Dual Air (oz2961)

About this Plan

Dual-Air. Twin CO2 sport model, 44in span. By Frank Ehling MAN Oct 1947.

Quote: "A highly practical twin motor ship using CO2 power.

THE Dual-Air illustrates one of the best ways that CO2 engines can be used because it will turn in one fine flight after another. The ship will fly with two cartridges, as the distributor will run the two engines for the same length of time and with the same power.

It was originally designed to use a re-chargeable fuel tank which would fit in the fuselage, fastened beneath the wing center-section. Such a tank had been used on a previous twin CO2 model and proved a huge success; so much so in fact that the ship (and more important, that special tank) was lost OOS in a recent contest.

A second tank was made and was, we thought, identical with the first. On a bench test however it exploded (we are still shaking!). While we feel that a rechargeable tank is the coming thing, much more research in materials and methods of fabrication must be done to assure safety. Meanwhile we have found the model to be a highly satisfactory flier with the dual cartridge hookup shown here.

The cabin was made in one piece and is removable so the cartridges can be refilled easily. The wing nuts on the cartridge holders extend through the bottom of the fuselage and can be turned at will when ready to fly. The wing is held in place with a rubber band; this is handy when the ship hits an object because the wing can shift and ordinary collisions will not ruin the model.

Construction: The fuselage is made in much the same manner as any other. However, the top longerons are 1/8 x 1/4 while the lower ones and the uprights and crosspieces are all 1/8 sq; this is done to keep the fuselage from going out of line and to assure a good base on which to mount the wing and tail. The fuselage has flat sides and a flat bottom but the top is built up to a rounded section with formers. The top longerons show on the fuselage drawing as the wide strip along the centerline. After the sides are cemented together and the formers cut out and cemented in place, the stringers can be added. These are laid atop the formers. spaced evenly and held in place with pins. then cemented. Add the nose block and sand the entire fuselage.

The wing is next, the ribs are cut out by using a centersection rib as a pattern. All center-section ribs are the same, but ribs in the outer wing panels are each different. Every successive rib, following the inner one (which is the same as those in the centersection), can be shaped from the next larger one by trimming it to proper length and shaving a bit off top and bottom.

Assemble the centersection using 1/4 in sq for the top and bottom spars as this part of the wing has to support the engines as well as the gear. The leading edge of the entire wing is of 1/4 sq strip, edge upward. Undercut the upper nose of the ribs as far back as the spar as shown on the plan and cover this part with sheet balsa. The outer tips can be made next. Complete the left wing and turn it over, then using it for a guide build the right. Cement these panels onto the center-section with each tip raised three inches for dihedral.

The nacelles are carved from balsa and hollowed as shown on the plan. The firewall for each is now cut from 1/8 plywood and cemented in place. Then the two halves of each nacelle are fastened together. Cowlings and spinners can be added if desired.

The landing gear is bent to shape. bound to a 1-3/4 in length of 1/4 sq and cemented into the nacelle, which can now be cemented to the wing. No tail skid was used on the original as the fins serve this purpose.

The stabilizer is made in the same manner as the wing except that there is no sheet covering - the lighter the stabilizer the better the ship will balance out. The rudders are cut to shape, sanded well, and glued to the stab.

The engines are now attached to the firewalls and the tubing run as shown on the plan through the nacelle to the distributor and then to the containers, which are fastened to the wing. The distributor is simply a block of 3/8 sq soft brass with two holes the size of the feed tubing drilled in it. Since both holes go all the way through, an X-connector is formed. This allows two tanks and two motors to be connected together. The cabin can now be finished and is held in place with pins.

The whole ship should be sanded and all cement joints re-cemented. Cover the ship with Silkspan and dope the entire ship with thinned out dope. Check the surfaces to make certain they are not warped If a warp is found, redope that part and hold in shape till dry.

Assemble the ship, holding the wing and tail surfaces on with rubber. Glide the ship with the two cartridges in place; add no more than 1/8 in elevation under the trailing edge of the stabilizer to flatten out the glide. If the glide is still too steep, move the cartridge back till a smooth flat flight results.

Screw in one full and one empty cartridge (this is done to test the ship, for in this way the duration is cut and you can see how the ship is flying); trim the ship with the rudder, then install two full containers and watch her fly. The propellers should both be the same 'hand' and of 8 in dia and 3 in pitch. When both props turn the same way, the ship can be adjusted to fly in the same direction of turn as it glides, which has been found to produce best results.

With oppositely revolving props (the engines run equally well in either direction), the ship can be made to fly absolutely straight but will usually dip quite a bit after the motors stop before going into a normal circling glide. "

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