Back Back to Heli articles index.

Third 'Whisper' up-date - Whisper one more time

May 2001

The story so far - Motors - Batteries - Chargers - Flying Time - Indoor flying - Further conclusions

The story so far

Following an email enquiry regarding the current state of the Kalt 'Whisper', I realised that it is now some 7 years since I last wrote anything about it! The original review appeared in the January 1991 issue of 'RCM&E' magazine. An up-date of that review appeared in the October/November 1991 'Model Helicopter World'. A further up-date appeared in the May 1994 issue of 'MHW'. At that time, the model had amassed a total of 329 flights on five different motors and four different batteries.

The current position (early May 2001) is that the model has a total of 1019 flights on the same five motors and a total of six different batteries. Very little has been replaced on the model itself, although I did eventually replace the original tailboom which had been dented and knocked back into shape so many times that the material had started to disintegrate. I have also replaced one undercarriage strut, which was broken in a particularly heavy landing.

The original Kalt gyro became very erratic in operation and was replaced by another (obtained second-hand). This was never as good as the original and was replaced by a Century piezo gyro. The complete gyro is virtually the same size as the mechanical part of the original unit, and the performance is all that anyone could wish for.

I'm still using the original set of weighted blades, although the heatshrink covering was replaced after many flights, when the original became very tatty.

Everything else is as originally supplied, including the speed controller (low frequency type). The current total of autorotations is 603, including the one that broke the U/C strut.


When I last wrote about the model I was using an Air Supply 'Red' motor, which gave good performance after advancing the timing. This suddenly stopped working after 212 flights. The brush gear had become very hot - enough to unsolder the connections - and the commutator was badly burned and pitted. There was no real explanation other than the failure of one of the brush springs.

I refitted the Kyosho 'Mega 20x2' motor which had already given good results and this motor is still in use, having completed 555 flights in all. At 535 flights there was a marked drop in performance. Investigation showed that the brushes were so worn that the springs were unable to apply any pressure to the commutator. A new set of brushes were fitted and the motor run-in for several hours on a low voltage. Initial performance was disappointing, but this has now returned to normal (as near as I can tell).

This is, to my mind, a quite unbelievable life and performance from what is, after all, a cheap motor. Other motors have given a marginally better performance - for a very limited time. The Air Supply 'Red' motor, which had around one third of the life, cost around three times as much!


The four batteries that had been used up to the time of the last writing were two examples of the 1100 mAh battery originally supplied, a 1400 mAh battery and a 1700 mAh battery, all of 8 cells. Since then, a further 1700 mAh battery has been used plus, very recently, one of the new 2400 mAh type. This last battery uses the new(?) type 'N' cells. All the others used Sanyo 'SCR' type cells.

The 'SCR' cells are well established and understood, but appear to be no longer available. This is due to the fact that it will soon be illegal to sell nickel cadmium batteries of any type in Europe (I believe the deadline is April 2002). From that point we will have to get used to Nickel Metal Hydride (NMH) batteries.

Based on this single example it would appear that the 'N' type cells are inferior to the 'SCR' type, despite having a nominally larger capacity. Cells up to 3000 mAh are available in this size.

More on that anon. For the present, back to the plot.

One of the original 1100 mAh batteries is still in use, but the average length of a flight is down to around 3 minutes with a generally poor performance. The other suffered a failed cell at around 200 cycles.

The 1400 mAh battery also had a cell fail at around 200 cycles. I replaced it, but performance was not as good and another cell failed shortly after. Like the 1100 mAh type, this size of cell has not been available for some time.

My first 1700 mAh battery is still in use, but has had two cells replaced (at different times). Flight time is down to around 5 minutes.

A second 1700 mAh battery now has 100 cycles on it and is beginning to show a reduction in flight time. This particular battery gave the best performance to date and would average over 7 minutes indoors with a very short drop off period at the end of the flight (a short 'dump'). The best outdoor flight was over 8 minutes with good power. Indoor flights have now dropped below 7 minutes, with an occasional flight below 6.

The new 2400 mAh battery was initially disappointing, with noticeably less power and indoor flights of less than 8 minutes, with a long 'dump'. This means 15% more flight time from 40% more capacity! The power has improved and the 'dump' shortened, but the flights haven't lengthened!

I recently saw 8 cell 1700mAh NMH batteries on sale, which were roughly half the size of ni-cads. I must investigate further!

Despite my earlier misgivings, the use of the larger capacity ni-cads gives better flights and a generally better performance. The down side is the greater lump of dead weight hanging under the model which tends to increase the damage if you make a mistake. I know I've been lucky so far...

Electric flight in all its forms really hammers ni-cads and it is clear that around 100 cycles is all that you can realistically expect. If you can reach 200, you are doing well.

BatteryCapacityNo of cycles to date
11100 mAh293 (failed)
21100 mAh212
31400 mAh208 (failed)
41700 mAh215
51700 mAh102
62400 mAh19

Battery 1 was used long past the point where it was any real use, with flight times below 2 minutes and low power. Yes, the figures do add up to more than the number of flights!


The original Union charger consisted of two separate units: a voltage booster and the charger itself. A couple of years ago, the booster section failed (I don't have a record of exactly how many times it was used). I can still use the charger section but for a maximum of 7 cells.

I replaced it with an RCLINE 'Multi Plus 12' charger. This is a one-piece unit capable of charging up to 12 cells and equipped with an ammeter. The absence of a meter on the Union charger had prompted me to fit an external ammeter. This showed that the 'Whisper' batteries (all capacities) were initially being charged at around 9 amps, with this falling to around 3 amps just before cut-off.

The RCLINE charger has a quoted maximum output of 4.2 amps. Monitoring this with the same external meter showed an initial charge of 5 amps or so, and this remained above 4 amps until cut-off. As you might expect, this lower initial charge rate does give a little less power at the start of the flight. It is clear that the booster voltage from the later charger is somewhat higher, thus explaining its ability to maintain the charge rate till the end of the charge. In fact, there is a definite crack from the connector when you insert it, confirming this.

I checked the actual output in each case and the original (separate) booster had an output of 15.5 volts, whereas the later (integral) unit supplies some 24 volts. Despite all of this, the actual flight times are very similar from the two chargers.

It seems that a properly charged ni-cad of the type used should be noticeably warm at the end of the charge. This is totally against most accepted advice that batteries should not show a temperature increase during charging. This may well be causing long-term harm to the battery (hence the life expectancy of 100 cycles), but you will not get the full performance if they don't become warm.

Flying time

This has already been covered at some length. It is clear that the flying time under a specific set of circumstances is more or less proportional to the battery capacity. Beware, however, that the stated capacity may not be the true capacity and that this depends on the batteries health and age.

Hovering uses up the battery faster than flying around. Indoor hovering gives the shortest flights. Forward flight outdoors gives the longest flights.

Autorotations don't make a big difference to flying time. The fact that the motor is switched off on the way down is balanced by the extra power needed to climb to height in the first place.

Indoor Flying

My general advice here is still that you should not fly indoors, unless you are a capable flyer and willing to accept the risks involved. However, people are going to do it anyway so here are some tips.

Ground resonence is a real problem and can seriously damage the model. The smoother the surface, the greater the problem. It is caused by the main blades moving as power is reduced. This causes a vibration which leads to the vertical fin hammering on the floor. Once it starts, the only solution is to add power and lift off. The extra power also straightens the blades and reduces the vibration.

If the battery is running down this may not be possible. In this situation, the only recourse is to hit the throttle hold and go to full positive pitch. This will, at least, reduce the damage.

Running the blades fairly tight in the blade holders helps, as does raising the vertical fin clear of the floor. There was a modification of the undercarriage which was supposed to help. This consisted of adding some damping between the undercarriage and the frame. It was not very successful and the only real effect was to raise the whole model slightly.

Flying indoors has all kinds of unexpected effects. One of these is a mental one where there is a tendency to haul the controls around to avoid surrounding objects and end up in a totally uncontrolled situation where the pilot is way behind the model.

Do be careful.

Further conclusions

In my earlier writings I stressed the need for a systematic start-up sequence for safety reasons. Despite this, I have had a couple of near accidents due to carelessness on my part. The most obvious possibility for error lies with selecting the wrong model on a computer transmitter.

Do remember to select throttle 'hold' on the transmitter before switching on the model and be sure that the throttle is set to 'low'. After going through all of this, I now hold the model well clear of myself before pressing the arming switch on the speed controller.

I have built and flown many model helicopters. This one I wouldn't part with willingly.

Coloured divider

Top Home C/L Heli Articles Links What's New