The following took place over three weekends and shows that there can be more than one thing happening at the same time to cause a failure. A club member arrived one morning with a model he had purchased off RC Trader. The model had been well looked after and was in good condition. It came complete with petrol motor and ignition system and 8 servos – motor, rudder, two for the elevator and two for each aileron. All that was needed was a receiver and new 2200 mA Enelope flight pack. The flight pack and ignition pack were charged the night before and everything was ready to go.

For the “maiden” the controls were checked for correct movement, the motor fired up and flight was achieved. The next flight was not so successful. The tank was filled, the motor started and then things went wrong. On take off the plane nosed over and flight did not take place. Back in the pits all the controls had gone to one end of their travel and did not respond to transmitter commands. A check of the flight pack revealed the pack was dead flat. How and Why?? Good questions.

When questioned the owner assured those present that the pack had been charged for two hours at 100 mA. The pack was assumed fully charged as the charger indicated it was. A discussion took place about how at 100 mA charge rate a 2200 mA pack could not be fully charged in two hours. At 100 mA charge rate it would take at least 22 hours to fully charge the flight pack from flat so two hours charging may not have charged the pack. Also some charges time out after a few hours and this is possibly what had happened so the charger needed to looked at and set up correctly.

An inspection of the servo wiring revealed each aileron servo had been connected to a separate channel. This could have also been a possible cause of high battery drain. With two servos per aileron operating from separate receiver channels, the travel of each servo would need to be perfectly matched. If there was a difference one servo would fight the other resulting in high current drain. A simpler method would be to run both servos from one channel on a Y lead – this assumes both servos have the same travel characteristics. With this in mind the owner said he would change a few things before flying the model again.

The next weekend the model was back. The charger had been set up correctly and the batteries had charged for 24 hours at 100 mA and indicated fully charged on the charger and battery checker. A good sign. Before the first flight the battery capacity was measured at 85% capacity. The small discharge could be accounted for by pre-flight checking. The aileron servos had been changed to operate on Y leads so great things were expected. After the first flight the battery capacity was again checked and it was down to 50%. At this rate you could only safely get two flights on a 2200 mA pack. Something was still not right. An LED flight pack voltage indicator had been installed and when the ailerons were moved the LEDs in the indicator would indicate a low voltage. An amp meter was connected just after the battery between the battery and the receiver to measure total current draw. The current draw was over 500 mA with all the servos centred increasing to over 1.5 amps with full aileron. These were analogue servos so the current draw seemed high assuming under 10 mA per servo at rest.

HS-645 MG Seervo Specifications
Performance Specifications
Operating Voltage Range (Volts DC) 4.8V-6.0V
Speed (Seconds @ 60C) 0.24 - 0.2
Maximun Torque Range oz./in. 107-133
Maximun Torque Range kg./cm 7.7-9.6
Current Draw at idle 9.1 mA

Disconnecting a pushrod from one aileron servo saw the current drop by 80 mA. All the servos were still powered so the servos were fighting each other. Disconnecting a pushrod from a servo on the opposite aileron again saw a current drop of over 60 mA. On closer inspection with both pushrods connected again and the servos centred they showed signs of being stalled – slight noise and vibrating. Time for a rethink on aileron servos.

The model is not that big - approximately 1.5 meter wing span so probably did not need two 6.5KG servos per aileron. The owner said he would replace both with one 13 KG servo.

Fast forward to weekend three and the model had been reworked to have one servo per aileron. The current measurement with the servos centred was 170 mA. This was what you would expect for a receiver, 6 servos and ignition module. With full aileron the current increased to over 1 amp while the servos were in motion and settled at approximately 500 mA at full travel. These readings were stationary with no load and would increase whilst in flight. However with these figures you could reasonably expect to have 4 or 5 flights and still have capacity left in the flight pack.

Things to remember:-

with a new model with a flight pack check the capacity of the pack before the first flight and after the first flight. If there is a big difference find out why.

even running two servos with a Y lead on the one control surface can cause a high current draw.

after charging a battery pack check the amount of charge added to the pack – does it seem enough.

until you are confident in the setup of a charger check the capacity of the pack before use.

use an amp meter to determine high current draw components

do the easy things first and if no success look further afield