Helicopter Body “Wobble”

Visible at low head speeds. Pronounced on Raptors, but happens to other brand machines as well.
(Not to be confused with "Bobble", the up and down tail movement initiated by hard maneuvers)

I have found that one of the main causes of the "Wobble", most pronounced at low head rpm (under 1700), is caused by the head itself. It can be more pronounced when other factors are present. The main problem is that the head has play in it on the main shaft. If you loosen the "jesus" bolt, rotate the main blades to “fore/aft position and the grab the main blades you will most likely find that the head rocks fore and aft. (blades spread out in "flying" position) If you were to tip the head on an angle to the main shaft, this would demonstrate how “static” tracking can to be off, one side of the head spindle lower than the other relative to 90 deg to main shaft.

The head will be sloppy 90 degrees to the "j" bolt and the slop will allow the head to pivot or rock on the "j" bolt. Over tightening the "j" bolt can actually make the problem worse, as it deforms the head block in precisely the wrong direction. In other words it makes the main shaft hole, in the head block, oval shaped and more slop will develop over time.

Questions:

Do the dampener styles have anything to do with this problem?
In general, no,…in specific situations, yes. In particular, the stock dampeners are very tight on the head spindle, and this can allow the spindle to be offset to one side of center until the head reaches a certain RPM, where the spindle will center itself from centrifugal force. At this point the wobble will “go away”. With a really sloppy head it won’t make any difference, the problem is still the head. If you grease the dampers this will mask this particular problem some, if there is only a little slop in the head, until the dampers dry out.

Can I fix the stock head?
It has been suggested that using JB weld in the Head Block is the way to fix this problem. I would say this....only if you think you are lucky enough to glue the head on perfectly in “static track”. Then of course, there is the problem of removing a glued on head. I recommend a new plastic head and just beyond snug for the “jesus” bolt, don’t reef the hell out of it.
Shimming the head to repair: Same goes here as for using JB weld. The question remains: How do you know that the hole is elongated equally on either side? It may improve the heli some, but you can bet if it is off at all (static tracking), you are creating a vibe that is/will cause damage. Buy a new head block.

Can I use the Stock head with a 90?
If you are going to do any serious 3-D style flying, no matter the engine, go to an aluminum head-period! (Click here for the replacement I recommend at this time.)

Note: A friend has some video He took of my machine, while testing, with a sloppy ALUMINUM head, I mean really sloppy, and it wobbled so bad I could hardly fly the machine. I will post it if I can get it from him. This video definitely defines the problem. Either style dampers made no difference. This same machine, with only a head replacement, had zero wobble. I tested with both stock and o-ring dampers. No problem with either on the new head.

Other problems that can contribute to the wobble (mainly if the head has the slop in the first place, but could contribute to worsening the slop as well as “showing” it where it is already present):

A crash, where the blades or the head have hit solidly – i.e. the boom, ground, etc and show any damage. This can elongate the hole in the head block. Check the head for this slop with the “j” bolt loose. (Remember to tighten "j" bolt when finished checking) Out of track or out of balance main blades will load the head unevenly and elongate the head block main shaft hole.

Tight links on or above the top half of the swash plate, in particular, if they are tighter on one side (all or most of the links associated with one blade tighter than the links associated with the other blade). This puts forces against the head in one plain unequal to the other.

Fore/Aft slop in the Upper Bearing Block (plastic version). You will find this in most machines that have any time on them, even more so if it has been flown aggressively. Check this by grabbing the head and rock it fore and aft. Watch for bearing movement in the upper main shaft bearing-block. Reason for problem: I believe the hole distorts after the part comes out of the mold, presenting an oval shaped hole in the bearing block. I have found this problem with new plastic bearing blocks. Also crash forces can do this or make it worse.
a.) Upper Bearing Block- Fix 1 –smear thin coat JB weld in the plastic bearing block, in the bearing slot, assemble with bearing back into machine, with main shaft installed, wipe off any excess. Allow assembly to cure for 24 hrs before flying. No. Tightening the frame bolts more will not fix or stop this problem.
b.) Upper Bearing Block -Fix 2 – Upgrade aluminum bearing block

:)

Pete Schmidt
AMA 6314
Pete’s Hobbies/Precision RC Products January 14, 2003

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