In this article we will be examining a small fixed wing Remotely Piloted Aircraft, designated as a Self-Guided Model Aircraft (SGMA) by the Model Aeronautical Association of Australia (MAAA) – and the technology incorporated into these little mini marvels.
A transmitter not suitably equipped for LiPos in which the battery caught fire. Fortunately the fire burned itself out before any real damage was caused, probably due to a lack of oxygen in the battery box and carrying case.
The choice of electric power as against internal combustion (IC) motors is a difficult one due to the severely limited energy density of batteries, even modern Lithium Polymer (LiPo) batteries.
Because of the severe power and endurance limitations currently imposed on electric powered RPAs, great care must be taken in the design and in-flight tuning.
However limited capacity not withstanding, the advent of the LiPO battery with its light weight and 3.7V terminal voltage has revolutionised electric flight for miniature aircraft. But they are still only suitable for short endurance flights at the moment, typically 15 to 120 minutes.
As we mentioned last month, electric powered aerobatic model aircraft are becoming a dominant force in international model aerobatic competitions, with over 50% of competitors now using electric power. This event only requires 15 minutes endurance and is thus ideal for electric power.
However, LiPos come with certain drawbacks including higher cost, they are easily damaged if not handled correctly and there is a higher risk of fire, especially in a crash.
In addition, charging is not a simple process, taking much longer than refilling a fuel tank and it usually involves multiple batteries to keep the flying session going.
Charging can occasionally be fraught with risk, especially when fast-charging. It is always a good idea to charge them on a fire-proof metal tray that can be easily carried outside in the event of a fire. And it is best not to leave them on their own when charging.
Fire is not a frequent occurrence but it does happen, particularly if the battery has been damaged in a crash. Models can be completely destroyed by the intense heat generated by burning LiPos.
When using LiPo batteries which can be damaged if the cell voltage falls below 3V so a low voltage alarm or cut-off is a must.
One word of warning here: there is a trend towards using LiPOs in transmitters which mostly (certainly older models) do not have low voltage cut-off. If that TX is left on inadvertently, then it is good-bye LiPo. So be very careful with this one.
However, this situation is rapidly changing, with faster-charging batteries and improvements in battery construction coming thick and fast.
Even so, a twofold increase in energy density or even more is required to lift the electric RPA into the really useful endurance category enabling it to begin to compete successfully with the IC engine. Rumour has it that this improvement is not far away.
Despite the foregoing, there are numerous advantages to electric power, including an almost complete lack of motor vibration (a boon for aerial photography), increased reliability over IC engines, ease of starting, the possibility of stopping and starting the motor in flight, (a great aid to increased flight times and further reducing vibration) and finally, an almost complete lack of noise.
In view of these advantages, the Author would use electric power exclusively were it not for the limited endurance.
Before we move on to an analysis of the electric motor and electronic speed controller (ESC) in the Cub, perhaps a few words on electric power are in order.