For those accustomed to tailed aircraft, the MB Raven fuselage appears truncated. Despite the appearance, the fuselage is streamlined. It's also easily built and incredibly strong. The short overall length is actually an advantage, as its contribution to inertia in pitch and yaw is exceptionally low. The recess in the bottom of the fuselage places the tow hook very close to the CG, so there is little leverage to drive the wing upward in pitch during winch launches.
Because we raised the wing to make its upper surface flush with the top deck of the fuselage, the template for the fuselage sides is different than what appears on the plans. The discrepancies are not much, but they are noticeable. The fuselage is a bit deeper beneath the wing and the wing saddle is of a different contour, the upper decking must follow the upper surface of the airfoil, and the former at the rear of the canopy must be moved a fraction of an inch forward to clear the leading edge of the wing.
As mentioned in our column last month, the servo to elevator connection for this version of the MB Raven is somewhat problematic. The elevator is in two parts, and each side is driven by its own control horn. We cut 1/16" plywood control horns to the outline indicated on the plans, then notched them so they fit below the elevator rather than above. A small length of brass tubing serves as a bearing for the clevis pin.
Installation of servos in the fuselage is easy as there is no tray, just rails. Due to the wing being higher on the fuselage, all of the pushrods are placed below the wing rather than curving over it. For this MB Raven, the receiver was moved forward to a position directly behind the battery. The servos are behind the receiver and in front of the leading edge of the wing, mounted at a sloping angle downward toward the fuselage rear. We exchanged the locations of the rudder and elevator servos in order to eliminate an unnecessary curve in the rudder cable. Mounting the rudder servo on the left side allows the cable to traverse the full width of the fuselage and have a relatively straight path to the rudder control horn which stays on the right side. The elevator hook-up utilizes a pushrod and so is very straight forward.
The elevator pushrod assembly must be connected to the control horns prior to mounting the wing on the fuselage, as all connections are on the under side of the wing. We used separate pushrods for each elevator half. A copper wire wrapped solder joint permanently connects the two pushrods to a single connection with the servo arm. Former 6, a compression block which also serves to join the plywood and balsa body sides, had to be carved out to clear the elevator pushrods and serve as a support for the rudder pushrod conduit. The elevators were temporarily attached to the wing using masking tape while all of the connections were fabricated. (MonoKote® hinges will be installed during the covering process.)
The canopy is cut and formed to match the fuselage framing and to feather into the top deck. The canopy cross-section is a semi-circle. We follow Dave Jones admonition and always tape the canopy in place, so there are no fancy internal latching mechanisms.
We fabricated several concave sanding blocks to assist in shaping the fuselage bottom and canopy areas. We used a Forstner bit and a drill press to cut a hole in a scrap 2x4, then ran the table saw blade so the edge of the kerf crossed the center of the hole. A mixture of five minute epoxy and microballoons was used to join the sandpaper to the inside of the cutout. The first, 2 1/4" in diameter, was used to shape the canopy once we had planed and rough sanded the outline. Such sanding blocks are very easy to construct, so we made several more, each 1/4" smaller in diameter. The smallest has a diameter of 1/2", to be used for shaping the leading edge of the fin.
The aileron servo cables exit the wing at the center of the top surface, right behind the spruce leading edge and rear canopy frame. From there they go down the fuselage sides, below the rudder and elevator servos, to the receiver. We seldom do anything within the fuselage cavity except change battery packs, so this additional wiring is not in the way.
Because of the assembly schedule, we started construction of the fin and rudder before the fuselage was complete. The vertical fin and rudder are lightweight structures with quite a bit of cross-bracing to prevent the covering from wrinkling. The fin is essentially a flat plate airfoil of narrow chord with a rounded leading edge. The rudder is large, and due to its shape it must be built using a piece of trailing edge stock as a jig.
To match the added wing tip extensions, we constructed the upper portion of the fin by laminating 1/16" x 1/2" balsa sheet around a scrap pine form. The combined fin and rudder area remains the same as on the plans. The FrameMaker® arc function produced a very nice elliptical template for this purpose, and we've included it here on a 1/4" grid for easy duplication. The top of the rudder is a piece of 1/4" sheet. The leading edge of the fin was sanded to a semi-circle cross-section with a 1/4" radius concave sanding block. The laminated curve of the fin was sanded to this shape as well. The top of the rudder was then sanded to match the extension template contour and the 1/4" radius of the leading edge, with a smooth transition to the trailing edge.
The wing is mounted to the fuselage by epoxying the wing to the two compression blocks and the wing saddle. This joint, since it has such a large surface, is strong enough to withstand winch launches without pulling the fuselage from the wing. Once the wing was in place, a balsa block was shaped to transition from the rear of the canopy to the high point of the wing, and a piece of 1/8" sheet decking was fitted to transition from the high point of the wing back along the base of the fin.
An Airtronics adjustable tow hook was installed as called for on the plans. This lightweight assembly consists of an aluminum alloy channel through which a small nut passes, and the tow hook has a nut placed on its threaded section. The tow hook is screwed into the nut in the channel and then locked in position anywhere along the length of the channel by using the second nut to tighten the assembly. We've replaced the original hook with a piece of 1/8" music wire, bent to a 90 degree angle and threaded with a 4-40 die. A 4-40 nut must be filed down in thickness and width to fit in the channel, but the resulting installation is more than strong enough to take full power winch launches without bending. Unfortunately, Airtronics no longer produces this item, and we have been unable to find a matching extrusion.
We tackle covering and flying in the next installment.
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.
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References and sources:
Bill Northrop's Plans Service, 2019 Doral Court, Henderson NV 89014-1075; PH (702) 896-2162 M-F 10A-5P, Pacific, FAX (702) 897-7775 any time.
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.
Model Builder Raven. Dave Jones. Model Builder, January 1982.
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.