Dirty Birdy Twin (oz11307)

About this Plan

Dirty Birdy Twin. Twin-engined modification to the classic Dirty Birdy design. For electric power.

This plan is a free download available from the Model Aviation site at https://www.modelaviation.com/e-dirty-birdy-twin where it appears along with links to the full build text and some great build pics. Also a free download of the plan in tiled format.

Quote: "The classic model reimagined as a twin. As seen in the February 2019 issue of Model Aviation.

If you had three glow-powered twin-engine models and lost two of them because an engine quit on landing approach, would you look for a more reliable power solution? The third model, a Goldberg Skylark Twin (oz1843), was retired before it met the same fate.

I’ve often thought of building an electric-powered twin. I lost my favorite airplane, a Dirty Birdy (oz4915) with 800 flights on it, and I wanted another.

Single-engine airplanes suffer to some degree from torque and yaw problems. It is easy to counter-rotate propellers with electric power, canceling some of those adverse effects. With this setup, could the Dirty Birdy be even better?

I’ve been flying an enlarged version of the e-Cobra (Model Aviation 2013) with an E-flite Power 25 1,250 Kv motor, an E-flite 60-amp ESC, and a four-cell 3,700 mAh LiPo battery. With a 10 x 6 or 10 x 5 propeller, its performance is outstanding, and I am getting 9- and 10-minute flights.

These 1,250 Kv motors, designed for Pylon Racing with an 8 x 8 propeller, are perfect for the Dirty Birdy project. They don’t need a large-diameter propeller, allowing the nacelles to be closer to the fuselage. The 10-inch propellers have not been a problem for these motors, and when I land the components, including the battery, are hardly warm.

Wing Construction: I ordered a short kit from Eureka models specifically for the wing ribs and canopy. I made many modifications to the wing, but only three are necessary: installing a wiring tunnel for the power and throttle wires to the nacelles, making an oversize aileron servo mounting area that opens to the belly for cooling and allows the wires to pass by the servo, and sheeting the top of the D-tube with stronger wood to carry the load of the nacelles and motors.

The wing root was extended 5/16 inch on each panel, and the wingtips were widened 1/8 inch at the edge near the last rib. This provides an additional 7/8 inch of wingspan to compensate for loss of area from the nacelles. The Eureka ribs are 1/16 inch short at the rear, so I used 5/16-inch material to maintain the wing chord.

Traditional aileron torque rods from my crashed Dirty Birdy were used. I felt that two aileron servos wouldn’t be beneficial. Dual servos are great for eliminating bellcranks; however, a single one weighs less. With torque rods, the wing remains clean, including no control horns.

The spars are 3/8 x 1/2 inch. Past the nacelles, you can trim the forward inside corner to a triangle shape between the ribs to save weight because this area is overbuilt. Traditional Dirty Birdy construction butts the wing halves together and joins them with fiberglass cloth..."

Note this is not a plan for a complete airframe.

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