BALSA SELECTION
The most important part of building a competitive EZB is the selection of the proper wood for each part. The wood is available at any hobby shop with a fair selection of balsa sheet. Special indoor wood is not needed. The wood used for the prop blades may be a possible exception.
The first consideration when choosing wood is weight. The density, or weight, of balsa is
measured in pounds per cubic foot. We say a certain piece of wood is "six-pound wood", and on some plans it may be marked "6# wood". Each component of an EZB is made from a certain weight wood. The very lightest wood is about 3.5 to 4 pounds per cubic foot. Wood with a weight of about 5 to 6 pounds per cubic foot is much easier to find at an average hobby shop, so this EZB is made mostly from this wood. Take a postage scale to the hobby shop and check each piece before you buy it. To check the density of a piece of wood first weight the piece to find its weight in grams. Then find its volume by multiplying its thickness by its width, and then multiplying that number by its length, in inches. We are mixing units here, but grams (metric) are easier to use for weight, while inches (English) are still what everyone used for small measurements. To use these together we take the weight in grams and divide by the volume in inches, then take that number and multiply by 3.81 to get pounds per cubic foot. A piece of 1/16 X 3 X 36 wood in the 5 pound range will weigh about 8.9 grams and a six pound piece about 10.6 grams. By figuring out what the wood will weight in a certain size sheet you can use a postal scale right at the balsa wood rack in the hobby shop to choose wood. You should buy "A" grain wood for EZBs. (see drawing)
Because the density of balsa wood can vary a great deal in any given sheet of wood the next step is to hold the sheet in front of a swing arm lamp with at least a 40 watt bulb. Turn off all the other room lights so that you can see the light coming through the balsa better. The wood will have a brown color that is lighter where the wood is the lightest in weight. The wood that you want is the lighter streaks or sections of wood that the most light is coming through. Mark these areas with small dots from a felt tipped pen while holding the wood up to the light. When you look at the wood when you turn the room lights back on you will probably notice that the wood you have marked is very light in color, almost white, and that it shows almost no grain at all. The areas marked are not usually very wide, yet you will not need much for several sets of wing spars, or ribs etc. When you cut these very small areas out leave a half an inch or so of darker, heavier wood to serve as a handle for the good wood. This will make cutting spars and other parts from this wood much easier. This method of picking out the wood will work even with 1/4 inch wood which you might use for motor stick wood. Cut the good wood out of the sheet and recalculate the density of the good piece. It might be as light and stiff as the special indoor wood and it has straight, smooth grain.
The next most important thing to check about balsa wood is its stiffness. Cut a test spar from each of the good pieces of wood and test them on the deflection meter. (see drawing) Use colored marker pens to grade the wood for stiffness so that you can tell which piece made the stiffest spars. If you do not mark them you will get them mixed up and have to test them again. You may be surprised at the difference in stiffness between one spar and the next, cut right beside the first. Simply selecting the stiffest wood from a given section will really improve the model.
SANDING
The sanding blocks are cut from pine, .75" X 1.5" X 5". Slightly round the long edges with sandpaper. Cut the sandpaper so it wraps up around both sides. Use 220 wet or dry paper for the fast cut and finish with 360 grit. To sand the wood for the prop blades, or any other wood that you need to be a certain thickness, the ends of the sanding block are spaced up to the height of the wood thickness. To do this shim stock is glued to the ends of the block. It can be made from metal, plastic or masking tape. It takes some experimenting to find the correct amount of shim for each application.
Glass makes a good surface on which to sand. I use a piece of double strength glass 10" X 24" which is mounted on several layers of foam board, painted flat black, (no lacquer). The glass was then taped to the foam board with duct tape to safety and protect the edges.
Sanding prop wood - It can be sanded to around .020" by carefully sanding with a back and forth motion. Once the wood is this thin you must start to sand in one direction only, away from the end that you are holding down on the glass Make sure to stroke the sanding block past the end of the sheet and to lift the sanding block completely off the glass before making another stroke. Start with 1/32" C grain balsa and take it down to. 008". This will take about 45 minutes, so be patient.
MOTOR STICK
| MOTOR STICK-------------- |
8.5"-------------- |
4.5#------------- |
AB GRAIN------------- |
.185 GR. |
Selecting good Motor stick wood is perhaps the hardest part of building an EZB. The wood must be light and springy. Punky wood will take a set, and the models flight characteristics will change making the model's flight unpredictable. Do not accept a motor stick that won't spring back after bending it noticeably to the right and in a downward plane when viewed from the front. When selecting motor stick wood cut them from 3/32" or 1/8" stock, preferably 3/32". For this project I found a piece of 3/32" AB grain. The sheet had several 1/2" wide sections of white wood sandwiched in-between wide bands of dark wood. I drew the outline of the motor stick right on the sheet and out it out with a new razor blade and straight edge. The sides were left straight. With no sanding this motor stick weighed .185 grams, and was just right for this model, I cut 10 sticks and found lighter ones, but felt that this weight stick was one that most modelers could find. If you are able to find a stick that is lighter and stiffer, use it.
Stiffness test for the motor stick- Coins are used to make the weights and spacers for this project because they are fairly consistent and available to everyone. Using new pennies, CA 2 pennies together. Make up several sets. Find a spot on the face of 2 sets that is .12" thick, and mark that spot with a magic marker. Take a nickel and quarter and CA them together to make one of the test weights (*See revision below). Cut a piece of balsa 1/8" X 1/2" X 1" long and CA that to the nickel as a handle. This is one of the weights used to measure motor stick bend. It weighed 10.67 gram Find a dime that is .051" high and CA a piece of thread to one edge. This will be used as a test spacer so don't get any glue or thread on the faces of the coin. The last weight to be used is a 5/8" coarse thread nut (hardware, auto parts store) that weighs 31.89 gram. The support for the nut is called the plank. Make it out of a piece of 1/8" X 1/2" X 4" balsa. On one end of the plank glue a 1/8" square x 3" long foot.
TESTING-Use any flat, hard surface to make this test. Place the motor stick flat on its right side across two sets of pennies with each end of the motor stick resting exactly on the center of one of the penny sets. Turn the penny sets to where the .12" thick area is under the end of the motor stick Use a ruler to find the center of the stick and place the spacer dime under the center of the stick. Place the test weight made from the nickel and quarter above the dime on top of the motor stick. The motor stick is a good one if it doesn't bend far enough to touch the spacer dime. If it is too close to see clearly, then gently tug on the thread to see if the spacer dime rubs the stick. Place the motor stick upright and place the plank end on top of the motor stick. Place the nut on top of the plank with the outside edge of the weight lining up with the outside of the motor stick. Again, the motor stick should not touch the dime. See drawing:
Revision of the Hobby Shopper EZB Motor Stick Test April, 2002
By Larry Coslick
There were several articles in the INAV ISSUE 105 directed toward my testing procedure for EZB motor sticks. After reading the articles, I decided to re-test my M/S's using the same procedure as described in the Hobby Shopper article, ISSUE 90.
The side-bending test in question uses 2 penny sets that are .12" high to support each end of the M/S. A dime is centered under the M/S and a 10.67-gram weight is placed on top of the M/S over the dime. The M/S passes the test if the dime can be pulled out without disturbing the M/S.
After the M/S's were re-tested, it was obvious that there was a flaw in the way that the original test was made. The Hobby Shopper M/S test was designed just for that article and for the M/S's that were cut for the prototype. The problem was that I didn't compare the penny test with my side-bending jig. My jig uses a mechanism that is not available to the public so I had to come up with an easier way to test the M/S's. I happened to select a stiff piece of balsa for the prototype and didn't realize it by using the new test. I would have detected the wood's stiffness by using my jig had a comparison test been made. After the article was complete the penny test was put away and never given a thought until ISSUE 105.
My side-bending jig uses a spring-loaded center post that drives a pointer along a numbered scale from 0 to 30. Readings in the range of 10 to 20 indicate a usable M/S, 10 being the stiffest. These numbers have no numerical value and are only used to compare one stick against another. I was able to measure the deflection and came up with a weight that would compare with my jig's reading. The new weight uses a dime, quarter combination with a balsa handle and weighs 7.9 grams.
I re-tested 10 M/S's using the penny test with the new weight and concluded that any M/S that clears the base will work.
The 10 M/S's that were tested range in length from 8.5" to 9" and weigh .16 to .19 grams.
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