The first of these monsters made itself known after we thought the wing was complete. We found the ailerons twisted a bit too easily. Given their length and size, we anticipated that the covering material would not add sufficient strength. Our solution was to add a 1/16" diagonal rib between each pair of existing ribs. This was a tricky proposition, as we did not want to build new ailerons or tear apart the existing set, but we did manage to install them successfully. These diagonal ribs have cap strips installed.

A second problem involved the elevator control horns and the upper fuselage decking. Since the wing was raised relative to the fuselage, the clearance between the elevator control horn and the 1/8" upper decking is very small. We had to cut down the control horn mounting lugs and make a rectangular hole in the decking. Once we were assured that there was sufficient up elevator travel, we sealed off the hole with a piece of 1/64" plywood. After some judicious sanding, the surface discontinuity is barely noticeable.

After much thought, we settled on covering the wing with transparent MonoKote®. Two of the four 6' rolls we purchased were defective. Replacement covering was extremely slow in coming from Great Planes, so we ordered additional MonoKote® from a local source, Hobby Town in Parkland Washington. Jon Packer, the manager of Hobby Town, had special ordered the 1/2" rocket body tubes for us, so we relied on him to solve this problem as well. Jon had the covering in stock, checked it for defects before mailing it out, and we got it in the mail the next day. Thanks Jon!

From the left wingtip to a point 24" inboard is transparent red; the next 36" is covered with transparent orange; the remaining 57" is transparent yellow. Aileron and elevator hinges were formed using the MonoKote® covering. A 3/8" wide black trim stripe separates each color. The vertical fin and rudder are covered with transparent orange MonoKote®. We used four Klett hinges we had in our parts box. The fuselage and canopy were first sprayed with gray primer, then gloss black.

With leading edge sheeting, ailerons, two extra servos and cabling, three ounces of nose weight, and a five cell receiver battery, the weight of our model compares favorably with the prototype MB Raven built by Dave Jones. That prototype MB Raven, built using "contest" balsa, weighed just 41 ounces and had a wing loading of 4.3 oz/ft². Our Raven weighs 60 ounces. With its slightly larger area, the wing loading is just over six oz/ft².

As mentioned in a previous installment, ballast can be added by means of inserting metal rods into the four nine inch paper rocket tubes in the center section of the wing. Half inch diameter metal rod is used as ballast. A 36" aluminum rod weighs 12 ounces and raises the wing loading approximately 1.2 oz/ft². Steel rod adds almost exactly two pounds to the overall weight of the glider, and raises the wing loading by 3.2 oz/ft². We cut both rods into three inch sections with an abrasion wheel mounted in our table saw. The three inch length makes the ballast easier to carry around - we have a small padded case for this purpose - and allows combinations to be used (12, 18.6, 25.3 and 32 additional ounces). See the included graph.

Since the ballast is placed somewhat outboard of the centerline, inertia in roll and yaw is increased. This is actually helpful in gusty weather as the glider does not bounce around quite so much. Thermalling with the wing loading nearly doubled (steel ballast) is not a major problem, but the ability to work light lift is significantly reduced.

Based on previous experience with plank planforms, notably Dave Jones' Blackbird 2M, we set up the ailerons for no differential and about 20 degrees up and down. (There is a full 60 degrees of upward travel available, as we want to experiment with spoilerons at some point in the future.) During flight testing, this proved to be more than sufficient. Elevator throws were adjusted for about 50% more than shown on the plans. We don't recommend this much travel for those flying a tailless airplane for the first time, but have come to be comfortable with these deflections during more than 15 years of flying this design. Rudder throw is 20 degrees each way, as recommended in the construction article.

Winch launching the MB Raven is an interesting experience. The line must be preloaded, and the best climb out angle is achieved by throwing the model at a 45 degree angle to the ground. The Raven should immediately rotate to about an 80 degree angle and climb strongly. Noticeable flexing of the wing takes place on tow. Zooming off the line is possible, but remember this airplane is flying on spruce spars.

Those of you who are using the MB Raven as a first excursion into the world of tailless sailplanes will find a few of the flying traits to be at first unique and somewhat disconcerting, then interesting, eventually endearing. These special characteristics are positive attributes to be used to advantage.

• Due to the short fuselage length and low inertia, control in pitch is rapid and achieved with relatively small control throws. Down elevator is very powerful, hence the seemingly minimal throw shown on the plans. With the short moment arm, large elevator deflections cannot place extreme loads on the wing. Still, you should always endeavor to make elevator control both as gentle and as smooth as possible.
• Flight speed is reduced as the nose is raised, just as when flying a conventional tailed glider. If a small amount of up elevator is held, the aircraft will continue to fly at reduced speed. As soon as up elevator is released, the aircraft appears to immediately shoot forward. There is very little loss of height.
• Due to the low inertia in pitch, recovery from a stall is rapid as well. Acceleration to flight speed and transition to level flight, even with the aircraft at a standstill in a nose vertical attitude, takes but a few feet of height. Once speed is above minimum, judicious use of up elevator can make recovery even more rapid. With practice, you can make a tight half loop using full up elevator with the aircraft coming to a complete stop just as it is inverted and at the peak of the loop. (The entry speed is critical.) Release up elevator while the aircraft is stopped. The Raven will quickly rotate to a nose down attitude, then accelerate through a large arc and transition to upright gliding flight in the same direction as at the start. The resulting maneuver traces the outline of a comma.
• If you've built the MB Raven according to the plans and without ailerons, you'll find that it turns much quicker if up elevator is applied before rudder control is input. Depending on how elevator and rudder are coordinated, flat or steeply banked turns can be made.
• Although the MB Raven may fall off onto one wing if fully stalled while in a turn, we've not been able to enter a spin.
• Thermalling is as easy as finding lift, setting the turn, and putting in some up trim. The Raven signals lift by pitching up. This motion is sometimes quite dramatic, and the airplane slows as it travels through the thermal. Centering is nearly automatic.
• The MB Raven does exceptionally well in light lift if the overall weight is kept down. At a recent contest, Bill twice set up his R/E Raven for a landing well short of the desired flight time. Both times, the Raven flew through a patch of very light lift on final approach, and was immediately turned back into the rising air. A series of flat turns in the low altitude thermal allowed the flight times to be met. Moral: Don't give up until you're on the ground.

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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.

Hobby Town, Jon Packer Manager, 402 Garfield St, Tacoma, WA 98444; (253) 531-8111.

Model Builder Raven. Dave Jones. Model Builder, January 1982.

Recommendations for first tailless, Part 1. Bill & Bunny Kuhlman. RC Soaring Digest, February 1991, and On the 'Wing... the book. B2Streamlines, 1993.

Differential and performance. Bill & Bunny Kuhlman. RC Soaring Digest, August 1992, and On the 'Wing... the book. B2Streamlines, 1993.

Aileron differential, Part 1. Bill & Bunny Kuhlman. RC Soaring Digest, May 1996, and On the 'Wing... the book, Volume 2. B2Streamlines, 1998.

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