Building the Balsa USA Stearman PT-17

By:  Bob Wideman 

Back when I lived in a much colder climate I became accustomed to building RC aircraft during the long winter months. Established habits are hard to break, so whenever the days begin to grow shorter, I start looking for a kit to build ----- preferably something that will occupy several months of on-and-off again puttering around. Having already built, flown, and absolutely enjoyed Balsa USA’s 1/3 scale Fokker DVII and 1/3 scale Nieuport 28c-1, I decided to try their somewhat more complicated 1/3 scale Stearman PT-17. This is a typical Balsa USA “builder’s” kit – the UPS guy delivers a lumberyard in a box. 

On the BUSA web page (http://www.balsausa.com/) prospective buyers are correctly informed that “this kit is for experienced builders”. If you take your time and patiently follow the well illustrated instruction manual while paying careful attention to the plans, everything goes together nicely. For example, you start the fuselage by framing up a rectangular box as the inner structure. This can be built straight and true using a flat building board, then the fuselage is “rounded out” by adding half-moon formers. Stringers then are attached lengthwise. This time I used small dabs of Gorilla glue to attach the stringers – it expands slightly to fill the joints completely and makes a very strong bond. The wings frame up quickly, but you’ll need a razor plane to help shape the leading edge. This kit features typical BUSA construction, and the results are outstanding!

Assuming you can interpret and follow the plans there is no need for a step-by-step repetition of the kit’s instructions. Instead, I’ll mention several key points during my construction.

1. I decided to give this plane plenty of “reserve” power (remember, the throttle stick goes both ways) by installing a Desert Aircraft 170 motor. I followed the kit’s “Large Engine Update Addendum Sheets” to strengthen the airframe, including: (a) reinforcing the center hard points for the forward cabane strut rigging wires; (b) using lite ply to reinforce all airframe attachment points for struts and rigging wires; (c) reinforcing the main landing gear support structures with 1/8” plywood and adding the recommended metal struts to the main gear; and, (d) reinforcing the tail wheel attachment hard point.  

2. The rear fuselage attachment surfaces for the horizontal stabilizer were strengthened by adding lite ply horizontally, flush between the upper rear fuselage sides. This surface was drilled for ¼” dowels that also will pass through the sheeted middle section of the horizontal stabilizer.

3. The very nice Ohio Superstar Giant Tailwheel (provided in the kit) required that the rear of the fuselage and the ABS tail cone be modified for an appropriate fit. 

4. For ease of access and to simplify the otherwise complicated control linkage for the elevators, I built two servo wells on the bottom of the horizontal stabilizer. The same was done for the lower wings. Not very scale in appearance, but I can see each servo and linkage absolutely clearly before and after each flying session.

5. I cut the ABS boot cowl in half and only glued the bottom to the front of the fuselage and firewall. I made the top half of the boot cowl removable to permit easy access to the fuel tank, smoke tank and throttle servo (located just behind the firewall). The batteries and ignition box also are located under the top half of the cowl, immediately behind the firewall (always good to move weight forward!). 

6. Initially I followed the kit instructions for installing the lower wing tube. However the bolts through the wing tube became difficult to remove and put back in (necessary for transport) after the rudder servo and rails for other electronics had been installed beneath the rear cockpit. I remedied the situation by gluing two hardwood blocks flush with the under-side of the fuselage, just beneath the spots where the wing tube exits the lower fuselage sides. These HW blocks were drilled and tapped for 8-32 bolts that act as set screws to positively retain the wing tube in place.

7. Hardwood blocks drilled and tapped for set screws also were installed in the upper wing center section and in the lower wings to make a positive attachment between the wings and wing tubes (copying the wing attachments for the BUSA Fokker DVIII – which does not use flying or landing wires). 

8. The frames for the windshields were strengthened by gluing carbon fiber strips (1.4mm x 4.5mm) to the inside of the rear frames. Be sure to cut the side pieces short to make room for flush attachment of the frame to the fuselage.  

9. A truly nice aluminum cowl is provided with the kit. I wanted to use this cowl to cover the DA-170 and give the model the look of the 450 HP “Super Stearman” upgrade (the original trainer came with an exposed 220 HP engine). The aluminum cowl was deepened to fully contain the DA-170 by adding a skirt of 1/64” plywood reinforced inside and outside with fiberglass cloth. Hardwood blocks were attached with dowels to the firewall, and these were drilled and tapped to attach the lengthened cowl. 

10. Fiberglass cloth and epoxy finishing resin (thinned with 1/3 by volume denatured 100% ethanol) were used to reinforce the D-tube leading edges of all wings, the upper center section, and the firewall-engine box area. The firewall and engine box also had been reinforced with 1/8” plywood extending at least 8” into the fuselage frame, with 1/8” dowels used to strengthen the attachments to the airframe and the engine box. I also did this with the Fokker DVIII; it has well over 200 flights and still is rock-solid throughout! 

11. The airframe was primed with white primer and finish-sanded prior to the enclosed photos. Balsarite will be used to insure excellent adhesion of the Solartex covering to the airframe. 

SPECIFICATIONS

Engine = DA-170 (initial break-in with 2 gallons on the test stand; 3W 32x10 prop; reliable idle at 1,800 RPM)

Wingspan = 116” (top), 111” (bottom)

Weight in the bones (with engine) 39.5 lb; Target weight = 48 Lbs.

Covering will be BUSA Solartex per US Army yellow & blue scheme (top-coated with gloss polyurethane after pinking tape and insignia are added)

U.S. Army stencils and lettering are from “Get Stencils.com”; Pinking tape from “Pink-It”

Pilot = Generic 1/3 scale Aviator from Aces of Iron

Stainless steel flying wires from Vogelsang-Aeroscale.com

Batteries = Fromeco Relion Li-Ion 5200 mAH (ignition & smoke) 2 each 2600 mAH (radio electronics).

Smart-Fly super switches HD, Power Expander pro, and Turbo Regulator.

Hitec 7955TG servos throughout (except throttle)

Futaba FASST radio system.