The following is a review of the HDS-877 digital micro servo. My thanks to Arshia of Ashtek Electronics for supplying the servo used for this review.

Overview

I did take some photographs of this particular servo but they seem to have got lost. Therefore the following are published and provided kindly by Holger Hemmecke.

The technical specifications for this servo (877) are as follows:

As can be seen from the above this servo is very similar in size to the popular HS-56, it also has similar speed and torque. The crucial difference between this servo and the latter is that this servo is a digital servo and therefore has a faster frame rate. This in theory should make it both more accurate and faster to react to commands from the gyro. One other thing to mention is that this servo also has metal gears and therefore in extended use these may wear and require replacement.

Flight Testing

After the initial installation and setup of this servo it was time to see what it was capable of doing in-flight. As with all digital servos this servo emits a high pitched noise whilst in operation. My initial thoughts about this servo were that it provided quite a good hold when coupled with a GY-401, speed and accuracy appeared to be good and its ability to hold the tail when under load was also good. Certainly backwards flight was handled okay, although on occasion the tail would let go.

It was not until something like my 15th flight that things started to go wrong. I had noticed that the tail was starting to react differently, particularly when performing any maneuver involving a pirouette. The tail seemed to be to lose and tail stops had lost their snap. This deteriorated over the following two flights until I was unable to fly the machine as the tail would not hold at all. It was almost like the belt was slipping as the tail lost all power. Some investigation revealed that under load the servo appeared to have lost all of its torque. It was sent away for testing and the results were that it had a partially burnt out motor. Further information was then found in that the servo will burn out its motor as the stall current on the servo is too high for the motor to withstand. The advice following this was that it was imperative to make sure that this servo does not have any binding in its range of movement. I wasn't aware that on my machine I did have any binding but was unable to check whether this was the cause of the servo failure as it had long been removed from my machine.

Conclusion

At face value this servo looks like a winner, however, due to the flaw in its design which may lead to servo failure I have some reservations. Initial flight performance was good but this did deteriorate quite quickly and ended in an unusable servo. I therefore cannot recommend this servo unless the manufacturer corrects the flaw in its design.