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Crash Autopsy

Old 08-19-2011, 02:42 AM
  #1  
MJtheSOLID
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Angry Crash Autopsy

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I have a BlitzRC Sky Surfer that has been a dream to learn how to fly on. I had a couple months of experience on the smaller Champ flyer, and everything went extremely well (after a few CG problems were worked out) for 20 or so flights. Then, our last flight had a catastrophic equipment failure.

During a slow turn, the DX6i transmitter lost control over the airplane, and it turned into a dive and broke the nose off. After the plane was inspected, we found that the 3s 1300mAh lipo battery had only 10.6V. The 6200 receiver was blinking, indicating that the system had lost power during the flight. In addition, I later found out that the default Throttle setting was incorrect, and when the receiver loses connection, the motor turns at about 25% throttle.

The ESC is supposed to have a voltage protection feature, that shuts down the power to the motor at 3.6V and is supposed to protect the receiver from a shutdown due to power loss. All of the connections between the motor/batter/esc/receiver were still intact after the crash.

Can anybody explain to me why the receiver might have lost power and had to reset with the transmitter? I believed that after the battery was drained to such a low voltage, that it could not deliver the needed amperage to the ESCs internal BEC and it cut out momentarily. However, I am still pretty new to this, and would appreciate any help a more experienced RC mechanic might be able to offer.
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Old 08-19-2011, 03:04 AM
  #2  
kyleservicetech
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Originally Posted by MJtheSOLID View Post
https://skydrive.live.com/redir.aspx...jn8Uj8yJ6s8%24

I have a BlitzRC Sky Surfer that has been a dream to learn how to fly on. I had a couple months of experience on the smaller Champ flyer, and everything went extremely well (after a few CG problems were worked out) for 20 or so flights. Then, our last flight had a catastrophic equipment failure.

During a slow turn, the DX6i transmitter lost control over the airplane, and it turned into a dive and broke the nose off. After the plane was inspected, we found that the 3s 1300mAh lipo battery had only 10.6V. The 6200 receiver was blinking, indicating that the system had lost power during the flight. In addition, I later found out that the default Throttle setting was incorrect, and when the receiver loses connection, the motor turns at about 25% throttle.


Take a look at the attached view of what size heat sink would be required for optimum performance. The penny gives an idea of size. Years ago I did build up several regulators using the LM7805 five volt regulator units. Radio Shack has them. They worked very well on models with an 8 cell Nicad battery, and 4 servos. But the CC 10 Amp uBEC is far lighter, far smaller, and has 5 times the current capacity.
The ESC is supposed to have a voltage protection feature, that shuts down the power to the motor at 3.6V and is supposed to protect the receiver from a shutdown due to power loss. All of the connections between the motor/batter/esc/receiver were still intact after the crash.

Can anybody explain to me why the receiver might have lost power and had to reset with the transmitter? I believed that after the battery was drained to such a low voltage, that it could not deliver the needed amperage to the ESCs internal BEC and it cut out momentarily. However, I am still pretty new to this, and would appreciate any help a more experienced RC mechanic might be able to offer.

I've got the uneasy feeling you got hit by the dreaded "brownout" problem. So what is that???

If you're using the normal ESC with the built in BEC (Battery Elimination Circuit), those BEC's typically use a standard "Linear" voltage regulator to operate your receiver and servos.

Normal design on these linear voltage regulators requires a very large heat sink on them to keep them cool. That heat sink would have to be several times larger than the whole ESC itself. And those ESC manufacturers don't use heat sinks.

So what happened Those linear voltage regulators get hot. The more you use the servos, or the more cells you have on your motor battery, the hotter they will get. When they hit perhaps 200F or so, they turn off to protect themselves. When you get to the scene of the crash, the regulator has cooled off, the regulator and connected receiver/servos has started working again.

I've seen this happen at our club field twice this year so far.

So what's the solution? IMHO, any model with more than a 2 cell Lipo battery, and/or more than 4 servos should use either a switching power supply uBEC, or a separate battery for the receiver.

As for me, a separate battery is a pain in the ***, so all of my models are equipped with a Castle Creations 10 Ampere uBEC, available for about $25 or less. These units simply are wired into the INPUT of your BEC, and their output connects directly to your receivers battery input. The uBEC does require that the built in BEC of your ESC be disconnected, so they don't fight each other.

These uBEC's weigh perhaps 1/2 ounce or so. I've got them on all of my models, and have been absolutely problem free.

Some of the newer ESC's such as the Castle Creations ICE series of ESC's are using built in switching power supply BEC's.

These switching power supplies are not new. Your computer likely has several of them, your cellphone charger has one, TV's, your chargers for your battery operated tools, even the electronics of your washing machine might have one for power. They aer cheaper to make than the old transformer type units, and are usually more reliable.

Years ago I used the common LM7805 voltage regulator along with a heat sink on several of my models with an 8 cell Nicad pack, and four servos. Did a lot of flying with them with zero problems. But the new switching BEC's are 1/2 the weight, 1/4 the size, and have 5 times more current handling capability.
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Old 08-19-2011, 04:07 AM
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mclarkson
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I dunno. 10.6V is only 3.5V per cell which is your LVC. It could be a simple case of no juice in the batteries, especially if the ESC didn't shut down the motor.
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Old 08-19-2011, 04:53 AM
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MJtheSOLID
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Thanks for your help so far guys, but it gets worse. I just received an email from the tech guys, and they state that because the plane only runs on a 3 cell battery, the ESC does not have a BEC. Now I'm slightly confused. Does this imply that the servos and receiver are getting the full battery voltage? What does this mean for the overall diagnostic?
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Old 08-19-2011, 05:07 AM
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If you remove your battery cover, what is the product ID on the label?

Are you using a DXMx transmitter and a DSM2 receiver?

Dave
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Old 08-19-2011, 05:33 AM
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the product id is a HA009X

The transmitter is a Dx6i and the receiver is the matching AR6200 Rx
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Old 08-19-2011, 05:57 AM
  #7  
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If your transmitter is a DSMx (it will have the DSMx logo on the front, also check your manual). If it is DSMx and not DSM2 then that date code is being recalled. Here is the recall notice with date codes:

http://www.horizonhobby.com/Products...0#quickSupport

My transmitter was DSMx and had that date code and I had an unexplained crash. I was in a turn and lost all control and I was on final so it stalled in the turn and I could not recover.

If it is not a DSMx transmitter then I am not sure what could have happened.
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Old 08-19-2011, 06:08 AM
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kyleservicetech
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Originally Posted by MJtheSOLID View Post
Thanks for your help so far guys, but it gets worse. I just received an email from the tech guys, and they state that because the plane only runs on a 3 cell battery, the ESC does not have a BEC. Now I'm slightly confused. Does this imply that the servos and receiver are getting the full battery voltage? What does this mean for the overall diagnostic?
???
That makes absolutely zero sense. Unless they think you're running your model airplane on a 3 cell Nicad pack, not a LiPo pack. You've indicated yourself that your battery was putting out over 10 VDC. Putting that directly into a servo would wipe out its electronics.

If your ESC does not have a BEC, either you have to have a separate BEC, or you'd have to be running your receiver/servos on a separate battery.

If not, me thinks someone is handing you a little line of BS.
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Old 08-19-2011, 06:32 AM
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I tend to agree with you kyle. Im an electrical engineer, and i have trouble believing that the receiver and servos would run off of the full battery voltage. It is a DSM2 tx/rx setup, so i dont think it was radio interference. I guess overall what im trying to find out is should i replace the esc with an aftermarket before i put my airplane in the air again. I dont like crashing!
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Old 08-19-2011, 06:42 AM
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Mr ,

For ESC's that gets powered up by a 2 cell or 3 cell battery does not have a BEC. Only ESCs using a 4 cell battery like a 14.8V Li-Po battery would have a BEC. The Sky Surfer brushless version uses an 11.1V Li-Po so it doesn't have a BEC.


Thank you,
Abby
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Old 08-19-2011, 06:51 AM
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Originally Posted by mclarkson View Post
I dunno. 10.6V is only 3.5V per cell which is your LVC. It could be a simple case of no juice in the batteries, especially if the ESC didn't shut down the motor.
That would be highly unlikely.. the BEC only outputs 5V and only requires slightly over 5V input to operate. If the LiPo is measuring 10.5V then the BEC had more than enough input voltage to work properly. Remember that standard ESCs are designed to work with 2 cell battery packs that would have much lower voltage than 10.5v, even when fully charged.

The default LVC voltage in a ESC usually set at about 3.2v per cell, so that's 9.6 volts for a 3 cell pack. That would of course be under load but it's perfectly possible that if you were not working the battery too hard you would not have hit the 3.2V per cell LVC with a 3 cell battery that has a resting voltage of 10.5V...

Bottom line is 10.5V is low but in itself would not have caused the radio to fail. I'm sure Kyle hit the nail on the head with his comment about the BEC shutting down.

It seems that most medium and larger ESC's you can buy these days have switching BECs, even the cheap Hobbyking type items... It's really only the smaller ESC's that still commonly have linear BECs.
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Old 08-19-2011, 07:01 AM
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Originally Posted by MJtheSOLID View Post
Mr ,

For ESC's that gets powered up by a 2 cell or 3 cell battery does not have a BEC. Only ESCs using a 4 cell battery like a 14.8V Li-Po battery would have a BEC. The Sky Surfer brushless version uses an 11.1V Li-Po so it doesn't have a BEC.


Thank you,
Abby
Abby,

Sorry but that is simply wrong. Virtually every ESC that's designed to run with 2 or 3 cell batteries has a built in BEC. If it didn't then you would either need a separate external BEC to power the RC gear or a separate 5v battery. If, on anything other than single cell lipo systems, the RC gear takes it's power supply from the ESC then the ESC has a built in BEC in order to reduce battery voltage down to around the 5v required by the radio gear.

Here's an example.. the cheapest ESC Hobbyking sell, designed for 2-3cell lipos.. and yes it has a BEC :http://www.hobbyking.com/hobbyking/s...k_10A_ESC.html

It's usually larger ESC's that are designed to operate on 6 and more cells that don't have BECs

Last edited by JetPlaneFlyer; 08-19-2011 at 12:29 PM.
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Old 08-19-2011, 07:17 AM
  #13  
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Thanks Jet, i think that pretty much clears it up. I'll just replace the ESC and consider it a lesson learned.
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