In a world with no friction or wear, you could put the bellcrank just about anywhere and it wouldn't make a difference.  What affects the dynamics of the airplane are where the leadouts are located, and the ratio between line motion and elevator/flap motion.

The reason for this is that all the airplane "sees" as far as line tension vs. pull on the airplane is what acts on the leadouts -- if the leadouts are forward of the bellcrank the bellcrank gets pulled forward to exactly the same extent that the leadouts get pulled back, and what's left over is the same no matter where the bellcrank is.

In a world with friction and wear, like this ol' one we live in, the big effect of having too much of a bend in the lines at the leadouts is probably wear, followed by friction, with maybe some issues with the leadouts putting a permanent kink in the lines.

So you shouldn't just put the bellcrank any old where -- but what you should be thinking when you do locate it isn't "is this going to make my plane fly crooked?", but rather "is this going to make my control system bind up or wear out?"

Tim says it well.

Good points all !  Thanks for your input Phil. 

Phil, could you draw a picture of what you're suggesting? I can't visualize it from your words.

Thanks.

Hi Bill,

I think at least part of what Phil is talking about is the fact that as a model travels around the circle in a blowing wind, there is a change in control input to maintain what is perceived as neutral control needed for level flight.  From the upwind side of the circle to the downwind side of the circle and depending on the distribution of fuselage side area, the airplane will yaw a different amount.  That yaw has the effect of needing to change the position of the bellcrank relative to the model itself to maintain neutral control/level flight.  The further the bellcrank is from the CG,  (or maybe a better statement would be the further the bellcrank is from the desired alignment of the lines given the "appropriate" line rake from the leadout guides to the bellcrank) the more pronounced will be effect to maintain neutral control during the change in yaw from one side of the circle to the other.  This is not a "significant" change in control input, but it is there.  Most pilots are not even perceptive or aware to this thing.  The pilot is just continually provided very minor control inputs for level flight as the model flies around the circle.

I do not want to speak for Phil, because he might have something else in mind.  And I did not explain my thoughts on this very well.  Life gets so complicated.

Keith

You can certainly get symmetric controls without moving the bellcrank away from the CG.  If you don't have Excel, you can just put the bellcrank input arms perpendicular to a line from the axis to the leadout center, then put the output arm parallel to the wing TE.  That will get you close.  If you have Excel, I can send you a program that calculates the 3D geometry. You can then see how to locate the controls to get the response you want.  

There's a decent leadout calculator on this board.  That tells you where to put your adjustable leadout guide.  The best you can do is trim the leadout positions to a reasonable average.  Due to the various changes that affect the plane(wind direction, gusts, accelerations, prop precession, side area, etc) it's kinda pointless to try and do more.  About the only things you mention would be whether or not there is enough drag in the controls to make them stiff or hard to move.  Generally, if the flaps drop down under their own weight(and there aren't any hidden flaws in the controls) that is sufficient.

The diagram shows the real problem.  We use a 4" bellcrank, lthen make the line guides about 1" apart.  No matter how you cut it, the leadout cables are going to bend at the wing exit.

Floyd

I think this speaks against solid wire lead outs.  I've for years machined my lead out blocks from 3/16 Delron and have found no  visible wear on cables or block-always working effortlessly.  If one was concerned about bellcrank movement with respect to yaw I would think over/under leadouts would negate the issue.

Dave

Yeah it probably does go against solid lead outs but the diagram is not to scale unless you are going to use a 4" crank in a 16" wing span model and the angle must be infinitesimal in a real model.
The angle also progressively minimizes during maneuvers as the crank flattens out to present a narrower profile.

Just hold a solid lead out model by the clips and wiggle them, and then compare it to a flexible lead out model and see if you can detect the difference. I would be surprised if a blind test lead to any conclusive results.

Thanks.

To Tim, Reply #1...

Right, but it almost seems to you're suggesting that misalignment betwen the leadout guides and the bellcrank arms, might affect the model's yaw angle. Claus Maikis published a nice article about this in (I think) the 1970's, in an Aeromodeller issue. Several photos of a cardboard cutout of the top view of a model with the bellcrank pivot, CG and leadout 'cables' marked with pins. The thread ran from the leadouts to the bellcrank pivot. The cutout was ballasted so the CG was at the positions of its pin, when that was changed. This rig was hung vertically...

Result: wherever the CG was located, the "leadouts" aimed at the CG as they passed through the "leadout guides." Moving the "guides" for a particular CG position yawed the "model" accordingly. Moving the CG for a given leadout "guide" position did the same thing.

Moving the bellcrank pivot did nothing. Any push fore or aft occurs inside the model structure, not outside, where it could affect yaw trim.

As several mentioned, a bellcrank pivot badly located makes leadouts bend through the guides,  INSIDE the wing, which can produce friction, possible wear, and perhaps even stiffer control 'feel.'

BTW, testing how free the controls feel, without the load of in-flight pull, could be misleading.

Very good point about crosswind yaw effects - it's a good thing we fly analog, not digital. We correct for the difference by reflex, if it is stronger than usual, or instinctively without noticing it, in less severe crosswinds...

 One guy told me it absolutly doesnt matter where it goes,,, he told me I could put thebellcrank in the stab if you had room & as long as you exit the leadouts in the proper area of the wing tip  ,, it has been proven to not matter.   I suggest you do your own thinking on this subject.
  John

Howard, to your #16...

Rattle, yes, and bend...

And are flexible lead outs immune from permanent bending?

Personally I have seen just as many kinks in both kinds of lead outs.

One guy told me it absolutly doesnt matter where it goes,,, he told me I could put thebellcrank in the stab if you had room & as long as you exit the leadouts in the proper area of the wing tip  ,, it has been proven to not matter.   I suggest you do your own thinking on this subject.
  John

In his Control Line Capers column, American Modeler, Oct 62, Bill Netzeband showed a test fixture that illustrated whereever the bellcrank was attached to a pendulum type apparatus, or as a lead weight was placed in different positions relative to the bellcrank and the leadout guide,  the leadout guide always aligned with the CG.

In a one page article, American Modeler, Jul 66, Walter Williamson illustrated a trainer type model (solid wing, box fuselage, upright engine) where it was flown with the bellcrank mounted in various positions, from behind the engine, above the wing and just forward of the stabilizer.  The different bellcrank positions made no difference in the manner the airplane flew.  The leadout guide always aligned with the CG.

As has been explained countless times on these forums, placement of the bellcrank location should be where it makes the most sense to do so given the space, structure, and the ability to best line up the bellcrank position with the expected line rake through the leadout guides to minimize wear and friction of those leadouts against the guide.  In general, to minimize the wear and friction (which is almost a non-factor for most configurations) and the angular change of the leadouts at the guide, this means that the bellcrank pivot would be slightly ahead of the expected CG.  (This would be about 2 degrees forward of the leadout guide, looking from the top, for mot "typical" stunt designs, slightly less than 2 degrees for heavier models, slightly more for lighter models.)  In this process, it should be understood that the CG will align itself with the leadout guide in flight and that as the CG is changed in the flight trim process, the optimum leadout guide will also change.

Keith

I most always mount the bellcrank bolt just behind the expected CG because the leadouts usually end up 1-2 degrees aft of the CG and that just seems right to me.  There is something I'm not sharp enough to cipher tho: on pull test that scale is pulling directly from handle to crank bolt.  It doesn't care about CG, leadout position or the fishing weather.  There must be some point, say in strong winds where the weight dynamic takes over and seeks to pull straight line which would sling the airplane into a yaw if the bellcrank is too far off.  Where is my logic wrong?

You are failing to see the leadouts as acting like a hinge.  In fact, on your pull test the scale is (probably) not pulling directly from the handle to the crank bolt unless you are making it happen, either by holding on to the crank bolt itself, or by lining up the plane thusly.

But don't believe me!  Grab a piece of board and call it a plane.  Now drive some nails into it, and do the experiment: put in an eyelet or a bent nail, and call it your leadout guide.  Now drive a nail into the CG of the board, and drive a few more nails into other spots on the board.  These are your "bellcrank pivots".  Grab a piece of string and call it your "leadout".  Tie a loop in the end, thread the string through your eyelet, and hook the loop on your CG nail -- your "plane" should hang with the "leadouts" in line with the "bellcrank".  So far so good.  Now hook the string on another nail -- it'll go through the eyelet, then bend, then go straight to the "bellcrank" -- but the "plane" will hang exactly the same way it did before, because it's CG will be in pretty much the same place (unless you use a really light board, and really heavy string).

I most always mount the bellcrank bolt just behind the expected CG because the leadouts usually end up 1-2 degrees aft of the CG and that just seems right to me.  There is something I'm not sharp enough to cipher tho: on pull test that scale is pulling directly from handle to crank bolt.  It doesn't care about CG, leadout position or the fishing weather.  There must be some point, say in strong winds where the weight dynamic takes over and seeks to pull straight line which would sling the airplane into a yaw if the bellcrank is too far off.  Where is my logic wrong?

   Because when you do a pull test, the pivot point  is wherever you put your fingers.  In flight, the pivot is the CG. Hold the airplane for the pull test with your fingers at the CG, and you can move the bellcrank all over and it doesn't matter.

    Brett