stunthanger.com

General control line discussion => Open Forum => Topic started by: Matthew Brown on June 23, 2016, 06:57:02 AM

Title: So why is a 4" bellcrank so much better than a 3"?
Post by: Matthew Brown on June 23, 2016, 06:57:02 AM
After spending a bit of time learning about stunt and related gear, 4" bellcranks seem to be somehow better than the old standard 3 inchers. I've thought about it a bit but have not come up with any real gain to be had. I know it's there or they wouldn't be so popular. Even one of the local guys used to make nice custom carbon fiber bellcranks to sell.
So, what are the big advantages?

Thanks, Matt
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Dave_Trible on June 23, 2016, 08:16:38 AM
There are at least a couple reasons why most would prefer a larger bellcrank.  A four inch bellcrank is closer to the common range of handle line spacing,  giving something closer to a one-to-one feel or response from handle to bellcrank deflection rate-  slowing the rate down considerably over a three inch crank.  The feel is smoother and more linear.  Also long arms give more authority over the control surfaces of larger,  heavier airplanes and under windy conditions.

Dave
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 23, 2016, 09:51:17 AM
Hey Matt:

I'm going to more or less say what Dave said, in a different way.

Any method that increases the amount of line motion for the amount of surface deflection is good, because it reduces the amount of force needed on the lines for that given amount of control deflection (search on "Netzband Wall").  That reduction, in turn, means that you have more control over your plane at all times, but particularly when the lines start going slack.

A 4" bellcrank allows for more total line travel, which -- with the right connections to the control system -- allows for this higher line-to-elevator ratio. 

A bigger bellcrank yet would allow for even more travel yet, leading to the obvious conclusion that you should use a two-foot bellcrank if you could only make it fit.  I don't know how many have tried even larger bellcranks or why they aren't prevalent (actually, I think some people have used 4 1/2 inch bellcranks, but I can't be sure).  Perhaps someone will comment -- one could certainly cram a 6" bellcrank into a full-sized stunter wing, if one were willing to make a handle to match.  I suspect that folks don't like the resulting exercise in shoe-horning, nor the angle of the rear leadout.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Dave_Trible on June 23, 2016, 11:21:33 AM
I know some have tried bigger cranks-Windy I know- but don't know their final result.  There will be a place going bigger where it would be much like flying much too nose heavy.  You'd have to move the stick too far to get reasonable square corners but there isn't enough time to do it effectively and you get a gallop at each corner because you couldn't neutralize quickly enough.

Dave
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 23, 2016, 11:49:24 AM
I know some have tried bigger cranks-Windy I know- but don't know their final result.  There will be a place going bigger where it would be much like flying much too nose heavy.  You'd have to move the stick too far to get reasonable square corners but there isn't enough time to do it effectively and you get a gallop at each corner because you couldn't neutralize quickly enough.

Dave

I was assuming that you'd compensate by making the handle spacing bigger.  At some point things get absurd, and 4" certainly seems to work.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Eric Viglione on June 23, 2016, 01:02:00 PM
I know some have tried bigger cranks-Windy I know- but don't know their final result.  There will be a place going bigger where it would be much like flying much too nose heavy.  You'd have to move the stick too far to get reasonable square corners but there isn't enough time to do it effectively and you get a gallop at each corner because you couldn't neutralize quickly enough.

Dave

Dave - Speaking of Windy, one thing I do really miss is his forked end carbon 4" BC. At the time, I liked them a lot even though I thought it was ridiculously over-built. Now... I have come to realize and appreciate that beastie all too late. The super thick carbon center section with large trumpet shaped eyelet support bearings made BC wobble almost impossible, and long as you installed the supports relatively snug and lubed everything before you buttoned it up. I had zillions (ok, like 1500 to 2K) flights on planes with music wire/arrowshaft/carbon sleeve/ etc. type installs using pounding ST60's, and those things never deteriorated into the BC wobbles with age.

I "think" some of the issue with modern BC arrangements and wobbles isn't so much the newer BC designs inasmuch as the additional side/top force loading of using ball links, which stand off above the BC, especially if you use the brass cone shaped spacer washer to give less angle to the taller horns. This gives less twist to the horn, but puts more load at the plane above the BC, no? I'll let a mechanical engineer chime in, I'm just guessing with my gut here...

EricV
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 23, 2016, 01:24:28 PM
Dave - Speaking of Windy, one thing I do really miss is his forked end carbon 4" BC. At the time, I liked them a lot even though I thought it was ridiculously over-built. Now... I have come to realize and appreciate that beastie all too late. The super thick carbon center section with large trumpet shaped eyelet support bearings made BC wobble almost impossible, and long as you installed the supports relatively snug and lubed everything before you buttoned it up. I had zillions (ok, like 1500 to 2K) flights on planes with music wire/arrowshaft/carbon sleeve/ etc. type installs using pounding ST60's, and those things never deteriorated into the BC wobbles with age.

I "think" some of the issue with modern BC arrangements and wobbles isn't so much the newer BC designs inasmuch as the additional side/top force loading of using ball links, which stand off above the BC, especially if you use the brass cone shaped spacer washer to give less angle to the taller horns. This gives less twist to the horn, but puts more load at the plane above the BC, no? I'll let a mechanical engineer chime in, I'm just guessing with my gut here...

EricV

Indeed it do.  I make bellcranks out of 1/8" phenolic, but then put tall aluminum bearings on them to control rocking.  The bearing is probably taller than it absolutely needs to be, but it's easy to make tall and one might as well use up the available space.  (Note that the bearing is bolted and glued with epoxy onto the bellcrank -- glue-and-screw is a pretty common practice in aerospace these days, because the strength lest by screwing things together is complimented by the strength of the glue, making a strong, light, easy-to-assemble joint).
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Dave_Trible on June 23, 2016, 01:37:34 PM
Yes Eric I'd sure have to think so.  I know that bothered me when working on take-apart ships.  I thought about mounting uprights on the bellcrank where the leadouts attach for a style of removable connection but the side thrust on the bellcrank bushing bothered me.  Should I have done that I would have mounted one over and the other under.  I chose to simply put them on permanently and cut them off when I had to.

See you in a couple weeks bud.

Dave
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Dave_Trible on June 23, 2016, 02:25:24 PM
Tim your arrangement looks like it would work fine as long as you don't get a crack-out around your bushing.  I might be inclined to build up a double or triple thickness in that area.

Dave
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Matthew Brown on June 23, 2016, 02:49:01 PM
Nice discussion! I can see that there is a degree of mechanical advantage as Dave pointed out in the first reply. I've always flown with the Fox handle with fairly narrow spacing. I'm not sure I'd ever be able to feel the difference but after my Twister, all my planes will get 4" bcs.
I notice in Tim's pics that his bc is mounted "backwards". Also the leadout holes are not in line with the bearing. Can you shed any light on these factors? These seem to me like they'd have quite a bit more effect on feel than going to a 4" bc over a 3" bc.

Matt
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 23, 2016, 02:57:14 PM
Nice discussion! I can see that there is a degree of mechanical advantage as Dave pointed out in the first reply. I've always flown with the Fox handle with fairly narrow spacing. I'm not sure I'd ever be able to feel the difference but after my Twister, all my planes will get 4" bcs.
I notice in Tim's pics that his bc is mounted "backwards". Also the leadout holes are not in line with the bearing. Can you shed any light on these factors? These seem to me like they'd have quite a bit more effect on feel than going to a 4" bc over a 3" bc.

Matt


There was a discussion about this a while ago and several of the top people were advocating putting the up line in front.  So, there it is.

The leadout holes aren't in line with the bearing, but there's plenty of area (IMHO) above and below them.  I suppose that in the long term it may wear unevenly, but I'm not concerned about that.

Note that Sig (and I think Brodak) both sell 4" bearings, so if you're not trying for all-out fancy you can get them fairly easily.  Having said that -- I'm trying to copy-cat the top guys, and they all want bellcranks with more or less these features:

Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Dennis Toth on June 23, 2016, 08:01:19 PM
Matt,
The 4" bellcrank, setup correctly, simply gives a little more linear control movements because of a bigger arc for the output arm. On smallish 35 size ships the leverage can be the very close with either size but the amount of control throw will be more limited with the 3" crank. On large ships because of the large size of the control surfaces the 4" crank has better leverage.

Best,   DennisT
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Target on June 24, 2016, 12:07:40 AM
What about a circular 3.5"- 4" bellcrank??
What advantages and disadvantages do the circular bellcranks offer?

Thanks,
Chris
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Air Ministry . on June 24, 2016, 05:44:26 AM
Got a Eather ' Firecracker ' plan , 112 mm bell Crank , almost 4 1/2 in .

(https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRyRhsTnxplShJMaC6DDaSaV4tuTjzc_rcH7fg2E7HKfW4OxUYtrw)

The longer levers on the 4 in. give less slack with wear / excess clearane , to the pushrod bush , too . Linearly - at the pushrod .

UM , as its longer its less % of movement . anyway.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Chris McMillin on June 24, 2016, 01:48:18 PM
Resolution. The ability to have the travel over as long a period as practically possible and still have adequate leverage. 4 inch bellcranks, over one inch tall horns, and 4 inch handles had made that possible.
One can believe that 3 inch are as good, but you'd be behind in resolution and in the top rank of competitors that would be the control issue changed to keep up.
Chris...
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Matthew Brown on June 24, 2016, 02:06:42 PM
Resolution.
One can believe that 3 inch are as good, but you'd be behind in resolution and in the top rank of competitors that would be the control issue changed to keep up.
Chris...

I can buy into that for the top ranks. At my age, I doubt I'll ever gain anywhere near that level of proficiency. True precision flying is something that is out of my reach physically. Between my reflexes and hand/eye coordination, I'll be doing good to make it to the middle of the pack in beginner class. On top of that I know any appearance points will be given away as well. One thing I was always in awe of was the finishes on the stunt planes and the pylon planes at the Nats. I found many years ago I am not a painter nor am I very talented when it comes to sandpaper. If I can't iron on covering, I won't finish it! I even pretty well suck at doing that, both application and color scheme.
Barring all that, I still have a great time!

Matt
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: FLOYD CARTER on June 24, 2016, 02:20:00 PM
Here is one problem with a 4" bellcrank.

We want the lines to exit as close together as possible in order to minimize the differential pull of the composite line tension from straying relative to the CG.

With a 4" bellcrank and a 1" exit line guide spacing, there will be a bend at the leadout guide, and this means a friction point.  It is possible to keep the bending friction low, but it is still there.

Floyd
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 24, 2016, 02:24:56 PM
Here is one problem with a 4" bellcrank.

We want the lines to exit as close together as possible in order to minimize the differential pull of the composite line tension from straying relative to the CG.

With a 4" bellcrank and a 1" exit line guide spacing, there will be a bend at the leadout guide, and this means a friction point.  It is possible to keep the bending friction low, but it is still there.

Floyd

Quite true.  But that friction is proportional to the line tension, and when the line tension is high the amount of friction is less important.  So, just when we want the friction to be minimal (i.e., when the lines are slack), the friction is minimal.

I think that if it really mattered, someone would use a pulley there.  I certainly haven't seen that done.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mark Scarborough on June 25, 2016, 12:06:30 AM
Quite true.  But that friction is proportional to the line tension, and when the line tension is high the amount of friction is less important.  So, just when we want the friction to be minimal (i.e., when the lines are slack), the friction is minimal.

I think that if it really mattered, someone would use a pulley there.  I certainly haven't seen that done.
*WARNING, WHAT FOLLOWS IS MENTAL MEANDERINGS FOR AMUSMENT PURPOSE ONLY ANY COMMENTS APPEARING TO BE FACTUAL OR BASED SOLEY,,  oh never mind, you get it right?

Tim , only commenting because ,,,, well because its me and I see a caveat here of sorts.
from another view there is an undeniable benefit initially to look at pulleys , HOWEVER
First pass~ mechanically, to do  pulleys, it requires an absolute minmum of two pulleys on each line.More likely three pulley wheels? constructed abstractly as light as possible. a mechanism stable enough to contain this in fairly critical alignment. lets say MAYBE you could do it in 1.5 ounces, now you will need an additional 1.4 ounces on the outboard tip to balance span wise. ( spitballed the factor for asymetric wings its a fudge factor for  eyeballing)* random guess gues I am to lazy*
so first blush, thats 3 ounces additional to gain an impercevable advantage in control geometry. Which begs the question, as Paul says, will it help you score better ? anyway, I was driving all over Washington state today and needed to flex my brain,,
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mike Griffin on June 25, 2016, 01:00:01 PM
The physical size of the model and your skill level plays into this also.  If you are a sport flyer only and are flying something the size of a Ringmaster or Flite Sreak,  the 3" is all you need.  If you are flying large stunters in competition , you will probably want to use a 4".

Mike
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Juan Valentin on June 25, 2016, 01:49:46 PM
    Interesting post,I`m building a profile P-51 and I already have the 3 inch bellcrank installed.I had looked at the 4 inch bellcrank and to me it looked too big for this plane. I wanted to slow down the flap and elevator movement so I went to a 1 1/8 flap horn.  the bellcrank arm is 3/4 of an inch. On a previous P-51 I had the flap horn was 1 inch and it felt good. Is my flap horn height ok?
                                                                                                                                                                                                                                                        Juan
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 25, 2016, 02:35:09 PM
Mark, I was not saying someone should try using a pulley in that application.  The phrase "over engineered" springs to mind -- and if I'm thinking it, you know that it's too much engineering!
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Howard Rush on June 25, 2016, 03:49:15 PM
   Interesting post,I`m building a profile P-51 and I already have the 3 inch bellcrank installed.I had looked at the 4 inch bellcrank and to me it looked too big for this plane. I wanted to slow down the flap and elevator movement so I went to a 1 1/8 flap horn.  the bellcrank arm is 3/4 of an inch. On a previous P-51 I had the flap horn was 1 inch and it felt good. Is my flap horn height ok?                                                                        

It is OK if the flaps move just far enough for the tightest turn you will make and the bellcrank moves as far as possible.  That will give you the most leverage over the hinge moment that you can get with the 3" bellcrank.  You can get this with different flap horn heights and bellcrank arms as long as the ratio between them stays pretty much the same.  The greater the flap horn height and bellcrank arm length, the less the pushrod force will be, and the less the effect that play in the linkage will have.  
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Target on June 25, 2016, 04:23:35 PM
All of that is almost directly comparable to a reasonable servo setup in a RC model, with the exception of speed of the control surface, which is decided at the handle for us....
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Howard Rush on June 25, 2016, 04:29:27 PM
Any method that increases the amount of line motion for the amount of surface deflection is good, because it reduces the amount of force needed on the lines for that given amount of control deflection (search on "Netzband Wall").  That reduction, in turn, means that you have more control over your plane at all times, but particularly when the lines start going slack.

A 4" bellcrank allows for more total line travel, which -- with the right connections to the control system -- allows for this higher line-to-elevator ratio.  

A bigger bellcrank yet would allow for even more travel yet, leading to the obvious conclusion that you should use a two-foot bellcrank if you could only make it fit.  I don't know how many have tried even larger bellcranks or why they aren't prevalent (actually, I think some people have used 4 1/2 inch bellcranks, but I can't be sure).  Perhaps someone will comment -- one could certainly cram a 6" bellcrank into a full-sized stunter wing, if one were willing to make a handle to match.  I suspect that folks don't like the resulting exercise in shoe-horning, nor the angle of the rear leadout.

Tim's got it.  

You are flying these airplanes with a spring (the lines) between the handle and the control surface load.  The bigger the bellcrank and the farther it moves, the more leverage you'll have over the control surface load, so the less effect the spring will have.  The spring characteristics are nonlinear and weird, and there's probably a point of dimishing returns for bellcrank size and rotation angle.  Hube Start gave a measure for that in Stunt News awhile back.  I have worked on calculating it, and I hope to publish a short monograph on it one of these days.   The airplane control system geometry and your physiology determine the line spacing at the handle: it's a dependent variable.  I flew today with 5.5" spacing.  

Leadout angle at the tips is a concern.  I've added that to my control geometry program.  Here's an example for a 5" bellcrank.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Howard Rush on June 25, 2016, 04:40:05 PM
The physical size of the model and your skill level plays into this also.  If you are a sport flyer only and are flying something the size of a Ringmaster or Flite Sreak,  the 3" is all you need.  If you are flying large stunters in competition , you will probably want to use a 4".

The physical size of the control surfaces in particular, especially flaps.  The bigger the flap area multiplied by flap chord you have, the bigger the bellcrank or the greater angle it needs to move.  Model weight is a consideration, too.  Curiously, the lighter the model, the bigger the bellcrank or the greater angle it needs to move.  Rich Porter flew 1/2A stunters on 70-foot lines by using a huge bellcrank. 

Skill level matters in that it takes a lot more skill to fly a plane accurately if its bellcrank is too small than if it's larger.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 25, 2016, 04:41:11 PM
Tim's got it.  

You are flying these airplanes with a spring (the lines) between the handle and the control surface load.  The bigger the bellcrank and the farther it moves, the more leverage you'll have over the control surface load, so the less effect the spring will have.  The spring characteristics are nonlinear and weird, and there's probably a point of dimishing returns for bellcrank size and rotation angle.  Hube Start gave a measure for that in Stunt News awhile back.  I have worked on calculating it, and I hope to publish a short monograph on it one of these days.   The airplane control system geometry and your physiology determine the line spacing at the handle: it's a dependent variable.  I flew today with 5.5" spacing.  

Leadout angle at the tips is a concern.  I've added that to my control geometry program.  Here's an example for a 5" bellcrank.

Clearly the answer to this is to revisit the Fancher Round Bellcrank, but to fix the spots where Ted obviously lacked imagination.

What you need is a drum, with the line wrapped around it several times so that it can rotate more than one turn in each direction as the handle is moved through its range.  This drum needs to be mounted on good bearings, and connected to the horn that actuates the push-rod with a gear or system of gears (alternately, it could have a scroll-and-pin arrangement).

Then, the only challenge becomes making a handle with a line spacing of several feet and as low a moment of inertia as Paul's current 18-gram handle.  But that's an easy enough task that we can let Mark do it.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Juan Valentin on June 25, 2016, 04:55:39 PM
Hello Howard
                            Thanks for the information in your response. I will try to keep system play to the minimum. A friend said to go to 3 inch line spacing in my handle.  The way I have it now I have 30 degrees of flap deflection at bellcrank full travel.
                                                                                                                                                                                                         Juan
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Russell Shaffer on June 25, 2016, 05:29:35 PM
Mark, wouldn't two pulleys work?  All you need to do is change the leadout direction to aim into the guide at the tip.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 25, 2016, 05:42:08 PM
*WARNING, WHAT FOLLOWS IS MENTAL MEANDERINGS FOR AMUSMENT PURPOSE ONLY ANY COMMENTS APPEARING TO BE FACTUAL OR BASED SOLEY,,  oh never mind, you get it right?

Tim , only commenting because ,,,, well because its me and I see a caveat here of sorts.
from another view there is an undeniable benefit initially to look at pulleys , HOWEVER
First pass~ mechanically, to do  pulleys, it requires an absolute minmum of two pulleys on each line.More likely three pulley wheels? constructed abstractly as light as possible. a mechanism stable enough to contain this in fairly critical alignment. lets say MAYBE you could do it in 1.5 ounces, now you will need an additional 1.4 ounces on the outboard tip to balance span wise. ( spitballed the factor for asymetric wings its a fudge factor for  eyeballing)* random guess gues I am to lazy*
so first blush, thats 3 ounces additional to gain an impercevable advantage in control geometry. Which begs the question, as Paul says, will it help you score better ? anyway, I was driving all over Washington state today and needed to flex my brain,,

Actually, now that I've made fun of myself for bringing it up, it occurs to me that if your purpose was to run a really big bellcrank, then putting a single pulley about mid-wing on just the rear line would significantly alleviate the problem of the rear line going through a sharp angle.  Basically, mount the pulley just far enough backward so that it never fouls the front line, and then angle the rear arm of the bellcrank to match the now-sharper angle of approach from the pulley, rather than the leadouts.

Note that I'm not going to run out and do this on my next plain while extolling it's virtues -- although I may give it a try.  I think you could make a pretty light pulley assembly if you worked at it -- but yes, anything that went into the pulley would also have to go into the outboard wingtip.

I did this drawing and then realized that one could (maybe) go even bigger yet.  This one, at any rate, shows a 5" bellcrank in a Twister wing.  Presumably one could go up to 6" in a "big" ship.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mark Scarborough on June 25, 2016, 10:55:29 PM
dont forget the complexity of cable retainage on the pulley, also the pulley would not be required to be a complete circle. only that part of the circle which the cable will actually touch, I have something niggling the back of my mind with the rear cable and the arc transcribed by the bellcrank arm being a different angle than that of the front line. I have not the math to calculate it, and my CAD computer is non functioning now
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 25, 2016, 10:59:59 PM
dont forget the complexity of cable retainage on the pulley, also the pulley would not be required to be a complete circle. only that part of the circle which the cable will actually touch, I have something niggling the back of my mind with the rear cable and the arc transcribed by the bellcrank arm being a different angle than that of the front line. I have not the math to calculate it, and my CAD computer is non functioning now

I've done the engineering.  Some drafting guy can flesh out the little details.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Dennis Moritz on June 26, 2016, 05:46:17 AM
On midsize planes. Magicians (48") even Orientals, I find a 4" crank too slow. Needing a wide spaced handle. I like small light handles. 3" or 3.5" cranks work better. My preference.


Sent from my iPhone using Tapatalk
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mark Scarborough on June 26, 2016, 10:50:10 AM
I've done the engineering.  Some drafting guy can flesh out the little details.
thats engineering, well hell , then I can do engineering too, I thought engineering involved like math, and calculations, and like, yeah you know,, engineering is the details,,
that sounds like the framer who says the sheet rock hanger will fix it,, the sheetrock guy says the tapers will get it, the tapers say the painters will get it, and the painters say th efinish carpenters will get it, and they say the owner wont ever notice,,
so yeah, engineering ok,, lol
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Target on June 26, 2016, 10:59:48 AM
That's pretty funny, right there!
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Steve Helmick on June 26, 2016, 01:27:07 PM
One thing there, Tim, is that the bigger the BC, the further aft the pivot point has to be, and then the shorter the BC > Flap Horn pushrod, the more rotational fubar induced. Tilting the BC would help some.

That pulley on the rear leadout is overkill, but if you were going to use it, I think you'd want to put it as close to the tip as you could. Then, you'd need more tipweight on the outboard tip, increasing the total weight. It creates a lot of problems, doesn't it? A 4" BC is fine, if the rest of the ratios are correct and allow a decent handle spacing. The wider the handle, the more accurate your input to the plane will be...right or wrong.   H^^ Steve
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Howard Rush on June 26, 2016, 02:21:14 PM
One thing there, Tim, is that the bigger the BC, the further aft the pivot point has to be, and then the shorter the BC > Flap Horn pushrod, the more rotational fubar induced. Tilting the BC would help some.


If you do the calculation (and somebody else has done the work: all you need to do is plug numbers in) , you'll find that the fubar is avoidable and tilt is unnecessary.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 26, 2016, 03:34:12 PM
That pulley on the rear leadout is overkill, but if you were going to use it, I think you'd want to put it as close to the tip as you could.

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.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mark Scarborough on June 26, 2016, 07:58:33 PM
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.
hopefully we are all just having a verbal exercise, I am pretty sure Paul would raise an eyebrow at the both of us were we to actually do this, right?
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 26, 2016, 08:29:17 PM
hopefully we are all just having a verbal exercise, I am pretty sure Paul would raise an eyebrow at the both of us were we to actually do this, right?

Both of us?  I thought you were going to go first!  Maybe we need to get the project manager to assign this one to Howard.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mark Scarborough on June 26, 2016, 09:45:13 PM
Both of us?  I thought you were going to go first!  Maybe we need to get the project manager to assign this one to Howard.
Howard already did it with his excell spreadsheet, and if you think Paul would give us grief? Howard would have it twice as bad LOL,,

Its an interesting excercise for sure, but I cant build accurate enough to be confident any improvment, or devolution of the response characteristics would be lost in the noise, not to mention my flying, well its getting better? but?

so Tim now that we have totally hijacked this thread, hows the shoulder, are you going to be contest worthy in August?
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: wwwarbird on June 26, 2016, 10:16:00 PM
    Interesting post,I`m building a profile P-51 and I already have the 3 inch bellcrank installed.I had looked at the 4 inch bellcrank and to me it looked too big for this plane. I wanted to slow down the flap and elevator movement so I went to a 1 1/8 flap horn.  the bellcrank arm is 3/4 of an inch. On a previous P-51 I had the flap horn was 1 inch and it felt good. Is my flap horn height ok?
                                                                                                                                                                                                                                                        Juan

 Juan,

 Where do the aluminum control horns come from? Nice stuff. y1
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 26, 2016, 10:46:38 PM
so Tim now that we have totally hijacked this thread, hows the shoulder, are you going to be contest worthy in August?

I think I have a shoulder again.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Target on June 26, 2016, 11:12:50 PM
Juan,

 Where do the aluminum control horns come from? Nice stuff. y1
I agree!
I was thinking of doing something similar in carbon fiber....
R,
Chris
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Juan Valentin on June 27, 2016, 06:50:48 PM
Hello Chris and warbird
                                                The aluminum control horns I made myself from a piece of 2 inch angle extrusion that I had left over from some shelves I was making for my Triumph motorcycle parts. I like that they are stiff and can be threaded so I dont have to use a locking nut, I use Tamiya threadlocker used in R/C cars. I had done a couple for my previous P-51B which I lost due brain fade. I put some pieces of dowel in the flap and elevator so when I tighten them I don`t crush the balsa. here are some pics.
                                                                                                                                                                              Juan
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Tim Wescott on June 27, 2016, 06:57:48 PM
Beautiful work, Juan.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Target on June 27, 2016, 07:29:29 PM
Yeah, that is looking pretty nice, Juan.
I especially like the latest version you did with the offset for the bell crank pushrod as well as the elevator push rod....
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Air Ministry . on June 28, 2016, 08:17:44 AM
Probly Being an old woman , but a bone shaker can crystalise Alluminum in no time .
I guess thats a ABC in there , not an old feral Er Ferous Piston Job .

Incidently Bently adopted alluminum pistons for the RNAS  in the first war .

Is Your Triumph an old Boneshaker or one of the Modern ABC ones .  S?P

The p.u. Bonevel shed Jap Parts indescriminantly , British mostly stayed put .

NOW , if youve got a 3 in. bellcrank in a 60 Oz ( odd )  .40 up airplane ,
the swines usually pulling sufficent that theres little Line Weighting Differance
between the two ( up'n down ) so theres a ' on ice'ish feel to them .

The Greater Spread of the 4 in give a bigger differential between em in manouvres ,
so the feedbacks better .

First Flight of the Whirlwind ripping round on 60 ft of .016 I think , had a bit of a porpise low .
Was going to close the line spacing as it felt touchy . Decided it was line stretch ' weaving '
in the breeze , OPENED the Line Spacing and it improved things no end , and no porpise .

If You Fly with Excessivly ?? ?? heavy lines , you CAN get virtually No Stretch . Thus Very Direct Controls .
This Is Why some cunning sods us single strand . Its Dirt Cheap ( if you buy by the pound ) so can toss
wobbly pigs tail ones no worries , as you need to. Theyre a bear to look after .

Got A REEL of .018 single strand , as I figure 70 Oz in a good breeze will need it .
18 Thou 7 strand steel can twang in the bottoms of squares when she's blowing .

And good british steel 7 strand .018 Laystrate is getting as rare as people with No Television .  mw~

(http://i.ebayimg.com/00/s/MTQ0MFg4MTA=/z/D7AAAOSwGYVXCKNf/$_57.JPG)

preume that mayt be the real stuff . Maybe .

(http://www.rcuniverse.com/forum/attachment.php?attachmentid=2010540&d=1404290146&thumb=1)

(http://www.f2d.dk/equipment/liner/pics/laystrate.jpg)

was 3 strand .012 , 5 strand .015 ( for COMBAT and light / 35 stunters . and 7 strand .018 . Dunno what the S.M.A.E. or N.Z.M.A.A. said that was for . But a  G - 51 without its liable to get interesting .

Id better get a picture or two of my packaging for it , as its vastly differant . A Tag . Modelair did lines in NZ into the 70s .
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Ted Fancher on June 28, 2016, 10:05:50 PM
On midsize planes. Magicians (48") even Orientals, I find a 4" crank too slow. Needing a wide spaced handle. I like small light handles. 3" or 3.5" cranks work better. My preference.


Sent from my iPhone using Tapatalk

Two things, Dennis.  First, the four inch crank is only one of three levers affecting the displacement of the control surfaces (which is what you're talking about).  i.e. the line spacing at the handle; the length of the BC leadout arms and the ratio of the BC output arm to the flap input arm.  The ratio between the BC output "arm" length and the flap horn input "arm" can be can be configured to allow the same amount of control surface deflection with the four inch crank as with the three inch.  IMHO doing so on airplanes like the Oriental and similar isn't worth the effort of squeezing the larger crank into an appropriate chordwise location in the wing.  I've never bothered to use larger bellcranks on my classic/OTS ships as the only advantage of the larger BC is not necessary...i.e. the increased leverage for a given amount of line tension to overcome the control surface loads.

It's worth noting that a moderately recent-past multi-year Walker Cup Champion who felt a mass produced nylon three inch BC was adequate for pretty much any stunt ship once published pictures of modified handles he employed to achieve the response he needed to fly competitively.  The handles were modified Hot rocks which extended the line spacing to as much as six inches.  Also of interest with respect to this thread is that he advocated large chord flaps and forward CGs on his designs which, mathematically, compromised the ability of the stock three inch nylon bellcrank to provide adequate leverage to deflect the control surfaces far enough to overcome the response deadening forward CG location by deflecting the flaps and elevators far enough with the available line tension multiplied by the shorter arms of the three inch B/C.  Wow!  That was a dam long sentence.  sorry.

Note also, achieving the necessary control deflection even with the large line spacing at the handle could even require inquire pilot inputs that momentarily increased line tension (pulling the handle toward the pilot during inputs) to deliver the necessary force to displace the control surfaces.  A classic case of the Netzeband wall conundrum.  Alternative means of achieving desired response might have included a more aft CG location, smaller flap chord and/or movement and/or a larger area tail...or, a larger diameter bellcrank which would multiple the line tension/arm multiplier enough to allow the surfaces to be deflected adequately to achieve the desired response without extra "muscle".

While our friend Howard can quantify and illustrate the details, the bottom line is that longer armed bellcranks with geometrically appropriate flap/elevator horn arms can be configured to permit all the necessary control input/output/response necessary for competitive performance using a 60+ year old Hot Rock handle applying comfortable pilot inputs.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Ken Culbertson on March 05, 2018, 01:41:13 PM
Quite true.  But that friction is proportional to the line tension, and when the line tension is high the amount of friction is less important.  So, just when we want the friction to be minimal (i.e., when the lines are slack), the friction is minimal.

I think that if it really mattered, someone would use a pulley there.  I certainly haven't seen that done.

I know this is about a half year late but I was scanning older posts looking for something on appearance points when I came across this.  I chuckled that no one had tried them and I know why. I tried pulleys in the late 60's.  I found one of them yesterday in a box of old hardware.  I had written "DO NOT USE" on the casing.  If memory serves me right they were no better than a standard bellcrank and had a fatal overcentering problem if you didn't put in effective stops.  You cannot over-center a standard bellcrank.  I think others have used them and some may still be out there.  My hat is off to anybody that is having success.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Serge_Krauss on March 06, 2018, 02:47:54 PM
That directs the line tension (full centripetal pull) at the control horn/hinge. That can't be good for large planes.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: phil c on March 07, 2018, 04:43:05 PM
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.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Steve Helmick on March 08, 2018, 08:53:28 PM
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.  y1 Steve 
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Brett Buck on March 08, 2018, 08:59:27 PM
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.  y1 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
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Steve Helmick on March 08, 2018, 10:09:03 PM
    ??? 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>   :P Steve



Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Brent Williams on March 09, 2018, 02:03:07 AM
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>   :P Steve

Here's the Primary Force plan showing the orientation of the bellcrank.
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Steve Helmick on March 09, 2018, 03:30:34 PM
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
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Brett Buck on March 13, 2018, 11:02:56 AM
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
Title: Re: So why is a 4" bellcrank so much better than a 3"?
Post by: Mike Haverly on March 13, 2018, 12:25:48 PM
Pretty close Steve, I did measure it and it is .270".