To start his Bench Blog workshop column this month, Tim Hooper looks at a capacitor powered micro model.
Just in case you’re wondering, ‘WSAT’ stands for Wooden Shoe Aviation Toys. Obvious, really. This Netherlands based company is headed by a gent named Vincent Merlijn, who has seen an opportunity in the market for a growing range of small free flight model kits.
The thing is, Vincent’s decided to specialise in the production and supply of kits that are powered by a rechargeable capacitor – the same principle that powers the flash unit on your camera, albeit on a much smaller scale. I don’t know enough about the theory behind the principle to dwell on it, but in practice if we wire up 3 AA cells in series (to yield 4.5 volts in total), then that’s all you need to rapid charge a 5F capacitor in a matter of seconds. The longer you charge for, then the longer the discharge.
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The capacitor can power a very small electric motor, spinning a tiny 50mm propeller. As the capacitor’s discharge follows a diminishing curve, we can exploit that nicely. The initial burst of power should see the model climbing well, then, as the power flattens out, our model can settle into a cruise mode, leading into a (hopefully) gentle descent.
Heading for Vincent’s website at www.aviationtoys.nl there are currently two kits available. The first is the foamie ‘Rookie’, which is aimed more at the younger generation. However, it was the other offering that caught my eye – The Speedster.
We’ve all seen lots and lots of model aeroplanes over the years, haven’t we? It takes something special to stop me in my tracks these days and the little Speedster did just that. Despite spanning less than 16″ this little model looks as though it came directly from one of those romantically fanciful art deco aviation posters of the 1920s.
With a sleek, curved pencil fuselage, elliptical wings and a T-tail it looks great. Best of all, it’s built from laser-cut balsa sheet and the kit is supplied complete with a pre-wired power system and a little charging box as well. The power system features a little circuit board, which incorporates the charging socket. To my mind the kit is well worth the twenty quid asking price (although shipping can add considerably to this).
There’s no plan to build over, but you can download the photographic instructions from the company’s website. This is a model for those who enjoy a bit of fiddly assembly work.
The fuselage is based around a full-depth, vertical balsa keel. Trapezoidal formers are fitted to one side of the keel and the motor and capacitor are fitted with a little blob of (in my case) UHU POR glue. The kit’s attention to detail is comprehensive; for instance, the top of the keel is marked ‘top’ and there are tiny recesses in the formers to accommodate the motor wires. It’s well thought through.
The undercarriage is bent up from the supplied bit of steel wire (0.76mm), which is passed through the hole in the keel and CA’d in place. Once dry the instructions suggest adding a fillet of UHU afterwards.
The fuselage turns out to be hexagonal in cross section. There are six longitudinal panels that, once their edges are gently bevelled, fit onto the formers and form the skin of the monocoque. I used aliphatic glue as it’ll sand nicely later on. The six panels are specific to their destinations on the formers. Starting with the lower pair and making sure the central keel isn’t distorted in the process, they’re glue spot glued in place using CA at their front and rear ends, and with a thin bead of aliphatic in the mid-section.
The middle panels follow next. Again, these need to be identified correctly as they each carry a tiny piercing and lasered guideline to enable correct fitment of the wings later. The two upper panels are last to be fitted; the right-hand upper panel features a little cut out to allow access to the charging socket.
Weight is the nemesis of any small model, so I attacked the fuselage with a small sanding block to get rid of the external corners. The result isn’t truly circular, but more of a softened hexagon. I decided to give a fuselage a thinned coat of non-shrinking dope before progressing any further.
Those gorgeous elliptical wings are simple 1mm balsa sheet. I feathered the edges down to half that measurement and toughened them up with more thinned dope, sanding smooth before and after.
Now, here’s where the design gets sophisticated. Flat plate wings are notoriously inefficient in my experience and need to operate at a significant angle of attack in order to ‘surf’ through the air. This causes drag. Some will argue about this, but I’ve built quite a few flat-winged models over the decades, and they all flew slightly tail down in order to deflect the passing air downwards.
The Speedster, however, turns its flat wing panels into ‘proper’ aerofoils by forcing them to curve at their roots, essentially transforming them into ‘Ritz’ type aerofoils, as used in some vintage free flight models. This curve is achieved by adding shaped balsa blocks to the fuselage sides, using the lasered guideline as a (you guessed it!) guideline.
The roots of the wing panels are shaped in such a way that when they are affixed with CA and aliphatic, not only is the required aerofoil formed, but the wing tips point upwards to induce the correct dihedral angle at the same time. There’s a lot of clever thought gone into that apparently simple procedure.
The T-tail is simple, made from skinny sheet, and there are halving slots in the fin and tailplane to ensure a correct fit. Getting them perpendicular to each other is down to the builder, as is assembling the fin to the top of the fuselage in a simple butt joint. This struck me as something of a weak point in the design, so I knifed a slot in the top of the keel to give the fin something of a home to sit in.
The wheels are each laminated from three layers of balsa (keeping the grain direction at right angles to each other) and little beads are supplied to install as bearings. Since I’d (cleverly) cut the axles too short, I slimmed down the wheels considerably and then fitted short lengths of plastic tube to act as bearings instead of the beads. The outer wheel retainers were cut from very fine brass tube and secured to the axle ends with a tiny drop of CA.
The undecorated Speedster tipped the scales at a mere 14 grammes, but I thought it best to see if it actually flew before I’d consider adding any additional weight in the form of decoration. The Speedster then sat on the hi-fi system until the wind dropped.
Cometh the day and the Speedster found its way to the field. To be honest, there was a noticeable breeze present and my test glides over the sun-hardened potato field resulted in a broken prop, as the (apparently) tail-heavy model stalled into the ground. A quick online chat to Vincent saw more props on the way, whilst I added a bit of nose weight to the model, in readiness for their arrival.
So, with an extra three grammes of lead taped under the nose, the little Speedster could now glide in a respectable fashion, although the need for all that redundant ballast didn’t seem right – especially when nobody else on the WSAT Facebook group was having the same issue.
Eventually, of course, the penny inexorably dropped… In carving out a slot for the fin to sit in securely, in the top of the fuselage, I’d altered the incidence of the T-tail, giving it a severe dollop of ‘up’ elevator in the process. I smartly cut the fin free and cut a tapered sliver from its lower edge. I also re-gouged the slot in the fuselage and then tack glued the tail surfaces back in place, as well as removing the nose weight.
Test glides in the garden confirmed my initial error. It’d been a bit of a vicious circle, in that the unwanted up elevator, called for unwanted nose weight, which made the up elevator necessary for stable flight. Removing the cause of the problem removed the need for the cure and reduced the ballasted Speedster’s 17g AUW by around 20% in the process.
In my pitiful defence I could point to the fact that there’s no recommended CG position in the instructions. But as tactfully pointed out by designer Vincent, anybody who builds the Speedster as he originally intended isn’t going to have any problems anyway!
Re-christened the ‘Flying Shallot’ by Mrs Hooper, the tiny Speedster has since been to the club field on several calm occasions and the assembled membership have been both intrigued and impressed by its performance and flight duration. Even with only a few seconds charge the Speedster will climb sharply and then cruise for over a minute, circling in right-hand circles.
In flat calm conditions it’ll circle contentedly overhead but be aware that if there’s even a hint of breeze it’ll circle downwind quite rapidly, and you’ll be taking quite a hike to retrieve it!
For me, it’s a winner, both for its stylish appearance and its surprising performance. Mind you, I can’t help wondering how it would translate to a larger version of, say, 1200mm span, fitted with a lightweight electric power train and simple R/C. Maybe, we’ll get to find out, eh?
Every now and then life can be very kind.
Not so long ago one of our club’s senior statesmen, Trevor, beckoned me into his august presence to have a word. Trev had noted that I’d become the club’s semi-official repository for old diesel engines and mentioned that he’d got an old engine from his youth, that had lain unused for decades. Would I be interested in giving it a home? Oh, go on then, twist my arm.
A couple of days later Trev handed over the as yet unidentified engine and I was intrigued to see that it was actually a variant of the holy Mills .75. I say variant because it sported twin exhaust stubs made of brass, each about an inch long, one each side of the cylinder jacket.
Puzzled, I asked Trev about the exhausts, and he patiently explained that the engine had originally been bought in a marine format, complete with flywheel, water-cooled head and those exhaust stubs (which allowed the exhaust gunk to be piped out of a model boat’s hull). The engine had later been retro-fitted with a conventional air-cooled head, but the screw-in stubs had stayed.
Obviously delighted, I gladly accepted custodianship of the little Mills. Refusing any payment, Trev then told me that the last time it had been running was 63 years ago, in 1958. That’s the year in which I was born, people.
Expecting it to be gummed solid, it was a surprise to find that the engine still turned over smoothly and had definite compression at top dead centre. Trev must have oiled it before laying it up for the duration. Good man!
Really, there was nothing else to do but to bolt it to a bench, fit a 7 x 6 prop, hook up a fuel supply and start flicking! As soon as the new fuel had displaced the old oil in the combustion chamber (after about 30 seconds) the engine started to ‘pop’ quite eagerly and within another minute it was running quite cheerfully.
There’s a little bit of video footage on YouTube that records this momentous event: www.youtube.com/watch?v=v7OGgCXZIsA
By now the Mills had leap-frogged my mental queue of upcoming projects, to usurp the current leaders and assume the top spot – and quite rightly too, in my opinion. So now the hunt began for a suitable airframe for the Mills to live in. Actually, it was a bit of a no-brainer as the answer was obviously the 36″ Veron Cardinal, which seemed to be destined to be the Mill’s spiritual home.
To be fair, there’s a full kit of the Cardinal currently available from the Vintage Model Company. But as I’ve been spoiled with laser-cut kits recently I thought I’d better don the appropriate hair shirt and work from the plan alone. The original Veron Cardinal plan is freely available on the internet, so it was downloaded, and tile printed onto fifteen sheets of A4. The sheets were carefully taped together and had their outer edges trimmed to yield a perfectly serviceable plan of the Cardinal.
As usual, I started with the wings and tailplane. Both of these items are parallel-chorded, so I traced the aerofoils from the plan onto tracing paper and then stuck the paper to a scrap of 1/32″ ply. The ply was carefully knifed out to give me two rib templates.
Each template was simply pressed on the 1/16″ sheet stock with my fingers and run around with the scalpel to produce all the ribs that I’d need for the build. If that sounds complex and time-consuming it all took less than a single evening in practice. So there.
All of the wood for the project came from the carefully hoarded Hooper stockpile; nice firm 1/8″ sheet to form the single spars and slightly softer stuff for the leading and trailing edges. The wing panels were assembled over the plan but minus their 1/8″ root ribs, which were added after the panels were joined with the 1/16 ply dihedral brace at the spar’s inner ends.
The Cardinal’s fuselage sides are each split lengthways into an upper and lower section, which is great as the top edge of the lower section is a straight line, which means that the fuselage can be assembled, inverted, on the board. I cut both of the lower sides from a single sheet of 4″ wide 1/16″ sheet and added the 3/32″ square longerons and verticals.
The heart of the fuselage’s structure are the two hardwood engine bearers and the two ply formers, which I built as a sub-assembly. There’s no down thrust shown on the plan, but I chose to taper the tops of the bearers to give a couple of degrees of down thrust when the engine was installed. Whilst I had easy access, I bent up the undercarriage from 14swg wire and fitted 2mm blind nuts to the second ply former to accept the little bolts that pass through the U/C’s p-clips.
This central crutch was glued to one of the fuselage sides and then the other side was carefully aligned and glued on top. When this was dry the assembly was installed in my SLEC fuselage jig to have the rear ends of the sides assembled on the centre line.
Once free of the board the fuselage had the upper portions of the formers, the rest of the sides and the wing seat fabricated. I opted to sheet the top, rear deck with 1/32″ balsa sheet, but left the bottom open to allow for fitting of the control runs and radio gear.
The venerable Mills .75 didn’t come to me with its familiar fuel tank. No bad thing, as I really wanted a tank with a larger capacity to allow longer flights under radio guidance. The works laboratory was relieved of a 5ml syringe, cut down a tad to 3ml volume, and I added a yellow, plastic tank top of unknown origin. The tank was trial fitted to the front of the firewall with a strap made from brass shim stock.
So, that’s it for another episode, I’m afraid to say. Never mind, next time we’ll get to the end of the Cardinal project and maybe backtrack a little to last summer and a pair of little free flighters that found their way onto the bench.
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