Cracker (oz14456)

About this Plan

Cracker. Radio control slope soarer model. Wingspan 57 in, wing area 460 sq in. For 3 or 4 servos.

Quote: "Try this smart, quick-build project from our pull-out plan. Cracker, by Mike Briggs & Steve Griffiths.

Cracker came about in an attempt to develop a windy-weather sport/aerobatic slope soarer, to complement a lightweight (27 oz) model produced a couple of years ago. The spur which eventually caused us to set pencil to paper was the increasingly tatty state of a 10-year-old Phase 6, which had fulfilled its role admirably but was getting a bit past it, like its owner.

The basic design criteria did not include light weight; if anything, the opposite. So, with complete disregard for weight-saving, we drew up and built the prototype, fitted it out with four servos, Rx and battery and carefully put it on the kitchen scales. It weighed - wait for the scales to settle - 30 oz? A whole three ounces heavier than its lightweight companion, with more wing area too. Must find a supplier of thin sheet lead. Never mind, we thought, it looks OK, so we might as well carry on.

Off to the slope, in a moderate blow, and the first flight was uneventful. Slight adjustments were made, including moving the centre of gravity back a little, before it was re-launched and what a little mover it turned out to be. Fast, agile (square corners really are square), possibly the most manoeuvrable model either of us have flown. It's even better in good lift, when it really shifts and has coped quite happily with winds in excess of 25mph. We always have trouble thinking up names for our designs, but when this one flew the comment - that's a right little cracker - clinched it.

Don't be misled, though - this model is more sport than competition aerobatic. It possibly doesn't have the inertia to carry it through uninterrupted sequences of vertical manoeuvres and perhaps has too little side-area for prolonged knife-edge passes. It is, however, great fun, and offers plenty of challenge for us average fliers.

For such a light model, we have been pleasantly surprised. Its speed we put down to it having low drag; there's certainly very little discernible noise from it, so it must be a fairly clean beastie. The agility is probably due to the fact that there is no heavy weight far from the CG. In fact, we had some difficulty in getting the CG back to where it is, and the RX and battery pack are right back against the bulkhead at the wing LE, with nothing in front but foam packing. As a result, we suggest that fairly dense wood is used at the tail, to give more scope for radio gear positioning.

We've also had the (inevitable) opportunity to discover that it's quite a sturdy model, providing you fit all the braces and fillets shown on the plan, and make sure all your joints are good (the model's joints, anyway). All that aside, it's an easy model to build and fly.

It will take three or four servos, according to whether you fit one or two in the wing (with two, you have the options both of using flapperons, and ailerons as brakes, if you have a suitable mixer in your Tx). The elevator and rudder servos in the fuselage are Hitec HS101 or others of similar size but standard servos could be fitted by deepening the fuselage slightly. For neatness, the rudder is driven by closed-loop but a snake exiting over the tailplane would be just as effective, although more obtrusive, if you are put off by that system.

The building notes which follow aren't of the detailed 'stick A to B' type (firstly, because there are no parts labelled 'A' and 'B', and secondly because even if there were, we'd make sure they wouldn't fit together, just for spite). Most of the construction follows fairly standard practices, so with even a little experience you should have no problems putting it together. However, they cover only the four-servo version, so if you want to go with a single aileron servo the mods are up to you.

Building the Wing: Cut out and line the servo boxes in the bottoms of the wings and fit the hardwood blocks to position the servos and for the screws for the ply lids. Bore a hole from the wing root to a suitable point in the servo box for the servo leads, and cut away the bottom skin at the wing root to bring the leads out Attach and shape the leading edge, then make the tip blocks, and glue them on so that the ply centre is exactly on the chord line.

Join the wing halves with their top surface flat on the building board. Fit the 1/16 ply braces across the centre joint from underneath, with the balsa block between two of them to take the wing bolt; bandage the joint as you prefer. Do not drill the hole for the wing dowel at this stage and don't build the wing-root fairing. Note that there is no spar in the wing..."

Cracker from Silent Flight, October/November 1975.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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