It's funny how ideas for new designs come about. Whilst using a can of Humbrol spray enamel I noticed that the cap was just the right shape to make a cowl for a small-ish FW-190, although closer investigation revealed that for true scale the model would actually be tiny, at only 15" long... scratch that idea, then. Mind you, it did set me thinking more about finding gainful employment for a HET Typhoon Micro 6/20 brushless motor and some small servos that were going spare.
I then moved on to consider the Frog Interceptor, a small, rubber-powered toy aircraft modelled on a 1930's monoplane fighter that was produced in its millions from 1931 until the 50's, indeed every rich boy had one. Thinking that I might scale that up, I discovered that it had already been done, so I had to start from scratch. The result of my drawing board doodlings is presented here as Funterceptor, which owes a bit to both types mentioned but with a vision of producing a cartoon period-type model.
CAP IN HAND
Funterceptors design originated around that Humbrol cap, which is also the starting point for the build (an alternative here is a streamlined balsa block nose, which can be added later.
First, take the cap in hand and remove the centre of the closed end to reveal the moulded tube inside (simple drilling and filing sees to this).
Next, cut out the formers F1 and F2 from 1/8 birch ply and the others from 1/8 liteply. The Humbrol cap cowl then needs a 1/2 thick liteply disc spacer mounted onto F1, to sit the reversed Micro 6 motor on. The disc fits the moulded tube section inside the cap, and the motor has running clearance inside it. To make the spacer I stuck a motor mount template onto two thicknesses of 1/4 liteply glued together, and drew a ring around it the same diameter as the cap tube. Subsequent fret-sawing and sanding then creates the correct fit within the cap tube. The disc glues onto F1 on the centreline and can then be drilled for the motor mounting and ventilation holes (if using a balsa block nose, the motor can be mounted straight onto F1 without the spacer).
With this you can cut out the two fuselage sides from 1/8 balsa sheet. Remove the curved wing positions carefully, as the off-cut pieces can be used to form the wing mount doublers. The sides also need cut-outs for the tailplane and the elevator servo (in my case a Hitec HS-55). Mark the positions of the formers on the inner face of each fuselage side, then add the curved wing mount doublers, triangular reinforcements at the F2 and F4 positions, and the liteply mount for the elevator servo. Lay one of the sides on the building board and glue F1, F3 and F4 in place on their marked positions. Once set, the other fuselage side can be added. Whilst the glues drying on these items you can bend the undercarriage wire then sew and epoxy it to F2. This former will later slot into the gap between the wing mount doubler and triangular reinforcement pieces - but not just yet!
Returning to the fuselage, pinch the sides together onto a small piece of 1/4 balsa sheet to form the tapered rear end, adding F5 forward of the fin. Whilst the fuselage top and bottom are formed from soft balsa sheet, note that the 3/8 front sections have a 1/8 rebate at the glued joints to allow a greater curvature when shaping the nose. You'll also need to form a cut-out for the cockpit in the 1/4 top deck. Similarly, the 1/4 rear sections need cut-outs for the fin, skeg and rudder servo. Before being glued in place, the upper sections must be prepared for the battery and receiver hatches. In this respect, then, cut the hatch positions only three quarters through the wood depth, being careful not to use glue in the hatch areas (the hatches will be completely cut through after shaping). Once glued in place, sand the sections to a smooth profile to give the fuselage some curves.
Back at the nose, work carefully to create a circular profile so that the plastic cap will just slide into position. If you aren't using a Humbrol cap, make a nose block from soft balsa that has running clearance around the motor and glue it to F1.
With the fuselage now sanded to shape, the hatches can be cut all the way through and removed. The front hatch will need some 1/8 square pieces to sit on, whilst the rear hatch has 1/8 square balsa strips glued to its underside to locate it between the fuselage sides. To hold the hatches in place I used 1/32 ply tongues at the front and micro magnets / tinplate strips at the rear.
The motor can now be installed, which is a bit fiddly as the speed controller wires have to be threaded through the motor mount before being attached to the motor wires. Once joined correctly, thread the wires back through the mount and fix the motor to F1. The undercarriage / F2 assembly (made earlier) can now be glued into its slot. To finish the fuselage, either drill the holes for the wing band dowels or fabricate the captive nut plate, according to your chosen method of wing retention.
With the fuselage done and dusted we can move on to the fin and tailplane, which are very simple to build using mainly 3/16 square balsa and sheet components. When set, sand to the rounded profiles shown on the plan and slot them for the hinges.
The wing is pretty simple, too, built in one piece and later divided to form the dihedral breaks. Set out the 1/4 square and 1/8 x 3/16 lower spars on the plan, packing them up with 1/32 scrap. The 1/4 x 3/16 t.e. / rear spar needs setting up 3/8 above the plan with more packing. Note that the two 1/8 centre ribs must be reduced by 1/16 for the centre-section sheeting that will be applied later. Glue each rib to the t.e. and onto the lower spars (cyano makes for a quick fix here, although I usually use white glue). Once set, the upper spars can then be glued into position. When all is firm, offer the 3/16 sq. leading edge to the front end of the ribs, and glue in place (again, cyano works well here). Finish the basic build by adding the 1/4 square pieces that will later take the aileron hinges.
Okay, remove the wing from the plan, cut the t.e. and spars flush to the outer ribs and add the 3/16 balsa sheet tips. Note that these butt up to the front of the l.e. but extend beyond the t.e., to match the ailerons. Add the 1/8 sheet gussets top and bottom at the spar positions, as per the plan. The wing can then be given a preliminary sanding to shape.
Cut out the 1/16 ply dihedral braces and then pin the wing back onto the plan, but only at the centre section. Cut through all the structure outboard of the centre ribs to separate the wing halves, then make props to go under the spars to support the tip ribs 1 above the building board. Trim the mating surfaces of l.e., t.e. and spars so that each wing fits snugly up to the centre-section, and glue them in place checking that the whole assembly forms a straight line viewed along the l.e. Once set, cut through the ribs at the l.e. and t.e. (a standard hacksaw blade works well) so that the ply braces can be slid into position and glued. Next, web the main spars with 1/8 balsa sheet (vertical grain) that's flush with the front of the spars, then repeat the ply brace insertion procedure.
Your wing will now be strong enough for handling and can be sanded as required in preparation for sheeting. Clad the lower centre-section with 1/16 balsa, followed by the upper section, but leaving the area between the spars open. Now add the servo box by putting in two part rib sections at an appropriate spacing for your servo (in my case a Ripmax SD200BB) and some suitable servo bearers. Finally, add the remainder of the centre-section sheeting.
Lengths of 2mm pushrod wire and mini-snake outers form the aileron torque rods, plus some commercial bits to connect the rods to the servo link clevises. The snake outers form the bearings for the torque rods, offered up to the t.e. and tacked in place with cyano. Note that the centre-section of the rear spar will need notching to allow the torque rod arms to move forward when operated. To complete the task, cut some 1 x 1/4 t.e. stock to length, groove as required to cover the bearing sections, then glue in place remembering that the t.e. centre-sections will also need notching to give operational clearance for the torque rods. The same t.e. stock is used for the ailerons, which can now be cut to size and slotted for their hinges (as for the tailplane and fin), and drilled to accept the aileron toque rods. Finish the wing with some soft balsa block pieces at the front and rear of the centre section to make the wing underside blend in with the fuselage. If you're retaining the wing via screw mounting, add a 1/16 ply reinforcing piece for the screws at the rear underside, and a peg at the front to engage with the hole in F2.
The component parts of the prototype were covered in Solarfilm, with Solartrim for the roundels and fin flash. The cowl and other openings for the servos etc. were painted using silver Solarlac. I decked the cockpit area with a piece of black Depron from a food tray, perforated with a couple of ventilation holes. Meanwhile, the pilot was whittled from a piece of pink foam, sealed with white glue and painted with acrylic.
With all the components covered, remove the film from the respective gluing contact surfaces of tailplane and fin and glue them into their slots, making sure that everything's square. Insert the 1/16 ply control horns onto rudder and elevator, hinge all the control surfaces and connect the servos up. I used plastic strip cyano hinges, which work fine but feel a bit stiff, so you might prefer to use a pinned hinge type. The aileron servo links are commercial threaded rod items with a Z bend at the servo end and a clevis at the other. Meanwhile, the tail surfaces use my favourite closed loop links, made from 20 lb fishing trace wire and ferrules. In terms of control surface movement, the Hitec HS 55 servo arm / horn combination I used gave me 3/4 left / right rudder and the same for up / down elevator, this reduced to 1/2 with the rates in. You'll also need to set up the ailerons to give 3/8 up and down and 1/4 with rates selected. The model flew well with these settings although it is worth noting my comments in the flight section.
The final piece of the puzzle is the pilots head fairing, which is shaped from soft block balsa thats then sealed and painted with silver Solarlac. It can be glued in place, having first removed the covering film immediately behind the pilot, on the hatch top. Note that the fairing is not glued to the fuselage behind the hatch. Once set, a razor saw can be used to cut the fairing at the front of the hatch, enabling its removal. Seal and paint the cut surfaces, then drill the section behind the pilot to accommodate the receiver aerial.
That's about it. Check the C of G with everything attached and the battery installed (the prototype balanced towards the rear of the range), make sure everything's secure, pack your sandwiches and head off to the flying field!
Ive read a few RCM&E articles recently where the maiden flights of new models didn't go so well, which tempted me into a pre-photo shoot flying session as I didn't want to waste the cameraman's time should the same fate befall my new charge. On a fine Sunday morning with range checks complete and what seemed to be reasonable (large-ish) control throws, my Funterceptor was away. She took off briskly, sped skywards and proved very roll sensitive - it took all my (fairly limited) skill to keep the wings level, never mind get the model trimmed out! I wished Id set - and used - aileron rates!
Funterceptor is quick, and she got rather too far away in a very short space of time. Now I'm not certain whether this was too much for the GWS Naro Rx or my lack of appreciation of model attitude, but either way, a roll into the deck resulted. Fortunately the damage wasn't too bad, being mainly caused by the NiMH pack exiting through the nose. A quick rebuild and reduction of aileron movement was needed for the next try.
Second time out on a breezy afternoon at Chedworth, with more conservative elevator and aileron rates set and ed Graham on hand to take photographs, I readied myself for a somewhat heart in the mouth take-off. But what a transformation! Despite the prevailing lumpy breeze, Funterceptor rose smoothly into the air and was easy to fly. So easy, in fact, that I soon switched the low elevator rate out, as it proved too soft. Seven or eight minutes of camera fly-bys were followed by some easy loops and rolls, all of which was pretty uneventful, albeit the model went downwind like a rocket, compensated for by slow upwind flight. Landing was slow, with lots of control. Perfect!
Okay, so I may be biased, but rest assured that Funterceptor will put a smile on your face every time you fly her. This model really does possess fun in spades! Shes small enough to keep rigged in the back of the car for those quick park excursions, but bear in mind that with an AUW of just 18oz (510g) you need gentle conditions. However, when you do get them, go and enjoy!
Sport aerobaticDesigned by:
36" (914mm)Fuselage length:
29 1/2" (749mm)Wing area:
2.25sq. ft. (0.2sq. m)All-up weight:
18oz (510g)Wing loading:
8oz / sq. ft. (2.4kg / sq. m)Control functions:
Aileron, elevator, rudder, throttlePowertrain used:
HET Micro 6/20 brushless motor, Jeti Advance 12 Plus brushless ESC, 7-cell 1050mAh NiMH; Graupner 8 x 4 Slim propPower (static):
Power / weight: 62 W/lb
Aileron: ±3/8 (9.5mm) high rate, ±1/4 (6mm) low rate
Elevator: ±3/4 (19mm) high rate, ±1/2 (13mm) low rate
Rudder: ±3/4 (19mm) high rate, ±1/2 (13mm) low rate
- Please note that the free plan from the June 2008 issue of RCM&E didn't have the C of G printed. This should be - Forward C of G - on the mainspar centreline Rearward C of G - 11mm back from the main spar centreline - Ed.