Scottish Aviation Twin Pioneer
It is as you can see very different from my usual canvas and wire jobbies. Wing Span is 38" and will be powered by two small brushless motors, two 6amp ESC, 300 mah 2 cell lipo and controlled by a flight profiler that works in a similar manner to a receiver but the settings are preprogrammed before flight. I have a feeling that the black art of free flight scale trimming will be a challenge on this one. A target weight of 16ozs would be good but possibly optimistic.
I am going to use a lot of blue foam to create the stressed skin appearance. I have not used this material before so the learning curve is pronounced.
1/32nd core balsa with the foam on either side. The foam is shaped and then coated with micro balloons filler. Sanded again. Tissue covered. The corrugated rudder is made up of thin strips of photo graphic paper. Panel lines are scribed into the foam. Two coats of white matt enamel.
22 grms so far.
The tailplane and elevators are made in the traditional sandwich method. I got a bit frustrated with the blue foam as it proved difficult to produce the fine edges and shapes needed for a contest model. I have made them more substantial than the tail feathers I use for the WW1 types as it has to support the three fin/rudders and there is no external bracing. There is a strip of carbon in the leading edge and carbon rib capping top and bottom on the main spar. Weight is creeping up and now totals 36 grams. The tail moment is not too long so I am relying on the batteries being enough to counter the weight of the beefed up tail end.
The wings look simple enough now but have proved to be a real trial to get right. They taper in every view and have to be strong enough to support the U/C and both motors.The main spar is two lengths of spruce laminated with a thin strip of carbon in between. Rear spar is a balsa carbon lamination. Wings plug together with piano wire in ali tubes. Wing tips blue foam.The motor mounts are ply discs supported by carbon tubes.The whole unit is a little heavy but has to do all the work. The fuselage is only there to hold the wings and tail surfaces in the correct positions.
1mm ply formers were fitted to the centre line of the nacelles and the main U/C leg fitted to this and the main spar.Blue foam blocks were cut to shape an adhered to the ply former. Sanded to shape, or so I thought. studying the photos showed there was a discrepancy of shape to a fair degree from the 3 view I am using so I adjusted the shape using Model Llite filler.The blue foam was then painted with thinned down Model Lite to fill the minor holes and blemishes. Then sanded to a smooth finish. Light with a solid finish that will, hopfully, make a reasonable impression of a stressed ali skin.
Stub wing / U/C wires are taped and epoxied to fus'
1.5mm balsa sides, with windows, are fitted to the fuselage
Fuselage double curvatures are formed using blue foam in a similar manner to the engine nacelles.
BMFA rules dictate that that a free flight model of this size, in competition, must be fitted with a device to cut out and disarm the motors when the flight terminates.
The micro switch is fitted with a strong magnet (under the lead weight) so that the arm of the switch stays down when depressed by the weight on the arm.
When arming the motors to fly, the pivoted weighted arm is lifted and set against the bulkhead. Then with a length of wire with a small hook on the end, the micro switch lever is pulled upwards disengaging it from the magnet so switching on the arming process. The timer fitted will then go through the flight as programmed.
When the flight finishes, be it a premature crash or a greaser of a landing, the lead weight moves forward with the inertia and shuts down the motors. They cannot be restarted by accident.
This also has the advantage that one can abort a launch by simply tipping the model forward so the inertia switch is activated.
Wings are fitted and the fairings made of blue foam.Fitted U/C fairings, wing struts and stub wings. This area will potentialy receive a lot of hard knocks so are made from a laminate of Lime and balsa for strength, flexability and lightness.Windows are taped up to prevent me blobbing thumb prints of epoxy on them.The 18 gram brushless motors have been installed with the E.S.Cs and plumbing.Initial tests with 3 bladed 5x3 contra-rotating quadcopter props and 2 cell lipos give 10,000 rpm on a 80% power setting. Pulls nicely.One thing for sure, it does not sound like two slow reving radial engines. But both engines are within a 100 rpm so I should not get problems with assimetric thrust. You know the story well enough, ok on the bench but at the field the gremlins can turn up as well. We will see.
Flying surfaces are covered in jap tissue over mylar and the remainder of the model jap tissue only.
Coloured with Humbrol enamels except for the white which is acrylic.
Surface texture was worked somewhat with a 3B pencil to produce panel lines, just, and weathered appearance.
Whole model will be finished in a mixture of glosscote and satincote to achieve final finish. The top surfaces are a lot duller than the gloss undersides.
Scale detail, I thought, would be minimal but the more you look the more you see and there are lumps bumps and sticky out bits all over the model. At this point there is only the ariels to fit, 5 of them, and fuselage nav' lights.
Just need some calm dry weather to trim the model to a safe flying pattern.
Regrettably trimming this model proved to be a challenge I could not overcome and in my efforts it slowly destroyed itself over many attempts. This is, so far, the only scale model to beat me.
The conclusion for this was for a number of reasons.
Model was too heavy. Not a lot of wing to hold it up in the air. Brushless motors were not able to be synchronized close enough.
The consensus of opinions by my fellow competitors was that the three fin rudders were the main cause. These were far too powerful to set to give a consistent flight pattern. Model climbed and then when the rudders took hold it would dive in left or right. "oh gosh"
Learnt a lot from this model and put the failure down to experience.