LAST MONTH WE modified and completed the Hobbistar 60 MK III's wing. During this process the fuselage servo-mounting areas were reinforced, as was the tray mounting. This month we will finish the airplane, and next month we will make a few test flights to get it properly trimmed.

    There is a lot of assembly work to do and not much space, so let's get to it.

The Hobbistar MK III will look like this after you have finished the work covered this month. It's ready to check out and fly!
Photos courtesy the author.

Every model builder has his or her own preferred assembly sequence. Mine usually involves starting at the front and working backward but stopping just before installing the main landing gear. Then I work forward from the tail, installing the main gear last.

    Since the fuselage is constantly being rotated and repositioned during assembly, I try to delay installing parts that "stick out," such as the main gear and horizontal stabilizer, as long as possible. This means servo installation is usually the first step. Then I mount the engine.

    However, there is one thing to check before starting power-plant installation. It is a good idea to reinforce the firewall from the inside. As with the servo-mount reinforcement from last month, the Hobbistar 60 is a poor example because the manufacturer has already installed firewall braces. But most ARFs, especially those with a nose gear, could use this extra help.

    A photo shows the installation of 1/2 spruce—not balsa—triangle stock to brace the firewall of a different ARF. Use five-minute epoxy—not cyanoacrylate—for maximum strength and extra positioning time. Make sure the triangle stock does not cover the blind nuts. Usually only two pieces of spruce on the sides are required.

    While you are in there, apply thin cyanoacrylate to those four blind nuts that will hold the engine mount in place. Make sure not to get any adhesive in the center holes.




Many ARFs use the popular clamp-on mount. How to install an engine in this mount was covered in the "From the Ground Up" installments about engines. (If you do not have this article, it has been reprinted in Sport Aviator's Pri-Fly section at www.masportaviator.com.)

    The basic idea is to center the engine in the mount and position the rear of the engine's bolt plates firmly against the two rear clamping bolts in the mount for proper alignment. If your ARF uses the reinforced fiberglass mount, adjustable or rigid, go to the previously mentioned area of Sport Aviator for mounting details.

The type of engine mount found on today's ARFs fits almost every engine and makes swapping engines easy. Be sure all eight bolts are "thread-locked."

Reinforce firewall using spruce triangle stock epoxied in place. If it is necessary to cover a blind nut, trim that part of spruce for clearance.

Install the mount onto the firewall. Use the bolts and washers provided, but make sure to use removable thread-locking compound. The clamp-on mounts are strong and allow easy installation.

    However, the mount's metal construction allows some engine vibration to be transmitted to its mounting bolts. Tighten them securely. Remove the two clamp plates and position the engine on the mount in roughly its final position.

Get the muffler and see if the fuselage sides will permit its installation. If there is enough room, skip the following few paragraphs and go on to the section on setting up the throttle. Chances are there is insufficient muffler clearance; I don't know why, but many ARFs do not provide enough. Now what do we do?

This model did not have adequate clearance between muffler and fuselage side, which is often the case, but the fix is not too difficult.

    Mount the engine in place. Plug the intake and exhaust holes, and cover the entire engine with a plastic bag. Use a straightedge and a sharp modeling razor knife to cut the covering. Make the cut slightly larger than required. Carefully peel back the covering and apply some low-tack painter's masking tape to protect the covering.

    Use a razor saw to make the vertical cuts that are visible in the picture. Score the wood for the horizontal cut and then use a 1/2-inch hobby razor chisel to cut through the tough plywood. Nudge the other end of that chisel handle along with a tack hammer. You finally get to hammer away on something!

    Remove the offending wood piece and masking tape. Carefully position the covering over the new slot and iron in place with a hobby covering trim iron. Cut any overlap away and use this excess to cover the inside vertical parts. Some wood has to show but is rarely noticeable once the muffler is installed. Coat any exposed wood parts with five-minute epoxy to prevent fuel damage.

    If you can locate new covering that is the same color, using it is preferable. But usually either the colors do not match or purchasing an entire roll is required, which can get expensive for 2 inches worth of material.

Protect engine, leaving glow plug in place. Carefully cut covering and peel away. Use razor saw to make two vertical cuts.