We like to begin construction of any airframe with the wing. There are a couple of psychological reasons for this. The wing usually has many more pieces than any other major part of the airframe, yet wings are perceived to build faster than real time. This may be because the skeleton looks to be a lot more than it really is - sort of like framing a house. But the biggest reason for building the wing first is (with apologies to TWITT) "The wing is the thing!"
After the preliminary decisions were made, and before actual construction commenced, we once again looked at the plans in detail. We wanted to make sure all potential problems could be resolved before construction progressed to the point where solutions would be forced upon us. One significant change to construction was a switch back to wing jigs made of balsa blocks and trailing edge stock. Despite having sufficient foam on hand, the utter simplicity of using scrap balsa was just too enticing.
Formal construction of the wing began with fabrication of the rib templates. The Raven S plans gave us a detailed drawing of Barnaby Wainfan's BW 05 02 09 airfoil. We scanned the drawing and then erased all of the extraneous lines, leaving only the section profile. The resulting TIFF image was then imported into Foil 1.2, and that application did a good job of deriving a coordinate set from the digital image. MacFoil was then used to generate a large on-screen image, and the coordinates were modified until the resulting profile was smooth and MacFoil prints of the section matched the image on the plans. The resulting coordinate set is included in this column, along with a print of the section profile.
MacFoil was then used to generate profiles with 1/16" allowance for sheeting and cap strips. These paper plots were glued to aluminum flashing material using a spray adhesive, and the rib templates were then cut out. Since MacFoil also printed out percent chord marks on each plot, locating the spar caps and control surface hinge points was relatively easy. Sharp #11 X-Acto blades made relatively quick work of the wing ribs and the partial ribs required for the ends of the elevator and ailerons. Eight of the sixteen center section ribs were drilled to hold the 1/2" inside diameter paper rocket body tubes used for ballast. (See the Photo 1.)
The wing was constructed in three parts - a center section and two outer wing panels. All three major parts were constructed in the same basic order. The bottom spar cap was pinned to the work surface and the two end ribs were pinned in place using machinist blocks to assure proper alignment. The 1/4" square leading edge stock was then blocked into place using a 45 degree triangle and the pre-cut notch in the rib leading edge. With the wing jigs in place, most of the ribs were glued into position. A 1/16" plywood plate seals the end of each ballast tube. Once these plates were glued in, the ballast tubes were pushed into place and glued securely. The remaining ribs were then glued into place. Balsa sheet spar webbing was installed using a plywood and spruce template to position the webbing relative to the lower spar cap. The upper spar cap was then glued in place.
The ailerons on this model are rather large, so standard JR 101 servos with ball bearing inserts, one in each outer wing panel, are used to drive them. Plywood was used to provide a mounting surface for the aileron servo and to strengthen the rib in the area of the cutout. See Figure 1. Some portions of the wing top surface required sheeting so that the aileron push rods would go through slots in solid material rather than through covering alone. Servo leads were routed through the wing such that a connection is made at the wing disassembly joint.
Simple dihedral dictates the addition of plywood bracing in the center of the wing. A straight wing with tips raised 2.5" gives 2.5 degrees of dihedral. We be made two 1/8" plywood dihedral braces for the wing center section. The brace at the front of the spar goes across one rib bay on each side, the rear brace goes across two bays on each side. There is also a sub spar dihedral brace at the elevator hinge line. See Figure 3 above.
The original MB Raven, which has a flat center section, maintains control linkage to the elevator even when the aircraft is disassembled for transport. We wanted to keep this positive feature, but, since we had settled on simple dihedral, the simple spruce joiner which connects the two elevator halves could not be used. We looked through our boxes of hardware and found nothing which provided an ingenious solution. One of the limiting factors is the narrow width of the rear fuselage. We finally decided to use one control horn for each elevator side, a single servo and a split pushrod. This is often done on pattern 'ships, so we're sure it will work for the Raven.
The original servo to elevator connection was "pull for up." This placed the elevator control horn adjacent to the top deck. Moving the wing higher on the fuselage eliminates even this small amount of clearance, so we'll be using a "push for up" connection with the control horns on the bottom of the elevator.
Allowance also had to be made for the aileron hinging, not at all part of the original design. We used 3/32" sheet for the sub spar at the trailing edge of the wing, and the leading edge of the aileron. This is similar to the elevator structure. For lightness, the ailerons use 1/16" balsa sheeting to form a C-tube at the leading edge, plus cap strips. The entire trailing edge of the wing has a 1/2" wide strip of 1/64" plywood where the upper and lower surface sheeting meets. This allows the trailing edge to be finished to a sharp edge. While the elevator is hinged from the top, the ailerons are hinged from the bottom. This has proven an effective way to eliminate adverse yaw due to aileron deflection. See Figure 1 again.
The wing tip extensions were constructed of 1/16" sheet balsa. We used the arc function of our desktop publishing software (FrameMaker®) to create the elliptical curve. Figure 4 provides the shape superimposed over a grid for easy duplication. Because of the upper and lower surface curves, and the angle at which the two sheets meet along the outer edge, this structure is both light and strong.
A note on the Raven wing construction is in order, as it directly affects final assembly. As can be seen from the plans and has been mentioned previously, the wing is composed of a center section and two outer panels. During initial construction, the center panel is made 48 inches long. The wing tips are built separately. Once complete, a wing tip is slid onto its dihedral brace and glued in place. The center panel is then cut six inches inboard of the resulting joint line, where two ribs have been placed directly adjacent to each other and the brass wing rod receptacles have been pre-installed. This construction method assures accurate alignment of the wing rod receptacles and mating of the wing surfaces. The shortened center panel (36" span) and fuselage, complete with elevator and rudder servos and linkages, form one major piece. The two outer panels complete the three piece breakdown for easy transport. See Photos 2 and 3.
Some parts of the wing construction were delayed because we lacked the ballast tubes. Your local hobby shop can order these either directly from Estes or from a wholesaler, but it's helpful if you have a description and part number and order these ahead of time. Relevant information has been included at the end of this column.
We'd like to recommend Superior Balsa and Hobby Supply, Hawaiian Gardens California, as a source for balsa, plywood, music wire and brass tubing, and other items needed for construction. We've ordered from a number of balsa suppliers over the years, and have found Superior to indeed live up to their name. The prices are more than reasonable, and we've never seen better wood. The balsa is light weight but not at all spongy, the grain is straight with no aberrations, and everything is cut true. If you've not ordered from Superior before, please give them a try.
The plan set available from Bill Northrop's Plan Service includes a copy of the magazine construction article. If you have specific questions about construction of the modified MB Raven, please contact us at either P.O. Box 975, Olalla WA 98359 or by e-mail from our home page.
Next installment: Building the fuselage, fin and rudder.
__________
References and sources:
Aileron differential, Part 1. Bill & Bunny Kuhlman. RC Soaring Digest, May 1996, and On the Wing... the book, Volume 2. B2Streamlines, 1998.
Bill Northropıs Plans Service, 2019 Doral Court, Henderson NV 89014-1075.
PH (702) 896-2162 M-F 10A-5P, Pacific; FAX (702) 897-7775 any time.
Estes Industries, 1295 H Street, Penrose CO 81240. Rocket body tubes, BT-5, 0.544" ID, four 18" lengths per package, part number EST 303084, about $6.00.
Foil 1.2 by Gregory Payne. Available here.
FrameMaker® is a product of Adobe Systems Inc., 1585 Charleston Rd, P.O. Box 7900, Mountain View CA 94039-7900. FrameMaker® information is available at the Adobe web site.
MacFoil by Dave Johnson, 58 Chenery St, San Francisco CA 94131. Available at the MacFoil web site.
Model Builder Raven. Dave Jones. Model Builder, January 1982.
Personal correspondence, Barnaby Wainfan.
Superior Balsa and Hobby Supply, 12020-G Centralia, Hawaiian Gardens CA 90716. (800) 488-9525, Monday through Saturday, 7 AM - 7 PM. Further information is available on the web.