Kyosho: Electric Learjet ARF

Model Airplane News, Jan 2001 by Ziroli, Nick Sr

A twin ducted fan for the Sunday flier

by Nick Ziroli Sr.

Only a few years ago, an ARF, ducted-fan Learjet would have been very unusual, and an electric one would have been totally unthinkable, but

The Learjet's fuselage, wing and tail are made of styrene foam with a hard, smooth outer surface. The fuselage halves are joined over a plywood structure to produce a strong, light assembly, and the wing and stabilizer halves are joined over foam-cores-very light Thoug, and strong. The engine nacelles are also molded foam with the ducted-fan shrouds and aluminum mounting sockets in place.

MODEL ASSEMBLY

Through the Learjet is an ARF, a fair amount of work is required to complete it. You'll need only a few tools and epoxy. To begin, use a razor and sanding blocks to remove the flash at all of the wing joints; use a razor plane on the starght sections. The control-surface hinges have been molded into the surfaces. You need to cut the hinge lines through the surface, leaving short uncut sections that become the actual hinge. Cut a 1/32-inch gap at the ends of the ailerons. The cotrol-horn torque rods are molded into the wings; nicely done. Use 30-- minutes epoxy (not 5-minute!) to join the wing panels. The double-plywood wing joiner allows a wide variation of dihedral. I sanded the ends to match with about 3/4-inch dihedral under each tip.

I used Airtronics* Microlite 94501 servos on all controls. These weigh about 1/2 ounce each and have 29 oz.-in. of torque. The instructions indicate the size of the servos but not the more important torque recommendation; the 94501 servos have proven to be adequate. The servo openings in the die-cut plywood mounting plates were a little large for the 94501s, so I glued -inch plywood strips over the plates on each end to close up the holes. The supplied clevis links that come with the preformed pushrods were too long, so instead of rebending the pushrods, I substituted small Du-Bro* links.

There's very little work required to complete the fuselage: remove the flash, cut a few holes, and epoxy some parts into place. All of the parts fit accurately. Trial-fit the motor pods onto the mounts to be sure the pylons fit into the fuselage; sand the pylon ends and mounting tabs until they are a flush fit. You'll need a 1.Smm Allen wrench to install the nosewheel strut; this isn't supplied, nor is it mentioned in the list of required tools. I managed with a regular wrench and a pair of pliers.

The strut wire is smaller than the holes in the steering arm and collar. I filed a flat on the back of the strut, parallel to the axle. This has kept the arm in place-even with the mismatched parts. Although the instructions don't mention it, you need to adjust the nosewheel-strut length so the fuselage will be level to the ground when it's on three wheels. I like the screw-on wheel retainers and installed them with a little thread-lock. I soldered a washer to the axle inside the wheel rather than use the supplied soft plastic tubing. Accurately aligned, freely turning wheels are important to a ducted-fan model; the fan performs at its worst during the takeoff run, so the model should roll as easily as possible.

Mounting the wing is the same as with any bolt-on wing assembly. I would have liked to have seen dimensions and hole sizes called out in inches as well as millimeters. Be sure to align the wing as shown, measuring from tips to tail. I decided to glue the wingtip tanks into place but then noticed that the instructions don't say to glue them on. It turns out that the tanks are supposed to be removed for flight. Mine are on, and as you will read in "Flight Performance," they're staying on. To hold them in place temporarily, you can use strips of the clear part of the supplied graphics.

The instructions tell you to epoxy the stabilizer into place, then cut the control surfaces free and mount the control horn, but it's easier to cut the hinges, mount the rudder horn and then epoxy the stabilizer into place. Position the control horn so that the screws are centered between the leading and trailing edge of the elevator; otherwise, the angle of the pushrod will not allow equal up and down control throws without offsetting the servo arm. The instructions do not mention where to place the horn.

One mystery plastic piece is not shown in the instructions: it can only be a cap for the top stabilizer fairing. I decided to leave it off.

After I had installed the servos and pushrods, I wanted to support the pushrod tubes in the aft fuselage. I cut a piece of 1/8 x 3/4 x 3-inch balsa and angled the ends so it fit across the fuselage on top of the pushrods, then notched a second piece of 1/8 x 1/2-inch balsa to fit around the pushrods and glued it on edge to the first piece. These are not easy to install, but they hold the pushrod tubes securely. Make the outer pushrod tubes' ends no more than inch from the servo case, and add a support at the ends to prevent the cable from flexing. The threaded end of the nosewheel steering pushrod and plastic clevis were a loose fit, so I soldered a clevis to the pushrod. Using the innermost hole on the servo arm for the nosewheel pushrod will make takeoffs easier. I also found using a small nylon clevis, like those on the ailerons, worked better on the rudder pushrod than the long clevis that is supplied. I set up all control throws per the instructions.