Atom 500 crash diagnosis

[npomeroy]

My Atom was in slow FF about 5 metres up and just dropped to the ground. I thought it rolled as it went down.

I posted details on the Skookum forum...
http://www.helifreak.com/showthread.php?t=313419
and it seemed the crash was unrelated to the SK720 controller, BEC voltage or uncommanded Tx inputs. It was possibly a servo or the servo cable. All the servos work, but 2 of them had the arms slip 90 deg on the shaft (presumably on impact but there is a question of whether one slipped in flight - I could force them around by hand against the servo torque under test).

Another possiblity occured to me hence this post:
After the crash the main shaft had about 1 cm vertical movement, i.e. the collar had been forced downwards on the main shaft by an upwards tug on the shaft. It is hard (but not impossible) to see how this could happen when the head impacted in the ground (you would think the shaft would be forced into the heli, not pulled out of it). It's possible the dying blade thrashes created "lift" against the dirt.
BUT what if the collar slipped by itself during flight? The shaft would rise; the blades would be pulled into zero or negative lift, and down she would go.

I've crashed dumb-thumbing before but an out-of-blue dump depresses me. If I can't find the cause it can happen again. Goodbye thoughts of a scale fuselage!

Any thoughts or similar experiences?

Nelson NZ

[chopstick]

when the bolt in the maingear is broken, there will be enough play to get 1cm as you described.
If not broken, there is not enough room between maingear/mainpulley and the frame.

You mentined a roll. I cannot figure out how an axial displacement will cause a roll.
When the main axis goed up a little bit you should notice this since the heli will probably have less pitch (FBL). With about 5 m of height, it will last only seconds before you hit the deck.

[npomeroy]

Originally Posted by chopstick

when the bolt in the maingear is broken, there will be enough play to get 1cm as you described.
If not broken, there is not enough room between maingear/mainpulley and the frame.

You mentined a roll. I cannot figure out how an axial displacement will cause a roll.
When the main axis goed up a little bit you should notice this since the heli will probably have less pitch (FBL). With about 5 m of height, it will last only seconds before you hit the deck.

Yes, I put the peices together again to check your comment, and indeed there is only about 2mm vertical play when the collar does not hold. The spindle bolt was not broken, so my memory must be bad and the vertical movement was much less than 1cm (I dismantled the parts before writing this thread).
I agree a roll is not likely to be caused by a sudden pitch loss. The SK720 playback was a bit confusing because although my memory was of the heli tipping sideways in the air as it fell, the playback looked as though it came down level (and much slower than reality) and only tipped over when it hit ground. But I am told the playback is not exact.
Thank-you for making me investigate further, because as I was playing with the main gear I felt that that the one-way bearing is gone: It feels smooth but it is now two-way! I will need to add that to my parts order.
Probably some sort of servo malfunction was the cause but it is difficult deciding what damage happened before or during the crash.

Nelson NZ

[chopstick]

Glad to be helpfull.