Airbus 400M by Tony Nijhuis
I covered the prototype in silver Easycoat Solarfilm and then keyed the surface using 800 grade wet n dry, ready for the paint to be applied. The only problem with this stuff is that, as a heat-applied medium, the spectre of wrinkles is ever-present - but it does at least give a strong, light finish. The alternative is to use either tissue / nylon and dope, or of course, lightweight glass cloth (17g/m2) and acrylic varnish (available through Falcon Aviation). Personally, I'd suggest the latter as the best alternative to Solarfilm. Oh, and if youre really feeling lazy and don't fancy painting, then J. Perkins stock a mid-grey Profilm that would suffice for a Royal Air Force scheme. Bearing in mind the full-size hasn't yet been built, a scale colour scheme doesnt effectively exist yet... so you could do it in black and white stripes and it wouldnt be wrong... might get some funny looks at the club field, though. The Airbus Military website shows some virtual colour schemes, but generally for this type of aircraft you can have any colour you like as long as its grey!
FINISHING
My decals were originated on the computer using AutoCAD, printing them onto clear, self-adhesive A4 labels (Avery). Clear is best used for dark colours, if a white background is needed then use white self-adhesive labels instead. Its also a good idea to seal the decals with a spray-on acrylic varnish before cutting them out, to protect against water / rain.
Installing the radio is quite straightforward, with the relevant hardware generally contained in the section of the fuselage thats under the wing. As such youll need to remove the wing to access the flight packs, as these fit just behind the balance point.
POWERING UP
After discussing various set-ups with John Emms of Puffin Models I decided to go with conventional 600-size motors, John suggesting the use of a softer 600-8.4V and connecting them in series / parallel. Series / parallel means wiring the two sets of motors in a daisy chain arrangement in each wing half, then connecting the two positives together and the two negatives from each wing half.
The benefit of this arrangement is that it offers operation at a higher voltage of 19.2V (16 x 1.2V per cell). The electrical input power to fly this model is around 500W, so as power equals voltage x current, its easy to see that the higher the voltage, the less current is needed to maintain the same power. So, in short, as all battery packs have an amp/hour (Ah) rating on them, the lower the current being drawn, the longer the battery pack will last. Using 16 cells and connecting through a wattmeter, the input power at full throttle was 750W, with a current draw of around 40A.
I used two eight-cell packs of Puffin Hi-Flow 3300mAh cells on the prototype and Im pleased to report these have proved to be a very good, robust cell with flights in excess of eight minutes regularly being achieved.
INTO THE UNKNOWN
There's an old saying that says: If it looks right, it'll fly right. I remember thinking this on a cold day in February with long grass, a 10-knot wind, puddles festooned all over the Hastings club patch and an uncovered model with a taped-on flower pot plugging the big nose opening (I always test fly electric models prior to covering and making the vac-formings). It didnt look very pretty and the looks right / fly right wisdom seemed inappropriate - but my hopes were high as I opened the throttle and started to puddle-dodge.
I must be blinking mad, I thought, as water sprayed up and the tiny wheels struggled through the long grass. I think the model thought this too, because it stayed firmly fixed to terra firma even after applying full up elevator. I was aware of the C of G being further forward than it should be, but this was a safety margin I employ for all my new designs.
So, three attempts later and the model still hadnt left the ground. The ground speed was part of the problem (not enough of it due to the grass and water), but also the model wasnt rotating. This was partly due to the forward balance, but not wanting nor being able to change this at the field I decided to bodge it and drastically increase the up elevator movement!
On the fourth attempt the nose finally rose on applying full up elevator, and she was airborne... creeping into the slowest climb-out I'd ever seen. Although the model felt incredibly sluggish, all the controls seemed very positive. Almost all the up trim was used, and flying speed was maintained only at full power. Not ideal. After a few circuits the Airbus was guided in for a landing (in the middle of a puddle, of course) where it came to an unceremonious halt. Pleased to return home with the model still in one piece, I allowed it to dry out for a few days and gave the project some more thought.
FURTHER DEVELOPMENT
Compared to its fuselage length the A400M has a very short wingspan, which, I understand, is incredibly efficient in providing lift. On the model, rather than getting involved with this complexity of aerodynamics, I'd used a fully symmetrical section and increased the wing area by around 15%. With the final weight of the aeroplane looking to be around 10 lb (4.5kg), and after such a lacklustre test flight performance, the wing size was starting to concern me. Alas, there was nothing for it but to redesign a slightly larger wing and use a semi-symmetrical RAF38, modified to a deeper section. Feeling reasonably confident with this, I went the whole hog and finished the aeroplane, complete with mouldings, covering, suitable paint finish and decals.
THREE MONTHS ON
For once I didnt hurry the finishing process, mainly due to the weather and the need for a runway cut with short grass... something only the summer months can provide. When that day did arrive and the A400 was rolled out in all its finery, the if it looks right it'll fly right saying seemed more appropriate. Incidentally, I'd moved the C of G back about 25mm, but the large elevator movement was maintained.
As the throttle was opened the A400M accelerated quickly, and after 30 metres or so with just a touch of up she rotated and climbed happily away at about 20°.
I was absolutely gobsmacked - the model was 1 lb (453g) heavier than when it first flew, yet this time I had to throttle back to slow her down! The combination of a new wing, correct C of G and improved aerodynamics (by adding the vac-formed nose and sponsons) had turned this tortoise in to a real hare!
LITHIUM HEAVEN
Now I have to say I'm a bit of a dinosaur when it comes to batteries and I've resisted going the Lithium Polymer route, not only because of the large number of conventional NiCads and NiMHs I already have, but also through lack of knowledge and perceived expense. The biggest bugbear with electric models (especially larger examples) is duration, and its something I knew Li-Po cells would cure.
So I made a call to the helpful guys at FlightPower and in particular their main man, Julian Cox. Conveniently, FlightPower can fabricate a pack to suit your exact requirements, which in my case was a 5-series item (18.5V) to give the equivalent voltage of a 16-cell NiMH. As for capacity, I wanted to match the weight of a conventional NiMH pack, so Julian suggested a 6.5Ah capacity. When the pack arrived it was beautifully put together, with carbon plates top and bottom to protect the cells from mechanical damage and prevent any distortion that might occur during charging or discharging. Better still, when the cells were weighed they were a full 8oz (227g) lighter than the NiMH I'd been using! FlightPower use top-of-the-range Li-Po cells, which are capable of unloading at 20c - over 120A in this case. Awesome!
WHOA THERE!
With the pack fully charged (approx 21.5V) the model was unleashed once again... wow! What a transformation. With the reduction in AUW and a touch more voltage available, I was able to reduce the throttle to a little under half stick. Subsequent flights have proved just how good these cells are, and flight times are edging towards 20 minutes, which for this type of model is fantastic. The best thing of all is that the carrying capacity has space for a second or even third similar pack. So in theory, flying constantly for one whole hour is conceivable - a real breakthrough.
In comparing cost on a like-for-like basis, Li-Po may be twice the price of NiCad / NiMH - but I'm now totally convinced that where duration and weight is concerned, the cost is well worth it.
- Log in or register to post comments