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Bell Crank or Servo?

Old 04-16-2017, 03:38 PM
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Default Bell Crank or Servo?

Hi everyone, I'm starting a new build. It's a Pica FW 190 D9 and my question is do bellcranks work as good as having two servo. Well it would actually be four with the flap servo's. The plans show bell cranks but I'm just wondering if you get better response with the servo's in the wing. And will the extra weight override flight characteristic. Thanks your help would be appreciated.

Regards, Tom
Old 04-16-2017, 04:41 PM
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If servos will fit in the wing they will always be better than bellcranks as far as mechanical advantage and ease of operation . I would only use bellcranks in a situation that required scale appearance or truly scale operation.
Old 04-16-2017, 06:28 PM
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Thank You, Ck1
Old 04-17-2017, 07:20 AM
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More parts equals more slop and more points of potential failure. I would only use them if absolutely needed on a flying surface. For flaps? If weight is critical I could see it being feasible.
Old 04-18-2017, 03:20 AM
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I'm not a big fan of dual servos. I've used bell cranks with 1 servo and never had a problem. When I've run a servo out in each wing half the extension wire required has caused a voltage drop that made the servo have trouble returning to center. To fix the problem I make up my own servo wires starting right at the servo circuit board. So there's that problem to deal with. You also want to orient the servo so the arm is moving in/out perpendicular to the hinge line. The quick and dirty method of dropping the servo straight into the bottom of the wing causes the pushrod to swing left/right and that wears down on the control surface clevis.

The downside of bellcranks is slop and they will wear down over time. Installation has to be spot on and tight. Plus there may not be a way to replace the bellcrank unless you make a hatch for it and who wants to do that.

Neither setup is perfect. I give the nod to bellcranks but it's not a one size fits all solution.

carl
Old 04-18-2017, 04:36 AM
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I haven't used belcranks for years. With smaller, more powerful, lighter servos, the dual ( or quad) servo approach is best for most applications.
Old 04-18-2017, 08:30 AM
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Thanks everyone for your input. Looks like its a mixed bag some no bellcrank and some servo's. After thinking about it I think quad servo's is the way to go as long as they fit.


Thanks Again, Tom
Old 04-25-2017, 09:28 AM
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The hands down winner for most of us is indeed in your case a four servo wing. Bellcranks are simply an holdover from former times when servos were expensive and heavy, folks would would go to any effort and chance for slop to avoid adding servos. There was even the half way attempt to still use only one servo on ailerons using big bends in ni rods to the ailerons, well this sorta worked on some but was never satisfactory for a number of reasons.

Naw the advantages of using the modern application of one surface/one servo far out weighs using longer servo wire.

I am currently working on another this one a jet with of course a four servo wing.

John
Old 04-25-2017, 03:08 PM
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I think it has more to do with your construction methods. I have some original models with years of flying time using good quality bell cranks that have no slop in them. I also use multiple servos without any problem. In the old days when a 1/4 scale servo was huge bell cranks were a fact of life. Sometimes you just don't want the ugly look of the servo or arm showing on the outside of the model. Good craftsmanship usually takes care of any slop no mater which method you choose.

Dennis
Old 04-25-2017, 04:17 PM
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Using separate servos also allows the use of a RDS. Here is a pretty good thread on the setup. https://www.rcgroups.com/forums/show...rive-system%29 The RDS makes scale actuation simple and easy.. Never used one myself but many praise the use and reliability.

Ken
Old 05-04-2017, 07:15 AM
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I have a Pica FW-190D-9 with bellcranks in the wings for the ailerons. When I bought it RTF, they were a bit sloppy, so I cut hatches into the bottom of the wing and tightened everything up. I have had zero issues with them since and have no reservations about using them on future builds.
Old 05-04-2017, 11:25 AM
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There you have it. Damn if you do and Damn if your don't I have used both methods and they work fine, if you don't want servo arms hanging off the bottom of your aircraft then go with bell cranks and 2 servos mounted in the center of the wing.

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