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Thursday, January 12, 2012

Blade mCP X - 100% Vibration/Controllabilty Fix

Update 3: See also:

Update 2:  mCP X with Z8RC truss upgraded to A+.  With the Z8 truss mod, my mCP X suddenly rivals my mCX in controllability. My stock mCP X grade was F, but my sample was one of the vibrators suffering from quirky tail boom resonnce, your mileage may vary. Once flying well, this copter is a blast and a great way to take your skills to a higher level with limited risk.

Update 3: At last, a major break through! Blade mCP X vibration problem solved and 100% fixed.  Additionally, the tail no longer blows out with quick throttle changes.  Unlike all the other "sort of" fixes floating around out there, this Z8RC exclusive is the real deal.
I'm not the best RC helicopter pilot on my street, but I'm not the worst either. Before the boom truss mod in this yellow update, just a few hours ago, this helicopter was a basket case that I couldn't keep within a 10 foot circle.  What a difference a truss makes!
I was convinced that AS3X was the source of my (and so many others) crazy mCP X vibration problem because the helicopter was fundamentally balanced.  Consider:
  1. With no blades attached, the main shaft spins smoothly and axially
  2. I balanced and straightened the blades exactly.
  3. Blade tracking seems ok at 50% (no) pitch.
  4. ...yet...
  5. With the blades attached, the main shaft and the copter wobble like crazy.
That pretty much leaves AS3X-induced vibration as the culprit.  

First, I tried to dampen the all-in-one main board using rubber grommets at the helicopter attach points.  Interestingly, that made the problem much worse, so even though I didn't have the solution I was encouraged that I was close to identifying the problem.  

Next, when the copter was running most smoothly in the air, I noticed there was still some high frequency vibration in the tail assembly.  Ah ha!  Maybe the problem is lower frequency resonance in the tail boom impacting AS3X-directed inputs and outputs.  That would explain everything:
  1. Why some mCP X's have this problem and some don't.
  2. The situation dependent, come and go nature of the problem for those that have it.
  3. Why changing the boom length fixes some problematic copters and not others.
  4. Why the copter only vibrates when there is torque present.
My boom truss mod, pictured below, finally fixed the problem, eliminating ground and airborne vibration and vastly improving control precision:

The flat side of the strip is oriented horizontally for maximum left/right rigidity when countering main blade torque.  I CA'd it to the frame just above the landing gear receptacle and to the boom itself.  It weighs .006 grams. 

Since my boom is modified to 5.875 inches long, I made the truss 2.5", or somewhat less than half the wavelength of the boom.  Those with a stock 5" tail boom should probably try a 2.25" long truss to prevent a sine wave with a mid point node from resonating in the boom even with the truss in place. 

I popped off the truss just to make sure the basket case devil heli returned - it did.  Mysterious mCP X  vibration problem solved!

Now to dress the brushes in the tail's brushed motor for faster responsiveness and a little more tail rotor power.  The minerals in the tap water act like a wet stone to dress and polish the brushes for better responsiveness and more rotor power.  The following video shows the first break-in for this motor.  You can also do it with the propeller off. 
 Do not do this with any brushed motor!!!

My original article follows:

I've been working a lot on my weak helicopter skills. I can finally say I can tool around in my (nicely improved) Blade 400, at least right side up.  So it was a real bummer to get an mCP X with all sorts of nasty vices.
After finally turning my attention to it, I've discovered a small collection of internet fixes that help a lot.  In order of effectiveness:
  1. Problem: Massive helicopter vibration at certain places in the throttle curve.  Partial solution: Wrap an elastic band around the main grips.  For some reason, wrapping a prop-saver sized O Ring around the main grips almost stopped the problem and seriously improved overall copter stability. Another option is to press a canopy mounting grommet onto the three swashplate ball links before clicking the links in place.  The tail link can only take a grommet that has been sliced in half.  Both fixes seem to work about the same, but the grommet fix seems more mechanically pure.
  2. Problem: Twitchy tail.  Partial solution: Lengthen and strengthen the tail boom.  2mm carbon rod is a direct replacement.  The original boom when removed is 5" long.  I made a new one that is 5 7/8ths inches long based on urban legend.  The original boom is hollow and sliced lengthwise to insert and house the tail motor wires, which makes it very weak and pliable.  Mine broke in half, which is what prompted me to try all of these fixes.  I don't really know how much the added length and additional strength contribute to the overall fix, but it didn't hurt.
  3. Problem: Play in pitch and roll plus higher throttle stick position than I like.  Partial solution: Increase the length of all three control arms one 360 degree spin.  Adjust the pitch sub trim to achieve 0 pitch at half throttle. I also tightened the main blades until they resisted movement as suggested by E-Flite.  I did that first but couldn't tell a difference if it helped at all.
  4. Problem: Loose motor.  Solution: Tighten and thread lock the screws.
  5. Problem: Poor blade tracking.  Solution: None discovered.  Feathering shaft is fine. Links are new. Main gear is seated. New blades track even worse than my old ones.  Out of ideas.
Taken as a whole, these fixes really helped calm my mCP X down.  It went from a very twitchy, darting bird trying to shake the fillings out of its teeth (yes, some birds have fillings in their teeth) to a less twitchy bird with less play that spins smoothly 90% of the time.
It sux that some mCP Xs seem to have major issues, but the fact that I invented none of the above proves that I'm not alone.

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