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[Motorman]

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[john e. holliday]

Supposedly they were kind of self centering.

[Ted Fancher]

Supposedly they were kind of self centering.

Which would be fine if, in fact, the point of such self centering happens to co-inside with the point at which level/straight ahead in maneuvers segments is achieved.  If they don't the self-centering, if significant, will result in either a climb or dive...ooops!

The way most bellcranks are designed and installed the latter case is probably the norm.  Bellcranks are almost universally designed so that the pushrod drive arm is at 90 degrees to the points of leadout attachments.  The vast majority of fliers install their bellcranks with  the leadout attach points parallel to the centerline of the aircraft.  They then install the leadout guide to accommodate the leadout sweep due to line drag simultaneously destroying the desirable overall relationship of all the arms and angle operating a 90 degrees to one another (the argument/discussion re a vertical handle position versus "relaxed pistol grip" being a classic example.

The consistent 90 degree  relationship in a stunt ship's control system is a functional step toward balanced inside/out side response rates.

Ideally, in my opinion the proper relationship of the leadout attachments to the pushrod output would be such that the leadout attach points would be at an angle 90 degrees to the leadouts as they enter/exit their guide.  Given the fact that final leadout location is seldom 100% predictable an angle of ~2.5 degrees would be a good basis on which to design a crank for a nominally "normal" configured aircraft.

The custom made circular bellcranks I used for several stunt ships back in my "Imitation/Excitation" era employed this design configuration; i.e. the pushrod attach point was.  The circular configuration automatically assured the axis of rotation would always be at 90 degrees to the  pivot...i.e. no "self-neutralizing"effect per the subject of this thread.

Ted

[Tim Wescott]

... The vast majority of fliers install their bellcranks with  the leadout attach points parallel to the centerline of the aircraft.  They then install the leadout guide to accommodate the leadout sweep due to line drag simultaneously destroying the desirable overall relationship of all the arms and angle operating a 90 degrees to one another (the argument/discussion re a vertical handle position versus "relaxed pistol grip" being a classic example ...

Hmmm.

[Brett Buck]

Supposedly they were kind of self centering.

   That depends on the angle the leadouts converge with each other. A "straight" bellcrank is actually slightly "uncentering"/unstable, assuming the leadouts are closer together than the span of the bellcrank.

   To get it neutral, the angle from the leadouts, to the attachment point, to the pivot, needs to be 90 degrees.

    The SIG bellcrank has much more angle than necessary for neutral, so, it really does "self center". And note well Ted's point - the bellcrank will want to center itself so that the angles noted above are equal - which is swell of that's where the aerodynamic neutral happens to be, and not so swell if that angle happens to give you up or down elevator or flap. It's just like having handle overhang, and the self-centering is exactly the same phenomenon.

     I wouldn't recommend self-centering bellcranks, but if you want to try it, make sure you can adjust it to put the bellcrank at its ideal position when the airplane wants to fly level. If you just line it up with the centerline of the airplane like it shows on almost every plan ever drawn, you will likely have to fight the self-centering tendency just to fly level, and expect different control pressure on "up" vs down.

      Brett

[Howard Rush]

"Self-centering" bellcranks might be a good idea if done right, but I suspect the sign is slightly wrong. 

[BillLee]

Self-centering bellcranks are quite useful in other disciplines..... The bellcrank in most F2A models looks like a big 'Y'.

[MikeyPratt]

Which would be fine if, in fact, the point of such self centering happens to co-inside with the point at which level/straigrht ahead in maneuvers segments is achieved.  If they don't the self-centering, if significant, will result in either a climb or dive...ooops!

The way most bellcranks are designed and installed the latter case is probably the norm.  Bellcranks are almost universally designed so that the pushrod drive arm is at 90 degrees to the points of leadout attachments.  The vast majority of fliers install their bellcranks with  the leadout attach points parallel to the centerline of the aircraft.  They then install the leadout guide to accommodate the leadout sweep due to line drag simultaneously destroying the desirable overall relationship of all the arms and angle operating a 90 degrees to one another (the argument/discussion re a vertical handle position versus "relaxed pistol grip" being a classic example.

The consistent 90 degree  relationship in a stunt ship's control system is a functional step toward balanced inside/out side response rates.

Ideally, in my opinion the proper relationship of the leadout attachments to the pushrod output would be such that the leadout attach points would be at an angle 90 degrees to the leadouts as they enter/exit their guide.  Given the fact that final leadout location is seldom 100% predictable an angle of ~2.5 degrees would be a good basis on which to design a crank for a nominally "normal" configured aircraft.

The custom made circular bellcranks I used for several stunt ships back in my "Imitation/Excitation" era employed this design configuration; i.e. the pushrod attach point was.  The circular configuration automatically assured the axis of rotation would always be at 90 degrees to the  pivot...i.e. no "self-neutralizing"effect per the subject of this thread.

Ted

Ted, Brett,
We have discussed this before, but I must be missing something in all of this.  I've ran this in AutoCad, Solidworks, and Pro E cad programs and they seem to work perfectly.  For every degree of bellcrank movement you get the same in the flap and elevators as far as degrees of movement, which is better than the old three inch bellcrank’s.  Can you explain this to me and why does Howard say there are advantages to them?  I've built all my models with this type of bellcrank with no ill affects what so ever.  I've never liked the name self centering bell crank, to be a true s/c bellcrank would take more than 10 degree of forward sweep, more like 20 degrees plus.

Thanks,
Mikey

[Brett Buck]

  I've ran this in AutoCad, Solidworks, and Pro E cad programs and they seem to work perfectly.  For every degree of bellcrank movement you get the same in the flap and elevators as far as degrees of movement, which is better than the old three inch bellcrank’s.  Can you explain this to me and why does Howard say there are advantages to them?  I've built all my models with this type of bellcrank with no ill affects what so ever.  I've never liked the name self centering bell crank, to be a true s/c bellcrank would take more than 10 degree of forward sweep, more like 20 degrees plus.

   I guess I don't understand the question, there's nothing wrong with CAD programs. The effect is entirely straightforward, if it's bent towards the leadouts at an angle that is greater than necessary to get a right angle with the leadouts, it self-centers.

     It's a weak effect, but its there, and a "straight" bellcrank is unstable, that is, it (weakly) wants to move away from neutral.

     Brett

[john e. holliday]

Look at the old Veco and Perfect bell cranks.   

[Howard Rush]

For every degree of bellcrank movement you get the same in the flap and elevators as far as degrees of movement, which is better than the old three inch bellcrank’s.

I don't think you care what the bellcrank is doing relative to anything else.  What counts is the relationship between leadout travel and control surface travel.  Not having the leadout attach points and the bellcrank pivot on the same line thickens the plot by making differential line tension a function of common-mode line tension.  Intuitively, I suspect that adding extra differential line tension to that caused by control surface hinge moment would make it harder to fly stunt accurately, but I haven't done the calculation.  My program assumes leadout attach points and bellcrank pivot are on the same line.  I might change the program to see if there's any advantage to a "self-decentering" bellcrank.  This analysis would be a lot easier to do with a kinematics program than with my VBA program. 

...why does Howard say there are advantages to them? 

Advantages to what? 

[MikeyPratt]

   I guess I don't understand the question, there's nothing wrong with CAD programs. The effect is entirely straightforward, if it's bent towards the leadouts at an angle that is greater than necessary to get a right angle with the leadouts, it self-centers.

     It's a weak effect, but its there, and a "straight" bellcrank is unstable, that is, it (weakly) wants to move away from neutral.

     Brett

Hi Brett,
Yes, my point is as you stated, when the bellcrank is straight (attachment holes and pivot hole lined up) it is slightly unstable (weakly).  That was the reason for my reason to explore the swept forward arms to eliminate this weakly effect.  I tested bellcrank’s with 5, 10, 15,  and 20 degrees of forward sweep to the bellcrank arms.  The 5 degree crank did almost nothing, the 10 degree swept arms was the best and the 20 was the worst. 

The reason that 5 degree crank didn't work very well was because our models fly with a 3 degree + - sweep in the lines effectively giving me a 2 degree sweep in the bellcrank not 5 degrees of sweep.  Also, the 10 degree B/C is a total of 7 degrees of forward sweep.

I have two FP 20 with iron pistons, do you know where I can purchase ABC set up for them?

It's good to chat with you again old friend,
Later,
Mikey

[Brett Buck]

The reason that 5 degree crank didn't work very well was because our models fly with a 3 degree + - sweep in the lines effectively giving me a 2 degree sweep in the bellcrank not 5 degrees of sweep.  Also, the 10 degree B/C is a total of 7 degrees of forward sweep.

     You are using sweep differently than I usually do, but I think by "sweep in the lines" you mean the angles at which the leadouts converge, which is also what I am talking about.

https://stunthanger.com/smf/open-forum/carbon-bellcranks/msg354907/#msg354907

    Ted was talking about the fact that you have to consider the issue described in the very last drawing in that post, too, or you will have to fight the self-centering action.

   I would also note that a circular bellcrank automatically gives you a neutral stability.

I have two FP 20 with iron pistons, do you know where I can purchase ABC set up for them?

     No, and I don't have any. I do have complete engines, PM me if interested.

     Brett

[Howard Rush]

Can you guys feel a deleterious effect from a misaligned straight bellcrank, or do you just intuit that you would?  I just did some crude ciphering using my control geometry program.  With bellcrank deflection of about +/- 70 degrees, it looks like the straight bellcrank would give about .4 lb. of differential line tension at max control in either direction.  Around neutral, though, the curve appears to flatten out, kinda like a cube function.  What did you get from your calculation?

I wouldn't think there would be any perceptible destabilizing because this effect would add to hinge moment, and hinge moment would dominate around neutral control position.  Mind you, I haven't calculated or measured hinge moment, so I don't know the relative magnitudes of this bellcrank effect and hinge moment on differential line tension.  I'd be inclined to use a self-decentering bellcrank to counter hinge moment, but the bellcrank effect increases proportional to line tension, which is opposite of what you'd like it to do to counter hinge moment.  Thanks for bringing this up (again, in Brett's case).  I shall fuss with it more before making the bellcrank for my next dog. 

[pmackenzie]

Can your program plot linearity (handle motion to elevator motion) around neutral and at the end points?
Those sort of differences (between up and down) might be more a factor than control loads.

I never really notice control loads, will have to pay attention to it next time I fly (which won't be for several months  )

In either case the human mind can easily learn to correct for these sort of differences, so you are unaware of it. (pistol grip style handles for example)
Then initially the "correct" setup will seem odd, till your brain re-learns.

Pat MacKenzie

[Brett Buck]

Can you guys feel a deleterious effect from a misaligned straight bellcrank, or do you just intuit that you would?  I just did some crude ciphering using my control geometry program.  With bellcrank deflection of about +/- 70 degrees, it looks like the straight bellcrank would give about .4 lb. of differential line tension at max control in either direction.  Around neutral, though, the curve appears to flatten out, kinda like a cube function.  What did you get from your calculation?\

   It's very weak compared to many of the other things (mostly importantly, the handle offset, which is always "self-centering" and very strong). That's how people got away with both Ted's issue (putting it in at the wrong angle) and the instability for decades without ever even realizing it. Since I was making my own bellcrank, I decided to correct it to neutral whether it made any difference or not. Since there are so many other things that also matter and are much larger effects, I couldn't say if I could tell one way or another.

     Brett

[Howard Rush]

Can your program plot linearity (handle motion to elevator motion) around neutral and at the end points?

Yep.  Well, leadout travel to elevator motion.  It leaves out the springy lines, but I'm working on that.  Send me your email address and I'll give you a copy.