Sierra Sportster (oz15381)

About this Plan

Sierra Sportster. Radio copntrol sport biplane model. Wingspan 36 in, for .20 to .25 engines and 4 functions.

Quote: "This month's full-size plan offering is in response to the survey carried out in RM some time ago. The survey highlighted that readers wanted larger or more complex designs as full-size plans, even where it meant spreading over a couple of issues.

The drawings have been prepared in such a way as to enable readers to commence building the fuselage virtually to completion this month, and to follow with the wings from next month's magazine. The model was inspired by a desire to build a small, attractive aerobatic biplane along the lines typified by such types as the Pitts Special, Mong Sport, Little Toot, etc. It takes the character of such aircraft rather than copying specific features, to enable the lines to be visually harmonized.

The author hates to see such matters as square wing tips with rounded tailplane or fin. A lot of commercial kits, monoplanes included, seem to suffer from this aspect - it's as if one person designed the wings and another the tail without consultation. The end effect detracts from the design.

The power requirements are basically any good .25-.30 two-stroke. Built lightly, a .20 two or four-stroke would fly it but it might lack power in the vertical plane. The type needs power for aerobatics, though of course for general flying around, the engine should be throttled down to cruising power.

It's a model to fly with much use of throttle, and not to blast around at full power all through the flight. This adds another fun dimension to piloting. Anything larger than a .30 would really be too much power and could cause take-off problems. Some experience with a tail-wheel aircraft is desirable as the model is relatively high-powered for its size, and care is required to keep tracking in a straight line at take off.

The engine has been mounted in the old fashioned way using a 'breaking plate'. This enables a quick and easy change of engine type if desired, but also allows easy access to the fuel tank through the firewall without the need for a separate hatch or removing the lower wing and R/C gear. Choosing this method enabled the centre-section struts to be simplified to a single central pylon mount cut from plywood. This eliminates nasty wire-bending and jig-setting the upper wing. In fact, the wire-bending has been reduced to that for the tail wheel only if a dural u/c is used.

There are examples of this central pylon arrangement amongst the small pylon racing biplanes in the U.S.A. such as the `Mongster', so it's well within character. The upper wing is held level by the strength of the pylon and by carefully trimming the interplane struts to equalise the gap on each side. As the model is small, permanently fixing the upper wing in place adds strength, saves bother and weight, but shouldn't cause any real storage or transport problems.

The lower wing is only really made removeable to provide access to the R/C gear. If you wish you could lower the fuselage sides to cover the root ribs position and butt join the lower wings to the sides. ,A bottom hatch and a means of locating the aileron servo would be necessary but not difficult to arrange. This would make the model a one-piece affair enabling the interplance struts to be glued into their slots, and making the wings into a very strong box form.

Fuselage construction: The fuselage is built around the usual sides-box format. Formers Fl, F2 and F3 are of constant width to simplify the initial assembly. Each of these should be glued to a side panel at exactly 900. Join the second side and invert over the building board to prevent any twist occuring in the side view. Note F1 and F2 will need to overhang the board. The top, cranked, part of F3 can be added later. Pull in the tail and join with a length of 1/8in sq. Do make sure the fuselage is straight; it is fairly critical on these small models, as any inaccuracy tends to be magnified. Add in formers F4-F7.

Epoxy in the engine bearers and 1/8in sheet infill between. The dural u/c should be temporarily fitted to the u/c plate and checked for true tracking. Remove the u/c after the plate is epoxied in position. Spot glue the 1/tin sheet lower nose section and sand to shape. Now cut away, re-fit the u/c and permanently glue the pre-carved nose sheet in position. Add the tailwheel wire, lower rear sheet, and wing bolt retaining plates. Now glue the completed tailplane in position and fit the tubes for elevator and rudder controls. The top decking can be added to complete the basic structure..."

Sierra Sportster from Radio Modeller, May 1988.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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