For years, decades in fact, model flyers have wondered just what’s happening to their aircraft as they streak across the sky. Models can be subject to stresses that are difficult, if not impossible, to appreciate from the ground while, stresses aside, engines and power systems can behave differently when they meet moving air.
Flyers have asked themselves just how much current the electric power system is drawing? Does the electric ducted fan jet draw more current in the air than on the ground? Will the combined effect of those servos strain the battery? How high and fast is the model travelling? And, not least, is the fuel tank empty yet!
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In seeking answers, they’re questions to which some flyers have developed convoluted homemade apparatus. The battery that becomes exhausted unexpectedly has been the bane of electric flight and this in-flight current draw question has prompted more than one pilot to leave a wattmeter connected in the model and fit a video camera specifically to film the wattmeter’s LCD screen in flight – a rather weighty solution that one assumes was reserved for a model large and powerful enough to carry the equipment.
Some commercial systems have been available; the Picolario system made by Renschler in Germany, introduced eight years ago, has been helping glider pilots by providing a link to a walkie talkie device that relays voice feedback covering height, gain, sink and battery state. At around £335, the system isn’t cheap but soarers who steer expensive airframes see it as a worthwhile investment and wouldn’t be without it.
Eagle Tree’s data logger systems have been growing in popularity with improving capabilities to match the emergence of telemetry equipped 2.4GHz radio systems. Initially starting as a data logger, where info could only be accessed after a flight, handheld displays are now available that receive information in real time with audio feedback. The added advantage is that systems like these can be used with any radio. For this article though, I want to look at the telemetry facilities that operate as part of a 2.4GHz radio system.
To know what was happening to your model in real time seemed the stuff of science fiction not long ago, then 2.4GHz came along.
2.4GHz radios form an unbreakable (bi-directional) link between the transmitter and receiver so providing a route for information to be sent back from the model. At the time of writing, the following mainstream manufacturers offer telemetry facilities:
Graupner, Multiplex, Spektrum, Jeti, Hitec
Futaba have been late joining the party although have promised that their high-end T18MZ radio, due later this year, will be their first to have a telemetry capabilities.
So how does it all work and what do you need? Well essentially most telemetry systems require a telemetry-enabled receiver (Rx) into which data collecting sensors can be plugged. Not all 2.4GHz receivers will accommodate sensors so it’s important to check before purchase. The Rx will look the same as any 2.4GHz Rx but with the extra socket. The sensor is placed in the aeroplane, plugged into the Rx and the job’s done. The transmitter (Tx) screen (or plug-in display) then reflects the data collected by the sensor.
Systems do vary and it's important to bear in mind that, at present, few transmitters are equipped with screens large enough to reflect the extra data – a quick glance when flying may not be sufficient to 'grab' the info required and so it's better to employ the services of a caller. Some systems have recognised this by offering alarms and vibrating alerts. Some even offer voice feedback via an earphone jack. The exception to all this is receiver battery voltage where most systems don’t require a separate sensor to reflect the state of the pack in the model – very handy.
Sensor placement will vary depending on the sensor type. For instance one of Hitec’s i.c. fuel tank level sensors must be attached to the fuel tank while a temperature sensor must be attached to the battery and an RPM sensor near to the prop. Placement is less critical for others so current/voltage sensors and height/GPS sensors just need a spare space – instructions direct their siting. Modern electronics mean that weight shouldn’t be an issue to the extent that most sensors are no larger than a postage stamp, weigh no more than a few grams and can be fitted into even the smallest of airframes including lighter park flyers.
Telemetry has some very obvious benefits although it's clear that the practical application of the technology is still in development. Many, if not all, the transmitters offering the facility are just 2.4GHz versions of older designs and even some of the newer transmitters introduced in the last year (such as Spektrum's DX8) still employ a small screen that may be hidden when neck straps are used.
That said, it's clear radio manufacturers see this new information stream as ripe for development so, for many, it looks like those age old model flying questions are about to be answered.
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