and bend to realign. The bearing must swing free on the prop shaft. This will not happen until the front of the bearing and the pig tail are in near perfect alignment.
Before mounting the thrust beating to the motor stick, make sure that the prop can be threaded through the bearing. If the bearing front end is not ground down far enough, or if the pig tail is not properly formed, the prop shaft will not thread onto the bearing Make sure that the front of the bearing is ground down to match the drawing. if the Problem is with the pig tail, you might be better off by just making a new beating Once the bearing is made and you have it aligned you can use it to help get the prop shaft square with the prop spars. Temporarily mount the bearing to a l/8" sq. piece of balsa, like a false motor stick. Do not mount the bearing on the real motor stick for this step, the pressure of getting the prop shaft straight might weaken the glue joint. At this time I have the prop shaft mounted to the prop spar. No blades. Put the shaft through the bearing and hook up a thin loop of rubber. Put in some hand winds and check to see if the spar is running true. If there is any wobble in the prop spars as they turn, make note of which spar is most forward, and then, grasping the prop spar where the wire shaft is bent and glued to the spar, bend the shaft until the prop spars turns straight. Go easy and make very small corrections.
Remove the thrust bearing from its temporary mount and clean off any glue. Cut a 1/4" deep slot in the front of the motor stick. Angle the slot to provide 2 degree left thrust. Place a piece of .010" wire 3" long through the bearing to check the thrust line. Slide the bearing into the slot. The reference wire should be .150" below the bottom of the motor stick. Do not place glue in the slot. The front of the bearing should intersect the lower right angle of the motor stick. (see drawing) Take a new razor blade and cut the front of the motor stick to match the front angle of the bearing. Recheck for 1 degree down and 2 degree left thrust. The front of the bearing must be flush with the motor stick. Apply two thin coats of glue, to the wire and wood. Build up a small glue gusset where the pig tail and the front of the bearing meets the wood. No extra glue is needed.
Cut a 1/64" slot at the rear of the motor stick. The motor stick and boom are joined by a scarf joint. Cut apiece of .009" wire 5/8" long and bend over one end 1/16" long. The 1/16" hook will be imbedded in the wood but the wire will be flush with the rear of the motor stick Tack glue the wire in place. Cut an angle on the tail boom to match the motor stick pre-glue both surfaces using Ambroid glue. Attach the boom and make sure the bottom of the boom is even with the bottom of the motor stick. Cut a gusset so that the end of the gusset is .125" below the motor stick The gusset is glued to the boom. Place a strip of Japanese tissue over the gusset and wire. You can angle the wire again where it breaks away from the gusset. Cut the wire to a usable length (see plans)
Boom
| Boom--------------------- | 9.80"---------------- | 6#------------------ | .04 gram |
I cannot stress enough the importance of a good EZB tail boom It needs to be fairly stiff and light. When they are not stiff enough the model will usually flounder under high launch torque.
To get a tapered boom start with a sheet of good clear grained 6# wood 11" x 1" x .062" (1/16" sheet), and sand it down to a taper from .062" at one end to .028" at the other, using a 220 grit sanding block. Once the sheet is tapered in one direction the boom can be cut to a taper in the other direction using a Harlan stripper or a good eye and a straight edge. This taper is from .075" to .028".
The boom is used on the model with the .075" side vertical so that the boom is stiffest in the vertical plane. If you build and use the deflection meter the boom is tested in the same position. Insert the large end of the boom into the hold down and adjust the pivot and the scale until the end of the boom is right at the 0 mark With a .270 gram weight trimmed from a paper clip hung on the very end of the boom, there should be less than 1-1/8" deflection. A deflection of around 3/4" is a good boom.
Stab
| STAB CONSTRUCTION | ||
| PROJECTED WEIGHT | ---------------------------------------- | .05 GR. |
| OUTLINE ------------- | .025" X 0.27" X 24"------------------- | 5.0 # |
| RIBS------------------- | .017" X .027" ------------------------- | 5.0# |
Make the template from .032 sheet balsa and coat edge with CA. Cut vee notches at the rib locations so that the ribs will clear the template.
| FIN--------------------- | .025" X .025"-------------------------- | 5.0 # |
Select from either 1/32" or 1/16" stock for stab wood. Use A grain with a density of 4.0# to 5.0#, and cut the sheets 24" long A 24" outline will wrap all the way around the stab template, but if you have trouble finding a good piece of wood this long you can cut the spars and splice to get the correct length When wrapping the thin outline around the template it's easy to put a twist in the wood. To keep this from happening mark thin black lines ever 4"or 5" along the edge of the sheet you will cut the span from. These lines act as a reference when pulling the wood around the template. To get the wood strip to wrap around the template without kinking you must hold a bit of tension while pulling the spar around the curve of the template. You can either sand the wood to .025" thickness or use Steve Gardner's stripper (see drawing). His stripper cuts the stab and fin outline at the same time from 1/16" sheet. If you sand 1/32" down to .025" it is best to use a Harlan stripper (see tools list) if you have one. The dry outline should not weigh more than .025 gram. A light one will weigh .015 gram. Do not cut the outline dimension any thicker, because it is over-built with the wood sizes shown.
The ribs are stripped .027" high out of A grain and then stacked on a form. See illustration for stab wood stripper and rib form
