Ohio R/C extra 300L

Model Airplane News, Nov 1999 by Kuper, Michael J

A winning ticket to IMAA aerobatics

I've heard many modelers say that if it looks right, it probably flies right. From the Extra 230 to the 300L, this principle is clearly illustrated in the aerobatic airplanes produced by Walter Extra. All of his designs have classic proportions and lines, and they are also proven performers. The 300L is the tandem-seat version that replaced the original 300.

I'm an active IMAC Scale Aerobatics competitor, and for this year's Unlimited class competition, I chose to build the Ohio R/C* Extra 300L. A large airplane that is part of Ohio R/C's Monster Series, the 300L features a two-piece, tube-mounted wing that spans 102 inches; this makes it 33-percent scale. This kit differs from precision scale in that the fuselage is slightly stretched to provide its excellent flight characteristics. Ohio R/C's planes have a proven track record in scale aerobatic competition.

All of the parts come nicely packed and undamaged. The kit includes a clear canopy; high-quality, fiberglass, twopiece cowl and wheel pants; aluminum landing gear and a hardware pack. The wood is all machine cut (no die stamping) and is also of high quality; Ohio R/C is one of the few manufacturers that still mills its own balsa. The Extra also comes with 6-inch sheeting that eliminates the need to join sheets in some assemblies. I weighed the provided sheeting and sticks and found a mix of balsa ranging from 5.5 pounds per cubic inch to 8.5; the heavier, harder balsa will form the longerons and other parts where strength is needed, while the lightweight wood is for other areas. A 28-page instruction manual accompanies the two rolled sheets of plans. This kit assumes a certain level of experience on the builder's part; you must pay close attention to the plans and not merely rely on the construction manual to guide you.

WING ASSEMBLY

The plans include full-size line drawings of both the right and left wing. The supplied plywood jig allows you to assemble the wing over the plans, but I decided to use my 3/4-inch melamine jig.

You must decide where to position the rib holders. First, cut the wing plan to separate each wing panel, then draw the rod positions on the plans to make sure that they won't interfere with the construction. To minimize any inaccuracy caused during setup, I tried to position the rods as far away from each other as I could. Next, draw a centerline down each rib from the leading edge (LE) to the trailing edge (TE) to mark the rod position. I positioned the forward rod parallel to the main wing spar and about 1 inch in front of the spar. After putting the rear jig rod parallel to the TE and behind the rear shear webs, carefully drill holes into each rib so you can position the rods.

Tape the plans onto the jig base and make sure that the spar is perpendicular to the end board; then thread the ribs into place by inserting the front rod, spacing the ribs over the plans and then carefully inserting the rear jig rod. The jig will support the rods, which should be clamped into place. The weight of the rods and ribs will cause the middle of the rod to sag slightly, so place the temporary supports under it. Now measure the rib height above the jig base to ensure that everything is straight. As you construct the wing, the spars and stringers will interfere with the 3x24inch temporary supports, so remove them and place the 2-inch supports wherever possible along the length of the rod.

At this point, you reach one of the most time-consuming parts of the wing construction. After inspecting and measuring several times, you can position and glue the spars and stringers. I used Ohio R/C SI 1500 industrial grade CA on the main spar, which is spruce, and Balsa USA* Gold CA on all of the balsa parts. The main spar is doubled for the first 18 inches; when cutting the doubler, taper the end to avoid stress concentration. The first four ribs (W1-W4) are lite-ply because they contain the wing tube. I installed the wing tube, and I also installed a servo guide tube through ribs W1 to W7 before clamping the jig rods in place. (Lightweight tubes are available from TNT Enterprises*.) Before you glue the spars into place, make sure that you have the root rib (W1) set at the proper angle. After the spars and stringers have been glued in, flip the assembly over and do the other side. Make sure that you have the wing bolt plates and hinge blocks installed before you sheet the wing. I don't recommend removing the jig rods until after you attach the shear webs and sheeting. When sheeting the wing, you are instructed to leave a l/2 inch of overhang at the root rib to be cut later, but I found liz inch to be inadequate. I recommend leaving at least 1 inch of overhang.

You are now ready to shape the wing's leading and trailing edges. I used a Master Airscrew* planer and a sanding block for this process. Cut out the ailerons, glue the balsa-facing material, and mark the hinge lines. Ohio R/C assumes that you have your own method for installing servos, so the position is marked (between W7 and W8) on the plans, but no servo trays are supplied.