So why is a 4" bellcrank so much better than a 3"?

[Serge_Krauss]

That directs the line tension (full centripetal pull) at the control horn/hinge. That can't be good for large planes.

[phil c]

Aside from the tipweight issue, I'd want it about there to allow for leadout movement.

I think that it's probably overkill, too -- I just tossed it out there because folks were talking about two or three pulleys, and I saw a way to do it with just one.

With all the inboard weight worries, a piece of slippery plastic tubing, probably HDPE(Teflon is too soft) mounted on a 2in. or so radius piece of balsa tied to the spar or tip could be used to route one or both leadouts exactly where you want them.  Any friction would be pretty negligible.  Keep the deflection low, near the tip, the tubing as short as needed, and the radius as large as possible.  HDPE is extremely wear resistant, especially with such a light load,  millions of cycles.

[Steve Helmick]

Ken, are you referring to the "TED Bellcranks" that were semi-circular, or the earlier Hi Johnson circular bellcranks? I have one of the TED units. It has holes for binding wire to keep the leadouts in the groove. I recall the Johnson bellcranks enough to remember that there was a small round one and a larger one similar to Ted's design. I'd guess that they had holes for binding wires to keep the LO's from jumping ship, but I never used one of those. I did make a lathe-turned aluminum round BC and used it on a .40 Rat. Of course, it needed a long control horn and still needed narrow handle spacing...not good for flying in traffic due to the resulting lack of precision guidance.

Note that if you use a 4" SIG bellcrank, Mike Pratt says to mount it with the "V" so as to avoid self-centering...I believe that means front leadout is "up"...I have one of those also, and can look at it if anybody demands it.  Steve 

[Brett Buck]

Note that if you use a 4" SIG bellcrank, Mike Pratt says to mount it with the "V" so as to avoid self-centering...I believe that means front leadout is "up"...I have one of those also, and can look at it if anybody demands it.  Steve

    It's either self-centering, or it's self-uncentering, there's no way to make it otherwise. It needs to be mounted with the point of the "V" to the outboard side, which will make it self-centering.  The self-centering is undesirable in general but if you are going to use it, you sure don't want to put it the other way around.

    Front up or rear up line, makes almost no difference. I think it will end up "front up" with the SIG self-centering bellcrank.
    Brett

[Steve Helmick]

    It's either self-centering, or it's self-uncentering, there's no way to make it otherwise. It needs to be mounted with the point of the "V" to the outboard side, which will make it self-centering.  The self-centering is undesirable in general but if you are going to use it, you sure don't want to put it the other way around.

    Front up or rear up line, makes almost no difference. I think it will end up "front up" with the SIG self-centering bellcrank.
    Brett

The SIG 4" bellcrank has been discussed previously, and Mike Pratt commented about which way to mount it. I'm sure that thread could be found with a Googly search. I'd bet that he made some prototypes and tried the concept before having the molds made. I'm pretty sure that he said he did not want self-centering at all, and intended it to be un-centering. That's IF I remember correctly. The pushrod lever is in the crotch of the "V", anyway.

It really isn't very much, whichever way it is mounted, but somebody with a set of Mike's plans for the Primary Force may be able to dig out their plans and tell us which. I have a set of those plans, but can't locate them right now. Shop improvements....<sigh>    Steve

[Brent Williams]

The SIG 4" bellcrank has been discussed previously, and Mike Pratt commented about which way to mount it. I'm sure that thread could be found with a Googly search. I'd bet that he made some prototypes and tried the concept before having the molds made. I'm pretty sure that he said he did not want self-centering at all, and intended it to be un-centering. That's IF I remember correctly. The pushrod lever is in the crotch of the "V", anyway.

It really isn't very much, whichever way it is mounted, but somebody with a set of Mike's plans for the Primary Force may be able to dig out their plans and tell us which. I have a set of those plans, but can't locate them right now. Shop improvements....<sigh>    Steve

Here's the Primary Force plan showing the orientation of the bellcrank.

[Steve Helmick]

Brent, thank you for posting the PF plans showing the direction for installing the 4" SIG bellcrank. I haven't done any precision measuring of one to get an exact dimension, but it eyeballs at under 1/4" offset between the pivot hole and the L.O. holes. I believe Mike said his intent was to get closer to equal tension on the two arms while deflected, not to induce any centering or un-centering. Basically, shortening the arm being pulled on most, while lengthening the arm that is being pulled on the least. I think it makes sense, but I'm not 100% sure I would notice the difference, either.  Steve

[Brett Buck]

Brent, thank you for posting the PF plans showing the direction for installing the 4" SIG bellcrank. I haven't done any precision measuring of one to get an exact dimension, but it eyeballs at under 1/4" offset between the pivot hole and the L.O. holes. I believe Mike said his intent was to get closer to equal tension on the two arms while deflected, not to induce any centering or un-centering. Basically, shortening the arm being pulled on most, while lengthening the arm that is being pulled on the least. I think it makes sense, but I'm not 100% sure I would notice the difference, either.  Steve

     Take a square, put it with one side along the leadouts, and the end aligned with the hole where the leadout goes through the bellcrank. If the other leg of the triangle falls in front of the bellcrank pivot, it's self-centering. Even on the little P-Force, this line falls about 3/8" inboard of the pivot, so it self-centers,. Larger airplanes have the leadouts converging at a shallower angle, and self-center more.   

      The effect in this case is that the line with the most tension has more tension for a given torque than the "loose" line. The lever arm for the "tight" line becomes shorter as you deflect it, and the lever arm for the "loose" line gets longer. That's *why* it self-centers. It's the same effect that causes heavier control effort with an offset handle (just much less of it).

   Note that this is not an indictment, we all flew with bellcranks at least this much *unstable* without even realizing it for years.

     Brett

[Mike Haverly]

Pretty close Steve, I did measure it and it is .270".