- The Wattsdog plan was published in RCM&E Dec 2006. The plan can be purchased here.
There I was on eBay, watching the final minutes tick away in an auction for a complete Multiplex Pico radio set-up that was going for a song. Well, it seemed churlish not to put in a last-minute bid at a fraction over the odds, and blow me, when the hammer fell I'd won a Brick (Rx / servo combination), charger and battery, a motor, and speed controller, none of which I needed!
Once I'd bought the gear, of course, I had to find some way to use it, so I decided to design a model around the radio kit, and then sell the whole lot on eBay again. A simple model was going to have to offer something unusual to make it sell, but what? Then inspiration struck - portability.
Usually, when one sells a model on eBay the buyer has to collect it because posting is too problematic. Making a model that can be broken down into small components, however, would mean that I could not only post it, but that the buyer can be keep it in a car, ready to fly whenever the opportunity should arise.
OUTLINING THE DESIGN
Those of you who remember Dick Schumacher's Little Ship, one of the finest vintage R/C designs that ever flew, may well recognise its influence in the design that now began to take shape.
In view of the planned radio installation, I reckoned that a simple high-wing cabin model would provide the best layout. To keep the weight down and make repairs easy, construction should also be kept very simple, with a conventional all-sheet box fuselage reinforced in the corners by square section balsa. The wing - made with four 3/16” sq. spars and the very minimum of sheeting - was to be a two-piece affair, simply assembled with wire and tube. The wing section itself would have a versatile profile - one that would allow the model to penetrate quite stiff breezes, yet glide well and even soar in thermals.
The tail, meanwhile, with surfaces made from 1/8” sheet, would sit beneath the fuselage and be attached by a couple of 3mm nylon bolts. The only other part that would need to be removable, I decided, was the undercarriage. I spent a bit of time working out a system for doing this that was all very neat and clever, but eventually I went back to vintage methods and used a simple plug-in undercarriage held in place by a single small screw.
Easy access is one of my watchwords, so the motor would be reached via the front former, while a hatch in the windscreen area would let me get to the battery.
And that's how the Wattsdog was born. The name? Well, these days it's getting very hard to coin a new play on words with electrical connotations...
GETTING STARTED: FUSELAGE
Once you've cut out all the fuselage parts (including the doublers for the nose), make up the undercarriage using 10swg wire, taking care to ensure that the tops of the legs are parallel. Fit the undercarriage into the tubing mounts, bind the mounts to F3 using copper wire, and then solder the wires together. To make the job of binding quicker, I drill 3/32” holes in the former, and bend the thin copper wire into four, which I then pass through the holes before twisting the ends together. When you solder them into place, make sure that the solder flows right round the wires and bonds to the tube. The undercarriage should now slide in and out easily.
Cut F1 from 1/8 liteply and make the hole to clear the motor; cut F1a to match but with a smaller hole to match the bearing on the motor. Now offer up the motor to F1a and drill the holes for the four mounting screws.
Build F4, F6 and F7 over the plan using hard 1/8 x 3/8 strip, and then bind the tail skid to F8, noting that the skid is offset to allow access to the front tailplane hold-down screw.
Glue the 3/32 sq. strip to the top and bottom of the fuselage sides as shown, and the 1/8 x 1/4” strip to the edges of the wing seat. The front of the fuselage either side of F2 is reinforced with 1/4 and 1/8” doublers as shown; you can also fit the doubler with the servo rail notches behind F3, remembering to leave a gap for the former itself. Now join the sides with F2, F3, F4 and F5 before pulling in the fuselage at the rear and fitting the remaining formers.
The balsa tailplane platform needs to be very accurate and so must be made to fit the model; the slight variations in former width and spacing that will occur between individual examples mean that it really cannot be shown accurately on the plan.
Glue the pieces of 1/8 liteply to the platform, fit the 3mm blind nuts, and then glue the whole assembly to the fuselage. While this is drying, you can make the tailplane and fin themselves using medium soft 1/8 sheet stiffened with anti-warp strips.
Now offer up the tailplane to the fuselage and - once you're sure that it's in the right place - drill down through the blind nuts. Now you have the position of the holes, you can cut out the 1/16” ply discs that are glued to the underside of the tailplane to take the load from the heads of the nylon fixing screws.
The simple clotheshorse hinge method. |
Similarly, to provide a hard point for the undercarriage fixing screw, you need to fit a strip of 1/8 ply, about 1/2” wide, across the bottom of the fuselage just behind F3. This will also serve to bolster F3 and prevent it being knocked backwards by any heavy landings. The wing dowel locations should also be reinforced as shown using gussets made from 1/8” sheet.
While we're busy bracing everything, you can, if you like, add a cross-strip to the built-up formers to support the snake outers. That said, with lightweight snakes this isn't really necessary providing that the outers are secured firmly at each end.
Now fit the fin into its slots, add the strips along the fin that will support the sheet covering, and then glue on all the top and bottom sheeting, noting the use of 1/16 ply at the undercarriage location. Leave the battery hatch for the moment. Instead, sandpaper the nose flat and glue on F1, not forgetting to build in the down-thrust. A touch of right-thrust is also okay, but any left-thrust is not. Finally, screw on F1a and shape the nose as shown in the front view.
As built-up wings go, they don't come much easier than this. The only complication is the wing joiner, and even that's not terribly taxing.
Once you've cut out all the wing ribs, find the hardest 3/16” sq. spars that you can lay your hands on and pin the lower spars onto the building board. Then add all the ribs except R2, and use the rib angle template to set R1. Once you've done this, add the leading edge and trailing edge and the upper front spar.
Fit the tube to the root of the left wing and pack it using tapered balsa pieces as shown; the tube locations are further braced by 1/32 ply webs on either side of the spars. Finally, add the front and rear of R2.
Now prop up the left wing to twice the dihedral and insert the 10swg wire joiner and right-hand wing tube. Turn to the right wing - which is still pinned down, and offer-up the left panel, joiner and right wing tube. Install this second tube, then pack as before with tapered balsa pieces and ply webs. This method ensures that the wings are at the correct dihedral and match perfectly at the root.
Sandpaper the root smooth and fit the 1/32 ply facing ribs, followed by the 1/64” ply centre-section sheeting on the top and bottom of each panel. Note that this sheeting carries right over the leading and training edges. Okay, having made up the tips as shown, you’ll be pleased to hear that the wing is now ready for wrapping.
Former F3 with the u/c assembly mounted in the tubes. Note that the tubes must be fitted before assembly. |
COVERING & FITTING OUT
I covered the model with translucent red and yellow Solarfilm, which is light, attractive, and gives the model a particularly pleasing vintage appearance. I especially like the bi-coloured wings - a look that's easily achieved by covering them in yellow from the front spar to the trailing edge, and then in red from the leading edge to the front spar. The fuselage trim, incidentally, is simply cut from Solartrim.
If you haven't already fitted the rails that'll carry the servos, you need to do that now, followed by the servos themselves.
The snakes can then be hooked up with pushrod connectors - the sort that have a hole and clamping screw, and are held to the servo arm with a sort of internal star washer. The control surfaces can either be hinged with sewn thread hinges, or with clothes horse hinges as detailed in the accompanying illustration.
The motor I used was nothing more than a standard 6V Speed 400 with a small GWS electronic speed controller (ESC) and a six-cell 600mAh NiCad battery. With the radio on board, the model's all-up-weight came out at a very respectable 21oz, spread over a wing area of 336sq. in. which gives a modest wing loading of 9ozs / sq. ft. This one, it seemed, had the potential to be quite a floater.
NOT QUITE ACCORDING TO PLAN
The first flights of the completed model didn't quite go according to plan: she flew alright, but with a lot of up trim, and the motor kept cutting out after a few seconds of full power. Although the motor problem was eventually traced to a faulty battery, I initially tackled it by swapping the Multiplex gear for a GWS Rx, Supertec Naro servos, and a new ESC. Of course, this negated my plan to use the eBay gear, but it did make some very useful savings in weight, which I hoped would cure the trim problem.
While I was in the workshop I also increased the wing incidence by about 1 degree. Naturally, once Wattsdog was ready to fly again, the weather changed from roasting hot to freezing cold, wet and windy...
Wattsdog is the ideal lazy cruiser, perfect for the park. |
About four weeks later I had a stroke of luck: the wheat field in front of my house was harvested, so I was able to nip across early one morning and take advantage of a break in the weather.
I switched on and launched the model. A touch of left trim, a little up trim, and she climbed away happily to a good height. Throttling back, I discovered that there was obviously some lift around as the model stayed aloft on tick-over alone, gliding back and forth over the valley below me.
Though she proved very stable, the Wattsdog does like to circle - she's not fussy about the direction, either way will do - and trimming for dead straight flight proved difficult. I don't really mind this trait as in the event of a problem (control loss etc.) she'll simply circle in the overhead.
Over the next few sessions, the Wattsdog performed faultlessly, and demonstrated the gliding properties of her wing when I was able to ride some wave lift created by a low earth bank.
Whatever the wisdom of my eBay adventure, I'm one very pleasant little model to the good. She's cheap and easy to build, and is stable enough for even a novice to fly in calm weather. What's more, she packs away into a small space so that she can be easily carried in a car, and then assembled ready for flight in just a matter of minutes.
Model type: Vintage style small electric
Designed by: Peter Miller
Wing area: 336 sq. in.
Fuselage length: 32''
All-up weight: 21oz
Wing loading: 9oz / sq. ft.
Rec’d motor: Speed 400
Control functions: Rudder, elevator and throttle