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Setting Up Dual Receivers
Low (er) dollar absolute redundancy
Text by Bill Rutledge and Tom Hintz
Photos and video by Tom Hintz
Posted - 9-20-2017
The idea of using dual receivers in a plane has been around for a long time. I expect the idea originated with the advent of larger planes that often needed more channels/servos than the radios back then provided. However, even with modern interference-resistant radio systems the dual receiver concept is still relevant, especially in terms of safety. For instance, in the United Kingdom, if a model weighs over 20kg (44 lbs) dual receiver redundancy is mandatory. The good news for us is that setting up a dual receiver system seems harder than it is. Honest.
It is important to recognize that a dual receiver system is not the savior from all problems. It is a concept to give you a better chance of getting a plane on the ground with less or even no damage if one receiver has a problem. Two receivers do not preserve the function of all control surfaces. The whole idea of separating the left control surfaces from the right with a dual receiver system is to give us a fair amount of control even if one side shuts off altogether.
For the dual receiver system to work it is crucial to have neutral failsafe positions set for all control surfaces. You should be setting these failsafes anyway. That way if one side fails all or most of the servos controlled by that receiver return to neutral which gives the surfaces on the other receiver a better chance of maintaining control. Again, dual receivers are not the answer for all failures but it does increase the odds of you saving a plane from destruction or at least minimizing the damage.
When I first heard of the dual receiver idea I wondered how we got two receivers bound to the one transmitter. After cruising the Internet on the subject, I found that I was not the only one wondering that. The answer is very simple once you understand that simply put the transmitter isn’t learning the receivers, the receivers are learning the transmitter. The transmitter still sends out the same commands but now both receivers execute them.
The two receivers do not have to be identical, they just need to both be same DSM2 or DSMX in the case of Spektrum receivers and be capable of the same frame rate (11ms vs 22ms). They can be different in terms of the number of channels. In other words, one can be an 8 channel and the other a 9-channel as we use in the new Hangar 9 Valiant. Keep in mind if you need the 9th channel, use it for something that does not have another side like ailerons or elevator halves. Single servo things like throttle, rudder (in many cases) and in my case a remote engine kill can use that stand-alone channel on the 9-channel receiver.
Batteries and Switches
Though I normally use LiPo receiver packs, twin LiFe 6.6V, 2100mAh packs will power the dual receiver system in the new Hangar 9 Valiant that is replacing the one I lost due to a receiver failure. I suspect that I will get plenty of flight time from these packs because of the smaller control surfaces and light servo current draw in the Hangar 9 Valiant. To error on the side of caution I will monitor battery usage closely until I get a good sense for receiver battery endurance under normal flying conditions.
Also used are two switch harnesses, Spektrum 3-Wire (charge) #SPM9530. I like these switches because they are small, dependable and have the charge lead built in which simplifies most of my installations. Each switch passes current from one of the LiFe packs to one of the receivers to keep them separated and to maintain redundancy.
I laid out the receivers I am putting in my new Hangar 9 Valiant along with the twin batteries and switches on a table. AR8010T and AR9030T Spektrum Telemetry receivers are being used. One feature these receivers have is a Bind button that is depressed while turning on the power to enter Bind mode rather than using a separate bind plug. I set up a new model in my Spektrum DX9 transmitter and then turned it off.
The Bind process will sound very familiar because it remains identical to single receiver setups. I turned one receiver on in Bind mode followed by turning the transmitter on, also in Bind mode. The transmitter goes through the normal Bind routine and announces when that process is complete. Shut that receiver and the transmitter off. Now, repeat the process with the other receiver. Turn it on in Bind mode, turn the DX9 on in Bind mode and let the process complete before turning the receiver off and then the transmitter. If the Bind process completed both times you now have two receivers bound to the one DX9.
Servo Port Usage
Using dual receivers must be considered from the beginning, when setting up the model in the transmitter. If we set the model up using the “single servo” selection for operating the ailerons, elevators and flaps we simply plug those servos into the Aileron, Elevator ports in both receivers. For instance, the left aileron goes into the Aileron port on the left receiver while the right aileron goes in the Aileron port on the right receiver. But, there is a down side with this arrangement.
When we select the “single servo” scheme in the transmitter we lose the ability to adjust individual servos through the transmitter. That means rather than a simple Sub Trim tweak we must make those adjustments with the linkages which is of course always a good way to do it. However, manual adjustments can be difficult on the bench in some planes and even more difficult to do at the field. This can also be somewhat less precise than a click or two of change through the transmitter.
If we choose dual servos for the ailerons, elevators and flaps in the model setup screens we can totally separate the receivers yet still make servo adjustments in the transmitter. In this case we follow the channel assign chart generated by the transmitter after we choose the number of servos driving the surfaces. In my situation, I plug the left aileron in the left receivers AUX1 port, the left elevator in the left receivers AUX4 port. The right aileron goes in the right receivers Aileron port, the right elevator in the right receivers Elevator port.
Remember, if the throttle servo is plugged into the throttle port on the left receiver we can’t use the throttle port on the other receiver for something else. And, we can’t reassign the throttle channel for something else because that must be done in the transmitter and would change that assignment in both receivers. The receivers do not know two of them are listening to one transmitter and the transmitter has no idea there are two receivers responding to it.
One thing we need to avoid is creating an asymmetrical situation. Say we have two flap servos and plug one into the right receiver and the other flap servo into the left receiver. If one receiver goes away having one operational flap could make the plane unflyable. In the case of the flaps it is better to have them both plugged into an open channel on one receiver using a Y-cord so they will work correctly or not at all.
The rudder can plug into the rudder port on either receiver as the throttle can plug into the Throttle port on either receiver. However, if you use a remote ignition kill that can be plugged into an open port that is not used otherwise on the other receiver, not the same receiver the Throttle itself is plugged into. That way you have two shots at controlling the engine if there is a receiver failure.
Despite using two receivers we can still run out of options if we do not have a sufficient number of channels to start with. A 9-channel receiver is a huge help with many planes but still may not be enough if the model has retracts, spoilers or other features that need a channel for operation.
Using dual receivers is an attempt at creating a type of failsafe situation where the loss of one receiver is not automatically fatal to the plane. It’s not perfect but does give you a fighting chance to get the plane down with far less damage (if any) than a crash would cause.
It is important to note that I am not getting away from the Spektrum Powersafe receivers. I love them for their security and simplicity of installation. I did have one go bad leading to the crash of my first Hangar 9 Valiant but that has nothing to do with the overall quality of that receiver. When I built the new Hangar 9 Valiant I already had the dual receiver story “on the bench” and needed a plane to put it in. The Hangar 9 Valiant was just handy and made a good platform for the story, plus I got to fly it even more which is always a good thing when you own a Hangar 9 Valiant.
The dual receiver idea is not for everyone but will be attractive to some that have concerns about the radio security of their planes. This dual system is used all over the world, including mandatory use in large planes in some countries like Great Britain. You must decide on your own if this is the right solution for your situation.
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