Chula Vistan PAA-Load Free Flight Article & Plans
August 1954 Air Trails

August 1954 Air Trails

Table of Contents These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

Here is unique and sophisticated 1/2-A free flight model designed for and flown successfully in Half-A payload, Half-A free flight, Class A free-flight, and it meets FAI gas requirements. By adding a set of floats it can be flown in R.O.W. events. Chula Vistan's elliptical dihedral and use of a diesel engine set it apart from many similar models of the era (mid-1950s). As with many F/F models, the Chula Vistan's framework is a work of art that is most appropriately covered in a translucent or transparent scheme with colored trim so that its structure can be appreciated. Not being a free flighter, I do not know if it meets modern specifications for current AMA or FAI events.

Chula Vistan PAA-Load Free Flight

Elliptical Dihedral Diesel Powered PAA-Load Free Flight Contest Model Plane for Five Events

By CPO Tom Henebry

This is the second of a series of small gas models with an elliptical dihedral wing form. It is unique for a payload model in that it retains the high performance of a true pylon, yet the simple cabin enclosure meets PAA's Half-A payload specifications. It has been flown in Half-A payload, Half-A free flight, Class A free-flight, and it meets F.A.I. gas requirements. By adding a set of floats it can be flown in R.O.W. events. All told, five events may be entered with this one basic model. The dummy is located exactly on the C.G., and flying with or without the dummy doesn't noticeably change the flight pattern.

Rather than repeat at length the construction of the wing jig, I refer you to page 37 of the January 1954 issue of Air Trails. The jig for Swayback and the Chula Vistan are the same in type and construction, the only difference being in the curve line. By comparison you can see the Chula Vistan is flatter in the center section and rises more sharply at the tips.

Assuming you have the jig, with the plan located properly in the concave form, install the inner piece of 1/8" sq. balsa that forms the leading edge. Outline it with pins and weight it down so it stays in contact with the form along its entire length. Cement the leading piece of 1/8" sq. along its entire length to the piece already installed; you now have a leading edge 1/8" high and 1/4" wide. Repeat these steps until you have the leading edge built up to 1/4" sq. and evenly cemented together at all points of contact. The trailing edge is made in the same manner except the inner piece of 1/8" sq. is installed a little later.

When the leading and trailing edges are set up, cement the slotted 1/8" sheet tips in place. Follow with the 1/8" x 1/32" bottom cap strips, cut to size and cement the 1/32" x 2" cross-grained sheet in the indicated position in the center section. After the cap strips and center section strips are in place, cement the inner piece of 1/8" sq. trailing edge to the trailing edge as well as to the tops of the cap strips and center section piece.

Detailed cutaway drawing is by Douglas Rolfe, nationally known aircraft artist. Mr. Rolfe's work appears regularly in this publication and in the forthcoming "Air Progress"

Cement a piece of tough 1/8" sq. from tip to tip, filling the slots in the tips and cement securely to all cap strips and center section strip. You will need to weight the spar down with any small heavy object at hand to assure even contact with all cap strips. The spar is built up by cementing successive pieces of 1/16" x 1/8" to the 1/8" sq. already installed. These pieces are progressively shorter as indicated, so when the spar is complete it is not unlike an automobile leaf spring, thicker in the center and thinner at the tips.

To complete the wing, cement in the ribs and the top cap strips, cover the three center section ribs with cross-grained 1/32" sheet and finally add the unslotted pair of wingtips on top of the installed pair. When the wing is thoroughly dry remove it from the jig, shape the leading and trailing edges and the tips with sandpaper - and of course an overall sanding with fine sandpaper. The wing is now ready to cover. I believe it is at this point that the builder realizes the beauty of this type of wing construction. No weak dihedral joints, no chance of misalignment, and even without covering its resistance to torsion is amazing. The secret here of course is that every strength member in the structure is pre-stressed.

Stab construction is the same as the wing except the leading edge and the spar are not laminated. The spar should be tapered as indicated before installation.

Fuselage construction is conventional; make the two sides over your full-sized penciled plan. You will note that the longerons are 1/8" square but the sheet fill-in is 3/32"; this is intentional. Place scrap pieces of 1/32" sheet under the sheet parts to bring them flush with the longerons on the first fuselage side. Make the other side directly over the first, but push the sheet parts down so they are 1/32" below the edge of the longerons. On assembly be sure the depressed sheets are on the outside; the longerons are then sanded smooth with the face of the sheet before covering.

The pylon is made from laminated pieces of 3/32" x 3" overlapping each other at 60 degrees. The pylon extends through the fuselage to the broken line on the drawing. After laminating the pylon is cut to shape, sanded, and lightening holes cut where indicated, and formers F-1, 2, 3 and 4 are cemented in position on the pylon. The complete firewall assembly containing the landing gear and engine mounting nuts is cemented in place securely. Insert a waxed PAA dummy in position between F-3 and F-4 - be certain he fits snugly.

With the dummy in place, cement the fuselage sides to the formers and firewall, being careful to maintain correct alignment. Cement the 1/16" x 8" cross pieces in place, then the completed rudder, fill in as indicated with 1/16" sheet. The 1/16" plywood stab platform is cemented in place with the fuselage lying on a smooth, flat surface and the stab in place. Be sure it follows the curve of the stab exactly and that the stab is in line with the fuselage bottom.

The headrest fairing is carved from soft balsa, hollowed out and installed as shown. The plastic canopy is cut to size, slotted to fit around the portion of the pylon that protrudes through it, and cemented in place. Cut the required holes for the engine timer but do not install it until after covering. Finish shaping the pylon and add the 3/4" trailing edge stock plus the two layers of 1/16" sheet, leaving a 1/8" wide strip on the center line for a wing key.

The completed structure is covered with Jap tissue, water sprayed and given four coats of butyrate dope. It is a good idea to double-cover the stab bottom; because of its low position it is prone to minor tears and damage. After covering install the engine timer, wing hold-down wires and the bent pin hooks that retain the payload dummy. Cement a piece of hard 1/16" x 1/8" on the centerline of the bottom of the wing. Before the cement dries, secure wing in place and check the distance from each tip to the top of the rudder - be sure they are equal.

The wire pivot bar and dethermalizer parts are formed and cemented in place. Cement liberally and use strips of Band-Aid backing (crinoline) to further reinforce the points of contact between the wire and the structure. Do not force the wire into the wood but drill holes for it, which results in a stronger joint.

The McCoy Diesel shown gave me the best Half-A and Payload results, but any other good Half-A engine could be used. The Royal Baby Spitfire tank shown needs only minor alterations to fit the McCoy. Filing off a small flange, a home-made gasket and drilling the tank mounting holes slightly oversize will make it work. If you care to compete in Class A events, a Cub .074 or a Royal Spitfire can be attached to the firewall with wood screws.

You may note that there are no provisions On the drawing for any form of glide turn control. You may use your own favorite method, a tilted stab, a floating tab on the left wing or maybe you prefer, like me, to use 1/16" to1/8" wash-in on the right wing. I feel this prevents the right wing from "digging in" in a tight power turn. Whichever method you prefer, the model is trimmed to glide smoothly to the left before power is applied. Start the power flights on low power settings and gradually approach maximum power, making minor thrust line corrections until a smooth, fast, right climbing turn and a floating left glide result. Remember the C. G. position is right at the dummy - if you shift engines, be sure the model balances at the same spot.

Posted April 5, 2014