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Newbies: Tips and Information Section of HF, specifically for Passing along info to newcomers to the hobby. Setup, tweaking, orientation practice, etc.


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Old 07-25-2021, 05:58 AM   #1 (permalink)
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Default Align T-Rex 300x has 1.3 play in swash plate

Hi all,

I've noticed that my Align T-Rex 300x has some play in the swash plate:
  • If I adjust the pitch to have 0 angle
  • Then I can move the blade to 1.3 just by slightly wiggling it, without any amount of force, and without the servo protesting.
(See the photos below)

This is my first normal size helicopter, so I don't know if this is normal:
  • Should I change the swash plate, or can I keep going?
  • Can I fly until I receive the new swash plate?
  • What is the normal / safe amount of play?
Also, if some play is acceptable, then should I take it into consideration when I level the swash plate? Like, I should measure the average angle between two extremities of the play (for example, 0 and 1.3, the angle is 0.7)?

Well, any hint will be helpful.

Have a nice weekend.

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Old 07-25-2021, 03:03 PM   #2 (permalink)
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You need to first find out where the slop is. It could be in the linkage around the DFC links (IE loose screws, cracked ball link, etc. It could be in the swash plate, but do check the balls first to make sure all are tight (going down to servos, up to DFC/Head.). Finally check linkage going down to servos, including ball links on the servo horns, the servo horns themselves, and then the servos.

I would not start buying parts without knowing which is giving problems...
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Old 07-26-2021, 01:08 AM   #3 (permalink)
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Default Checked for play from DFC/Head down to servos

Hi BobHyatt,


Thanks for the advice. I did check before posting, but I did it again, being consciously thorough, and looking for play step by step:
  • The blades
  • The rotor grip arms
  • The linkage of rotor grips
  • The ball links
  • Then the swash plate
  • Continuing on the servo linkage (balls, links, ball at servo horn)
  • Continuing on the servo (horn, the servo itself, and its attachment to the body)
The play definitely comes from the swash plate. The red disc has some play with the silvery disc. When measured as blade angle, it translates to 1.3. The play is consistent at all orientations. Meaning that I can turn the blades in any direction, and still get the same amount of play.

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Old 07-26-2021, 05:15 PM   #4 (permalink)
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Was just checking on mine.
What you see is normal, all heli’s will have a little play.
Be aware, Align did not loctite the balls.

As far as set up goes, do it at rest.
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Old 07-27-2021, 08:10 AM   #5 (permalink)
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"What you see is normal" is the sweetest answer. Also, thanks for checking on your own helicopter.
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Old 08-01-2021, 01:36 PM   #6 (permalink)
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No problem.

If you use a digital pitch gage, it’s mass may not sit directly over the blade, and due to slop you may see a tiny difference if your mass is forward vs rearward over the blade, but it will be minuscule so setup doesn’t matter, but if you wanted to be technical then the mass should be at the rear.
Me, I like to practice good methods, so keep it the same each time.

Some of these professional pitch gages can be rather bulky.
But a 300 size, it’s whatever works.
Still prefer the old leveling bar method, myself.
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Old 08-13-2021, 02:30 AM   #7 (permalink)
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Default The center of lift and the feathering axis

Thanks for this tip.

I would like to understand why.

Let me ask some questions, just to be sure where if I'm on the right track:
  • For all assertions below to be true, the play has to be small (we're speaking of less than 2), and consistent at all angles.
  • When the helicopter is flying normally, the blades will be forced at one extremity of the play, either max or either minimum.
  • This is why the play is not that important - it is only noticeable when you switch to inverted flight, or do some adventurous manoeuver that requires a change in pitch more important than instant lift.
  • In normal flight or hover conditions, if the pitch axis of the blade (the one parallel to the feathering shaft) is before the blade's center of lift, then the blade will be constantly forced to the minimum extremity of the play.
  • On the contrary, if the pitch axis is after the center of lift, then the blade will be constantly forced to the maximum extremity of the play.

So, if my reasoning is correct, then your advice to let the mass of the gauge behind the blade is consistent with the feathering axis to be after the center of lift.

I have two questions:
  • Is my reasoning correct?
  • Why is it a common design decision to place the feathering axis after the center of lift?


Last edited by jmgonet; 08-13-2021 at 04:35 AM.. Reason: Consistent wording with the image
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Old 08-13-2021, 08:53 PM   #8 (permalink)
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Sort of,,,,,

What happens, rotational force is going to center the blades to track a least resistance.
If you could,,, you could spin up your head, take off and hover and uncouple the heli from the head, the head will sit stationary in space all by itself for what seems like forever until enough speed bleeds off.

Modern fbl blades, good ones, have the weight forward of the blade root.
In the early days of fbl this mattered. Today, not so much as the algorithms have improved.

So, feathering shafts are centered over the main shaft for 2 reasons.
1, rotational force. It takes less energy to move.
2. KISS,,, keep it simple, stupid.
No need to reinvent the wheel.
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