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TRI-F-O

Flying Saucer Design by George Gassaway

Published in the NAR's Sport Rocketry Magazine and the NAR Member Guidebook.

 

Plan below, followed by instructions

 

Tri-F-O

© copyright 1996, 2004 by George Gassaway & the NAR

Commercial rights reserved

The name Tri-F-O stands for Triangular "UFO". This model is a relatively easy to build balsa model which flies somewhat similarly to the circular cardstock Flying Saucer introduced by Centuri in 1977 and also in plastic from by Estes years later.

The model is primarily built from 3/32" balsa. Non-wood parts required are one 2" length of 18mm tube for the engine mount, an engine block, and a launch lug for flying off 1/8" launch rods.

The plan shows full size templates for the struts and spin tab joint reinforcements, photocopy them so as not to cut into the magazine to use them. Due to space limitations the triangular body side and spin tab shapes are shown 1/2 size, but those are easy to draw out. Cut out all parts, being careful that the wood grain runs in the right direction.

Prepare the wood parts by sanding the leading and trailing edges of the struts, the leading edges of the side plates for the triangular body, as well as the trailing edges of the spin tabs. Sand the 45 degree diagonal edges of the side plates to a bevel of about 45 degrees, so the joints will match up well when the three side plates are glued together.

Finish: If you want to have a smooth finish on the model, it needs to be done before assembly as most of the balsa surfaces cannot be easily reached by sandpaper after assembly. A coat or two of sanding sealer is all that's needed for a somewhat reasonable finish. However, the prototype had no sealer, it was sanded smooth then after assembly sprayed flat white for an undercoat and fluorescent orange for the final color.

Assembly: Use tape to hold the edges of the side plates together, to create the triangular body. The tape should go across the top of the corners, and the placement adjusted so that the three side plates are held aligned in the proper position for gluing. Use thin Cyanoacrylate glue (CA) along the inside of the corners (opposite of the side with the tape) to tack-glue the side plates together. After the CA has cured, remove the tape, careful not to pop the tack-glued joints loose. Than use more CA along the inside of the joints for a solid bond. Be careful not to use so much CA that it runs. After the CA is cured, use sandpaper to round off the sharp outer corners of the joints.

Use the guide in the plan to mark onto the engine tube the location for the 3 struts. Use a door jamb as a guide to redraw those marks into parallel lines for the strut roots.

Insert a standard size 18mm engine into the engine tube, using tape for a friction fit so the engine top is flush with the top of the engine tube. This should leave exactly 3/4 of the engine protruding down from the tube. Place this on a flat surface. Hold one of the struts in position alongside the motor tube, with the trailing edge of the strut root even with the bottom of the motor tube. The tip of the strut should just barely touch the building surface, as the strut has 3/4" sweepback. Apply a little CA to tack-glue the strut into place on the engine tube, being careful not to let the CA run to accidentally glue the engine into the tube. Repeat with the other two struts. Remove the engine. Since the struts may not be exactly 120 degrees apart, lay the strut/tube assembly upside down into the triangular body assembly (also inverted). With the strut/tube assembly centered, carefully pry the struts if necessary to get them seated into the inner corners of the triangular body. Then apply CA for the root edges of the struts, along the tube, to permanently glue the struts to the tube (do not glue to the triangular body yet).

Insert the engine back into the engine mount tube, leaving it hanging out by about 1.5", with a slip-fit that is not too tight or too loose. Set the strut/mount assembly onto a flat surface, with protruding engine holding the assembly above the work surface. Carefully slip the triangular body down over the strut tips, keeping it centered. Press so that the engine tube/strut assembly also slides down the engine, until the triangular body touches the flat surface. If you have done this right, the engine bottom is still flat to the work surface, assuring the engine tube is properly aligned. If not, repeat the process to get it right. When you're sure it is aligned, apply some drops of CA to the strut tips to tack glue them to the inner corner joints of the triangular body. After curing, lift off the model from the surface and apply more CA to make the strut tip to body joints permanent.

To help even out any errors in alignment, as well as add some extra flair to the flying, the model uses spin tabs which also look like landing pads (OK, sort of...). Making sure the spin tab grain is in the correct direction, glue the spin tab root directly to the trailing edge of the side plate, so that the tab is the same angle as the side plate. Make sure all three tabs are located to the same corners in order to produce the desired spin. The prototype had a problem with spin tabs getting broken in transport, so the plan shows a 1/2" wide reinforcement strip. The reinforcement can be 1/32" plywood, or 1/16 to 3/32" balsa. Glue the reinforcement strip to the underside, half of it across the spin tab root and half across the side plate, as shown in the bottom view.

Remove the engine from the tube, and glue in an engine block (or 1/8" tall piece of burned out casing acting as an engine block). Glue a launch lug to the engine tube, at the root of one of the struts.

The model is now complete, other than adding a final finish. Color as desired

Intended engine power is B6-0 and C6-0. B6-2 or C6-3 engines are not recommended due to the potential to eject too close to the ground. Tape the motor in for a friction fit.

When flown the model will climb to about 100 feet on a C6 motor, spinning slowly. When the motor burns out, the model will come to a stop almost immediately, then flip over and descend while rotating. Due to the very high drag, the model will land relatively softly. With no recovery system to prep, reflights go fast.....until you run out of engines!