Blunderbus (oz675)


Blunderbus (oz675) by S Cal Smith 1953 - plan thumbnail

About this Plan

Blunderbus. Old timer plan for FF IC. Plan shows a Cub 0.049 motor.

Quote: "Blunderbus, by S Calhoun Smith.

This little straight line special will never win any contests, but for Sunday afternoon fun it can't be beat. If ever a design has been flight proven, this one has. It is a veteran of nearly a year of weekend flying sessions. Damage to date: punctured wing covering from roosting in trees.

Blunderbus was inspired by a Doug Rolfe sketch of an ultra-light German ship built by Albatros in 1924. This 'Sportflugzeug' was a tiny single-placer powered with 30-40 hp Bristol Cherub engine, and from its appearance in the sketch was simplicity itself designwise.

Because no complete plans were available of the real aircraft, the model was designed with typical free-flight proportions merely to resemble the big ship. The comma-shaped rudder was added just for appearance.

Model specifications are: span 30-1/2 in, length 21 in, wing area 150 sq in, stab area is one-third of wing area; power with any .049 engine. Weight is 8 oz., pretty heavy for a small free flight, but since contest performance was not wanted, much durability of structure was gained by building fairly heavy.

Fuselage construction is simple box-type. Cut out the fuselage sides of 1/16 sheet. Mark former positions on inside surfaces. Since fuselage is rectangular no former drawings are given on the plan. Formers can be made by referring to the plan top and side views. Cement 1/16 x 1/8 longerons along the top and bottom edges of the fuselage sides. Notch former corners to fit over the longerons. The firewall is a piece of 1/8 plywood backed with pine strips opposite engine mounting toles. The pine furnishes some thickness for wood-screw mounting.

Cement 1/4 in sheet front, firewall and next two 1/8 sheet formers to one fuselage side. Add other side. Work upside-down over plan top view. Sides are parallel in this front section, so good alignment here is important. Add formers progressing toward the tail, checking against top view for straightness and squareness. A block of pine or plywood is next added across bottom for land-ing gear attachment. Block in nose and let whole fuse-lage structure dry thoroughly.

The scale-like wire wing mounting is the only fussy part of the model, and unless you are handy with pliers and a soldering iron, the alternate pylon mount should be built. The wire wing mount enhances the appearance of the ship and has proved rugged and practical for flying. The isometric sketches show the makeup of the wire pieces. Center pieces are bent to shape first; note difference in height in side view for wing incidence. The center pieces are joined at the top corners by binding with thin soft wire and soldering. The bottom ends pass through notches in formers at side former corners.

Next bend outer wire pieces and diagonal braces and join these to center pieces. This operation is best done upside-down over a board with center piece blocked up 3/8 in. And the work must be done with parts carefully aligned. The top pieces pass under the center piece top corners and touch the inside edge of outer piece corners. Bind all joints and solder well. This complete wire mount is then pushed into place at the front of the 1/8" formers. Double-cement where wire passes down corner.

If you prefer the pylon mount make the pylon of three layers of 1/16 sheet joined with criss-cross grain, like plywood. Cut to outline shown in phantom lines on plan side view. Notch formers to receive the pylon base. Pylon top is made of 1/8 sheet and can be rectangular or oval in outline. Run grain spanwise and gusset corners where top pieces join vertical member. With wing mount in place, cover top and bottom of fuselage with 1/32 sheet, grain running cross-ships. Add plywood strip for stab mounting and gusset bottom surface. Add tail hold-down wire hook and tail skid. Double-cement over the wire.

Landing gear is about as simple as you'll find, consisting of a straight length of wire soldered across a piece of tin can stock. Use four wood screws to fasten the tin plate to the fuselage bottom at the pin block..."

Update 15/02/2019: Added article, thanks to RFJ.

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Blunderbus (oz675) by S Cal Smith 1953 - model pic


Blunderbus (oz675) by S Cal Smith 1953 - pic 003.jpg
Blunderbus (oz675) by S Cal Smith 1953 - pic 004.jpg
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User comments

Blunderbus is a delightful freeflight and I have built 4..
2 per Cal's drawings for DC Dart diesel power and 1 scaled up with an OS FS .91 4 stroker and the 4th just completed per the Albatros plan variant with a Saito 60T twin. The 2 enlarged ones both use spoilerons and the last one, I tried an all flying rudder per the Albatros plans. All 4 are fun flyers and very relaxing to putter around with.
Al Robinson - 13/12/2020
the design of the small blunderbus is probably based on the german 1924 "Sportflugzeug" Albatros L66 !? See:
Kind Regards,
Helmut Koenig - 29/12/2022
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