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Post  andrew Sun Jul 08, 2012 11:32 pm

After looking at a lot of C/L plans, there seems to be no standard location for the bellcrank. Assuming typical counter-clockwise flight, some mount the pivot point on the centerline, some to the right and some to the left of centerline. Do any of you guys have a location preference and why?

I tend to locate mine either to the left or on the centerline.
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Post  Kim Mon Jul 09, 2012 6:58 am

I'd always figured it to be based on where the designer wanted the pushrod to be routed (if it was to be hidden in a built-up fuse, or to allow a "Scale Side" on a profile model), and maybe to make things simpler in running the leadouts from a sheeted centersection. Never really thought about it much....so now I am !

More Coffee !!!!

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Post  Mudhen Mon Jul 09, 2012 9:18 am

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Last edited by Mudhen on Fri Nov 12, 2021 9:03 am; edited 1 time in total
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Post  Ken Cook Mon Jul 09, 2012 10:21 am

I'm not sure but I believe I do understand the initial question. I believe Andrew was asking how important the bellcrank location is in regards of wingtip to wingtip not so much nose to tail. In other words if you have a profile plane where would the ultimate spot be for location. As Kim stated, it's in regards to pushrod exit and control horn connections on stab. The bellcrank location has no affect on CG. Your leadout placement on your wingtip is the critical factor. Most plans show leadout guide exit locations. This doesn't mean that they're correct because the designer put them there. You physically need to fly the plane and this is the reasoning behind swing adjustable leadout guides.

When I build, I tack glue the leadout guides to the wingtip with a drop of ca. I then hold the plane by the leadouts and let the plane hang like a plumb bob. You want your plane to track correctly and having too much offset to the outside of the circle is going to cause a real problem not only with speed but also crabbing as the plane is working against itself. Having adj leadouts allows for fine tune adjustments. While the plane is hanging, the nose should point slightly nose high to the ceiling . This is a good starting point. Having a plane nosing too far outside of the circle with the addition of rudder offset can cause real problems especially with your tank pickup. This causes erratic runs at the end and extremely lean runs. Keeping the plane tangent to the circle is optimal and ideal for all situations. It's a good idea to shim the rear of the tank outboard slightly to prevent problems like I explained above. This also promotes a clean shutoff in the end and reliable fuel feed throughout the maneuvers.

You can certainly refine the way the plane flies using leadout sweep better than rudder or engine offset. Even though most plans call for rudder offset, it's not needed. Look at a modern combat plane, it has no rudder. They also use little or no engine offset. I only put offset in 1/2A's due to flying into the wind. What little performance they will suffer from slowing down due to offset they will make up with line tension. I wouldn't just assume that moving your leadout exit either forward or backwards shooting from the hip will make your model fly ok. You really need to test it by holding it as this is the way the model is going to fly in the air. I've seen many racing planes even use 1 deg of engine inset to help with keeping up the speed of the model. For those that fly old time stunt, the builders need to have the leadout exits as per plan as this is key in how the model was designed and how they flew the models back then. This doesn't mean that they're in the correct location and the model would probably fly better if they were relocated. I know many that cheat them forward. Bellcrank positions from nose to tail are generally placed about 1/3rd of the chord from the leading edge which is close to the CG. This is also within the widest parameter of the airfoil. Location from tip to tip, (left to right) are usually placed where everything functions properly. Ken
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Post  andrew Mon Jul 09, 2012 11:39 am

shawn cook wrote:I'm not sure but I believe I do understand the initial question. I believe Andrew was asking how important the bellcrank location is in regards of wingtip to wingtip not so much nose to tail. In other words if you have a profile plane where would the ultimate spot be for location.

Ken

That's correct --- and here's why I posed the question. We usually concern ourselves with the general term center of gravity. This is literally the center of mass, but almost always expressed in the pitch axis. But, there are two others, one in the roll axis and one in yaw. For R/C, pitch CG is most important; roll CG can usually be trimmed out; yaw CG seems to have little impact.

C/L is different, and as Ken pointed out, poor placement of the bellcrank can lead to trim issues. We need line tension, but excessive crabbing scrubs off speed which is counter-productive to good line feel. He also talked about the plane working against itself -- which we want to avoid.

That brings up lateral placement. If I took a 12" ruler and drilled holes at one inch increments for tying a string, at every location except the 6" mark, when I swing that ruler around my head, the heavy side will flip to the outside of the circle. If I mount the bellcrank to the right of the centerline (probably close to the lateral center of mass), the plane will try to roll the heavy side out and only the leadouts will keep this from happening. If the pivot point is to the left of the center of mass, flying counterclockwise, the heavy side is on the outside and the plane should be more stable in the roll axis and hopefully, be more inclined to fly level in the horizontal plane.

Consequently, I would think (dangerous situation here) that inside placement would reduce the tendancy of the plane to self correct. But, does it matter enough to worry about?

andrew



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Post  gcb Mon Jul 09, 2012 8:17 pm

There has been extensive testing and the result is that it does not matter where the bellcrank is located. It is very important where the leadouts are placed because that affects flight characteristics.

George
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Post  Mark Boesen Mon Jul 09, 2012 11:53 pm

...is this Stuka Stunt, for a moment I forgot where I was? Lol, great posts guys.
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Post  gcb Tue Jul 10, 2012 7:09 am

Mark Boesen wrote:...is this Stuka Stunt, for a moment I forgot where I was? Lol, great posts guys.

Those tests were done by a notable flyer many years ago...can't remember who did it. They moved the bellcrank to different extreme positions and it did not have a notable affect on flying.

Andrew brought up a good point in that you need to pay attention to how the leadouts are positioned vertically as well as horizontally. For a sheet winger this is not usually a problem but if you consider a high winger or a wing with dihedral, it could be if you are doing more than just round-and-round.

I believe most designers try for a bellcrank position that yields close to a straight line through the leadouts and the bow created by line drag. The primary consideration, of course, is that the leadout position where it exits the wing provides best flight characteristics for that plane. the straight line thing lessens friction on the leadout guides.

For some designs, the bellcrank is placed in the fuselage and the leadout guides are external of the wing. Also I have read where guys put weights in the wheels of a stunter to help the vertical balance.

WHEW! Enough for now. :-)

George
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Post  andrew Tue Jul 10, 2012 7:36 am

gcb wrote:
I believe most designers try for a bellcrank position that yields close to a straight line through the leadouts and the bow created by line drag. The primary consideration, of course, is that the leadout position where it exits the wing provides best flight characteristics for that plane. the straight line thing lessens friction on the leadout guides.

George

Line bow is a factor that I hadn't really thought about -- good point, George.

For stunt planes that are tuned to fly a consistant speed, the bow should be about the same throughtout the flight, but it still supports the argument for adjustable leadout positions to get the best possible performance. With a lot of folks switching from dacron to Spiderwire or Spectra for 1/2A's, we've been able to reduce line drag and improve feel.

One thing about getting old, you seem to have more time to sit and think rather than runnin' and doin'. Neutral

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Post  gcb Tue Jul 10, 2012 7:08 pm

[quote="andrew"]
gcb wrote:
One thing about getting old, you seem to have more time to sit and think rather than runnin' and doin'. Neutral


How true, How true!

George
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