Fact Sheet No. 7. Installing Mechanical Retracts

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1 Fact Sheet No. 7 This Fact Sheet is primarily aimed at the newcomer to Building R/C Model Aircraft Installing Mechanical Retracts There are a number of ways to attach the units in the wing, all of which, are very much depend on what type of unit is to be used. Air versus mechanical or whether you want to make your own, (See Sketch 1) or purchase readymade mounting boxes (see photo 1). The latter will save you quite a bit of building time for very little cost. Wings manufactured at Probuild can have reinforcing ribs for the retract mounting blocks built in the wing structure before the wing is skinned, plug-in wings have the ribs ready built in, which are an extension of the ribs supporting the spar system. For this example we will be using the readymade mounting boxes that suit the Supra DX60 mechanical retracts, which result in a very light but strong installation. Sketch 1. The ply box is interlocking and is made from 3mm lite-ply with 6mmx12mm hardwood mounting rails Installing the Retract units in a plug-in wing 1. The holes for the retract mounting blocks and wheel wells are marked out on the wing panels to the dimensions given in the kit you are building. The balsa veneer and foam core is cut away to allow the insertion of the mounting box, the wheel wells are also cut out at the same time. (At Probuild ply ribs are built in to the foam cores prior to the wing being skinned to suit the type of mount you propose to use.) See Photo.

2 2. The boxes simply fit neatly into this void and between the two wing ribs that were built into the foam core prior to balsa skinning. They are then glued in position by using 101-epoxy mix with some micro fibres. Picture 1 The readymade retract mounting box at 18 grams each they are extremely light. 3. The control push rod is run in a straight line from the retract operating plunger out to and through the wing root. This is done by cutting the foam away using a sharpened end of 3/8 brass tube or similar, angling up slightly to avoid fouling the wheel well.

3 4. The wheel wells should be lined with either 1/6 balsa or if you prefer lined using a pre-formed carbon fibre/nomex well, which is extremely lightweight. (These can be purchased separately from Probuild.) Once you are happy with the fit, the linings and wells can be glued in position. The cutouts for the legs can be lined in a similar way to the wells. 5. When the retracts have been fitted the surface of the wing around the retracts back to root should be reinforced using 100g glass Cloth. Retract servo for a plug-in wing 1. The position of the retract servo in the fuz is determined by marking off the centreline on the side of the fuz where the retract push rods will protrude through from the wing. 2. A slot is made on each side of this centre-line, 11mm back on one side and 11mm forward on the other. (This is to allow for the throw of the servo as it is actuated.) 3. At this point the exit holes for the push rods in the wing roots can be elongated to match the holes in the fuz. 4. The retract servo is mounted on a former across the fuz, inline with the centre-line of the retract push rods.

4 Probuild Tip The ends of the push rods are terminated on to Sullivan spring loaded ball link, with the rod ends cranked to avoid fouling on each other. The pushrods are made from Tetra Rod Adjusters part No A blank servo disc is drilled out to suit these ball links at 11mm from the centre 22mm apart. 2. The pushrods are connected to the retract units, then cut to length and finally soldered to the Sullivan spring loaded ball links. 3. The servo mounting former is lined up with the push rods making sure all works without fouling the fuz or binding, it is then glued in position using 101 epoxy mix

5 with some micro fibres. A balsa fillet can be glued on each side of the former to give extra reinforcing. Installing the Retract units in a one piece wing Before joining the wings together the retract units are installed in the same way as detailed for plug-in wings. Where the two differ is how the retract servo is installed in the wing. Installing the Retract servo 1. Before the wing is joined, the servo position is determined by the push rod exits in the wing roots. 2. Provision for the servo is made by cutting a slot in the root of the upper surface of both wings of 15mmx65mm to a depth of just above the bottom of the root. Which, when the wings are joined will give you a width of 30mm. 3. The wing is then joined as detailed in fact sheet 6, but not glass reinforced at this stage. 4. The slot can now be lined with 3mm balsa at each end. 5. Two 6mmx6mm hardwood or ply blocks are glued in at both ends of the slot for the servo mounting. 6. At this stage provision must be made for the servo wire, a hole is bored out from the back of the retract servo to the outlet for the aileron wires, or if this is not convenient a separate hole next to the retract servo well. The position very much depends on the size and set-up of the wing you are building.

6 7. After you have mounted the servo, a 45mm dia circle is drawn from the centre of servo spindle over the surface of the wing. This cut away down to the level of the top of the servo to allow for the retract actuating arm and pushrods to rotate. (See Photo) Probuild Tip To attach the retract pushrods to the servo arm, MK make a very neat set of cranked arms to avoid the pushrods fouling the servo spindle. (See Photo) 8. The wing root joint can now be Glass reinforced as detailed in fact sheet A blank servo disc is drilled out and shaped to suit the MK cranked arms at 11mm from the centre 22mm apart or use a 4 point servo arm with two legs cut off.

7 10. All that is left to do is measure and cut the retract pushrods to length and solder them on to the MK Fittings. 11. The servo, pushrods, cranked arms and retracts are tested making sure all works without fouling the wing, binding in the retract wells or stalling the servo. Operating the Retract servo on a separate battery. To give yourself added security, mechanical retracts can be operated on a separate battery to prevent the retracts stalling servo and draining the receiver battery. Here is a simple wiring diagram which enables mechanical retracts to operate on a separate battery. The live wire form the RX is removed from the plug and insulated. Power to the retract servo is then drawn from the additional battery, with the RX battery unaffected by any excess current loads from a possible stalled servo.

8 Retract Servo Y Lead Additional Switch Additional Battery 250/300 mah On the plug going into the RX remove the RED wire and insulate. Do not replace the RED wire. RX