SK720 Gain Relationships - HeliFreak
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Old 07-02-2010, 03:49 PM   #1 (permalink)
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Default SK720 Gain Relationships

As I had promised, I spent some time with the SK720 to figure out exactly what the relationships between the Cyclic and Hiller Gains are and how they affect each other. I have tried every conceivable combination and it looks like the rules I have come up with hold true.

It does not matter whether we are talking about Cyclic Gain set remotely from the TX or have a value "Locked" in the software. It behaves the same with either. When I refer to Cyclic Gain, it is the value in the "Lock Cyclic Gain to:" box or via Travel Adjust endpoints in your TX (Not sure if all of this holds exactly true for Futaba ATVs as I fly JR. I also do not use the GYRO menu as I feel this has less resolution than using endpoints.) When I refer to Hiller Gain, it is the values in the boxes on your Cyclic tabs. What your heli actually uses, I will refer to "Flight Gain" and is the Hiller Gains in the Flight Gains Matrix. This is nothing that you really need to know to get great performance from the 720, but is for the guys like me that want to know exactly how something works.

Hiller Gain Range: 0 to 100

Cyclic gain Range: 0 to 100 via the software, 15 to 150 via TX endpoints. (Below 15
endpoint will not affect Flight Gain, although 0 will switch you to the other Bank.)

Maximum Flight Gain is double Hiller Gain up to 200%

Flight Gain changes in porportion to the formula of:
The (difference in Cyclic Gain from 50%) x 2 x Hiller Gain + Hiller Gain.

For example: If Cyclic Gain is 60% and Hiller Gain is 40%, Flight Gain will be 48%.
60% - 50% = 10% x 2 = 20% (or .2) x 40% = 8, 8 + 40 = 48%.

The only way you can have different value on Elevator and Aileron Gains is to set different values in the Hiller Gains boxes. Once you do that, the higher value is controlled as per the above formula. The lower value the maintains the same percentage difference. For example, if you have 40% Elevator Hiller Gain and 60% Aileron Hiller Gain set in the Cyclic tab, 40 divided by 60 is .66666. If you then raise your Cyclic Gain to 70, your Flight Gains will be 56% and 84% respectively. (.66666 x 84% = 56%) Again, the higher value is affected by the formula and is controlling. The lower value is a percentage of the higher value.

I have not downloaded and read the new manual, but is seems at least in the new firmware the the "Cyclic Gain Channel Also Scales Damping Gain:" formula in the original manual is not correct. This feature is always active, even though there is not a check box for it as was discribed in the original manual. The Damping Gain Scaling appears to follow the same formulas I listed above.

Just for those that have not seen my posts on Tail Gain relationships, they behave differently from Cyclic Gains, so I will include them here.

Tail Gains are a multiple of the Tail Gain set either by your TX Travel Adjusts or the "Lock Tail Gain to:" box and the values in Rate Gain, Hold Gain and Accel Gain boxes. For example, if you have 80% Rate Gain, 60% Hold Gain and Travel Adjust for the Tail Bank set to 50%, your Flight Gains for the tail will be 40% Rate Gain and 30% Hold Gain. The range of values for the different tail gains are:

Rate Gain: 0 to 100
Hold Gain: 0 to 100
Accel Gain: 0 to 20

For a Tail Bank to be a Heading Hold Bank, you must have a value greater than 0 in the Hold Gain box. A value of 0 in the Hold Gain box makes it a Rate Mode Bank. Rate Gain is the primary gain for the tail. It would not matter how high you have the value in the Hold Gain box, if you set the Rate Gain box value to 0, you will never get the tail to hold. Rate Gain determines how strongly it holds against an angular displacement, Hold Gain determines how strongly it holds against a change in heading.

Sorry for the long post. Wanted to get all of the info in one place. Hope this helps some or at least is of interest to the hardcore tinkerers like me!
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Old 11-10-2010, 05:08 PM   #2 (permalink)
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nice write i noticed on my cyclic gain in flight when i do quick elevator inputs i get a small bobbing but on aileron its all good no bobbing at a;; do u think i just lower my HILLER gain on ELEVATOR to stop the bobbing? or just lower my GENERAL cyclic gain and then lock it when i get the right % so it stops bobbing?
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Old 09-08-2011, 11:48 PM   #3 (permalink)
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The tail gain set in the Tx or "locked":
Does this ONLY scale rate - hold - accel?
Or does it scale those mixes on the RH side of the GUI as well?

Nelson NZ
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Old 09-09-2011, 11:02 AM   #4 (permalink)
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=] the gyro gain or common gain that is set in the GUI is scaling only rate & hold gains. all other mixes are adjusted independently of this.

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Old 10-09-2011, 06:41 AM   #5 (permalink)
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Does the "Display Flight Gains Matrix" feature help ?

[This may be new with V3.]

It displays the effective Hiller & Damping gains; and the various tail gains. These change with Tx input, as the banks switch, etc.

Seems neat, and they don't change the way that previous docs may have lead us to expect. eg 50% Cyclic & 50% Hiller gives effective 50% Hiller. 50% & 60% or 60% & 50% gives (IIRC) effective 60%.

Also, Hiller & Damping gains seem permanently linked - which previoulsy wasn't the case ?

Anyway, this view seems very useful.
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Old 11-25-2011, 11:19 AM   #6 (permalink)
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Hello ROtOR,

I cannot seem to get rid of a faily constant bounce (up and down) in the tail boom. I have tried locking the gains in positions of any were from 40% to 65% with chnges of 5% each time. It seems like it should be a cyclic gain issue but nothing seems to help. I have not done anything with the Hiller gains.

Any suggestions?


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Old 11-02-2012, 05:04 AM   #7 (permalink)
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"""""For example: If Cyclic Gain is 60% and Hiller Gain is 40%, Flight Gain will be 48%.
60% - 50% = 10% x 2 = 20% (or .2) x 40% = 8, 8 + 40 = 48%.""""""

Hey rotor where you get the 50% and 40% from? The 50% is form formula? And 40% is its Current hg that on the software Or it's formula too?
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