In searching out natural wood formations from which to construct boomerangs, several criteria must be met. It is essential that the arms of the boomerang be in the same plane and that the angle between the arms be proper.
I soon discovered that even with the abundance of gnarled trees and exposed roots along the river bank, that finding a bent limb or a trunk and root combination that would meet the above criteria was rare. Rather than return home nearly empty handed, I decided that I would collect other trunk parts of trees that even remotely resembled the boomerang shape, and slice thru them to study the orientation of the grain.
For maximum strength it is essential that the wood grain follow the shape of the boomerang. Otherwise, the 'rang may break upon impact (Figure 2).
After slicing thru the various formations, I discovered that it is possible to find wood suitable for boomerang construction from a trunk-limb configuration and from a Y formation. Figure 3 illustrates the regions where the grain follows the outside contour of the tree.
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SPECIFICATIONS FOR THE OPTIMUM NRB
Material - Variable
Length - Variation: 16 - 36" (40 - 90 cm)
- Suggested optimum: 27" (68.5 cm)
Width - Variation: 1 1/2 - 4" (3.8 -10.2 cm) (l/w ratio = 24.00 - 8.00)
- Suggested optimum: 2 1/4" (5.7 cm) (l/w ratio = 12.00; w = 1/12 length)
Thickness - Variation: 1/4-3/4" (.6-1.9 cm) (l/w ratio = 16.00-4.00)
- Suggested optimum: 3/8" (1.0 cm) (w/t ratio = 6.00; t = 1/6 width)
Circumference - Variation: 3 1/8-8 1/2" (8.0-21.5 cm)
- Suggested optimum: 4 11/16" (11.0 cm) (equivalent of rounded stick 1 1/2" in diameter)
Cross-section - Variation: plano-convex to lenticular to convex-piano and flat
- Suggested optimum: lenticular
Weight - Variation: 5 oz -1 lb. 8 oz (140 - 680 g)
- Suggested optimum: 12 oz (340 g) (2.25 inches/ounce or .44 ounce/inch)
Angle of Bend - Variation: 123 degrees - 145 degrees
- Suggested optimum: 135 degrees
Curvature - Variation: 115 - 175 percent - Suggested optimum: 140 percent
Radius - Variable
Expected Forward Speed - 30 m/sec. (plus or minus)
Expected Rotational Speed - 10r/sec. (plus or minus)
Expected Distance - 100 - 200 yards
Expected Flight Patterns - Variation: curve left to right, straight, "S" curve to reverse "S".
- Suggested optimum: straight
Note: The above range of variations and suggested optimum specifications are based on experimental research and checked as possible against ethnographic and archeological data.
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Armed with this knowledge it is possible to increase your chances many fold of finding a natural piece of wood with proper grain orientation for boomerang construction. I found that a bow saw, sometimes referred to as a Swede saw and available at all hardware stores, was the best tool to take with you to the woods. Be sure to cut your logs longer than needed, because the ends may develop shrinkage cracks. It has been my experience that hardwoods produce the nicest blanks for making boomerangs. So far I have gathered samples from cherry, locust, oak and dogwood.
Ideal Grain Orientation
Figure #2
Once you have brought your logs home you must take proper care of them. Wood taken from a living fallen tree contains moisture in the form of sap or rainwater, and is referred to by woodworkers as green wood. Once cut and stored out of the weather, wood begins losing moisture until it reaches equilibrium with the moisture in the surrounding air.
Dotted Lines Enclose Regions Suitable For Boomerang Construction
Figure #3
As wood loses moisture it begins to shrink. In a log the wood on the ends loses moisture more rapidly than the wood in the interior of the log. With the ends trying to shrink while the interior remains swollen, enormous stress develops within the log, resulting in radial cracking known as checking. To prevent rapid end drying of a log, the ends should be coated as soon as possible. Any impervious material may be used such as wax, aluminum paint, etc.
Be sure to leave the bark on the log until you are ready to slice thru it. This helps to cut down rapid moisture loss. Some woodworkers even store freshly cut logs in sealed plastic bags to slow moisture loss. I have had a birch elbow crack completely through in less than 24 hours because it was not end-coated.
The easiest way to tell if a log has reached equilibrium with the moisture in the air is to periodically weigh it. When the weight becomes constant over a period of weeks, the moisture loss has stopped. This, however, can be very time consuming; for instance, to air dry a 1 inch thick piece of cherry may take from 2 to 7 months.
I, for one, am too impatient to wait this long. The alternative is to start slicing thru the log and see if the grain pattern is suitable. If the wood moisture content is not at equilibrium with the atmosphere the slices are going to warp (Figure 4). The easiest method is to cut them to permit good air circulation, and let them dry. Once the wood has dried and warped, simply flatten one side with a plane, joiner or sander and then plane the other side to the desired thickness.
How Green Wood Slices Warp As They Dry Figure #4
Figure #4
Do not cut the piece of wood to the boomerang shape until this time, because if edge cracks do develop and the slice is oversized, they can be cut away.
The process I have just described is by far the simplest method of obtaining usable blanks for boomerang construction. But with this ease comes a cost. You will find that since you must slice extra thick to allow for warpage you cannot get as many blanks from a log as you would like.
Originaly printed in Many Happy Returns -Quarterly Newslatter of the USBA, Spring 1985. Reprinted by permission of the author and Editor.
CROSS-SECTIONS
FLIGHT - PATTERNS
Tuning A Throwing Stick
By Robert Foresi
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Excerpts from Some Modern Ideas On The Construction of Non-Returning Throwing Sticks. An Unpublished paper submitted by Ben Ruhe.
Tuning and Throwing
As I have advanced the theory of tuning non-returners only incrementally beyond its primitive beginnings, I have not found a "cookbook" formula for tuning them as yet. The suggestions presented here are the most successful approaches I have taken to date. Perhaps I can at least save the aspiring stick-thrower from some of the least prolific throwing sessions I have experienced.
The first rule of tuning non-returners is to forget everything you know about returning boomerangs. Forget about the bottom side of the stick being flatter than the topside. Forget about adding extra undercut to the leading edge. And forget about leading edges being blunter than trailing edges. All these factors will tend to produce a stick with too much lift. All that is desired is a slight amount of lift, enough to negate the force of gravity.
The following is the airfoil I have used with the most success. Note that the faces are flat for the central 2/3 or so of the airfoil section, and note that there is a slight built-in negative skew to the airfoil section. The skew may not be essential to obtaining the desired flight.
ACTUAL SIZE
Make the throw stick with this airfoil section throughout its length, except no skew near the elbow of the stick. Test throw it, easy at first of course. If it rises too much, taper the topside of the leading edge a bit, or try bringing the whole camber line up a bit, as shown, or try more negative skew.
If the stick is dipping in flight too much. try tapering the trailing edge on top just slightly, or the leading edge bottom, cr lry bringing the whole camber line down a hair.
It should be mentioned that the throw itself has a lot to do with the flight. Launching with more layover off vertical will of course tend to make the stick rise more, and vice versa. The desired layover angle is usually 45°-90° off vertical.
Sometimes it is helpful to flip the stick over an
d throw it upside down. This is admittedly sneaky, but produces results more often than one might guess.
A final rule for the enjoyment of throwing non-returners and for their advancement is safety. Any criticism of non-returners typically is centered around their potential danger and their past use as hunting implements. However, if a throw stick does not strike anything or threaten to do so, there is no danger, and throw sticks can then be regarded in the same manner as returners. Therefore, the most important safety rules are to select an expansive throwing area, and to know the flight potential of the stick you're throwing. Throw the stick easy at first, and recognize the possibility of errant throws. Especially important is to use caution when a throw stick has just been re-tuned. Vastly different ranges and curvatures of flight are often obtained from only slight adjustments to airfoiles. For long-range sticks an open field of a size 350 yds. by 250 yds. is required.
REFERENCE ARTICLES
1) "The Non-returning Boomerang - Evolution and Experiment." Errett Callahan, Virginia Commonwealth University, 1975.
2) "Australian Throwing Sticks, Throwing Clubs, and Boomerangs." D.S. Davidson, American Anthropologist, N.S., 38,1936.
MAKING THROW STICKS BEHAVE
By Norm Kern
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If the throw stick had never been developed, it seems quite unlikely that the boomerang would ever have been developed and we wouldn't be having all this fun. I believe the first boomerangs were probably defective throw sticks. Even though throw sticks are probably the parents of the boomerang, very little is written about them. I have purchased or made 13 throw sticks. Like boomerangs, each has its own personality, but throw sticks have their own set of aerodynamic behaviors, and challenges. I set out to make a few throw sticks for demonstrations. Following is an account of this learning experience.
Since the throw stick was intended for hunting, the most desirable flight patterns would be straight, long, and would be nearly parallel to the ground for sustained periods of the flight. To increase the opportunity of striking the target, the plane of rotation near the target would need to be nearly parallel to the ground. It would also be necessary for the stick to retain significant radial velocity at the time of impact. Most animals would be alarmed by the scent of the hunter if approached from upwind, so the stick must be able to fly upwind or at least cross wind. It all seemed so simple until I tried to make a throw stick behave.
My first attempts were plagued by "fatal flutter". To have the above listed properties the throw stick must spin in a plane with the flat surfaces basically parallel to the plane of rotation. When a throw stick flutters, it twists somehow along its long axis, and the flat surfaces are no longer parallel to the plane of rotation. Once the flat surfaces are exposed to the oncoming air, the throw stick quickly loses its spin, and drops to the ground.
The other type of throw stick misbehavior is soaring, and/or veering. At the end of the flight, such throw sticks curve off the straight path. For right handed throwers, the throw stick veers to the right. The throw stick may also gain altitude at the end of the flight. A variation on the soaring theme, is the throw stick which likes to gain altitude from the moment of release, and does not come close to the ground until it has lost almost all forward momentum, and so hovers down.
Figure A shows outlines of four throw sticks. I call them the Club, the Large Banana, the Small Banana, and the Hopi stick. I made all four from half inch 9 ply plywood. You should be able to obtain such wood from lumber yards or cabinet makers. I throw the Club, and the Large Banana with two hands, basically parallel to the ground, much like swinging a baseball bat. I throw the Small Banana and the Hopi stick one handed and launch with about 45 degrees of layover.
Throwing Stick Profiles
Air Foils
When I began to make throw sticks, I read what I could and the common wisdom said the cross section should be that of a lens. The first throw stick I purchased had the lens cross section (Please see the cross section diagram above). The lens cross section has B and H equal to zero, C roughly equal to A, and D roughly equal to L, and likewise G equal to I, and F equal to J. This is how I made my first three throw sticks. All of them had "fatal flutter". Every throw was plagued by flutter, and there were no stable flights. My first success came on the Small Banana when I made C larger than A, and D larger than L. I also made I larger than G and J larger than F. (B. and H were zero.) This unequal lens made the Small Banana very stable, and enabled it to fly about 60 meters, but it tended to soar at the moment of release. Using the unequal lens on the other two throw sticks did not cure the flutter.
Upon discussing the flutter syndrome with Gary Broadbent, he showed me a very blunt airfoil with A much smaller than C. and I much smaller than G. and D, L, F, and J all very small. (B and H were basically zero.) I tried this approach on the Club (See the Blunt airfoil diagram). Both the leading and trailing edges were alike, and were quite similar to the leading edges of a normal returning boomerang. This provided a very stable flight but my intuition said that the blunt trailing edges must be creating drag which must be shortening the flight. I then increased F to decrease the drag. The effect was to cause the Club to soar and veer to the right at the end of flight. I lengthened the flight time, but it was certainly now a poor hunting device. I increased I and J to the current shape to reduce lift. This cured the soaring and veering, and Club had a nice level flight again.
I have four other throw sticks which have gone through similar changes with similar results. From these experiences I concluded:
1. On airfoils where B, and H equal zero, as you increase A and L (commonly called undercut), you will induce instability, and cause "flutter ".The more C exceeds A, the more stable the throw stick will fly.
2. As you increase F, you will increase soaring and veering.
3. As you increase I and J, you will decrease soaring and keep the throw stick close to the ground in flight.
4. As you increase D, you decrease drag, maintain spin, and lengthen flight times and distance.
My first throw stick was made by Bob Foresi. I asked Bob to review these conclusions. He said on his current air foils he tries to "keep A and C equal, I equal to or slightly larger than G. and D greater than L and greater than F. I also keep E & K small, not so much at the center of the stick." Bob keeps B and H equal to zero. He said he agrees with the above airfoil principles with the possible exception of number 1.
Gregg Snouffer is another active throw stick maker. Gregg favors a blunt lens. It has B and H equal to about 50% of the face of the airfoil. A, C, G, and I equal about 25% of the face. The transitions are rounded. Likewise L. D, F, and J are each equal to about 20% of the cord. I tried the blunt lens on the Hopi stick and the Large Banana with excellent results. Neither stick flutters.
For all of the throw sticks I have made or purchased, the air foil shape is the same on both arms, of course taking into account which edge of an arm is the leading edge. I have not intentionally experimented with different shaped airfoils on the same throw stick.
Shape & Weight
After all of the above events, I had most of my throw sticks working without fatal flutter at least when thrown downwind. Some were not stable when thrown up wind. I had operational sticks with 4 different airfoils. I concluded there must be something about the basic shape and weight distribution of the various shapes which also affected flutter, etc. Following are the dimensions I found to be important.
Z = Length - The distance between the extremes of the tips. Y = "Height" I placed the tips of the throw stick on the bench and used a carpenter's square to measure the highest point on the elbow region. U = "Height" of the center of mass. I taped one end of a string to one of the tips. I let the throw stick hang from my fingers at the point the string was attached. The other end of the string was weighted so it hung straight down. I taped the other end of the string to the other end of the throw stick at the point it naturally crosses the stick. I repeated the process with another string with t
he other end of the throw stick on top. The center of mass and rotation is at the point the strings cross. I put the throw stick on the bench again as before and used the square to measure the "height" of the center of mass. Weight = weight in ounces.
The sticks were made of a variety of materials, and were of significantly different sizes, so I could not just study the raw dimensions. I calculated ratios of these critical dimensions. Bob Foresi had indicated that the angle of the arms would be important. He believed that the lower (or more acute) the angle was, the more stable the throw stick would be. Since many throw sticks have one or both arms quite curved. I could not determine a satisfactory means to measure actual angle. The ratio of "Height" to Length relates to the angle. The higher this ratio is, the lower (or more acute) the angle will be. Based on my stick collection, the other critical ratio seemed to be weight to length. The throw sticks which weighed more per inch of length were more stable. This seemed reasonable. The wood used by the aboriginal peoples of Australia are heavier than the plywood I was using.
There was only one problem with these conclusions. One of the most stable sticks (Bob Foresi's) was the lightest and had the largest angle (as indicated by the ratio of Height to Length). It seemed there must be something else. This stick also had the "highest" center of mass as indicated by the ratio of Height of center of mass to Height. This observation took me back out to the field to experiment To "pull the center of mass up", I added lead tape to the elbow region near the balance point of the throw sticks which tended to flutter. They became more stable. To "push the center of mass down", I added weight to the tips of stable fliers. They tended to flutter more when thrown upwind, but had less soaring and veering when thrown downwind. I believe that a good compromise can be reached for any given throw stick.
Primitive Technology Page 33