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HomeAircraftHelicopterHelicopter Main Discussion › Servo Arm distance calculation - Any math wiz out there ???
01-27-2012 09:48 PM  6 years agoPost 1
wrathofkhan

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LA, CA - US

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Hello

I am trying to use a Mikado flybarless system on my Swift 16. The servos have too much movement, causes major binding.

I need to 'mechanically' cut down the movement. Currently I am using servo arms which have the balls 12.5 mm from the center (please refer to attached photo - aileron/pitch servos). I used the distance as per Bob Finless' video. Using the 12.5mm distance, I can use 19mm servo arm for the Elevator servo - which would give me equal travel for all three servos.

BUT, I need to reduce the throw (for example ... use 10mm distance from center, instead of 12.5mm). My question to you is, what is the ratio between the servo arms ... I need to figure out the 'NEW' length of the elevator servo arm (so then all the servos move the same distance).

Thank you ... Khan

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01-27-2012 10:12 PM  6 years agoPost 2
GMPheli

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W. Bridgewater, MA USA

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I think you want to reduce the elevator by the same percentage. 2.5 is 20 percent of 12.5. 20 percent of 19 is 3.8. So 19 - 3.8 = 15.2

You will also have to slightly offset the 10mm balls as they are in a push-pull setup. They will need to be 90 degrees to the pushrod at neutral to prevent binding of the PP setup

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01-27-2012 10:15 PM  6 years agoPost 3
AirWolfRC

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42½ N, 83½ W

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I'm not understanding your question.

If you want to reduce the travel, why don't you just change the limits in the TX ?

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01-27-2012 10:35 PM  6 years agoPost 4
rexxigpilot

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Florida

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Your problem is harder to fix due to having bellcranks instead of a direct to swash setup. Also, you need to provide more information to give you a good answer on what needs to be changed. We need to know the distance from the center of your swash to the swash balls and the perpendicular distance from blade grip balls to the spindle centerline.

In general FBL systems need much less servo throw than with a flybar. So your goal in to reduce total throw.

The V-Bar was designed to work with the Logo helis. on my Logo 600 I have the balls on my servo arms at the 17mm position. The links from the servo arms connect directly to the swash balls which are at about 34mm out from the swash center. The blade grip balls are also about 34mm out from the spindle centerline. This gives a 1:1 ratio between the swash and the grips. If I turn my swash 5°, my blade grips twist 5° as well. Now since the balls on my servo arms are at 17mm and the swash balls are at 34mm, this gives 1:2 ratio between servo arms and swash. I need to move my servo arms about 10° to rotate the swash 5°. So because I have a 1:1 ratio between grips and swash, my total ratio of servo arm to grip is 1:2. Does this make sense sense?

You will need to get close to this 1:2 total ratio for the V-Bar to work its best. You can probably get away with a 1:1.5 to 1:2.5 range.

The first thing I would do if you have too much throw is move the servo arm balls on the aileron and pitch servos in from 12.5mm to about 9mm. That will give you about the equivalent of about a 13.7mm arm to the swash. Next move your elevator servo arm ball to 13.7mm. 13.5 or 14mm will be close enough. I can see that your elevation link will be at a fairly sharp angle but that's probably OK. You could try flipping the servo around to see which makes a straighter connection at mid collective stick (0° pitch).

If this fixes the binding and still gives adequate throw you are good to go. If not, the next step is to get longer swash balls to reduce total swash throw more. This will also help the elevator link line up better. Let me know how it works out.

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01-27-2012 11:52 PM  6 years agoPost 5
wrathofkhan

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LA, CA - US

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The first thing I would do if you have too much throw is move the servo arm balls on the aileron and pitch servos in from 12.5mm to about 9mm. That will give you about the equivalent of about a 13.7mm arm to the swash. Next move your elevator servo arm ball to 13.7mm. 13.5 or 14mm will be close enough
thank you all for all the posts

I pasted the above mentioned, how did you come up with the calculation ... 13.5mm to 14mm for elevator? changing the pitch/ail distance isn't an issue, but i couldn't figure out the elevator distance. how did you calculate it?

khan

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01-27-2012 11:57 PM  6 years agoPost 6
wrathofkhan

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LA, CA - US

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AirWolfRC

i can change the travel using Dual rate, ATV, from the transmitter. i can also cut down the throw using several ways within the flybarless program - these are all done electronically.

problem with that is, i will not be getting full throw/resolution from the servos. basically you want the servo to travel it's entire arc, otherwise the heli might be unstable

i flew the heli, using the vbar program to cut down the throws a bit. it flew ok doing basic aerobatic. but, i don't think it felt fully locked in ... that is why i am looking at ways to maximize servo throws.

cheers, khan

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01-28-2012 01:21 AM  6 years agoPost 7
AirWolfRC

rrProfessor

42½ N, 83½ W

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Then GMPheli had the answer.

If you're going to use 10mm on the aileron servos then use

10 / 12.5 * 19 = 15.2mm on the elevator servo.

. . . leave the "T" bell crank alone at 12.5mm

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01-28-2012 07:01 AM  6 years agoPost 8
rexxigpilot

rrProfessor

Florida

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I pasted the above mentioned, how did you come up with the calculation ... 13.5mm to 14mm for elevator? changing the pitch/ail distance isn't an issue, but i couldn't figure out the elevator distance. how did you calculate it?
The servo arms should give the same amount of throw whether it's your elevator of your aileron. So using a servo arm length of 9mm for aileron and pitch and the 12.5mm you have on the bell crank input arms of 12.5 and bell crank output arm of 19mm, the math is 9/12.5*19 = 13.7mm. The elevator has no bell crank. It is direct to swash, so to give the elevator servo equal throw it must have the same effective length as the calculated servo arm lengths for the aileron and pitch servos, 13.7mm.

The V-Bar will do all the proper mixing for the 120° swash internally once the servo arms are correct to give equal throw.Again, do not use travel/endpoint or Dual Rate adjustments in you TX for setup. No subtrim either. You'll understand what I'm saying once you start to setup the V-Bar.

You could also try the recommendations of GPMheli - 10mm on aileron and pitch servo arms and 15.2mm on the elevator. This will also give you equal throws. My conversion of a flybar heli(ElyQ 50C) to FBL needed the smaller servo arms. Your Swift 16 should be similar since it is a 30 size heli. Try both recommendations and see which one works best.

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