Dave Jones of Torrance California was a prolific designer of tailless aircraft. Western Plan Service, which he owned, served as the distributor of his many full size plans. His basic philosophy was that models should be relatively easy to construct, with little sanding and shaping needed, yet they should consistently perform well. The Raven series (Raven, Mini Raven, Raven 2M, Raven NFF, and Raven S) exemplified these goals. The last in the series, the Raven S, had several advanced design features, yet used simple materials and construction methods.

The Raven series was formulated to provide a competition soaring machine with different capabilities than those of conventional tailed sailplanes. All of the aircraft in the series relied on a plank planform. A reflexed airfoil section provided pitch stability. The resulting aircraft were stable wings with no sweep or twist. Because of low pitch inertia, the elevator on these models is quite powerful, despite a short lever arm. Dave designed the Raven so the center wing panel, fuselage, elevator, fin and rudder are a single unit, and all controls are permanently connected. The wing loading of these models is very low - just 4.3 oz/ft² for the prototype MB Raven - yet penetration in windy conditions is not a problem. Ballast can be added to bring the wing loading up to around eight oz/ft².

Our first tailless model was Dave's MB Raven, which appeared as a construction article in the January 1982 issue of Model Builder magazine. As newcomers to the world of proportional radio equipment, we both used this model to learn to fly. In fact, we had so much fun with it that a second MB Raven, this with ballast tubes, was built a short while later. Both of these aircraft, each with several minor and a few major repairs, are still being flown.

Having already built two MB Raven, we had initially planned to build a Raven S this year. When the idea of a "build-along" construction article for RCSD was presented, however, we decided to delay building the Raven S and instead build a modified MB Raven. Continued availability of MB Raven full size plans from Bill Northrop's Plans Service was the deciding factor in making this decision. The plans are printed on two large sheets, and a copy of the original construction article is included. (Full size plans for the Raven S have not been available since Dave's death and the demise of Western Plan Service in 1991.)

There are nine modifications which are to be incorporated into this new model. Most of these changes are derived from information gathered from the MB Raven construction article, from the Raven S plans, or from personal correspondence with Dave.
 

• The original MB Raven was designed for a two function radio system. Control surfaces shown on the plans consist of a centrally located elevator and a large rudder mounted on a slender fin. Miniaturization of on-board radio equipment allows for a far more sophisticated control system to be installed, so our first modification will be to add ailerons. This will mandate some connections be made on the flying field, but the added maneuverability is a fair trade-off.
• To remove the ailerons from the adverse effects of being in the tip vortex, the outer end of the ailerons will be some distance away from the end of the wing. To maintain aileron length, new wing tips will be fabricated.
• The ailerons will be driven by direct linkages to wing-mounted servos, so we'll be adding servo mounts in the outer wing panels at the inboard end of the ailerons.
• The outer wing panel turbulator spars will be replaced by balsa sheeting. This will increase torsional rigidity so aileron deflection at high speed does not twist the wing.
• Cap strips will be added to all ribs where sheeting is absent. This is being done because it adds strength to the ribs, improves appearance, and makes the ribs easier to cut out.
• The MB Raven construction article contains very much more information than simply how to build the model. Dave included suggestions for modifying the airframe for specialty applications like electric power and slope flying. Several other possible minor modifications are outlined as well. One of the presented modifications is to straighten the wing so dihedral can be used rather than the polyhedral shown on the plans. As this makes the aircraft appear a bit more realistic, we've decided to build a straight wing. This modification simplifies construction of the joint between the center section and outer panels of the wing, but necessitates fabrication of a dihedral brace for the center of the wing.
• The Model Builder article also mentions the dihedral angle can be lowered significantly, promoting flatter turns. We're not looking for flatter turns, but we are looking for aileron deflection to produce the rolling moment rather than the aerodynamic interaction of yaw and dihedral. The original MB Raven had a flat center section and five and a half inches of dihedral - about ten degrees. We're going to use four degrees of dihedral, the minimum recommended by Dave in the construction article.
• We're going to take advantage of the reduced dihedral to also raise the wing root position on the fuselage so it is flush with the upper deck of the fuselage. Because of the reduced dihedral, the distance between the CG and the tow hook will remain nearly the same, so there should be no noticeable increase in pitching during launch.
• The MB Raven uses the CJ-3309 section (3.3% camber at 30% chord, 9.4 percent thick), while the Raven S plans show two airfoils: the CJ-25209 (2.5% camber at 25% chord, 9.5% thick) is denoted as the standard airfoil, the BW 05 02 09 as the option (2.0% camber at 27.5%, 9.4% thick). In a 1984 letter from Dave, he said, "This airplane is still in the design stage. Plan two identical models except for the wing sections. The other section is by Barnaby Wainfan, well known free flight flying wing designer. He is also an aerodynamicist and won P30 at the last NATS with a flying wing." The BW 05 02 09 beautiful, very much like an EH section in appearance. We contacted Barnaby Wainfan and discovered there are no published coordinates for the BW 05 02 09 section. We've derived coordinates from the printed sections on the Raven S plans and will be and will be using it for this model. The coordinate set will be published in the installment covering wing construction.

The radio system will be either our trusty JR Century VII or a JR PCM 10. As the entire control system is relatively straight forward, the Century VII is the logical choice. We do, however, want to do some experimentation. One thing we'd like to do is be able to utilize the ailerons as spoilers; the central elevator would then be used for trimming out any pitch change as the ailerons are deflected upward. Another option we'd like to try is to use the ailerons as elevons. This would free the elevator to be used as a trimmer only, promoting more efficient thermalling. These two experimental setups would require use of the PCM 10, as it has a multi-position flight mode switch and five separate channel mixing functions. The Century VII lacks these functions.

As with the second MB Raven we built, provision for ballast will be incorporated. Half inch paper tubes, as used in model rockets, will be placed in the center section of the wing. Ballast in the form of half inch steel rod can be inserted into the paper tubes by removing the outer wing panels. Filling the tubes adds two pounds to the aircraft weight, raising the wing loading by about 3.4 oz/ft².

We begin construction in the next installment, starting with the wing. Stay tuned!

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References:

Notes on planks. Bill & Bunny Kuhlman. RC Soaring Digest, October 1989, and On the 'Wing... the book. B²Streamlines, 1993.

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

How planks fly. Bill & Bunny Kuhlman. RC Soaring Digest, March 1992, and On the 'Wing... the book. B²Streamlines, 1993.

Sections with near zero pitching moments - good choices for plank planforms? Bill & Bunny Kuhlman. RC Soaring Digest, August 1996, and On the 'Wing... the book, Volume 2. B²Streamlines, 1993.

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

Bill Northrop's Plans Service, 2019 Doral Court, Henderson NV 89014-1075.
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