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View Full Version : Negative Lift, no positive - Halp!


joelseph
06-09-2009, 10:38 AM
Ok folks, forgive me if this has been addressed elsewhere here, but I can't seem to find it if it has. First post here. w00t! :thumbup:

I'm having trouble with my axe cpv3 with no lift. Tracking is good (though my blades are admittedly a big dinged), mixing arms are level, linkage spacing is good. However, when I spool up the heli gets a bit of thrust around 60-75% and then the blades get 'turbulent' above that and I get almost no lift whatsoever. I have checked for 0 pitch at neutral collective in idle-up and pitch appears to be as it should at +/- 100% collective.

What is strange is that I get strong downward thrust at -100% collective when holding the heli in my hand for testing, but the same symptoms as normal mode with positive collectivity - a tad of lift then turbulence (I can see it in the blade tracking also at high positive collective).

I've tried toying with linkage spacing, swashplate raising/lowering, and nothing seems to help. The one thing I'm missing is nuts on the bottom of the blade screws - I'm headed to the hardware to pick them up here in a few minutes. Not sure if the grips/blades being loose vertically would cause this behavior.

Let me know your thoughts - I'm a n00b, still learning this stuff. I should be able to come up with pics/vids of my setup if that would help. I'm running with CNC swashplate, CNC grips (actually I have stock mixing arm installed - I lost the bushings for the CNC mixing arms and am waiting for replacements).

Any help would be much appreciated. :)

halley
06-09-2009, 11:04 AM
Is the rotor rotating clockwise as seen from above?

Are you trying to spin up without nuts on the blade bolts!? Or are the blades really loose in the blade holder? The turbulent "flutter" at zero pitch sounds like they're loose and able to alter their own pitch independent of the blade holder.

joelseph
06-09-2009, 12:59 PM
Is the rotor rotating clockwise as seen from above?

Are you trying to spin up without nuts on the blade bolts!? Or are the blades really loose in the blade holder? The turbulent "flutter" at zero pitch sounds like they're loose and able to alter their own pitch independent of the blade holder.

Yeah, clockwise and blades are properly installed. Flybars are parallel, balanced, and snugged down also.

The bolts screw pretty tight into the CNC grips by themselves, so I wasn't tremendously worried about the nuts missing from the bolts. There is a good half inch of the bolt exposed through the bottom of the grips so I'm not too worried about them walking out either. I picked up some 1-72 nuts but unfortunately I didn't notice that they're coarse threaded. Bleh.

The flutter doesn't happen at zero pitch - they're nice and smooth until you apply near full positive pitch. It's weird. I can't figure out why I have plenty of negative push but little to no positive lift.

joelseph
06-09-2009, 01:03 PM
Another thing I've noticed is the difference in head speed when switching from idle-up to normal, when I'm able to get it in the air. I have to slam the left stick up to full throttle to get a soft landing. Bleh.

halley
06-09-2009, 01:45 PM
they're nice and smooth until you apply near full positive pitch

At about +20 or +25 degrees, you'd be at stall. Less than +15 should be fairly smooth.

joelseph
06-09-2009, 01:51 PM
At about +20 or +25 degrees, you'd be at stall. Less than +15 should be fairly smooth.

So maybe I have too much pitch? How do I adjust for that? Tighten the linkages? Drop the swashplate?

joelseph
06-09-2009, 02:04 PM
So maybe I have too much pitch? How do I adjust for that? Tighten the linkages? Drop the swashplate?

Ok I noticed someting very interesting... I broke a servo arm and stripped servo gears a while back. I ordered a spare servo and a set of arms. I rebuilt the servo with the gear set I ordered and was back in business, but I think that may have been when this problem started.

I hadn't noticed before but the new arm doesn't 'zip' to perfect horizontal on that servo like the others during power up - (it's the front servo btw). Maybe I need to compensate with length of the the rod on that servo? I moved the arm to the spare servo and confirmed that it doesn't go to horizontal on that one either. Must be geared differently? Wouldn't that make perfect sense why I have more collective one way than the other? Hrm... Thoughts? I'm going to try compensating with the length of that servo arm and see what happens.

joelseph
06-10-2009, 10:49 AM
Adjustments to servo arm didn't help much if at all. I also put the spare servo in because I noticed there was some gear wiggling going on inside the servo chassis. Bleh. I can get it off the ground now but lift is still very low. I'm waiting for CNC bushing/screw parts so I can rebuild the blade grips, mayhaps I'll give it a go after that and see how it works.

I need tail rotors and a few new sets of main rotors. Any recommendations on a good place to shop? Tower Hobbies seems to be out of stock all the time.

primestick
06-10-2009, 10:35 PM
Amain hobbies has a cheap set of carbon blades for the trex 250 which will fit the axe i think there like 7 bucks or something like that

joelseph
06-11-2009, 11:22 AM
Amain hobbies has a cheap set of carbon blades for the trex 250 which will fit the axe i think there like 7 bucks or something like that

But isn't it better to let the blades absorb the impact of a blade strike rather than the head/shaft? I think I am happy with balsa wood main rotors. Now tail a carbon fiber tail rotor, that I would love. Or at least some type of shield for the tail rotor would be nice.

jag8864
06-15-2009, 12:15 AM
Hey Joelsph
It sounds like your feather spindel in slightly bent and also I would check the o rings the could be worn make sure the spindle is ok remove it then also the other screw the roll it on a flat surface ( a table ) to see if it is good ( not bent) if ok replace it ,next check the Orings good bet there are worn and need to be replaced this has happend to me then of course re trim your bird this should fix the problem when the head is loose the blades worn stay at he right pitch and track as you increase power I hope this helps good luck and keep flying
JAg8864

joelseph
06-15-2009, 01:15 AM
Hey Joelsph
It sounds like your feather spindel in slightly bent and also I would check the o rings the could be worn make sure the spindle is ok remove it then also the other screw the roll it on a flat surface ( a table ) to see if it is good ( not bent) if ok replace it ,next check the Orings good bet there are worn and need to be replaced this has happend to me then of course re trim your bird this should fix the problem when the head is loose the blades worn stay at he right pitch and track as you increase power I hope this helps good luck and keep flying
JAg8864

Thanks for the info. I got my CNC rebuilt parts in so I'm back to all CNC grip parts (mixing arms were stock, probably not a good idea to mix stock and CNC parts I suppose). I also got a new set of blades. If it's still acting up I'll look at the feathering shaft and o rings. ;)