Backyard Special (oz12295)
About this Plan
Backyard Special. Radio control sport model for PAW 80 Classic diesel engine. Wingspan 990 mm.
Quote: "Backyard Special. This model was designed and built at least 15 years ago before the current range of light electric powered slow flying models were available. For people who are not attracted to electrics and/or happen to like oily diesel power, this is an alternative. In contrast to electric power, overall weight is probably less as only a very light battery is required (150 mah, 4.8V). To some extent the saving in weight allows for a stronger, more durable airframe.
I called this design the 'Backyard Special' because it is a model that can be flown in many small areas with little risk of damaging either the model or anything it may make contact with. I was asked by a school to demonstrate R/C aircraft and the only space to fly in was a quadrangle, but this proved to be no problem. The thick, high drag airfoil, low wing loading & large prop turning at low rpm result in low airspeed & inertia. While having 'hands off' stability, the controls are sensitive & the model is very manoeuvrable. All this adds up to an aeroplane that can be flown close in with very little noise, which means it can be flown in many local parks, sports fields etc (subject of course to local authorities approval) quite safely. I am sure it could also be flown indoors in a reasonable size hall. A small diesel running at a fast idle makes so little noise that the inevitable nuisance spectators are not attracted by the sound as they are by the usual sort of R/C model. I have flown it around cows and horses in a field and it did not seem to bother them in the least - the only problem was avoiding the cow pats when it landed.
The ideal engine for this model is the PAW 80 Classic diesel which has enough torque to turn an 8 x 4 prop quite comfortably, and the power out put can be varied over a wide range for the style of flying you wish to adopt. A throttle is not necessary & needs an extra servo which just adds weight. Keep it simple! A Mills .75 with the long range tank is also good and I am sure there are many other small diesels that would be quite suitable. Any radio system weighing under 50g or so is suitable, the lighter it is the slower you can fly.
CONSTRUCTION: The construction is quite simple but attention should be paid to keep it light- the design is quite strong enough and should not be beefed up. Light weight radio with 250 mah nicad and micro servos are essential. Finished weight should be about 11-12 oz without using extra light balsa. Solarfilm or a similar light covering material should be used. I made engine mounts from a piece of aluminium tee section and soldered up a small fuel tank from brass shim, but commercial units are ok if available. Dacron thread was used for the rudder control cables - light and positive.
Starting with the fuselage, build the sides on the plan, one on top of the other. Let the glue set then remove them and add the .4 mm ply to the outsides. Pin down one side and glue in position F2 and F3 making sure they are vertical, then glue the other side on top checking with a square at the tail end that the sides are aligned one over the other. When set remove and mark the centre lines on F2 and F3. Working from the front, glue in the tail end cross pieces, top and bottom, and the ends of sides together with the tail joiner pieces in place top and bottom ( note there is a gap at the tail for the elevator pushrod) making sure that the join lines up with the centre marks on the formers.
Make up the box nose section, sand the rear end flat and glue it to the fuselage at F2. Add .4 ply to the underside of the forward fuselage. Install 3mm diameter wing dowels, ply tail skid and front skid made from scrap circuit board with a piece of rubber hose as a spring, after covering the fuselage. A servo tray can be made from 1.5mm ply to suit your equipment. Leave the positioning of the servos etc until the model is finished so that they can be used to adjust the C of G position.
Build the tail components over the plan using medium straight grained 3mm balsa sheet. The diagonal bracing method is light and quite warp resistant. After covering the rudder and elevator are hinged with thread sewn hinges as the frame is too thin for conventional hinges, which aren’t as free anyway.
When ready to build the wing first collect the components. Use very hard straight grained balsa for the main spar, top spars and leading edge. The ribs, braces and trailing edge can be medium balsa. Pin the LE main spar and pre slotted TE in position then add ribs 1 to 8, using the wing joining angle template to set the angle of rib 1. When the glue has set add the top spars and the braces between ribs 3 and 8. Complete both wing panels to this stage then remove one panel from the plan and put aside. Trim the wing panel joining ends to the correct angle with a sanding block. Cut the spar dihedral braces from ply, put the wing panels butted together at the centre with the raised wing propped 130mm above the board at the tip while glueing the panels together at the same time adding the ply dihedral braces. After the glue is set install the top sheeting on the wing centre ribs, the diagonal bracing between ribs 2 and 3, the ply TE stiffener strips and the corner gussets. The wing is now ready for covering.
Cover the wing, tail parts and fuselage then glue the tail to the fuselage. Install the engine and tank, front skid, battery and Rx in the nose then position the servo tray to give the correct C of G location.
Connect the elevator and servo with a light pushrod then install the rudder control thread links. Set the control throws - about 10mm up and down on the elevator, 15-20 mm each side on the rudder.
FLYING: Wait for a calm day to fly as this is not a model that is at its best in strong winds. With the engine tuned to give a bit more than the minimum thrust required to maintain flying speed the model flies at about 10 - 15 mph and you don’t have to worry about hitting the ground - I have had great fun flying it around below knee level with the prop just clipping the grass. Turn up the wick a little, gain altitude and perform tight turns, loops, stall turns and any thing else you can do with rudder/ elevator control.
The Back Yard Special will virtually fly out of your hand with the easiest of launches then enjoy flying it around using very little space and fuel while creating almost no noise or risk of damage.
This minimalist micro monoplane is truly environmentally friendly!"
Direct submission to Outerzone.
Update 29/06/2020: Re-scaled this planfile now to correct fullsize at 990mm wingspan.
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