Description

The 4-120 Bipe was a kit manufactured by Ace RC.  In 1993 RCM published a plan called the Ultimate 120.  To use the plan, the builder would purchase the 4-120 kit from Ace, modify some of the kit parts as directed, fabricate some other parts, and assemble the parts over the Ultimate 120 plan.  The result was a big plane that looked a lot like an Ultimate Biplane.  It’s not really a scale project, but it’s a pretty convincing fake.  My short kit is a partial substitute for the Ace 4-120 kit that you were supposed to have.  It’s pretty much like my other short kits, with fuselage sides, doublers, 48 ribs, bulkheads, and a few other doo-dads.  It has a reasonably high parts count.

The plan has some issues that should be noted.  The scaling of the plan that I found online was way off, so I rescaled it to what I believe to be the correct dimensions.  However, even after I corrected the dimensions of the plan there were still some serious drafting errors in the original plan.  In particular, on the fuselage sheet the top wing is smaller than the bottom wing, and the bottom wing is smaller than what’s shown on the wing plan sheet.  Also, the bulkheads are not depicted correctly.  If one were to cut out the bulkheads as shown on the plan, some problems would be encountered during fuselage assembly.  So I made some executive decisions and designed the parts to produce a workable fuselage.

• Build the wings over the plan.  Trim the point off of the leading edge of the bottom wing so it’s flat and fits in the wing saddle.  You shouldn’t have to sand off more than 1/4 inch.  Fill in the gap at the trailing edge with scraps.  If you’re wondering what all the little lite ply rectangles are for, they are shear webs mentioned on the plan.
• Build the tail entirely of 3/8 inch balsa sticks.
• Don’t refer to the plan for the locations of bulkhead 3.  It has been moved back a little bit to accommodate the wing.  Just glue the doubler to the fuselage and trust the notches.
• I didn’t use the nomenclature from the plan, where the parts are referred to as N or K to tell whether you’re dealing with a kit part or a new part because that’s not relevant to builders of this short kit.  The three top fuselage plates are just called 1, 2 and 3, and they key into slots in the fuselage doubler.  The 3/4 x 1/2 inch strut anchor beams go in between these pieces.
• The cowling is your call.  The plan shows thick planks that I can’t cut with a laser, so you can cut them yourself, or make a cowl out of something else.

Other than that it’s pretty straightforward.  If you’re doing this project you should be able to figure it out.  If you have questions, feel free to ask.

I submitted my corrected version of the plan to Steve at Outerzone, and he posted the updated plan there.