One of the fundamental challenges of flying RC aircraft is that you are separated from the machine you are controlling. You must assess the health and status of your vehicle from a distance using only limited visual and aural cues – rarely an easy thing to do. Sometimes the first symptom of a failing system is a trail of smoke that inevitably leads to the ground.
RC telemetry systems provide the means to accurately gauge certain parameters of your model during flight. Think of it as a remote dashboard. Do you want to know how hot your motor is running? How about an alarm that can warn you when your model reaches an altitude of 400 feet? Telemetry devices can provide those things and more.
What Telemetry Requires
There are several different ways to receive telemetry data. Some telemetry systems are standalone units with a transmitter/sensor package in the model and a receiver on the ground. For FPV flyers, On-Screen-Display devices take the data from onboard sensors and overlay it on the real-time video feed. The result is something like a heads-up display found in many modern full-scale aircraft. An increasingly popular form of telemetry system is the type integrated into the model's radio system. The pilot's handheld transmitter sends flight commands to the aircraft while also receiving downlinked data. The same onboard receiver that interprets commands also transmits telemetry data. In this way, both the transmitter and receiver are actually transceivers.
The majority of radio manufacturers offer telemetry-capable systems in their lineups. The example that I've chosen to highlight in this guide comes from Futaba. As of this writing, there are three Futaba aircraft transmitters that are telemetry-capable (10J, 14SG, and 18MZ) as well as a handful of receivers. With these systems, their telemetry features are embedded in the S.Bus2 circuitry of the components. That nuance begs a brief explanation of S.Bus2.
What is S.Bus2?
Traditional radio systems are configured with parallel connections. This means that every commanded device (whether it's a servo, ESC, sound system, switch, etc.) has a dedicated 3-conductor wire linking it to the receiver. There are instances where you might use a splitter to connect two servos to a single output of the receiver. But those servos cannot be controlled independently, so they are essentially a single actuator in the eyes of the radio. With large and/or complex models using many devices, all of these wires can become an unmanageable bird's nest. The wires may even be a significant contributor to the model's weight.
Several years ago, Futaba introduced the S.Bus system which allows the onboard components to be connected in series – greatly reducing the potential clutter inside of a complex model. For instance, a single 3-conductor lead from the receiver could lead to a 4-way node (Futaba calls it a "hub"). Extensions from this hub may be routed to additional hubs in the nose, tail, and both wings. All of the servos in each of those locations are simply attached to the local hub. The end result is fewer wires and connectors to worry about.
While there is more to S.Bus, a thorough look at the capabilities it offers is well beyond the scope of this article. A recent article by Matt Gunn of RC Groups takes a deeper look at the control side of S.Bus. The primary point to be made here is that the newest version, S.Bus2, supports telemetry functions. Just as with the control devices, the various telemetry sensors can be connected in series as well.
There are many types of sensors to choose from. Those such as voltage meters and temperature sensors can be helpful on any type of aircraft to help avoid a system failure. Others such as RPM sensors and variometers can be useful for fine-tuning the performance of your bird.
To exercise the Futaba telemetry system I used the following components:
SBS-01RO Optical RPM Sensor
SBS-01TE Temperature Sensor
SBS-01A Atmospheric Sensor (variometer)
SBS-01G GPS Sensor
In the interest of removing variables, I installed all of this hardware into one of my tried and true models, the Sig Kadet Senior. If there's such a thing as an RC minivan, the Kadet Senior is it. It's not very stylish, but it has lots of room. There was plenty of spare volume and lifting capacity for all of the added telemetry components.
In stock form, all four servos of the Kadet are centrally located. Mine has an additional servo to operate a glider towline release, but it is in the middle of the fuselage as well. With all of the servos located together, there is really no advantage to connecting them via the S.Bus2 port of the receiver. So I plugged all of the servos in parallel to their normally-assigned channel outputs.
The four sensors, however, must be connected to the S.Bus2 port. This worked out well with one hub near the receiver that leads to an additional hub near the nose. In addition to data from the sensors, the receiver automatically transmits the voltage of the onboard battery powering the receiver.
The first device that I installed was the optical RPM sensor. It must have a clear view of the propeller arc to do its job. I decided that the best location would have the sensor peering through one of the cheek air inlets from inside the Kadet's fiberglass cowling. I added a thin balsa spacer between the sensor and the inner surface of the cowling to get the correct orientation. I then used GOOP adhesive to hold the sensor in position.
Next I installed the temperature sensor to the Rimfire .46 brushless outrunner motor. Most of the motor body rotates along with the shaft, so I had to carefully mount the sensor's thermocouple to the base of the motor. A small piece of aluminum tape is provided for this task. I secured the main board of the sensor to the motor mount using a pair of zip ties. Before replacing the motor cowling, I connected the signal wires from the RPM and temperature sensors to a 3-port hub with a 300mm lead. I routed the hub lead into the inner fuselage through a hole in the firewall. The lead was then plugged into a 6-port hub (Futaba calls it a 'Terminal Box") located just forward of the receiver.
The area between the firewall and the cabin provided plenty of mounting options for the GPS and atmospheric sensors. I used GOOP to hold them in place on a perch near the terminal box. The instructions note that these sensors have to be placed out of the breeze, so I made sure that they were in a sheltered area. The leads from these sensors were plugged into the terminal box. As soon as I powered on the system, the radio recognized all of the sensors and I could read their output data on the telemetry screens of the transmitter.
Flying with Telemetry
Having telemetry data on the screen of the transmitter is handy when performing ground testing, but it's actually somewhat of a hazard when you are flying. You don't want to have to look away from the model to read the screen--not even for a second. To overcome this, the transmitter can be configured to provide tactile or aural cues to the pilot.
For parameters such as battery voltage or motor temperature thresholds, you will likely want to configure a vibration or sound alert.
For parameters where you only want to be notified when a defined threshold is exceeded (battery voltage, motor temperature, etc.) you will likely want to configure a vibration or sound alert. The 14SG has four different vibration patterns and four different alarm sounds which can be assigned to individual alerts.
There are other data parameters that you may want to monitor continuously, such as speed or altitude. For these cases, the radio will provide spoken voice reports of your selected data at set time intervals. You may choose to have the data reports throughout the entire flight or tie it to a switch that you can activate as you wish.
While the alarm sounds are played over the transmitter's built-in speaker, the voice reports are only routed to the headphone jack. Using ear buds (as opposed to headphones) will typically allow you to hear the voice reports while still being able to maintain situational awareness on the flightline. I suppose you could attach a portable speaker to the headphone jack, but you should be considerate of neighboring pilots who probably don't want to hear your data while they fly.
The times that I've flown the Kadet with telemetry active, I shared the field with only electric powered models. So the ambient noise was rather low. I had no trouble hearing the alarms and voice reports through ear buds. It will be interesting to see if that holds true when louder gas/glow-powered models are also flying.
Saving It for Later
There are several cases where you may want to save your telemetry data for detailed post-flight analysis. Data logging is a built-in feature of the 14SG. You can choose the datapoint time intervals and the transmitter will write the data to an SD card. When you're ready to view the data, you must view the file through Futaba's File System Utility tool. It's a free download, but it appears to work only in Windows operating systems.
The Futaba tool outputs a .csv file that can be viewed in Excel or other compatible spreadsheet tool. The file contains the telemetry data as well as the commanded position of every channel. That will most likely provide all of the data you need to determine whatever performance answers you seek.
As I was researching the data logging process, I came across a tool developed by Arseni A. Timofeev that converts .csv telemetry files to .kml format. This allows you to plot your flight path on Google Earth. Fun stuff.
As I think back over some of the crashes that I've had during my modeling adventures (there are many), I can recall several instances where telemetry would have saved me. Likewise, there have been many times that I wondered "How fast is it going?" or "How high is it?" Telemetry systems such as the Futaba devices shown here can provide that crash-preventing lifeline and answer those nagging questions. Of course, telemetry isn't practical for every model. But for those times when telemetry makes sense, modelers will find that their options and potential applications continue to expand.