BP-100

Model Airplane News, May 2001 by Daniels, Ron

MODEL AIRPLANE NEWS CONSTRUCTION

A giant-scale sport plane with a bush-plane flavor by Ron Danniels

In spite of its large size and appetite for aerobatics, the BP 100 looks every bit a 1960 to 1970s-era high-wing tail-dragger. "BP" means "bush plane," and the model is based on what I think the ultimate modern bush plane would be like. In other words. it is a make-believe, sport-scale high-wing monoplane with an all-aluminum stressed-skin fuselage, fabric-covered tube-frame tail and aluminum D-tube fabric-covered wings that never existed. I hope you agree that it sure does look as if it could be scale.

With its trainer wing airfoil, the model is very gentle at the stall, and when powered with a large engine, it will lift an immense payload. It can easily be flown by anyone who can safely fly a trainer. Even on a paved runway, it does not have the strong torque reaction that's usual with a powerful tail-dragger.

The BP 100 can be assembled easily at the field, and with the spinner removed, it will fit into a short-bed mini pickup with the tailgate closed. The wide cowl can easily swallow a large array of big engines without having a cylinder sticking out of its side. To allow easy access to the powerplant for adjustment and maintenance, I hinged the top half of the cowl. Basically, the model is a simple 4-channel job, and because of its low airspeed, only the elevator requires a giant-scale servo; standard servos can be used on all the other control surfaces.

CONSTRUCTION Detailed, step-by-step instructions are included with the plan, so I will only go over the highlights here.

* Wing. Construction starts here. The prototype's wing is fully sheeted, but the plan now shows an open-bay wing with D-tube sheeting front and back. The panels plug into the fuselage and have a wooden-box web built into them; this accepts two wing-joiner blades. The panels have two I-beam spars with spruce upper and lower spars and vertical plywood webs. The box web is built into the panel before you add the wing sheeting, and for additional strength, I wrapped the box with carbon-fiber tow.

The plan shows all the details for the ailerons, the web box and blades, the strut attachment points and all the rib patterns. It also includes a complete bill of materials for all the wooden parts you have to make and all the hardware you'll need. The tail surfaces are made using lh-inchsquare and 1/2x1/4-inch balsa stick stock. The horizontal stab is reinforced with a 1/2x1/4-inch plywood brace at its TE. For strength, the rib strips in the vertical fin and in the stab that support the functional tail-support wires are made of A-inchsquare spruce.

* Fuselage. The four main fuselage longerons are 1/4-inch-square spruce, and all of the vertical supports and diagonals are made of 1/4-inch-square balsa. The main fuselage structure (truss box) is sheeted on the sides with %-inch balsa. Once the two sides have been assembled, you start to join them at the tail and work forward, adding the crosspieces as you go. Do this with the sides placed upside-down over the top view on the plan.

After installing the horizontal diagonal braces, add the upper fuselage formers and the plywood cabin side frames. The bottom of the fuselage is sheeted with 3A2inch balsa, and the forward portion around the main landing gear is extended with formers. The plan also shows a secondary landing-gear support well aft of the main gear support. If you would like to attach floats to your model (and which bush pilot wouldn't), just bolt the float struts into place. If you don't want floats, simply sheet over that part of the fuselage bottom.

In the bottom of the fuselage, I made a small access panel that allows me to get to all the servos and the radio gear. The wing blades are glued permanently into place. They make great tie-down points for transportation, and-most important-- they set the wing incidence every time you bolt the wing into place. During construction, set the wing to the correct incidence while it's bolted into place but before you've epoxied the wing blades into place. When everything is right, glue the wing blades in permanently. It is very important that they be sanded and then sealed with resin so that they will not swell or bow with changes in humidity or moisture or be saturated with oil. The lift struts are made of 5A-inch-wide, airfoil-- shaped aluminum tubes available from Commander RC Models.

FINAL ASSEMBLY

The rest of the construction is pretty straightforward. I attached the tail surfaces to the fuselage using 15-minute epoxy. I decided to use Aeroglass composite main landing gear, but if you prefer, you can use some other commercially available gear or form your own using 1/4inch-thick 6061 T-6 aluminum. The wheel pants and engine are also from Aeroglass.

Except for the elevator, which uses one giant-scale servo, I used standard servos (one for each control surface). Each aileron also has its own servo. The throttle servo is installed in the front corner of the cabin area just aft of the 32-ounce fuel tank.