GY401 failures people have been discussing (401 hardover)

[ImRich]

I have been reading about people warning about a possible issue with the Futaba GY401 causing the tail rotor to lock on hardover (full right or left tail command).

There have been reports of this on the rotory forum as well as a warning from Ricks to watch out for it (see http://www.ronlund.com/current_draw_and_what_it_does_to.htm ).

Basically what everyone is warning is that digital servo's will put a heavier drain on your battery, you can suck your battery dry a lot faster than one might expect. This is because digital servos can draw a lot of power.

If you have any control binding in your heli, then the current draw will be even larger.

The reason why people think the 401 gyro is going into hardover tail command is that when you put a load on your digital servo, there will be a large current draw, this can cause the voltage of your battery to dip below the drop out or reset voltage of the 401 gyro electronics and cause it to 'reset' itself. At this point it will recalibrate itself while you are in the air. This is not good and it is thought this is what causes the 401 to go into 'hardover' mode.

I started to think about this more and decided to do some experiments. I have two 401 gyros. One is over a year old and the other is just a few months old.

I have two heli's with 401's in them, my Gas X-cell has the older 401 and a Futaba 129DP receiver and my R50V2 has the newest 401 in it and a Futaba 149DP receiver.

One at a time I powered each heli to my adjustable power supply, monitoring the output voltage with a DVM. I started at 5.2 volts and slowly worked my way down until the receiver stopped working.

One thing I can say, WOW! I'm impressed with the 149DP receiver. It kept generating a signal down to 1.67 volts! That is well below the point where the servos stopped working. The older 129DP receiver didn't do as well, it stopped working about 2.76 volts (still below the point where my servos stopped working).

Here is the results of my voltage tests with the G401. This is not indicative of all 401s, but just my sample of the two that I own.

5.02 volts, all ok!
4.80 volts, all ok!
3.87 volts, the GV-1 low battery fail safe circuit kicked in and dropped the throttle servo to idle (or it could have been my PCM receiver, I did not disconnect my GV-1 for these tests).
3.20 volts, the servos stopped operating.
2.30 volts, the receiver was still sending a signal (but no servos responded to it). Note, I detected the receiver sending a signal by watching the LED on the 401. it changes state when in Hold mode as opposed to rate mode. I could still control the hold/rate mode and determined that the receiver (and gyro) was still working.
1.67 volts, the 149DP receiver finally stopped working.

I then brought the voltage back up to 5.2 volts. The 401 never lost it's lock on the 'center' tail signal. It never recalibrated itself. I watched closely for the 401 recalibration, when it occurs, the LED on the 401 blinks rapidly for a few seconds. This never occured, although I did not lower my voltage much below the 1.6 volt level as I wouldn't expect anything to work at this low a voltage.

So I came to the conclusion that if there are different 401 gyros out there (good and bad batches??), I think I have two good ones!

As I stated earlier, I'm very impressed with the newer R149DP receiver. To keep functioning down to almost 1.67 volts is incredible, even if no servo could work at that low a voltage.

Now back to the reports at hand. Do not discount this issue. Be sure you have good battery packs. If you use digital servos, I advise you to use nothing but the newer heavy duty switches and wiring. Try not to use any extension wires. If you do, use heavy duty wires. Also try to get a battery pack with heavy duty wires (not many people make these yet).

Current draw will cause voltage drop. Digital servos draw more current than analog servos, so the voltage drop with digital servos will be much greater than with non-digital servos. Be sure that your servos do not bind at all, even at maximum control throws.

If you are flying a 401 gyro (note, this could happen with any gyro, not just a 401) and your tail suddenly goes into the 'hardover' state, if you can hit your heading hold/rate mode switch and get the gyro into rate mode, you may be able to recover as the rate mode may not need the self calibration procedure to hold your tail.

If anyone else makes voltage measurements on your 401, please post them here so we can compare the differences.

My next test would be to measure all signals and voltages with a scope (which can react much faster than a DVM) and then put my digital servos under a large load to see if the current/noise spikes bother the 401 more than a simple voltage drop does alone.

[rob_jones]

Very informative test. But I think you overlooked one important factor. In your test the heli was sitting on the bench. Even if the gyro does reset, it will still be oriented the same and resetting won't make any difference. I think the problem comes into play when the heli is in motion. Too bad there isn't a safe way to test it in flight. I feel that would make a big difference! :shock:

[ImRich]

Hi Rob,

Thanks for mentioning this. But I kept an eye on the Gyro closely. If it rebooted, it would have gone through it's reboot sequence. Which normally has the LED blinking quickly until it's done. This never occured.

I do realize that this is a static test (no heli movement, no digital servos drawing current and making electrical noise), but I did not see the CPU in the Gyro drop out and restart its initialization sequence.

So in my test, the heli did not move, but the gyro did not enter the reset sequence either.

I'm not saying my tests are anything more than what they are. That is that my particular G401s seem to be working and holding their setting down to an acceptable voltage level.

Of course, I watch my battery voltage with an on board voltage monitor and check it before and after each flight. I encourage everyone to do this. The hobbico monitor works quite well and is very light.

[ImRich]

Here's an unconfirmed/unofficial update on the GY401 issue.

I had a conversation with a Futaba Rep on this. He confirmed that the issue was not a voltage issue at all as is being reported on the internet (like right here). This confirmed my test findings! However the details warning of digital servos drawing a lot of current and draining batteries faster than analog servos is correct, so always check your batteries before and after each flight, and you'll get better results if you use heavy guage wire from your battery pack, through your switch and to the servos also.


Futaba USA seems to know what the issue is but is not releasing the details until they fully work it out with Futaba Japan. He conformed that it's not a voltage issue as being reported on the internet. He also confirmed that not all GY401s have this issue. He says if you get past several flights on your 401 then you will probably never see the issue.

To me this sounds like an 'infant mortality' issue. He sort of confirmed that but couldn't report more to me until the meetings with Futaba Japan were completed.

Hopefully Futaba will solve this issue soon and then report on how we can screen our 401s for this issue safely.

So if your 401 is past a dozen flights, then you are probably out of the danger zone.

If you have a new one which you haven't flown yet and are concerned about how to test your GY401 to see if it may be affected by this issue, I'd reccomend that you turn on your receiver and simulate a dozen flights on the ground without the engine running. That means to operate your servos and have the gyro operating (as well as the other servos) for about 10 to 12 minutes at a time. Be sure to yaw the heli from time to time to that the gyro is making some corrections. Then stop, shut off the receiver wait 20 minutes and then repeat this procedure. Charge your battery as needed. Do this until you have about a dozen 'simulated' flights. Then do a range test and if all is ok, start your engine and then perform a careful test flight, if you can be prepared to switch out of heading hold mode. This 'break in' procedure may help you to get past the 'infant mortality' period while safely on the ground. Again, this is my personal opinion, Futaba did not mention this procedure at all.