FOAM WINGS (VI)

1971 R/C Modeler

Column Article

FOAM WINGS
A STEP-BY-STEP INSTRUCTION
MANUAL AND REFERENCE
HANDBOOK

By
J. ALEXANDER
Edited By Don Dewey

VI. LANDING GEAR AND CONTROL SYSTEMS

Unless the wing is intended for a rudder or rudder/elevator controlled aircraft, consideration must be given to landing gear, aileron, and control linkage systems. These installations are, or should be, somewhat easier in commercial than home produced wings as provision should be made for them. The home craftsman, on the other hand, has no such help. His is the sole responsibility for lay-out and installation of these items with satisfactory results dependent on properly doing so.



Figure 9 illustrates the various components of a typical foam wing. Note that the chord would be shorter for a strip than in-set aileron design. Note particularly that rounded tip blocks are installed after the wing has been sheeted. This facilitates in-set aileron installation.

Consider first the landing gear. One and two-piece blocks have been used with both working equally well in service. However, the one-piece block adds strength to the center joint and helps panel mating and joining. If the wing is of such a design that weight is a major factor, the two-piece block may be better. But, regardless which is used, the core must be channeled to receive them and the channels must be accurate.

Channeling can be done either before or after sheeting. Most commercial cores have pre-cut channels, so the builder has no choice but to sheet over the cavities. Blocks can, and have, been installed permanently prior to sheeting but the covering operation may become a rather aggravating affair, especially with the wrap-around method where one is working with a large sheet.

Gear block channels are usually made in one of two ways. The first, and perhaps most common, is to use a sharp X-acto or equivalent to make vertical cuts sized to whatever gear block is to be installed, then rooting out the foam to the proper depth. Though perhaps satisfactory, it is difficult to obtain a smooth bed and adhesion is not as good because of the rough surface inherent in this method of channeling.

The preferred method is a tool such as described in Chapter II. Because it operates as does the foam cutter, i.e., melts instead of gouging the foam, smooth cutouts result. As with any tool, it will be only as accurate as its working surface, in this case the formed wire tips. Equally important is the manner in which it is used. Though the tool can be employed for free hand smoothing or trimming of foam surfaces, it is best used with a guide or jig for gear block, servo, or linkage cutouts.



Landing gear installation is begun by laying out its shape and location on the panel as shown in photo #60. Though a one-piece block is shown, the following steps also apply to a two-piece installation with the exception of a separate template for channeling the two-piece bed might be used.



After lay-out, remove sheeting to expose the foam as shown in photo #61. This step, of course, does not apply if the unsheeted core is being channeled.



Photo #62 shows completion of the gear block channel. Note the block is being used as a template and reversed with its tip at the core root and its thick center section where the block end will be on installation. This is done to provide the angled bed required. In this way the cut is deep at one end (the root) and shallow at the outer end to conform to wing dihedral.



Photo #63 shows the block being installed. It should be slid in from the root rather than pushed down from the top to prevent crunching the covering as damage may result because of what should be a tight fit between core sheeting and gear block.



In photo #64 the block is in place. It is sometimes a good idea to forestall glue application until the cores are trial joined to reveal any discrepancies. When proper mating is assured, the block can be epoxied in place and, due to the close fit, helps hold the two wing halves in place for epoxy curing.

Aileron and servo installation in a foam wing is much the same as that in a built-up balsa type with the strip aileron being simpler. If in-set ailerons are to be used, some means of transmitting servo movement through the panel to the aileron horn must be provided. Figure 10 illustrates three ways of doing this.



In figure 10-a, the pushrod is passed through a hole cut span-wise through the panel. This is accomplished by drilling holes in the template large enough for the pushrod to fit through. A thin slot is cut from the bottom edge of the template to intersect the holes. Coring is begun from the bottom edge of the template, through the slot, around the hole, back out the slot, and on around the template. A neater job can be done by inserting a dowel or piece of 5/32" music wire through the panel in the desired location, fishing the cutting wire through, installing templates with holes to the core, hooking the wire to the bow, and coring out the holes.

A quicker method is to heat a piece of steel rod, longer than a panel, to foam melting temperature, then run it through the core in the desired location. This is an excellent method but care must be taken to prevent over melt. A rifle or shotgun cleaning rod is ideal for this application. It is recommended that jigs be made and used to prevent wandering as the rod passes through the panel.



In figure 10-b, a channel extending from the servo cavity to the bell-crank is cut into the bottom of the core just wide and deep enough to accept the pushrod without binding. After pushrod and bell-crank installation, the channel is sheeted over.



In figure 10-c, a channel is cut from the servo to a point near the aileron horn. A length of flexible nylon control cable, such as NyRod, is inserted into the channel. After the linkage is completed the channel is sheeted over. This method offers the advantage of a narrow channel and eliminates the bell-crank.

Still another method, not illustrated, is a combination of that in figures 10-b & c. A bell-crank is installed, except the cavity is not as deep as usual. A groove just wide and deep enough to accept a length of nylon tubing, such as the inside tube of a NyRod, is cut into the bottom of the panel, one from the servo cavity to the bell-crank, and another from the bell-crank to a point near the aileron horn. The linkage is completed by insertion of a length of 1/16" music wire from the servo to the bell-crank and another from the bell-crank to the aileron horn. This method has the advantage of an exceptionally frictionless operation and eliminates the usual holes and relatively large channels required of typical pushrod installation.

Pushrods

An excellent set of pushrods, for fuselage as well as aileron linkage, can be made from fiberglass tubing, similar to that used in the manufacture of arrow shafts. These pushrods will not flex or break in service and make for an exceptionally neat installation.



The first step is to bevel each end of the tube to about a thirty-degree angle so the very outside edge acts as a cutting surface. Refer to photo #65. A reamer does this job quickly but an X-acto or knife will do as well.



Push the rod into a two-inch block of soft balsa, with the grain, rotating it back and forth as you do so. See photo #66. The rod will cut into the balsa and jam itself full of wood. Repeat for each end.



With an ice pick or awl, prick a starter hole in the center of each wood plug as shown in photo #67.



Now apply epoxy cement to a Kwik-Link or equivalent shaft and insert it into the starter hole as shown in photo #68. Keep pushing the shaft into the wood plug until it has penetrated about an inch and a half or so, then apply more epoxy around the shaft and plug. Filing a point on the shaft end will make its insertion into the wood a bit easier. When the epoxy has cured, the pushrods are ready for installation.

In-set Ailerons

Radio control aircraft, like their full scale counterparts, generally utilize one of two aileron configurations - strip (full span) or the increasingly popular in-set type. Strip ailerons, as the name implies, are simply long strips, usually of pre-cut balsa stock, hinged to the wing trailing edge and running the full span of each panel. Because of simple installation and linkage requirements, they have been, and are perhaps now, seen on more aircraft than any other design.

From the standpoint of design and flight efficiency, the in-set aileron could be correctly said to be superior to the strip, though in practical flight conditions the average miniature aircraft pilot would probably be hard pressed to discriminate between the two.

On some aircraft, particularly those designed for high speed, most obvious would be an increased turn and roll rate as the in-set aileron, being nearer each wing tip and exerting more control at that spot, tends to cause quicker response and somewhat less drag. But the chief advantage of in-set ailerons, again from the standpoint of the average flyer, is that of appearance.

However, their installation is more difficult than strip, and requiring more precision in fitting and hinge installation. In this respect it should be noted that the builder capable of coring his own wings is most certainly able to handle in-set aileron installation and encouraged to do so.

Because strip ailerons are extremely simple and their installation requiring little more than alignment of hinges with the wing trailing edge and easy servo linkage, detailed instructions for them are not included, as the builder should have no trouble with them. In-set aileron installation in a foam wing, however, may present the newcomer with problems, so the following step-by-step method is likely to be of help.

Aileron installation begins with the hinge mount, which is inlaid into the core prior to the covering operation. Because of flight loads imposed on an aileron, it is advisable to provide a stronger hinge mount than that offered by the sheeting and foam of a core.

Begin by laying out the aileron shape on the foam core with a fine tip felt pen. If working from plans, be sure the shape and size is correct as ailerons often make the difference between proper and mediocre flight characteristics of an aircraft.

Now select a piece of 1/8" balsa stock and cut a piece 3/4" by about an inch shorter than the aileron length. This is the hinge mount, so take care the cuts are accurate and straight.

Using the mount as a guide, saw out a channeling template from plywood. Be sure to mark the hinge mount center line on the template. Sand the inside cuts smooth so the channeling tip will pass over them easily.



Matching the center line on the template with the front aileron lay-out line, draw the hinge mount location onto the core as shown in photo #69.



Align the center line of the template with the lay-out on the core. Now, taking care the template doesn't slip, use the channeling tool and make a bed for the hinge mount. It may help to tape the template in position to prevent slipping. Photo #70 shows the channeling operation.



It is important the channel be neither too shallow or deep. The top of the mount should be flush with the core surface. A low mount will cause a dip or low spot after sheeting. A high mount will cause skin bulge. Using Tite-Bond, glue the mount in place and, when dry, sand as necessary to match the core contour. Tite-Bond is advisable here as the mount will be sawed in two later. Photo #71 shows the hinge mount in place.



If an aileron linkage system utilizing a bell-crank is to be used, the cavity to receive it should also be channeled at this time, or at least prior to sheeting. Figure 11 shows a typical bell-crank installation. Note the cavity floor is angled to conform with the aileron horn mounting angle to permit free operation.

Prepare a bell-crank platform channeling template similar to that used for the hinge mount and sized to allow adequate crank arm movement but no larger than necessary. A two-by-two inch cavity is usually ample. Now channel the cavity, being sure here, as in all channeling, to make vertical entry and exit of the tip from the template. Do not force the tip in its cut as the wire may be deformed with consequent cutout distortion and inaccuracy.Make up a bell-crank assembly per the manufacturer's instructions. Drill a suitable hole in a piece of 3/32" plywood to receive the bell-crank bolt. Tighten and either mutilate the threads, coat with epoxy, or both to ensure the assembly will not vibrate loose. Epoxy the platform in place.



After the core has been sheeted, trimmed and rough sanded, ailerons are cut from it. Begin by laying out the aileron with a fine tip felt pen as shown in photo #72. Be sure the lay-out is correct. The leading edge (hinge line) of the aileron must match the center line of the mount previously inlaid in the core.



Ailerons should be cut with the core laying flat. This is best done by returning the sheeted core to the block it came from which then becomes a sawing jig, as shown in photo #73.



Cut the ailerons from the core at the hinge line. This splits the hinge mount in two pieces so one side is in the wing and the other in the aileron as shown in photo #74.



To allow for down aileron position, the aileron must be bevel cut as shown in photo #75. Use the smallest bevel angle, which will allow adequate down position. Twenty degrees is usually ample.

To permit proper fit of the aileron into the wing, allowance must be made for sheeting thickness along the mounting edge and inside end. For example, if 1/16" sheet is used for the core, the top of the beveling cut is in-set 1/16" along the hinge line and an equal amount cut off the inside end of the aileron. If the wing has been covered with cardboard, little practical difference is involved and can usually be disregarded.



With beveling and trimming cuts completed, both the core and aileron exposed foam surfaces are sheeted. Photo #76 shows the cap sheeted aileron and core. Contact cement is adequate for these surfaces.

The next step is hinge lay-out and installation. It is important here that all hinges operate freely to reduce servo loads. This is accomplished by carefully measuring and installing hinges in as near perfect alignment with each other as possible.



Photo #77 shows hinge lay-out and installation. Begin by plotting locations - both vertically and horizontally - as required by whatever hinges are selected. An X-acto is then used to slot the wing and aileron for hinge tabs.



Photo #78 shows an aileron with hinges mounted and pinned. Flex the aileron to check for free operation and, should any binding or roughness be detected, find and remove the cause. As stated earlier, it is important that ailerons move freely.



With the aileron properly mounted the control horn is attached per the manufacturer's instructions. For correct operation, it is essential that linkage from the bell-crank to aileron horn run in a straight line, with provision made for the slight vertical movement of the wire link in its travel. Photo #79 shows the horn mounted and control wire clevis in place.



Photo #80 shows the aileron being activated manually with the servo link prior to wing joining. This is the final acid test. Final because this is where it counts, and acid because if any previous binding was not corrected, it is now necessary to tear into the sheeting to fix it.

Servo installation is similar to bell-crank and landing gear installations except for size and that the cavity is sheeted. The servo itself is mounted with hardware or double back tape. The cutout can be made either before or after sheeting, but linkage channels must be provided prior to sheeting along with the bell-crank cavity. The main requirement here is that the installed servo arm and throw, lines up with the bell-crank arm.



Figure 12 illustrates an aileron and linkage system that, though more difficult and requiring precision fitting, is superior to the more commonly used exposed horn systems. The installation is similar to that used in full scale aircraft and lends itself to RC duplicates. An additional advantage is that multi-linkage control systems are eliminated, as are core channels and bell-crank cavities.

In this system, formed trailing edge stock is used and sheeting performed as for the usual aileron installation except the hinge mount is eliminated. The aileron is then laid out on the core as before and cut out. The aileron is then discarded.

A new aileron with rounded leading edge is then made up of either balsa or foam. If foam is used, a set of templates minus whatever sheeting thickness was used on the core is prepared and the aileron cored and sheeted.

Form a piece of sheet brass or equivalent metal to match the aileron leading edge. Use heavy enough stock so it will take and retain the form. This metal form is then heated to foam melting temperature.

Alternately apply and quickly remove the heated form from the foam surface in the wing until the proper concave cavity is formed for receipt of the rounded aileron edge. Then sheet the cavity with either plywood, balsa, or cardboard.

The inboard end of the aileron, as well as the mating wing surface, is sheeted with plywood. These surfaces act as a bearing and retaining mount and must be strong enough to support the loads imposed on them without deforming or tearing out.

Select a piece of brass tubing heavy enough to act as a torque member and hinge mount and attach a fitting similar to a bell-crank arm with holes to receive Kwik-Link clevis pins. The installation is similar to that used for strip ailerons and their servo links.

A hole for this shaft must now be provided for it to run through the wing, aileron (the shaft is secured permanently to the aileron, and into the wing tips. As rounded tips are recommended with this installation, they are attached after the aileron and shaft are inserted into the wing. The tips therefore act as an outer aileron shaft mount.

A variation, should the shaft aileron mounting system not be desired, is to simply utilize it as a transmission member by reducing its length and securing it to the inboard end of the aileron. In this case, the aileron could be hinged at its leading edge center line with multiple hinges as for the beveled aileron installation.

A somewhat easier method, providing the core is of suitable dimensions, is to use the recently available concave-convex hardwood in-set aileron fittings. Here the aileron cutouts are made in the core as for a beveled installation, allowing for proper fit of the fittings. The concave aileron mount is then epoxied to the core cutout. The aileron leading edge is cut to receive the convex hardwood piece being sure the cut is located in such position that the aileron will mate properly into the wing. Multiple hinges are installed and linkage set as before.

As mentioned earlier, the above system is demanding of greater precision and therefore more work than the usual exposed horn aileron installation. However, the additional work is justified by a very smooth and responsive control system and does away with an unsightly aileron horn.

All Contents Copyright © 1971. R/C Modeler Corporation. All Rights Reserved.

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