Century Hummingbird

After flying the Logo 10 for over a year, it was becoming clear that I was coming up against some sort of psychological barrier. Whilst I could hover okay and drift the helicopter around in lazy figure of eights, I was getting no closer to turning it nose-in or setting off into a circuit. If I was to progress further, I needed to take some action. The obvious action would be to grit my teeth and just do it. However, the prospect of a £300 repair bill and a major setback to my confidence were enough to inhibit me from this approach. Another way would be to put some time in on a computer simulator. To be honest, this would be my preferred route but unfortunately there are no suitable simulators available which run on the Mac. A third option would be to take a few classes with one of the commercial training organisations, utilising their dual control facilities.

I had just got as far as enquiring about this third route when a clubmate told me how much fun he had been having in his garden with a Hummingbird helicopter. Now this particular flier is traditional i.c. pilot, not particularly enamoured by all things electric and miniature so, if he rated this helicopter, that must be high praise indeed. Up until this point, I had seen many fliers trying to get to grips with the Piccolo helicopter indoors, with variable degrees of success. However, a few enquiries on the web seemed to suggest that the Hummingbird was somewhat more stable and also less prone to scattering its component parts around the floor in a heavy arrival. On the basis that a single mistake through being too brave with the Logo 10 would probably cost me more than the price of a Hummingbird, I persuaded myself that, with the new indoor season coming up, it would be false economy not to buy one!

The Hummingbird is widely available and I bought mine off the shelf at our local model shop. I opted not to buy the 'Birdseed' 7-cell NiMH battery and instead bought a 2- cell 1400mah Li-Ion pack. This I hoped would enable me to get at least three 5-6min flights in an evening without having to take along a charger. In the event, this proved to be a vain hope.

The instructions for the Hummingbird are very stangely variable in scope, covering some things two or three times over and omitting equally important steps completely. There have also obviously been changes made along the way, so the box art differs from the illustrations in the instructions and neither aligns quite with what you get in the box. There is little point in me going through these discrepancies in detail because the helicopter has recently been upgraded to the Hummingbird 2. I gather though that the instructions still leave a bit to be desired, so you may well wish to consult other Hummingbird owners at some point in the build. Having said all that, there is nothing really difficult involved, it is just that there are lots of wires to be plugged in the right places and the positioning of the various bits and pieces has to take account of the lengths of these wires - and the need to get the canopy on. The following two pictures (both of which will enlarge) will give some idea of how I fitted out my particular model. Note that the red lump is the Li-Ion battery, fitting of which involved cutting a chunk out of the canopy.

First flight attempts with the Hummingbird were within the confines of the lounge. After a few hops it was clear that more space would be prudent, so it was outside to the garden. This time I was brave enough to get off the ground and into a low hover. I barely had time to congratulate myself though before the helicopter embarked on a rapid pirouette and settled back on the ground. Inspection revealed that the pinion had come off the tailrotor motor and, tiny item that it is, was impossible to find. Century UK quickly provided a replacement motor and pinion, and flight tests were resumed. Once trimmed out, the model proved to be perfectly flyable in the lounge - although I did inadvertantly prune a couple of pot plants in the process. Success was again shortlived though, because after less than two minutes it was no longer possible to get the helicopter out of ground effect, so the flight ended having used little more than 10% of the battery's capacity.

Over the next few weeks, I tried a number of things to get around this duration problem. The fundamental cause of the difficulty is the lower voltage of the 2-cell Lithium pack compared to the recommended 7-cell NiMh one. The first step was to change the main drive motor for a Multiplex Permax 280BB. This is a better quality motor with a high rpm/volt figure so hopefully would enable hovering flight to be maintained on a lower voltage. This changeover involved removing the pinion from the stock motor, for which purpose I bought the rather neat special purpose puller, pictured below.

The motor change effected some improvement, but only extended the duration to 3-4 minutes, so the next step was to change the gearing. The pinion puller was again pressed into service and the 10 tooth pinion swapped for a 12 tooth. This time I was really confident that I had the problem licked.

As the next flight approached the 5 minute mark, the helicopter was still staying clear of ground effect but suddenly I became aware that my left thumb was pushing further and further to the right. As I felt the endstop, the helicopter embarked on an unplanned pirouette to the left and this time the rotor blades tried to take on the stoneware pot housing the previously pruned houseplant, and inevitably came off second best. After a bit of head-scratching, I realised that, whereas the change of motor and gearing had enabled the main rotor rpm to hold up in spite of the lower battery voltage, I had done nothing to help the tailrotor motor cope with what was essentially the same problem. In the early stages, the gyro copes with this problem by increasing the power to the tailrotor and so it is only when there is no more power to give that the model begins to turn and by then, moving the stick doesn't achieve anything either. I drew the line at messing about with the tiny tailrotor and its already troublesome pinion and, although I did do a few brief experiments with a seven inch GWS propeller in place of the tailrotor, the time had come to admit defeat and revert to the standard gearing and buy a 'Birdseed' NiMH battery.

Since then the Hummingbird has been trouble-free. Flight durations are usually six to six and a half minutes and I can get four flights (i.e. three re-charges) in a two hour flying session. If you do fancy doing a Lithium conversion on this model I am told that, although the instructions advise against it, the electronics will cope with the higher voltage of a three cell Lithium battery. Unfortunately though, the readily available three cell pack of the right size and weight is only a 700mah which would not sustain an evening's flying and of course the re-charge time is a lot longer than the NiMH battery.

At the time of writing (January 2004) I am beginning to coax the nose of the Hummingbird further round and creeping up on the dreaded nose-in hover position and I am confident that the model will help me to overcome this particular barrier. As far as flying circuits are concerned, the jury is still out. I have seen only one serious attempt to do this and the Hummingbird seemed to suffer from a tendency to pitch up in forward flight. This seems to be a general characteristic of these small helicopters and may be a consequence of some of the design measures taken to achieve a stable hover. There are though many modifications and upgrades available and I am sure that, with enough persistence and experimentation, circuit flying should be achieveable. For now though, I must just try and bring that nose round just a few more degrees...