Lyka jet, The

Model Airplane News, Nov 1997 by Zimmanck, Jack

THE FIRST TIME I saw the fast, smooth flight characteristics of a ducted-fan model. I was hooked, but I did not want the temperamental operation and expese that often go with these exciting and graceful birds. I tried a couple of the prop/jet look-alike kits. They looked OK, but the performance and flight characteristics were not even close. What I wanted just was not available. That left me just one alternative.

I decided the best way to achieve the shape, balance and weight for the Lyka Jet was to design the airfram, "from the inside out," around the components. I weighed the engine, pipe, tank and radio gear and dumped them on the workbench over a blank sheet of drafting paper. I began to develop the long-nose, shortcoupled jet silhouette that distinguishes the Lyka Jet. The swept wing and the tall, swept fin further define the Lyka Jet profile. The final touch of jet realism was to route the exhaust out through the rear of the plane. I chose a .40-size engine since it is fairly inexpensive and can easily be set up to turn the necessary rpm. The power system for the prototype was an O.S. .46, rear exhaust with a tuned pipe, a 9X9 APC* prop and an 8-ounce tank. To keep the cost down, all servos were Futaba S148s with ball-bearing kits added to keep the controls slop free.

The name? Everyone who saw the prototype said it really looked an flew like a jet.

LOW-TECH APPROACH

To keep building simple and inexpensive, low-tech materials are employed wherever possible. You can get a lot of the Lyka Jet materials at yoru local lumberyard. Perhaps I went a bit overboard on the low-tech when I chose 2-1/2 inch-square PVC downspout materials for the fuselage. Where wood is joined to the PVC fuselage, I use Plumber's Goop (lumberyard). The fuel tank is an 8-ounce shampoo bottle (grocery store) with a Sullivan* tank stopper kit. The aileron stock, wing sheeting, tail feathers and plywood will require a visit to your local hobby shop. The canopy/pipe tunnel on the prototype is from the Escape kit by Bridi Aircraft Designs*. The foam wing-core templates are shown on the plans, and foam wing-cores can be ordered from Wing Mfg.* Prototype one had no cowl. For the second prototype, I formed a fiberglass cowl.

CONFIGURATION

Before you start, take some time to decide just how much performance you want. A hand-launched airplane gives the lightest weight and best performance. It is also easier to build and looks great in the air with no landing gear hanging down.

Fixed gear can be installed but adds 1/2 pound to the total weight. Retracts area 12- to 14ounce weight penalty and will bring the wing loading up pretty high. Retracts may also require a fuel pump so that the fuel tank can be moved over the wing to make room for the nose gear. A long, flat, shampoo bottle can be used for a tank in this type of installation.

Though the Lyka Jet was designed specifically for a tuned pipe, it will be a good performer with a standard muffler. The muffler up front may require a bit of rebalancing to get the CG correct.

If you set it up with a tuned pipe and no landing gear, you will have a plane that will outperform many ducted-fan jets. At full throttle, you really have to think ahead of it, but it will slow down for gentle landings as long as the weight stays below 5 1/2 pounds.

CONSTRUCTION

Wings. The wings are standard foam-core construction with 1/16-inch balsa sheeting. Start by lightly sanding the cores while they're in the foam cradle. Cut all the necessary servo or landing gear openings with a sharp paring knife. The servo boxes are at the high point of the wing and will accommodate a standard servo standing straight up. Servo output arms are on top of the wing in hand-launched models. Install 3/4-inch filament packing tape (or 1/2-inch carbonfiber strips) for stiffness and strength.

Prepare the wing skins by trimming the edges of the 1/16-inch balsa sheets to join without gaps. Join the sheets along the full length of the seam with 3/4-inch masking tape, then turn the sheets over and open the seam by folding it back over the edge of the workbench. Glue the seams with medium CA. and close the seams on the bench top. Sand immediately with 100-grit sanding block to drive balsa dust into the seam. Repeat until all seams have been completed. then remove the tape. Cut the skins to shape leaving 3/8 inch all around the wing platform. Spread a very thin coat of twohour epoxy on the inside of the skin, and lay the skin epoxy-side-up in the foam cradle. Put the wing-core on top of the skin in the cradle. then apply epoxy to the inside of the top skin and apply the skin to the core in the cradle. Put the top cradle on top of the skinned core, lining up all the corners, and set the cradle stack on a straight, flat surface with a flat board covering the top. Evenly distribute 50 to 60 pounds of weight on the stack. (Books and magazines work well.) Repeat with the other wing-core and leave the cradles weighted for 36 hours.

Remove the cores from the cradle, trim off the excess skin, and add the LE and TE and wingtips per the plan. Sand the roots of the panels and then join them to produce 7/8 inch dihedral under each panel tip. Use 6-inch fiberglass cloth on the top and bottom of the center section for added strength.