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Home✈️Aircraft🚁Helicoptere-SABGoblin 700 › Wobble at lower head speed
08-20-2020 02:30 AM  42 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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FNFAL Next up will be working with the Proportional, Integral, and Differential valued of the control system for the head and see if anything will help.
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It is a time consuming task but it is necessary to do.

Hysteresis... Very difficult to model on FBL controller.

Wobble only on pitch axis or wobble on roll axis or wobble on both axis.

You need to isolate the problem.

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Saludos cordiales,
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08-21-2020 02:25 AM  41 days ago
Chris.C

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Hong Kong

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I ran into similar situation with my Urukay WC before. The head wobbles when head speed dropped to 1450 or below.

What I did, as always, is to deal with mechanical first. Since the heli is all brand new, I replaced the damper o-ring with softer. For F3C flying, the soft dampers have minimum impact on the aerobatics. Further, I don't mount my FBL controller directly on the metal chassis plate. My CGY760 is mount sideway on the carbon frame.

Then I tuned the FBL controller. I want my FBL controller works its best for me in F3C. When I know the gains level are at its best, e.g. to work my heli to sit still over a flag, I hate to alter the gains because of wobbles, even though a little compromise is inevitable.

I can run 1380rpm in hovering with VBar Neo without wobble. It has Futaba CGY760R now running the same head speed.
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08-21-2020 08:32 PM  40 days ago
FNFAL

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Dayton OH

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Thanks all.

Short answer - Subtle deviations in blade manufacturing tolerances resulting in some differential AoA or washout simulated an out-of-tracking condition introducing a harmonic resulting in a wobble at low speed. Vibration analysis showed nothing unusual. Setting to a pitch gauge did not suss this out. Adjusting the FBL controller by reducing P and I and increasing D solved this issue, but the affects on other forms of flight are still unknown. New blades on order.

Long answer - Unlike the graceful soaring airplane, helicopter fight is unnatural. They are unstable and wanting to explode in a hail of parts at any moment. They could be considered a sin against physics. The FBL controller is like the Sin-eater of folklore, compensating or making-up for the helicopter's deficiencies. At some point, the "Eater" can no longer cover for the host and instability starts. It's best to make sure your heli sins not.

Cheers
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08-21-2020 10:41 PM  40 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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Ok... I would like to know this wobble behavior prior to mess with the flybarless controller.

In my drawing:

the yellow movement is on pitch axis. You got a triangle shape movement.

The blue movement is on the roll axis. You got a triangle shape movement.

The gray movement is a combination of the two axis.
(IMO, the most difficult to control)
you got a cone shape movement.

So, on the flybarless controller we need to adjust things according to this three different behavior.

I propose that we need to isolate the axis in order to solve the problem.
First, to work on one axis.
Then, work on the other axis.

It is very difficult to work on both axis at the same time because It is imposible to predict contribution per axis on the problem.

This is my general procedure to do the adjustment on the flybarless controller.

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Saludos cordiales,
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08-27-2020 03:41 PM  35 days ago
FNFAL

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Dayton OH

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A switch to some new SAB 750TBS blades. The variable chord on these blades pushes the center-of-pressure further out to where you'd put your gauge which has only helped more with tracking. However, the "rocking" still remains, albeit now with less amplitude and a little faster hertz. Manual does say to "break-in"dampers, but will continue my attack on the mechanical and electronic fronts.
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08-27-2020 08:28 PM  34 days ago
JLF92677

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Southern California

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I had similar experience with head wobble caused by "mast bumping." I discussed this phenomenon with Dr. Ben and his input is as follows:

"Mast bumping isn't related to damage. It's an aerodynamic phenomena caused by the combination of blades, dampening stiffness, head speed, blade type, blade grip tension, model weight, model mechanic stiffness, main shaft length, and probably a few other things too."

Solution for my situation was to slightly loosen blade grip tension which completely resolved the wobble.

Hope that helps
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08-27-2020 08:43 PM  34 days ago
FNFAL

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Dayton OH

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Thanks for the info. I wish I could visually what mast bumping looks like. I agree all those parameters are "baked into the cake", like the taller main shaft the 770 has and the part solid dampeners. SAB suggests the HPS2 head with the independent spindles may aid in this issue, but at +$245 bucks will not be going that way soon.
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08-27-2020 10:39 PM  34 days ago
JLF92677

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Southern California

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FNFAL I wish I could visually what mast bumping looks like. I agree all those parameters are "baked into the cake"
Yes, I recognize it is difficult to discern what exactly is causing the wobble when there are so many variables "baked into the cake". So my suggestion is to start adjusting/changing the least intrusive factor to make change one by one until you get it resolved. In my particular case, I had observed that there was no wobble at the beginning of flight, but after 3 to 5 minutes into flight the helicopter would wobble on hover at around 1550 rpm. Upon inspection after landing, I noticed that the grip tension was stiffer than originally set prior to flight, which was caused by blade lead/lag effect over tightening the blade grip. So slightly loosening blade grip and using bees wax on the drag bolt to reduce friction to the blade root bushing resolved the problem caused by blade lead/lag and hence, resolved the wobble mystery.
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Home✈️Aircraft🚁Helicoptere-SABGoblin 700 › Wobble at lower head speed
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