Thinking out loud.... DS760 / 3400g / Rave 450 = meh... - HeliFreak
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Spartan RC Spartan RC - Quark, DS760 Gyros and other Spartan RC Electronics Factory Support

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Old 05-01-2011, 10:02 PM   #1 (permalink)
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Default Thinking out loud.... DS760 / 3400g / Rave 450 = meh...

I stole the Hitec 5065 cyclic 5084 tail servos off my Rave for the Mini Protos as they were better suited for FBL. I replaced the servos with JR DS285s and 3400g (my 3500g is down with a stripped gear). I had previously been flying the Rave with a Quark but stole that gyro for my Gaui 200 so I replaced the Quark with a Spartan DS760 which I have heard is just as good. Anyway... long story short... the tail sucked on the Rave now. Bounces to a stop in one direction and slides to a stop in the other.

I borrowed a buddies Flash Link cable and I updated the firmware from 2 to 2.1. I played around with settings with no real spectacular results. I decided to move the ball out one hole on the servo arm which should effectively speed up the servo at the loss of some TQ. All this really did from what I could tell was make me lower the gain. I am down to 25% (Spektrum gear EPA) for gain. I was still getting a bounce on CCW stops and a slide on CW stops. I decided to keep messing with the settings. In the end, I used the Flash Link to get the servo arm at 90°, reprogrammed the servo limits which were very skewed now, turned deceleration all the way down, turned acceleration half way down, turned stop gain up to 150 on CW and down to 50 on CCW. After those adjustments, it seems that the tail is livable. I really need to get the 3500g on the tail..... but now I am curious about the effects of ball position on gyro performance.

The Spartan manual states:

We recommend that the ball-link is mounted 13-17mm from the servo
shaft unless a different distance is deemed essential for a particular
helicopter. Ultimately, the most important factor is the servo travel which
ideally should be around 45° on either side of the mid-point (+/-15%).
For most servos this will result to gyro endpoints in the range 85-115%.
This combination gives a good balance between tail pitch correction
speed and available torque. If the servo travel or endpoints are
significantly different than this recommendation you may wish to change
the ball-link position to restore this balance.
The 45° servo travel I can understand but that is gauged against full slider movement. Now not all tail are the same. I imagine that some may provide just enough pitch to get the job done and others more pitch than you could ever use. I wonder what an acceptable tail blade pitch range might be and if you are overshooting it by maxing out the slider, is that detrimental to gyro performance? With the low gain numbers I am seeing, I wonder if moving the ball in would help it work better.... although the Spartan manual says:

Always set the gyro endpoints for the maximum available tail rotor pitch.
This defines how much pitch the gyro can demand and has no effect on
how responsive the gyro feels in flight. If you find the yaw very
responsive or very slow for your preference you may adjust this via the
transmitter’s endpoints or Dual Rate setting for the rudder channel.
Hmmm..... For reference, here is a selection of tails maxed out on the slider. I think it is pretty clear that there is quite a bit of a difference in available tail blade pitch. How does that effect things? Whatcha think?

Rave 450

Beam E4

Outrage G5

Furion 450

Mini Protos

Trex 450 Pro
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Old 05-02-2011, 11:39 AM   #2 (permalink)
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Join Date: Aug 2010
Location: Chicago 'burbs

These are just some thoughts about what you are saying.

I think what Spartan is saying, is that the heli manufacturer is the one that designed in the total travel on the tail rotor shaft, the mechanical linkages etc that determine the limits of pitch. You would hope (!) that they thought this out a bit ). If the rotor shaft is too short, you might not get enough pitch, and if too long, there is more than you need, and in addition, the shaft can flex more than if it were shorter (and adequate for the job).

Now the tail is doing two things--compensating for the main rotor torque, and secondly giving you rudder inputs to yaw the heli. Both are just applications of thrust on a lever arm. As far as the heli is concerned, it is the total torque that matters, not how it is being made.

The level arm is pretty much set by your tail boom length, so for that it is pretty much fixed unless you are really into modding the heli. So the thing we are varying all the time is thrust.

I know I have made this story too long, but the thrust (more or less) is a combination of 3 things, pitch, rpm, and blade size. Increasing any of these 3 things will produce more thrust.

What I am getting at is that simply looking only at pitch is ignoring these other variables. It is true that pitch is the thing the gyro is changing, and is the largest factor, but my guess is that the difference you are showing in your photos of different helis is probably on the scale of the differences we have from running different head (and hence tail) rpm, and using non-stock blades on the tail.

Now what was the point I trying to get at????

Oh yes, probably doesn't matter that much---except if you are at the extremes (high or low rpm, large or small blades, or too much or too little pitch). As long as you are in a nominal middle area, the gyro can probably cope ok.

Why does your signature have so much meaning to me?
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