ScaleAero

Under the title of Don't Do This with either 2.4 or 72. Futaba and "probably" every other brand:

We recently observed a Futaba customer who desired to create a redundant configuration for his onboard system by placing a second receiver in the plane, It was an identical receiver to the first. In this case a pair of 7~8 channel FASST receivers in a multi-engined model with the following configuration:

Into each channel on each receiver was plugged one side of a "Y" harness. Channel 1 was on one end of the "Y" and Channel 1 on the second receiver was on the other end of the "Y". The third end of the "Y" is plugged into either a single, or multiple servos by way of additional lengths of wire or "Y" harnesses to the servos to control flight surfaces, throttles and retracts.

The modeler had used this configuration successfully with all his multi-engine aircraft operating on 72 for the last 25+ years without problems. However when he replicated this servo and receiver configuration using his brand new 2.4 FASST it functioned properly for a number of days as he placed servos into a model as he finished up its setup prior to initial flight then the servos began to behave strangely without consistancy.

Turn the system on, link/bind it to the transmitter and everything would work fine then with no jittering or sudden glitch a servo would not move.

Obviously this had to be resolved so I contacted Steve Helms asking that he present this to Futaba's engineers. The answer came back from the Japanese in "clear and easy to understand" English. "Do not do this!" ; )

If you do...the timing will be either cancel signals to both receivers, or partial (as we had observed) or even worse. A configuration like this could result in the Fail Safe failing to function in one or both receivers.

The engineer speculated the configuration in a 72 system was slow enough that contention of the receivers and mutual channels was never realized. Each 72 receiver was off in their signals and problems were never observed on the ground...though they may have occurred in the air with minimum consequence. (that's called luck).

The customer is now configuring their multiple receiver setup as a left and right half with the rudder on one receiver and the nose steering on the other in the rudder channel but reversed to achieve proper ground handling.

The image below illustrates the improper multiple receiver "Y" harness split to servo wiring configuration.



I received a request to illustrate the "proper" configuration. Its simple... never plug any "Y" harness between the two receivers...period. Is there any part of do not do this which is confusing?

To properly use multiple receivers in a right and left half configuration be sure to set Fail Safe in "both" receivers to move to a 0/0 position to permit operable control surfaces to control attitude of model well enough to land in case either of the receivers were to fail.

Campgems

Interesting. My first thought was that it would be a mess, I'm susprised that the 72 didn't show up the problem also.

I suspect that you could use that logic by using some "and" circuits that would requie a coincidence of servo pulses from both receivers, but the down side is that it would only send the pulse width of when both were on. It would still give you some control, but I would hate to think about flying that way. The hard wire would be a mess as you would have the recivers fighting for the buss and what you would end up with is a jaged LE and TE on the pulse vs a nice square wave that you would hope for.

Using the old Space shuttle method, you woud need three receivers, because with two, if there was a differece, you wouldn't know which one was wrong. With three, you would have a tie beaker.

I don't really understand the need for the second receiver in todays world though. I've seen servos go bad, and two receivers wouldn't help this. I've had a battery go open and shut down everyting. Here, a reduntand battery setup would be nice and is something I seriously considering in a couple planes on the bench. What I've not seen is a receiver going bad in flight. I suspect that is has happened, but the complexity of handeling two receivers and deciding which was really the one in control just seems like a complexity you don't really need, or even want.

Don

BuschBarber

My Feedback: (2)

I know fellow pilots who are using Dual Rx's to put Half the servos on one Rx and the other Half of the servos on another Rx. So in the simplest case, one Aileron and one Elevator servo would go on one Rx and one Aileron and one Elevator would go on the other Rx. This could be used to avoid a crash if one Rx failed, to reduce the servo load on each Rx, or just to avoid having to make a number of servo connections when putting the aircraft together or taking it apart, at the field.

Binding multiple 2.4 Rx's to the same Tx is easy.

ScaleAero

To repeat the circumstance and configuration which should not be attempted...per Futaba engineering.

Never use a "Y" harness into two receivers (PERIOD) for any common control function regardless of it
being either narrow band or spread spectrum because the control signals will "0" eachother out except
in rare circumstances.

This was being done to provide a receiver back-up...not servo distribution which is more commonly the
issue being addressed. This has absolutely nothing to do with "daisey chaining" schemes for multiple Rx.
Those schemes are engineered into the product and described in owners operating manuals or online in
the manufacturer's and distributor's web sites.

The problem is SS receivers are not "listen only" devices and this configuration is expected to cause the
failure of one or both receivers and per the engineer will negate the function of any Fail Safe set up on
either receiver. Basically it can result in control signals being cancelled out.

The additional reason the individual wanted to use this configuration was to increase the reception % as
the airframe moved from one position to another. However, the pairs of antennas were not placed in a
recommended vertical axis. The two antennas were mounted in the recommended "V" shape, but the
pair of "V" were placed in a horizonal plane 180 degrees out on their widest dim.

It had been assumed this 180 out "V" arrangement provided greater reception...however considering the
shape of the effective reception field this is in fact not the case because either one or the other receiver
is receiving the greater % of signal instead of both receiving the same %.

It is typically best to have the "V" in a vertical plane and its placement in either a "V" up or "V" down
position...which ever be exposed to line of sight during the majority of the flight routine. Typical
flight plans do not present issues for models constructed of traditional materials and do not present
a challenge to reception.

Finally, if you are using more than one receiver to distribute the servo load or to increase the number
of servos in use beyond the number of channels on your transmitter through the use of programmable
servos, etc...

BE ABSOLUTELY SURE YOU SET THE FAIL SAFE POSITION ON THE LEFT AND RIGHT HALF
RECEIVER TO "CENTER" OR "NEUTRAL".

This will enable the receiver not in fail safe to control the other half of the plane without having to
compensate for the last position of servos plugged into other receiver which is in FAIL SAFE.

Hope this post contributes to your flying pleasure and safety by preventing this from happening to you.