Accurate foam wings require the block from which they are cored be square. A block 1/8" out-of-square could result in a cumulative error of one-quarter inch in panel mating, which may throw the leading edges out of alignment. This condition could also adversely affect dihedral angles. A combination of both, if uncorrected, may result in a resounding crash when the wing becomes airborne. Therefore, a square block and precise template positions cannot be over-emphasized. However, these precautions are easier taken than described.
A metal square and wide yardstick are valuable tools for squaring. A quick check of the block can be made with the square. Do not force it too hard against the foam as resiliency may give a false indication. Use only enough force to support it. You'll usually find one corner, and possibly one end square. If not, the job is somewhat more difficult. At any rate, draw lines across the top of the block to indicate any cuts necessary to affect squaring, and then extend them down the sides.
The block shown in photo #14 measures 24" x 36" x 4". Squaring lines have been drawn and after squaring the block will be halved. Since the wing is to have a sixty-inch span and eleven-inch chord, only half the block is used. Two panels thirty inches long will be cored from this half.
For ease of coring and accurate template positioning, elevate the working surface to near eye level to eliminate bending or stooping. Your first cores may be somewhat of a tense business and anything you do to ease it will help. In trimming cuts, be sure to make provision for the severed portion to fall away and wire withdrawal. This can be done by placing boards under the block or extending the block over an edge of the working surface.
Align the numbered edge of one of the trimming templates with the squaring line drawn down the side at one end of the block. Insert nails through the template and push firmly into the foam. Repeat for the other side. Lay the yardstick across the top of the block and flush with the template edges. The line drawn across the top of the block should match. If not, the templates are not aligned properly or the squaring lines are off.
Two persons are required for hot-wire coring - one we'll call the pacer, the other a follower. The pacer sets the speed of cut and compensates for any possible wire lag. His instructions are passed to the follower by counting aloud the marks on his template as the wire passes them. The follower adjusts his speed to match. Any change in speed by either operator must be gradual. Sudden jerking or stopping will cause the wire to jump or lag which will melt grooves and may cause a reject.
Pull the bow down so the wire is suspended just above the foam as in photo #15. If the bow is hung properly and counter-weights adjusted correctly, the wire can be pulled through and out the bottom of the block with very slight effort.
Block trimming cuts are from top to bottom. A suggested procedure is:
1. Operators place the wire against the template edges and just above the foam.
2. Pacer throws the switch and allows a couple seconds for the wire to come to temperature.
3. Pacer says "To the foam . . . now," and begins counting as the wire passes the marks on his template. At the word "now," the wire should contact the foam and cut begun.
4. As the wire clears the bottom of the block, pacer says, "Through the foam and... out." At the word "out," both operators withdraw the wire from the bottom of the block and out, being sure the hot wire does not touch the foam.
In Photo #16, the wire has completed its cut and the severed block half is being removed. Note the wire is being held in position to prevent further foam contact. When the severed portion is out of the way, the cutter will be raised. While making block-trimming cuts, some idea of the proper speed for the readers' particular cutter installation will be gained. A similar rate will be used for panel coring, and this should be reduced to about half, as the wire passes around the template leading edge, depending on the wing being cored. Reduced speed around the leading edge will help keep the wire straight, span-wise, and prevent center-lag, which will result in bowed leading edges.
Photo #17 shows the root template being attached to the block for the first panel. Notice the back of the template extends beyond the block edge. This assures a square trailing edge by providing a resting place for the wire as the core is begun as well as a supported exit. This method is recommended for the beginner. The trailing edge can be made by squaring the template and running the wire around it. In fact, some designs must be cored this way, but a certain amount of experience is required to prevent wire jump or lag, which will cause rounded or bowed trailing edges.
Photo #18 shows the tip template being attached. It is essential the template be positioned exactly as the root. This is accomplished by transferring position measurements from the root template to the tip. Accuracy requires relative positions of the two templates be identical, with neither further ahead, back, higher or lower than the other. The one exception to this is swept designs where template offset is intentional.
Photo #19 shows the wire in position for coring. The wire is resting on template edges just short of the foam. The pacer begins the operation just as with block cuts by counting, adjusting speed, and signaling withdrawal of the wire on completion. Coring may be started from either top or bottom of the template and is largely a matter of preference.
In photo #20, coring has started from top rear of the template and will proceed on around and out the bottom. Very light pressure--vertically against the template edge, and horizontally through the foam is sufficient. Excessive pressure against the template may cause movement with consequent airfoil distortion. Forcing the wire in its cut will most likely cause wire lag. Allow it to melt its way through the foam. If using the cutter described in Chapter III on six volts, switch to twelve volts if cutting speed seems too slow. But remember that with power increase comes also higher wire temperature and reduced error tolerance.
In photo #21, the cut has been completed and core withdrawn from the block. Immediately upon removing the core, sight along its leading and trailing edges. They should, of course, be straight. Any tendency of the core to curl is due to heat and stress relief and normal. The slight curl can be remedied by flexing slightly in the opposite direction if desired, but it will soon straighten itself out. Replace the core in the block and repeat for the second panel.
Photo #22 shows the dihedral cut. The foam block, with cores inside, is inserted into the jig previously made. One panel is pulled out so that the wire will make the cut as it passes along the templates. In photo #23 the cut is completed and severed portion removed.
Tip cuts are made in the same way with suitable (usually 45 degree) templates. Take care that cuts are made on the correct end of each panel. It is somewhat embarrassing to end up with two identical panels, or worse, cuts in the wrong direction.
Dihedral cuts can also be made during coring, by elevating the tip template the required distance above the root. This method requires extremely accurate block squaring and, because of increased error possibility, is not recommended.
Coring Tapered Wings
Coring a tapered wing is a bit more difficult than the constant chord type shown in the photos because of different size templates used. The tip being shorter than the root means the wire must move faster around the root template than the tip. To prevent wire lag it is necessary the wire remain parallel with leading and trailing edges while coring.
The principle is somewhat similar to pendulum action. If the tip is considered an anchor point, and the root an arc a weight would follow, it becomes apparent the wire must remain straight at any given position between these two points in order to maintain linearity. Non-linearity of the wire during a taper cut, especially at the leading or trailing edge, may result in a scrap panel. Figure 6 illustrates the relative position the wire should assume between templates at various stages of the cut.
In any coring operation where one end of the wire must move faster than the other, it is easier to feel and correct wire lag from the faster moving end, and this is where the pacer should be. Though the marks serve as a reliable guide to wire position, the pacer must be alert to sense lag and vary cutting speed accordingly. It is extremely important for the follower to keep the wire moving over his template at a smooth, constant rate. Any variation other than signaled by the pacer may transmit a false lag indication and result in erroneous correction by the pacer.
Because of the angles and sometimes severe template offset, swept wings can become extremely complex and difficult to core. This is especially so if the sweep is combined with a taper and thin-sectioned tip. An example of this is the full scale jet fighter in which the wing may vary from a near constant chord and gentle sweep to the dart wing of sonic aircraft.
Figure 7 depicts a foam block and the placing of templates on it for a swept panel. The numbering system is similar to that for tapered wings as are wire positions throughout the cut.
For accurate root mating of swept wing panels, it is imperative that center joint angles be identical. This is dependent on initial block squaring and template position, and will be assured by careful measurements before coring.
One limitation of the hot-wire method of coring is that the wire must remain in a straight line. Therefore all cuts must be linear. We cannot, for example, make a foam ball in one pass. But we can make a ball. Simple geometry and the orange show us the way.
By making two half-circle templates and cutting a sufficient length of foam in the form of a half-round piece of wood molding, then sectioning with angular cuts, we have a number of pieces the shape of orange sections. When the pieces are glued together - a ball.
Though it is not suggested the reader begin madly cutting foam balls, the illustration is apt in that by sectioning, we can develop wings such as gull, double swept, dual tapered, or any other configuration desired. Sectioning is also employed for fuselage components.
To illustrate the designs available and how sectioning can be used to core them, the following hypothetical wing is presented. Such an inverted gull wing, with its taper and thin-sectioned tip, would be virtually impossible to core without sectioning. It would, in fact, be somewhat troublesome with balsa.
Figure 8 illustrates a three-view of the wing, with circled numbers indicating the parts required to make it. The completed wing requires six foam sections (three per panel) and two balsa tips.
To make these six sections two identical templates are used for the center section, one for the middle, and another for the outer, or a total of four templates.
Figure 8A illustrates the templates and parts for one wing panel.
Figure 8B illustrates the center section (1) and the two templates and dihedral cuts to form the inverted gull shape.
Figure 8C illustrates the middle section (2). Note that one template for the center section ("A") is combined with the middle section ("B") in this cut.
Figure 8D illustrates the outer section (3) and how templates ("B") and ("C") are combined to core it.
The wing is completed by sheeting the sections, joining, installation of tip blocks, mating the wing halves and finishing. Each joint should be reinforced with glass cloth and resin or epoxy and tape.
As with the usual RC wing having two panels, a square block before coring will ensure accurate panel mating. Because of the additional joints in a sectioned wing, accurate angles and template position becomes even more critical. All Contents Copyright © 1971. R/C Modeler Corporation. All Rights Reserved.