Gasser dropout, HELP!!!

[Hank]

I have had this plane with the same setup for over 2 years now. I flew it once last week with no issues. Then on the second flight as I was about to take off, the radio started dropping out. The dropout only happens when I move the elevator(s).
Here's the setup:
1. 52cc gas engine
2. PowerHD LF-20 digital servos on all except throttle and flaps, those are generic analog.
3. Frsky RX8R Pro and backup XM+ backup receivers.
3. 5-cell Nimh for radio and 4-cell Nimh for ignition.
4. Frsky QX7 transmitter running old ACCST (not updated to 2.1 yet).
5. Dual elevator servos, channels 2 and 6.

What I tested so far:
1. Checked with another RX8R Pro receiver with same results.
2. Tested with XM+ connected and disconnected, no difference.
3. When I disable one of the elevator servos whether by disconnecting the servo from the receiver or by programming, the problem goes away.
4. I swapped out the servo extension wires (servos mounted in the rear) with no diffence.
5. I tested the same transmitter on another plane with no issues.
6, I replaced the receiver and ignition switches with no changes.

I have a noise suppression capacitor that will try next to see if that helps When the dropout happens, some of the servos go hardover, the engine quits (because of the opto-kill switch), and then the control returns. The receiver LED's completely blank out for an instant.

Any ideas what to check? I can't believe that the elevator servos would go bad in just a few minutes of sitting unused on the ground. This plane has never had a bad landing or crash.

Thanks for any inputs.

 

[Konrad]

Sounds like a classic Vss issue. That is the batteries are far too weak. Check your charger's output and each cell. Also check the amp draw. It isn't unusual for a servo to fail early in its life when subjected to IC vibrations, brushes, winding and armature often break distort, move or bind.

But 2 year old cells, really check that the battery can withstand the current load. 6V does put more strain on the battery cells from the added current draw.

 

[Hank]

I think you are right Konrad, I swapped out the servos, receiver, battery, even the receiver switch. Nothing fixed it. I noticed that when the battery is fully charged it seems to fault out less frequently. I also realize that the servos being high voltage variants, high torque, and digital, the battery output capacity has to be higher. I ordered some large LiFe batteries that I'm hoping will solve this issue.
But for 2 years I did not have this happen at all, I am surprised that it started happening suddenly. It went from a perfect flight to a complete malfunction in a few minutes. I'm lucky it happened while the plane was still taxiing.

 

[Konrad]

Battery chemistry is fickle that way.

I try to load test my batteries, all chemistries but NiMh in particular.

 

[Hank]

I finally got the dropout issue resolved. I flew the plane several times this Saturday without any issues.
The culprit was the the battery instantaneous current draw capacity as Konrad mentioned. I replaced the old battery with another one of the same rating but that didn't help. So I put a LiFe 4000 mAh battery with 25C rating and that fixed it.
In the meantime, I updated my radio to the latest OpenTX and ACSST, cleaned up the layout of wiring and components internally, and adjusted the CofG to my liking. Happy ending it seems.?

 

[Konrad]

Thank you for getting back to us with your findings. This feedback really helps us help others in the future.

 

[Hank]

This issue was a new experience for me with my gassers and glow planes. It happened so suddenly I didn't know what to suspect. At first it seemed like bad servos or even a bad battery. But the battery was good for most applications except for this one using several high torque digital servos.

 

[Fireflite88]

Glad you got your issues resolved, thank you for giving us the resolution. Can I ask where you got your new batteries?

 

[Hank]

Value Hobby for the LiFe batteries.
For Nimh batteries I get them from Aloft.
But it seems that the servos I am using actually need the 2 cell LiFe batteries.

 

[ululi1970]

The PowerHD servos I have all draw a lot of amps. niCad do not age well

 

[Hank]

The PowerHD servos I have all draw a lot of amps. niCad do not age well

It all has to do with impedance matching. The input impedance of the large servos is low, thus the large current draw. The output impedance of the battery has to be low enough (low output resistance or high discharge rate, or high C rating) to match the input impedance of the servos.
The ideal impedance matching (purely theoretical) is the source, the battery, to have zero output impedance and the load (servos) to have infinite input impedance.

 

[Alois]

Hello
This is a well known problem of the RX8R pro.
My German dealer and Frsky confirmed that all RX8R pro do not run stable with a Voltage of 6V.
There are also videos on youtube concerning that problem.
See details in the link below.
I recon that because of the noise reduction for gas engines there is to much internal resistor in this receiver.
(In my case the current of the signal lines was to high if I used more than 3 older analog servos)

I did some measurements with RX8R pro, RX8R, and a Jeti receiver.
The RX8R pro was the only one, where the telemetry value rxbat dropped to 2.9 V.
All other receiver showed a constant rxbat value of 5.0 to 5.2 volt.
Also the measurement of the voltage outside the receiver was always stable (tested with 4 cells NIMH and two different BECs)

For a RX8R pro you will always need a 2S lipo.
It may also help to use a powerbox or sbus instead of PWM (because of the lower current on the signal lines)

Best is to check the rxbat telemetry value with all servos in motion

see the link below

FrSKY Taranis X9d Plus and X8R Telemetry Signal Lost Problem

Sounds like an antenna nule problem. Does this happen as you are coring a thermal? Or does this happen while flying in a straight line? Are the antennas 90° to each other. While the fuse maybe glass is there any conductive material near the antennas? The antennas come out on the sides of the...

forum.alofthobbies.com

 

[Fireflite88]

Hello and thanks for your post. I currently run (2) 6.6v 2s LifePo batteries in my gasser and I haven't had any issues. It's been just over a year. I am concerned with your finding and I may just switch out to lipos. What chemistry battery did you find these results with?

 

[Alois]

Hello and thanks for your post. I currently run (2) 6.6v 2s LifePo batteries in my gasser and I haven't had any issues. It's been just over a year. I am concerned with your finding and I may just switch out to lipos. What chemistry battery did you find these results with?

Hello
I originally used a 4 cell NMH (2200mAh). Then I tried two different BECs (5A) with 5V and 6V.
EngelMt (the German dealer) used a 6V laboratory power supply.
But keep in mind that the effect depends on what currents your servos draw on the signal lines. (not on the red plus wire).
Do a check on the ground: move your sticks so that all servos move at the same time and watch the telemetry value rxbat.
I can not recommend a specific value (because in the air with load on the servo your 6.6V may also drop to a lower value)
All I can say is that in my configuration (five rather old fashioned analog servos) the internal drop of voltage was from about 5.2V to 2.9V
which made the receiver reset.
I do know that in other cases the receiver seems to work with 5V.
reason may be: A friend of mine does use of a powerbox (seems that there is less current draw on the signal lines of the receiver)
or: I recon use of sbus instead of PWM (which means only one signal line. (Signals are serial and not parallel all channels at the same time)
or - and this is only a guess: digital servos may require less current on the signal line (but they do need more current
on the normal powerline)
So do the test and look whether your rxbat shows a significant drop in voltage.
To be sure you could do a sdcard-logfile during your flight and have a look at rxbat in companion.
Or at least look at rxbat- (minimum rxbat) in the telemetry values.
Regards Alois

 

[HENNY]

I did some measurements with RX8R pro, RX8R, and a Jeti receiver.
The RX8R pro was the only one, where the telemetry value rxbat dropped to 2.9 V.
All other receiver showed a constant rxbat value of 5.0 to 5.2 volt.

I don't dought what you are saying but there is some missing information here. (Maybe) I am using the RX8R and have not seen what you describe. As far as the receiver is concerned the 5v or 6v feeding the receiver at the PWM pins is a parallel line also feeding 5 volts to any servos currently connected to the PWM outputs. The receiver measures this voltage and if it falls below 3 volts the receiver most likely needs to rebind before continuing to operate. To me this is a sign that the receiver voltage is actually the culprit. Just a question how are you feeding the receiver voltage, via a spare PWM output connection. Its possible your servos are pulling the receiver voltage down, perhaps the battery or becs needs to be carefully looked at as the real issue.

 

[Hank]

In my case, I was running 3 analog and 5 digital servos. I wasn't using a BEC, but a 5 cell Nimh battery. So, if the current draw was high enough, the battery voltage drops because of the battery internal resistance (impedance).
So ultimately, to avoid the voltage from dropping too low and subsequently causing the RX from dropping out, either the battery voltage has to be high (lipo) or use a battery with low internal resistance (impedance). A battery with low internal resistance would have a high C rating.
Since I avoid lipos like the plague , I opted for a LiFe battery with a high C rating (25C). Even if you start with a high voltage battery but with high internal impedance, the voltage drop could still be enough to cause an RX dropout.
There might be issues with the RX8R Pro needing higher operating voltage due to the extra filtering. But I think this issue surfaces more frequently with the RX8R PRO simply because it is used on big gassers with high current draw servos.
Incidentally, testing for battery dropouts would require a relatively high bandwidth instrument like an oscilloscope.

 

[HENNY]

Yes I agree, but if the battery voltage is going below 3 volts this is your primary issue you have, not the RX8R. An easy way to check this is to stall several servos while measuring the battery voltage at the receiver and see if this is whats happening. A battery does not have impedance, just internal resistance. Another way to power the receiver is to run a 3S lipo and 10 amp Bec with the number of servos you are using. No sure what the LiFe battery voltage is you are proposing but remember what ever your LiFe battery voltage is this will be passed onto the servos which may damage them.

 

[Alois]

I don't dought what you are saying but there is some missing information here. (Maybe) I am using the RX8R and have not seen what you describe. As far as the receiver is concerned the 5v or 6v feeding the receiver at the PWM pins is a parallel line also feeding 5 volts to any servos currently connected to the PWM outputs. The receiver measures this voltage and if it falls below 3 volts the receiver most likely needs to rebind before continuing to operate. To me this is a sign that the receiver voltage is actually the culprit. Just a question how are you feeding the receiver voltage, via a spare PWM output connection. Its possible your servos are pulling the receiver voltage down, perhaps the battery or becs needs to be carefully looked at as the real issue.

Hello
I have no experience with the RX8R.
But EngelMT assured that the problem is only with the RX8R pro because of the increased noise reduction.
I have tested with my plane (with 4cell NIMH, BEC 5V and 6V) on the ground.
Test included RX8R pro, RX6R, Jeti R9 (which was in this plane for some years)
I've tested two different types of connecting the Servos. First they were connected standard via the connectors on the receiver.
After that I connected them (plus and GND) via extra Pin-line (not via Receiver) also with additional capacitor and only the signal lines were connected to the receiver (and plus and GND to supply the receiver of course)
I also measured the drop of voltage outside the receiver (due to servo movements).

In all combination the receivers reacted the same way with nearly identical voltage measurements.
________________rxbat_________________________voltage measured at (plus and GND) pins of receiver
___Jeti R9:_____4.9 to 5.1 Volt_________________4.9 to 5.1 Volt
___RX6R : _____4.7 to 5.1 Volt_________________4.8 to 5.1 Volt
__RX8R pro:__2.9 to 5.1 Volt_________________4.9 to 5.1 Volt

EngelMT confirmed my results not only for the returned receiver but for their complete stock at that time.
They wrote for a save use of the RX8R pro the Voltage must not drop below 6V (and advised to use 2S lipo or at least a very strong 5cell NiMH).
I decided to go for 2S Lipo because 5cell NiMH and 2S Lion may come close to the 6V in flight condition.

I can confirm that the Archer R8 pro I got as replacement works find under all these tested condition and has no drop in the rxbat value.
I even tested the Archer R8 pro (on the ground) with a very lousy/scrap old 4 cell NiMH without any problems.
But I have to admit that from that point on I do use 2S lipo to directly supply the receiver (while my old servos are on 5V BEC now)
I'm sure that this layout would also have worked for the RX8R pro but I could not test anymore because it was replaced at that time.

The installation is different from plane to plane (concerning Voltage supply, wiring, servos and also load on servos during flight).
Therefore I strongly recommend to have a close look at the telemetry value Rxbat which is measured receiver internally and does clearly show if you might run into that problem on the RX8R pro.

I admit that all our experience gathered over the years says that this kind of problems is based on bad voltage supply/BEC or old accu.
And normally we are right.

Regards Alois

 

[HENNY]

OK thank you for the reply

 

[FlyBoyDVIII]

Late to the party I know. I personally have LiFe packs that have been in service for 10+ years and many cycles with no reduction in capacity. LiFe also have a 80% to 100% DOD with no ill effects to battery life. LiFe batteries are the safest chemistry available today. I also personally preffer the cylindrical cells over unprotected/non hardcase Prismatic. On another note my wife and I only have 512, A123 cells in 8 packs of 4S16P hooked parallel, for a modest 238AH of 12v battery to power our camper during boondocking at flying fields. A123 cells are rated for 2000+ cycles without degrading. I used used medical cart packs to make my Battery

For Henny enjoyment:

LiFe (lithium iron phosphate) is a lithium-based battery technology that is more stable and safer than LiPO. A123 batteries are based on LiFe technology. LiFe packs can be charged very rapidly and be discharged all the way without damage. Their lower voltage make them suitable for receivers that use 6V receiver batteries. On the downside, LiFe packs have lower voltage and smaller capacity than LiPOs of the same weight. LiFe packs have 3.6V per cell when fully charged.

• Lighter and smaller than NiCad and NiMH battery of the same capacity
• Lower voltage than a LiPO, can be used with 6V receivers without a voltage regulator.
• Includes two universal RX connectors, compatible with both Futaba and JR RX.
• More cycle life than LiPO, up to 1000 charge cycles.
• More stable and durable than a LiPO. LiFe rarely catch fire and would not be damaged in over-discharge.