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Old Jun 20, 2007, 11:50 AM
3 Sons - Legos and Lift
2motheus's Avatar
Grand Rapids, MI
Joined Apr 2004
565 Posts
Discussion
Berg 4L diagnostics help

Let me say right off the bat that I'm not blaming the receiver for the control failure I experienced over the weekend. I'm just asking for help in narrowing down the problem.

Setup:
2m R/E sailplane (Fling)
Berg 4L, antenna taped to outside of tail boom.
700 mah NiMH pack
(2) Hitec Hs-101 "mini" servos
Hitec Eclipse 7 with frequency module
No on/off switch; battery plugs right into a small "extension cord" that goes to the received. This connector has always been pretty stiff.
Battery is in the nose with the receiver on top. Antenna runs past the servos and then out along the boom.

Weather:
85+ degrees, full sun, humid

Although the electronics and plane are second-hand, I have had them about a month and have had no problems. On the day of the crash I had at least 4 successful flights prior to the failure.

Preflight:
While checking control function, the elevator servo turned about 45 degrees clockwise and locked in place. I think this corresponds to "up" elevator. No further control of either servo was possible, even after cycling power on the transmitter. Suspecting loss of power to the receiver, I unplugged the battery and plugged it back in. Everything returned to normal. I did not perform a range check prior to this flight.

Flight symptoms:
Launch seemed normal at first and I was able to steer the plane to a normal height hi-start launch. But the glider began to fly fast and oversteer near the top, and once off the line I realized I had no control at all. I estimate the plane was up at leat 350 feet. I had time to cycle power on the transmitter to no avail. The plane seemed to have the elevator locked in a "down" position, corresponding to a counter-clockwise servo rotation, I think. I can't be sure about the servo position, but it tucked all the way under to an inverted stall a couple times before gravity declared victory.

Post-mortem:
Battery voltage is fine, better than 5 volts with a load.
After disconnecting and reconnecting the battery, both servos still work normally.
Suspecting the connector between the battery and receiver, I tried to recreate the lock-up situation I saw during preflight by "barely plugging in" the battery and wiggling the conductors at the connection point. The closest I can do is get a slight twitch of the servos when the receiver loses power. It is nothing close to the 45 degrees I saw prior to the flight.

Any advice is appreciated. I plan on replacing the battery-to-RX line and possibly include a switch, but I still am not sure this is the problem. I have some questions.

Can high temperatures affect the Berg 4L? It was only 85 degrees, the fuse is white and the canopy is painted grey. The batteries weren't working hard, unless there was an unknown high load like a servo stalling.

If loss of power is the problem, why would a servo rotate and then lock... and why just one servo?

If a servo is the problem, why did the one locking up during the preflight prevent me from controlling the other?

Any advice besides replacing the power lines from the battery to the receiver?

Thanks,
Tim
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Old Jun 23, 2007, 10:17 PM
3 Sons - Legos and Lift
2motheus's Avatar
Grand Rapids, MI
Joined Apr 2004
565 Posts
What else?

No replies?!

I guess everyone agrees with my plan to replace the battery cable.

Someone from Castle - is there anything else I can do to test components or try to recreate the problem?

Thanks for any advice.

Tim
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Old Jun 23, 2007, 10:42 PM
Gravity is a harsh mistress.
Tim Wolff's Avatar
United States, MI, Temperance
Joined Sep 2002
6,584 Posts
If I had a sailplane that failed the preflight as you describe, I'd make darn sure I had found and corrected the problem before flying.

Why are you power cycling the TX? You did so twice in your description of the events.

If you have means to measure the current draw of the servos, check the current draw of the elevator servo, or just replace it. A bad servo can draw too much current and create back EMF interference that can affect other servos and/or the RX.

Low and intermittent voltage in the TX or flight pack can cause all sorts of bizarre behavior. Is there somthing about the battery to RX wiring that makes it suspect (asking because I can't beleive some of the stuff I've seen guys fly).
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Old Jun 25, 2007, 09:04 AM
3 Sons - Legos and Lift
2motheus's Avatar
Grand Rapids, MI
Joined Apr 2004
565 Posts
Tim,

Thanks for the reply. I agree that flying after a preflight problem is not wise. The reason I did is in the answer to your other question about suspecting the power connection to the receiver. Instead of an on/off switch, the plane when I bought it had a simple connector in the line between the battery and receiver. You unplug the connector to turn off the receiver and to gain access to the battery line for charging. This connector has always been a little stiff, even for plugging the battery into the charger, but once it's fully seated everything has worked fine. I though my preflight problem was due to not having the connector fully plugged in. I could not recreate the problem after re-seating the plug.

I cycled power on the transmitter just to "reset" the communications to the receiver. I know this is a long shot, but it's all I could think of when I lost control.

Thanks for the suggestion to test the servo current. I can do that. If the current seems normal, is there anything I can look for inside the servo to verify it might be the culprit - such as evidence of a poor connection on the wiper?

Thanks again,
Tim
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Old Jun 26, 2007, 08:11 AM
Gravity is a harsh mistress.
Tim Wolff's Avatar
United States, MI, Temperance
Joined Sep 2002
6,584 Posts
A dirty pot is enough to make a servo go berzerk. But, they are usually consistent once they go. I suspect that a low voltage condition that would cause one servo to lock over would do the same to the other. You just can't be sure about that though. If that servo has/is going south, you should be able to get it to fail on the bench. Be sure try applying a bit of a load (resistence) on the output arm when you test the servo. Some that look great with no load go bats#*% crazy with a pushrod connected.

Those connectors are not really designed to be plugged/unplugged all the time. A switch is a good idea and I'd replace both the connectors involved too.
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