MINI ULTIMATE BIPLANE

Model Airplane News, Aug 2005 by Tritle, Pat

A SMALL ELECTRIC AEROBAT WITH SPUNK

MANY YEARS AGO, I DESIGNED A 20-INCH-SPAN, free-flight Ultimate biplane, and when it proved to be a solid, stable-flying model, I immediately began to think about how I could convert it to RC. Back then however, such a conversion would have required the use of very specialized-and by today's standards expensive-equipment, so I put the idea on hold for a while. Now, with the availability of mainstream sub-micro radio components and the little GWS IPS drive systems, the time is perfect for this RC Mini Ultimate Biplane.

ABOUT THE DESIGN

The idea behind the Mini Ultimate Biplane was to design a park flyer that would be appropriate for novice "small airplane" builders yet be sufficiently appealing to experienced builders and fliers who would want to add ailerons and eliminate the dihedral. With only a little extra effort, pilots who want serious performance can easily produce an all-out hot-rod! Although the model was designed for the IPS 4:1 drive unit, it has plenty of room up front for a high-performance brushless motor that will offer noteworthy aerobatic performance.

The Ultimate is basically a stick-and-tissue model-strong and light! The model is small enough that the wings need not be removable, and it's also very easy to set up; the cabane-strut arrangement is self-aligning, which makes it a breeze to mount on the top wing. All you have to do is decide whether you want a simple park flyer or a full-blown aerobat.

BUILDING THE MODEL

Start by cutting all the parts out of the patterns provided. Use the appropriate wood sizes or the laser-cut "Short Kit" available from Pat's Custom Models.

Frame the vertical and horizontal stabilizers using the wood sizes shown. When the glue has dried, remove the parts from the plans, and sand the edges round. Cut the hinge slots with a sharp hobby knife. The hinges are made of 1/8-inch-wide strips of light CA hinge material. Fit the hinges, but don't glue them into place until the final assembly. Bend the tailwheel bracket out of 0.032-inch steel wire, and glue it into the rudder.

WINGS

Frame the wings directly over the plans. Pin the spars on the plan, and glue the ribs into place at each location, except at the dihedral breaks. Shape the trailing edges before you glue them into place; it's a lot easier that way. Then glue the leading edges into place to complete the basic wing structures.

Raise the bottom wing's tips, and block them up 5/8 inch; then glue the spar joints and the remaining two ribs into place. Block up the top wingtips ¾ inch, and glue the center rib into place. Remove the wings from the plans, and sand the leading edges to shape.

Build the interplane struts over the plans and then sand their leading and trailing edges round, but leave the top and bottom edges squared off for gluing.

FUSELAGE

The fuselage sides are built directly over the plans. Pin formers F1, F2 and F3 into place, and frame one side around them. For the opposite side, only F1 and F2 are used. Now is a good time to bend the 0.046-inch wire landing gear to shape, and assemble it into formers 7, 7A and 7B.

With both fuselage sides complete, pin them upside-down over the top/bottom view, and glue all the bottom formers and cross-pieces into place. When these have dried, remove the fuselage assembly from the board, and glue formers 1 through 6 into place. Next, add all the hard-balsa stringers to the top of the fuselage between formers 1 and 3, followed by F4 and F5 on both sides. Locate F4 and F5 carefully, as they set up the location of the cabane assembly later.

The aft turtle-deck stringers extend all the way to the tail post. Before you glue in the stringers, make a 3/32-inch balsa shim and tack-glue it to the top longeron behind F6. This will be used as a spacer to align the upper stabilizer supports. Once those have been glued into place, glue the centerline stringer into place, and allow it to extend just past the tail post. Next, fit the vertical balsa filler between the top stab support and the stringer, and glue it into place. Then add the rest of the stringers. Trim off the centerline stringer, and sand the aft end to shape. The stab shim can now be removed.

INSTALLING THE RC GEAR

Glue the servo rails into place, and space them to accommodate the servos used. I used Cirrus CS-4.4s, as they have more than adequate torque to control the model. The rudder servo is mounted on the centerline, and the elevator servo is mounted on the outside end of the rail. It doesn't matter which side the pushrod exits, but the servo needs to be on the same side as F3.

The elevator is controlled by a 0.025-inch wire pushrod running through a Sullivan no. 507 pushrod tube supported at both ends and in the middle. To avoid the added weight of the hardware, the pushrods are connected with a Z-bend at both ends. (Don't make the aft bend until final assembly.)

The rudder will be controlled by Kevlar pull-pull cables. Each cable is a single piece of string that runs from the rudder control horn through the servo arm and out the other side to the control horn. Tape the cables to the control horn, and mark the exact spot where the cables exit the fuselage on the plans.