Recently I have been having an intermittent issue that I cannot figure out.

This has happened twice now and each time it results in a severe crash.

My heli is suddenly taking a left turn in flight and pyro's to the ground. First time it happened it was immediately after doing a full throttle/pitch punch out to about 50/75ft. It didn't start until I leveled of and was very sudden (no prior indication). It was so fast that after the fact it was hard to determine what direction it spun but I am pretty sure it was nose left. The second time it happened I had been flying for about 5 min and had done several punch outs been flying around in forward flight and even fast backward without any indication of anything wrong. Again it was fairly sudden and If I remember correctly it spun to the left again from a fairly level position (high hover).

After picking up the heli, the first time, I notice the rudder servo had been stuck where the servo horn was in the aft position (left turn). As soon as I unplugged the battery and freed up the servo with the power off and then reconnecting the battery everything seemed to work fine. After the second time it happened I couldn't find anything wrong and all servos seemed to work fine.

Sorry for you Align guys but its and EXI. I am using the xmhelicopter 401B gyro, AirTronics RDS8000 TX/RX and JR servos (DS825/DS3400 combo). One item to note, the bearings are starting to go on the main drive motor but I do not think this is the issue.

My belt tension is good and no missing teeth. All of the gear train is in perfect working order so I am ruling out a mechanical issue.

I am thinking I got a defective 401B gyro and it intermittently decides to do its own thing.

Has anybody had anything like this come up and did you ever figure out what the issue was?

It's difficult to say for sure, but if you can rule out mechanical causes and from what you describe, I would first suspect the gyro is sending a hardover signal to the servo. That being the case: why? Has it had a low voltage induced shutdown and subsequent inflight reset or is it just defective? If it's low voltage, it could be the BEC being overburdened which could be due to, say, a defective servo pulling too much current. See where this is going, it could be anything. This is where it is really nice to have some spares to swap out electronics one at a time, starting with the best guess.

If the motor bearings are not the cause, they likely will be in a flight soon to come. Get them replaced. But I do not see how they would be involved in this hardover situation after the heli is down on the ground and motor off.

Yes. Check your pulleys at the tailbox and the secondary 'main' gear. They have been known to slip on the shaft over time and aggressive flying.

.... been there, done that [:@]

Thanks 321. I have been suspecting a brown out but I am not sure. The rudder servo (JR DS3400) is brand new but to add to that, I went through 3 esky servos on the rudder with each of them burning out in mid flight. One of those wound up giving me the same spinning issue but that time I suspected the rev/nor switch on the gyro was defective and removed it (hard wired the gyro PC board).

I will be replacing the gyro the motor shaft and bearings in the rebuild as well as a new ESC/BEC fora more powerful unit (currently using 25amp). Waiting to sell my Shuttle on the bay before I can put more $$$ into this one.

same thing happened to me on my b400, belt pulley gear was slipping on the shaft, would cause it to go into pirouettes out of no where. replaced servo, belt, and tail shaft b4 i found the actual problem.

Ok. I will look that again but When I checked it, I could turn the tail rotor shaft while holding the main gear and with enough force it will slip the belt over a tooth on the pulley and it takes a lot of force to make it slip over a tooth. Because of that, I have not suspected the pulley's are slipping on the shafts.

Sounds like a bad gyro or you end to end travels are binding the slider to he extreme sometimes. You have to back off total travel so the slider does not bottom out. If the servo was bond over to one side and releases when unpluged could be that the gyro at rest is moving the servo to one side and that is nornal for heading hold. If you are using FM/PPM receiver than I would say you got a radio glitch. I had that happen twice. I went to PCM and then on to 2.4 with no further problems.

Don't down grade your EXI it is fine. Take that money you saved and up grade your radio/heli/gyro/servos or take you lady out to a nice dinner and if the Trex boys want to make fun of your helicopter just take note that most of those posting don't even own one or have been in the hobby long enough to know the difference.

Just take everything as suggested by the guys one at a time and you will work through it. You may want to move some of you radio gear around. I want my gyro and receiver as far away from the motor or ESC as I can. Check that pully again. It may test ok on the ground but in flight they could heat up and the tail shaft pully and /or front pully can spin on the shaft. Front pully if it is plastic needs to be CA to the shaft or replace with a metal up grade. The Trex SA was known for that that problem. EXI is a SA copy.

Good luck,

Jim

Grawyl, you may want to upgrade to 30amp or 40amp ESC. Some of the servos or the gyro may not be getting the power it need during to the 25amp ESC. These thing need at less 30amp to power 4 servo and the gryo. When you lost power to the gyro, you no controll of the tail then. That will the first thing I would do before get back in the air. Yes upgrade to 2.4 tx and rx can solve lost signal problem. I have the EXI also, it fly great.

The ESC built in BEC's current handling abilities (or lack of) are a real problem, often with factory recommendations of up to 3 servos when they're really needed for 4 servos and a gyro. Going to a larger ESC doesn't necessarily give you a stronger BEC, so check the specifications on the BEC part of it before you buy for the purpose of getting a better BEC. There are also dedicated BEC's on the market which are more than sufficient for the job and they will work with your current speed controller.