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Solved ESC/BEC or Rx Issue?

Matagami Designs

Well-known member
#1
So I ran into an issue with my Aircruiser and not sure if I have a ESC/BEC issue or an issue with the new Rx. I recently got Frsky Taranis Qx7s and a RX8R pro reciever. While at the field I was getting things set up and had the system reboot twice. I am running with 5 channels. Motor, 2 aileron, rudder, elevator. I am using the flite test c pack radial and 35 A esc. Apparently the esc has a 5v/3a BEC so how do I know if I have a BEC/ESC issue or a bad Rx. I guess I could set it up with the rx6r which was working on the simple scout. Is there a way to see the draw of the motor and servos? Hopefully I can recreate the issue to help understand. Any suggestions? Thanks.
 

Merv

Well-known member
#2
Not good when an Rx reboots. Four 9g servos on a 3 amp BEC is the limit, some would consider it over the limit. You could try unhooking one of the servos to see it made a difference. You could have a bad connection some where, try wiggling the wires from the BEC & Rx to see if you can make it fail.

Are any of you servos binding? They make a grinding noise when they want to move farther but can't.
 

BATTLEAXE

Well-known member
#3
. Is there a way to see the draw of the motor and servos?
The servo channels are on a separate circuit from the throttle as far as your ESC goes, so throttle should not read as an amp draw taking away from your control surfaces. If it says 5v/3amp on your ESC then all your servos have access to all 3 amps. Four 9g servos on 3 amps Would be the max like @Merv said. There are 5v/5amp ESC's you can order up for that extra level of protection. My ESC is a 5amp BEC and I have had up to 6 servos running off it, four 9g and two 5g
 

Matagami Designs

Well-known member
#4
So I unhooked the right ailerons and it would appear the problem is gone. Plug it back in and reboots when given a bunch of crazy stick action. Seems odd as I didn't have it rebooting while running on the spektrum Rx? Also the scout works fine with x4 9 gram servos on a 2a 5v BEC? I will say that the hinges on the tail are definitely not as freely moving as the scout though not impossible to move. @BATTLEAXE what esc do you run on your rimfire 0.15 motor. Is this the one with 5A/5V BEC? Thanks
 

quorneng

Well-known member
#5
Matagami Designs
If you have four spare 9g servos you could always simply plug them on the rx but not linked to anything. If they operate with no rx reset then you know it is the linkage/hinges in the plane that is causing the higher current draw.
Different rx have different sensitivities to a low voltage condition and is probably why the Spectrum worked ok but obviously your setup is operating close to the BEC's current limit. Not a good situation.
 

BATTLEAXE

Well-known member
#6
So I unhooked the right ailerons and it would appear the problem is gone. Plug it back in and reboots when given a bunch of crazy stick action. Seems odd as I didn't have it rebooting while running on the spektrum Rx? Also the scout works fine with x4 9 gram servos on a 2a 5v BEC? I will say that the hinges on the tail are definitely not as freely moving as the scout though not impossible to move. @BATTLEAXE what esc do you run on your rimfire 0.15 motor. Is this the one with 5A/5V BEC? Thanks
I run the Eflite 40amp 5v/5amp 2-6s Pro Lite
 

Matagami Designs

Well-known member
#7
Matagami Designs
If you have four spare 9g servos you could always simply plug them on the rx but not linked to anything. If they operate with no rx reset then you know it is the linkage/hinges in the plane that is causing the higher current draw.
Different rx have different sensitivities to a low voltage condition and is probably why the Spectrum worked ok but obviously your setup is operating close to the BEC's current limit. Not a good situation.
If I had four spare servos i would be building a new plane around them. 😂 I do believe it is the hinges. I hooked up the servo tester and noticed the elevator has a zone where any increase in ppm only causes more noise and no movment near full deflection about the last 5%. In contrast the ailerons are smooth and no noise and full deflection on both sides. I tried cleaning up the hinges but not much improvement. I may need a more robust BEC to push me back above the limit.
 

Bricks

Well-known member
#9
Make darn sure all control surfaces are free moving for the entire range of movement if not work the surfaces back and forth until they move freely, or find out why they are not. Next check that your servos are not stalling, give full stick movement and listen to your servos are they buzzing if so they are stalling either from mechanical set up wrong or surface movement is still restricted. Some digital s may have a soft buzz no matter what you do.

None of my smaller planes running as many as 6 9 gram digital servos have ever for me caused a brownout on a 3 amp BEC, and that is flying 3D.

I do believe some ESC`s are not what they are suppose to be, especially the cheaper ones but I have not noticed that but could be just plain luck.
 

quorneng

Well-known member
#10
My one and only true foam RTF (a 2 m powered glider) had moulded hinges with 9 g servos in conjunction with rather poor linkage. As a result the servos made serious buzzing noises and did not even reach full travel. I never measured the amps they needed.
To resolve the situation I simply cut away the moulded hinge and reshaped the control surface to allow a simple top tape hinge.
HingeTop.JPG
HingeUnder.JPG
With short direct link rods the surfaces now not only move freely but they also manage quite happily with tiny 3.7 g servos.:eek:
ServoLink.JPG
Incidentally this type of servo has a stall current of just 0.6 A so even with all the servos stuck the total current would not overload the BEC.
Maybe a bit of an extreme modification but the plane is still going after 5 years! ;)
 
#11
So I went an orderd a new esc with a 5v/5a BEC should be here monday but I'm having this same issue with other planes. Is there a way to see the power drawn by just the servos? If I put an in-line watt meter like this one
https://store.flitetest.com/racetek-power-analyzer-watt-meter-100a-60v-rtk6007/p776158
will I be able to see the draw? I'm guessing this wont work since that's seeing battery voltage? Is there something similar that can go in line with the RX and ESC? Mulitmeter?
 

quorneng

Well-known member
#12
The power analyser will measure the peak current of the servos and rx - provided you don't touch the throttle! ;)
If you use an servo plug in any spare channel and connect the red and black wires to a multimeter it will show the voltage that the rx is seeing when all the servos are 'exercised'.
 
#13
Not good when an Rx reboots. Four 9g servos on a 3 amp BEC is the limit, some would consider it over the limit.
There are two limits: the instantaneous current draw, and the linear regulator heat-rejection limit. The latter depends heavily on the battery voltage; with a 2S battery the BEC dissipates ~1/3 the servo's power; with a 6s battery it dissipates ~3x the servo's power. On a 6S battery even running just a receiver on a linear BEC dissipates ~1W of heat.

tl;dr do not use linear bec on >2S. use this instead:

https://www.amazon.com/dp/B01MQGMOKI/?tag=lstir-20
 
#14
So the new Hobbywing 50A Skywakler ESC with 5V/5A UBEC is working for the previously problematic models. :) The only downside is the size, about twice as big and an additional 30g. Plus i had to do some soldering :rolleyes:. For the size of these models the extra weight shouldn't be an issue. I think i'll order one of those capacitors @Merv is talking about since it appears to only have issues when i give all the servos full power at once otherwise it seems if i take them up slowly they may be OK. I'm also curious if anyone knows if the BEC on the FT ESCs is a linear BEC or switching BEC? Thanks for everyone's help.
 

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