Medevac Bell 47

As many will already know, I’m a bit of a helicopter fan on the quiet and a sucker for anything that remotely resembles a scale job. The detail doesn’t have to be minute but if the overall scale impression is good and a model’s flown with some appreciation of the full-size, I can sit and watch for ages. No, really! Clever as it is, 3D helicopter flying does little to inspire me. Indeed, given the choice between watching a 90-size hot ship thrash its jolly old nuts off in Sustained Chaos or seeing a 30-size Bell 47 fly a sedate figure eight, I’m afraid the old whirlybird gets my vote every time. You see, I have a basic fascination with helicopter flight that needs no frills to keep me amused, and that, perhaps, is the reason I chose to sample this rather attractive little package from Model Engines (Australia).

To begin, let’s discuss the advantages and disadvantages of the twin rotor design. Contra-rotating (coaxial) helicopters are particularly suited to beginners for two very important reasons: firstly, they offer greater stability in the hover and, secondly, they’re more docile to operate.

For the budding novice who might end up frozen on the sticks wondering how to correct a rapidly deteriorating flight pattern, this inherent stability provides a second or two of valuable thinking time. It doesn’t sound much but every second counts in the learning game and often that’s all you need to realise what’s going wrong and correct the mistake before the model hovers uncontrollably into a curtain, chair, lampshade or other unforgiving household feature. Now that’s a pretty important asset for learners, but there’s more! The twin rotor set-up also eliminates the torque effect that, on a conventional helicopter, has a tendency to spin the fuselage in the opposite direction to the rotor disk. Ordinarily, of course, this would be counteracted by a tail rotor using a regular tail drive and pitch change mechanism. However, on something as small as the Twister Medevac the mechanics would be fiddly, difficult to manufacture and highly prone to damage. So, the advantages are clear but, as you might have guessed, there’s a trade-off and in this case it comes in the form of appearance (real Bell 47s don’t have contra-rotating blades) and the helicopter’s behaviour in forward flight, which we’ll discuss later.

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The Mk.1 Bell 47 had a few shortcomings and whilst these were generally minor, it’s interesting to see how this latest version has been improved. First and foremost, I guess, there’s the obvious fact that the model now looks far more realistic than the early version. Starting at the front, the two-piece vacuum-formed canopy has been replaced with a one-piece bubble that’s glued to a plastic rear cockpit moulding and floor pan. The flimsy vac-moulded original always seemed a bit of an afterthought, especially with the very distasteful centre joint that, inevitably, was smeared with glue.

Further cosmetic enhancements are evident in the form of stretchers mounted atop the skids, a more convincing undercarriage detail with dummy ‘walkie’ wheels, a painted scale-effect tail rotor and re-shaped fuel tanks. Most significant of all, though least noticeable perhaps, is that the mechanics of the new one have been redesigned. Again, the reason for this is purely cosmetic and with the twin motors now mounted side-by-side directly behind the canopy, the overall fuselage width has been reduced to echo that of the full-size. The two cyclic servos, incidentally, have been positioned behind the mainshaft in an easily accessible and much tidier configuration. Finally, the last discernible innovation is a purpose-made underslung battery cradle that replaces the rather hit-and-miss banded version of before. This should (but doesn’t quite) locate the Li-Po in the right place to achieve the desired C of G – more later!

In all, the various tweaks make this a very cute and attractive representation of a US Army Bell 47 Medevac helicopter of the type flown by M.A.S.H. (Mobile Army Surgical Hospital) units and, of course, featured in the popular TV series – cue Suicide is Painless and hum quietly to yourself for the remainder of this article! Anyway, as Bell 47s go this is the version that most enthusiasts will relate to and, for the general public, it’s probably one of the few helicopters with which most can identify. Clearly, converting the existing, rather inconspicuous, Twister into a Medivac ‘47 was a very shrewd, yet seemingly obvious move that’s sure to pay off. Medevacs will doubtless find their way into the homes of existing owners who want to treat themselves to an upgrade, whilst also titillating the hordes of people out there who’ve always fancied a model helicopter but have never quite had the time or motivation to learn the hard way.

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Okay, we’ve established that the concept is a good’un, but what exactly does the Medevac package contain? Well, in a word: everything! Yes folks, you’ll not be left wanting if you buy this package for it’s all in the box. Beautifully packed so that it reaches you in tip-top condition, you’ll find:

  • A fully assembled model Bell 47
  • A pair of clip-on stretchers
  • 35MHz transmitter
  • Eight AA batteries (for the above)
  • A 7.4V 800mAh Li-Po battery
  • A Li-Po battery charger / balancer
  • Mains power supply
  • Four spare rotor blades
  • Introductory DVD covering assembly and flight tuition
  • Printed assembly and flight training guide
  • Preparing the helicopter for action involves simply clipping the flybar and stretchers in place, charging the Li-Po and installing the batteries in the transmitter. The charger supplied is specifically designed for the 2-cell Li-Po pack and, running from either the mains or a 12V lead-acid type battery it’ll charge the cells in just over the hour. A further improvement worth noting is the fact that the Li-Po pack is now automatically balanced during charging. Here each cell is charged to within a very tight tolerance in order that it can reach its fullest safe voltage. Given that accidents and fires have occurred as a result of unbalanced Li-Po charging the system is not only welcome but, arguably, essential.

    As you can see from the photos we took delivery of both the Army and Navy versions and in each case getting the little model airborne was remarkably uneventful. Having two choppers proved quite interesting in as much as both needed almost identical amounts of trim in order to maintain a relatively hands-off hover. I say relatively as in both cases full right rudder, full rear cyclic and three-quarters of the available left cyclic was required to stop them wandering off. Even then both needed a small amount of pressure on the rudder stick to keep the tail in line. As it happens this latter point can be dealt with very easily by adjusting the ‘yaw trimmer’ potentiometer on the 4 in 1 mixer unit that’s housed in the cockpit. Labelled ‘proportional’ and located to the right of the gyro gain pot’ you’ll need to turn it clockwise to trim the nose back to the left and centre the trim accordingly.

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    When I get a minute I plan to address the cyclic problem by altering the pushrod lengths (they’re adjustable in the usual way) and shifting the battery carrier very slightly forward.
    Once airborne and trimmed this is a stable little machine and very docile in comparison to any single rotor indoor job I’ve ever flown. As a regular Sunday morning i.c. chopper jockey I’m rather embarrassed to admit that I had a bit of trouble at first with what seemed like a total lack of response from the controls, the heli’ wandering a little before I realised the amount of input required to keep her still. However, I soon got the measure of it and thereafter began to relax and thoroughly enjoy the coaxial experience once again. As a matter of interest I swapped my first Twister Bell for an ARTF warbird shortly after reviewing it and have often regretted the decision, particularly on the occasion of a recent school talk to a hall full of eight year olds. They’d have loved it. Oh well…

    Test flying took place here in the office, starting with a photography session for the front cover and continuing with various lunchtime and evening sessions in which I eventually mustered the courage to pilot my little heli’ over and around all manner of modern workplace obstacles: plants, desks, filing trays, etc. Finally, having flown the heli’ on half-a-dozen occasions I bit the bullet and sent it from one end of our open-plan office to the other (and back again), navigating the narrow ‘corridor’ between computer screens and the polystyrene suspended ceiling. With a gentle turn at each end I soon had some very rewarding circuits going and, truly, I didn’t want to stop. Landing every so often on a bookcase or cabinet, then rising into the air, blowing paper everywhere (oops!), and continuing on its way the little Bell looked a real treat and prompted many ‘oohs’ and ‘aahs’ from one’s somewhat distracted colleagues.

    Now in truth, twin rotor helis are not ideally suited to circuit work. Whilst they’re superbly stable hovering machines, when you push the nose down they fight to return to the hover and you need to really work the cyclic to keep the forward speed constant and progressive. As such, and unless you have bags of space, flat rudder-induced turns are the order of the day. Rudder authority, incidentally, is very positive and she displays no discernible torque-induced yaw when rapidly throttling up or down.

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    After eight flights – to cycle the Li-Po a bit and let things bed in – I timed a full hovering flight at a gnat’s whisker under nine minutes. Not quite the ten-to-fifteen suggested but very respectable and plenty enough for a beginner to learn something with each go and build on their experience. If you plan to use one of these as a learning tool, buy an extra battery and charge them both up for a session. Moreover, if you run the model continuously using more than one flight pack it might be worth considering the optional aluminium heat-sink upgrade that straddles the top of the motors. The little brushed one-eighties are certainly pretty hot after a flight.

    For the raw beginner, the Twister Bell 47 Medevac is perfect – it’s cheap, robust, relatively easy to fly and very forgiving of the mistakes that new pilots tend to make. Moreover, the package comes with a foolproof DVD that guides the owner from opening the box, to setting up and, finally, flying.

    Compared to the earlier Twister Bell this new job is prettier, more refined and an altogether better package. For what it is, it’s a cracking product that’s sure to encourage many people to try their hand at helicopter flying whilst providing a good deal of light relief to those who have already mastered the art. My considered opinion is that they’ll sell like hot cakes. Go and buy one without hesitation.

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