Two wires.

Interesting because it's a positive no slop set up.

Could weigh a bit more?

Guy's thinking.

CB

Randy, do you know what the action of this thing is, and why it's supposed to enough better to justify all the extra pivots?

It's kinda interesting.  It gives you more mechanical advantage over hinge moment.  I'd use a value of a/b of .3 or .4.  Behold that b could be greater than a or the output arm could stick out the other way.  Another option would be to implement Igor's mechanism using this to drive the flaps and a conventional pushrod from the bellcrank to drive the elevator.  You'd have to do a 3D analysis (you could use my program) to get them symmetrical.  This might weigh more than doing it Igor's way, but it would eliminate the evil of mass behind the flap hinge.  How do you like that sign reversal in the slope?

This from Yves Fernandez via Jan Skrabalek (I think that's the name). kinda of a new twist on the Expocrank.

   Inverse expocrank, faster around neutral than the ends.

    Brett

This is logarithmic bellcrank of Jan Skrabalek (slovakia). He had it in Hungary if someone remember him.

He did it to make easier flying in corners - lower feedback and lower sensitivity in corners - means exact defelection of handle in corners is less critical

I think logarithmic function (unlike expo) is much better for large models, while expo is better on small models. I am very confortable with my logaritmic device on flaps, while I wold like to have expo on small indoors, because it makes troubles to fly perfect level and still enough sensitivity in corners.

However I prefer to have logarithimic device on flaps only, not on elevator, because it makse level and rounds with nice reponse with lot of flaps, while corners are still easy to do. This bellcrank makes flap-elevator linkage 1:1 and only linkage to handle is logarithmic. I think its function can be done with circular or eliptical bellcrank with lines, instead of fixed point leadouts from arms.

Variation on the theme, as designed by Fred Bachl in the early 1960's  I think this is a somewhat softer transition than the opposing crank arm would be.  I'll find out this summer if I like it...

Looks like the opposite curve.  According to Igor you should keep it indoors.

Golf Dome here I come...! 

That was my first impression.  A negative expo-crank.  This would be the wrong way to go for me.  Personally, I would love to have just a little desensitized neutral and the normal amount of deflection for corners.  That would be a matter of experimenting on the geometry of the Expo crank.  Denny, I have a feeling that you may like this one.  Don't see why it matters what sized model it flies in.  The Expo concept is something I would like to try.
Pat Johnston
Skunk Works

Seems to me the only way to get what Pat is talking about is to put slop in the controls at neutral and for me that's a no-no.

Seems to me the only way to get what Pat is talking about is to put slop in the controls at neutral and for me that's a no-no.

    I think that what Pat wants is pretty much exactly what the Expocrank does. I wouldn't think it would be a good match with his general design approach (which would seem to me to be pretty slow around neutral already - although not as much as the Cardinal/Patternmaster type designs) but there's only one way to find out for sure.

   I hadn't previously considered the mechanical advantage issue that Howard notes. I guess if you are slow around neutral you have to catch up at some point and if you are catching up the control rate has to be faster and thus give you less leverage. That might be a double whammy on the giant flap models. But it might good on short-coupled airplanes that tend to be a bit twitchy in the first place - sort of like the Humbug.

     I would think it would be more advantageous/less of a disadvantage on the Imitation-style models that tend to be a little quicker around neutral to begin with as opposed to the giant flap models that already take two hands to get out of level flight. The original Imitation and designs around that era also used circular bellcranks which are actually faster around neutral when you use a straight handle, but have more mechanical advantage at the extremes - where you NEED more mechanical advantage.

     If anyone is worried about slop, a better solution might be a "circular" bellcrank that *isn't circular*. If you make it elliptical or some other shape you think will work better, it can have whatever response you want, and no slop. If you have a take-apart it would be a relatively simple matter to make the bellcrank removable and then you can experiment (and then probably go back to a standard bellcrank when you give up).

   All of this can be pretty easily controlled without mechanical monkeying around. If you have enough TVC and enough tail moment you can freely alter the CG and handle spacing to get it slow around neutral or fast around neutral. If you have adjustable flap/elevator rates you have even more freedom. That's what I do - and I have to tell everybody that my problem has been getting it to be FASTER around neutral without being crazy in the middle of the corner. I realized about halfway through the 2011 NATs that I was grossly overshooting the controls to get the corners started and stopped - hitting as much control as I could get to start, backing off in the middle, and then overshooting the other way (i.e. briefly putting in down in an inside corner) to get it stopped. That picture in SN showing my wrist about bent double is almost certainly at the start of an outside corner, and if it had been 10 ms later it would have been a third of that.

   Getting them to track in level flight is easy enough as long as you don't listen to 70's style trimming notes.

     Brett

I ended up coming back to an Adamisin type bellcrank with the flap arm in front and the bent arms.

Bent how?

Searches here and SSW were for naught, but Google has Dennis writing of bellcranks with 10-degree angles.  I presume that is with arms bent 10 degrees each toward the inside of the circle.  Do a lot of people use these?  By "front" do you mean toward the inside of the circle?

Howard,

Yes, each arm is "bent" about 12° toward the inside of the circle and the control arm is on the inside (as opposed to the outside) side of the circle.

I've seen those.  I suspect that the control arm being on the inside doesn't do much to the action one way or another, but the offset holes would.

So, why say you that it isn't self-centering?  It would tend to pull toward the center any time it's offset.  Or is it just that you don't feel that the "self centering" action is what really makes a difference?

The idea (as I understand it) was to help the controls to neutralize when line tension was lost (Dennis will probably jump in here and beat me with a club). My point was that without line tension, it wouldn't do anything. What it does do is make it easier to neutralize the controls in areas where that's advantageous. Such as when exiting a corner. I like that a lot. I tend to bounce fewer corners with this sort of bellcrank. I saw a real advantage to it, at least for me, and that's why the last 3 or 4 planes I've built have had one.

I only keep stuff that it seems like is working for me, not because it looks cool or whatever..

Wait, you mean Brett and Howard are not the same guy?   

Brett,

OK, I suppose that's true. But in the end, it's simple and it works so I've kept it in.

If one wants to mess with non linear controls, all kinds of responses can be made by whacking out a handle with an insert between the lines. For instance, a square shape with 3 inch line spacing at neutral would widen out to ~4.25 at 45 degrees handle movement.. Too much?? make it only 2 inches wide etc. By cutting different shapes you can get about anything you want and it requires no changes to your test mule. Just whack out a temporary handle and try it! John Wright did this using a round insert so that the line spacing would not lessen as he tilted the handle. I think I did a piece in Stunt News about this a long time ago.

Does anyone have photo of the "Expocrank design"? Which issue in Stunt News?

Thanks

Roger.

Does not look very intelligent, strenghtwise. L

Design something better.

Just close the near end of the slot.

I have been doing some ciphering on control systems, although lately it's been sidetracked by sanding.    Don is probably on the right track.  Think of Dolby noise reduction. 

I'm not sure what that would contribute to strength ............

From what I can see you have in effect the jaws of a pair of pliers being pulled shut by line tension onto your push rod mounting post.

Closing off that open end would definitely stop this effect from happening and could simply be effected by adding a brace over that gap after wards if you wish to retain the simplicity of manufacture.

I once made bellcranks out of 1/8" PC board. Worked out pretty well, actually.

So the advantage of the expocrank is??

So the pushrod doesen't work in such an arc?? Am I right???

Roger

Yes Ken, teoretically correct, but unlike R/C we feel what is happening in the model, so that exponential function eliminate feedback in neutral and magnify in deflections, just opposite we want on large models

so it can make so strong feedback in corners, that it coul make model insensitive in corners "forcewise" ... means large model can make just opposite feeling than you want, that was why I wrote that expo crank will be benefical on indoors, and that is reason why we fly rather logarithmic functions on large models.

I once made bellcranks out of 1/8" PC board. Worked out pretty well, actually.

We would cut up PC boards and use them for servo trays in pattern ships.

Charles

Larry,

Ya know, most that know me will admit that I've never been opposed to experimentation. From high aspect ratio planes to goofy engine setups to bizarre control systems, I've tried a lot of stuff. Real fliers such as Paul Walker or Ted Fancher have often told me I was, ah, probably not using my time as wisely as I might. I've had it explained to me any number of times that just going with proven components and designs are more likely to result in success. And it's even true. But I'm an experimenter at heart and have continued to try new stuff. I've done expocrank systems, a sort of strange logarithmic crank not unlike your illustration above, a cable system with sliders and pulleys in place of a bellcrank (Ted even flew that plane - the idea being to develop a linear system), circular bellcranks and one really odd direct cable system with pulleys in place of both bellcrank and control horns. That one was fun. Over the course of 10 or 12 planes (mostly in the 80s and early 90s) I tried all kinds of stuff. Upshot was, I never saw any significant gain over the boring old standard bellcrank and horns setup. In some cases, I saw a significant degradation compared to a standard system. Once or twice I tried to convince myself the the slick, new, never before tried system was superior and it may have been, but not enough to put up with the hassles and point failures that such a system presented. About the only thing I've kept in was much longer control horns and bellcranks just to increase the mechanical advantage. It's the only thing that seem superior to the more conventional setup.

I'm all for innovation, but it's also nice to have a system that you just know is going to work. There is something to be said for consistency and predictability.

Right you are, Larry. My point was just that I've been down many "less traveled" roads and what you usually find is a cliff at the end. But it depends on what you're looking for, I suppose. I had an awful lot of fun on those roads.

In spite of a normal human tendency to believe we have discovered the Holy Grail of features for our stunter, it almost certainly isn't. However, if you really look at the progression of construction, finishing and power techniques and features, you can see major improvements. Like nearly all real things,progress occurs in small incremental steps, and the summation of features is really what makes a prime competitive model airplane possible.

I remember the first time I flew a decent flapped stunter with a quality engine. I remember how smooth and sweet it felt; it was so refined over Fox combat engined Flight Streaks I had built and attempted to learn {outside loops!} on in high school. (The difference was astounding, but I really wish I had back one of my old Streaks for nostalgic reasons - I understand why we have such groups as the Brotherhood of the Ring!)

Witness the cumulative effect of minor improvements on any modern CL stunt ship! The simple fact we want to know about things, even less important or obscure things, is what keeps CL stunt such a rich and interesting hobby for me.

L.

"The greatest homage to truth is to use it." -Ralph Waldo Emerson

I can say that my results on EC and WC are significantly better with logarithmic crank and worse with ususal 1:1 linkage. And I used it mixed, sometimes yes and sometimes not. "With" was visibly better than "without" ... but I can say that I did not feel difference myself, just results was much better :- ))) ... may be judges see the difference

The exponential and logarithmic bellcranks produce opposite effects. With a conventional simple bellcrank, sensitivity is maximum around neutral, and the logarithmic bellcrank will increase this effect. The exponential bellcrank has decreased sensitivity around neutral and more sensitivity at deflection.

I'll try to attach a PDF of a reprint of my old PAMPA Stunt News article, which includes plots that should clarify its effect. The plots are for the same geometry (bellcrank drive radius = .75" and swingarm drive radius = 3") as my current version.

L.

"All virtue is summed up in dealing justly." -Aristotle

Hey, actual calculation.  That's a step in the right direction.  Now if somebody wants to go further and show handle torque vs. deflection, he could answer Ken's question about feel. 

I have some quibbles:

1. I don't think the names "exponential" or "logarithmic" are right: the names don't describe what the mechanisms are doing.  I did the figuring on the  "exponential" crank (page 1 of this thread), and it came out sines and cosines without the imagination required for exponential.  Correct me if this is wrong.

2. Many folks discussing this issue disregard the relationship between hinge moment and line elasticity (both from shape of the curve the lines take and the stretchiness of line material).  That's probably the most important consideration in control geometry affecting how accurately you can fly your airplane.  I'm trying to figure it out.  It's hard to do (for me).  From what I can tell so far, for stunt planes with flaps, it looks like the Slovakians' nonlinearity favors being able to fly accurate stunt.  Igor's application to flaps only looks even better to me (as shown below), but for all I know Dave Fitzgerald could be using an Expocrank.   

Howard, I know that guy from somewhere :- )))))) .. my wife has lot of pictures with him 

:- ))) ... so another proof of its function

I have a strong desire to implement a mapping ROM, with the look up table controlling deflection of right flap, left flap, elevators, and RUDDER with an optional field for ENGINE RPM (!) vs the bellcrank transducer output, so we REALLY can play with this stuff.

(You guys left out differential flaps! Maybe we want an exponential inboard and logarithmic outboard flap.. did you think of that?)

Also we may want a "pilot feedback" field which will stiffen the bellcrank transducer according to deflection.

And we definitely need a 16-position rotary switch for Pilot Select:
0) {1-1 Conventional Bellcrank Emulation}
1) Howard Rush
2) Igor Burger
3) Brett Buck
4) Ted Fancher
5) ..et al
:
E) ..et al fin
F) .. GEORGE {auto-pilot}

Hand me that PIC16F68 - I'll get right on it. ;->

L.

"The price one pays for pursuing any profession or calling is an intimate knowledge of its ugly side." -James Baldwin


 

I have a strong desire to implement a mapping ROM, with the look up table controlling deflection...

That's essentially what Keith Trostle did on his Bearcat's rudder, and it's mechanical and programmable!

All this time I thought it was Carl Goldberg, not Rube Goldberg models.. ;->

L.

"If you don't want to work, you have to work to earn enough money so that you won't have to work." -Ogden Nash

"If you don't eat your meat, you can't have your pudding!" - Pink Floyd

Hi again Larry.

I think it's still not good. In order to make the thing stiffer, you are just adding unnecessary material to the bellcrank. But the basic design fault is still there. If I were you, I would do as Howard suggested and close the other end of the slot so that it becomes an oval- shaped hole. Only that way you can have harmonic load paths on both tension- and compression side of the bellcrank.
 With your current shape, the compression side is problematic because the loads must go through a Z bend which eventually will flex and tend to close the slot.
 If you do as I suggest, you will end up with a lighter, stiffer and more stabile crank, and you have no need to use titanium. Just a good aluminium or correct type of plastic will do. And they are a lot nicer to work with.

From cold Moscow, Lauri
 

If a little is good, then more is better, and most is best of all.

By increasing the thickness of my overkilled expo bellcrank, as well as molding it from a new 3D cubic matrix composite structure from elbonia (which combines the strength of carbon fiber and the natural bearing properties of nylon), just look at the masterpiece of engineering and material construction I have created!

Stand back, you rubberneckers, this ain't no car accident.. ;->

L.

"It is only one step from the sublime to the ridiculous." -Napoleon I

So, Mark, you didn't buy the new 3D cubic matrix composite structure from elbonia (which combines the strength of carbon fiber and the natural bearing properties of nylon) part?

L.

"Only those who will risk going too far can possibly find out how far one can go." -T.S. Eliot

Here's a real Rube Goldberg expo setup mounted (grafted) to an existing flying tail airplane.  Its built to allow changes to the length of the swing arm, thus changing the gain of the system.  The overall throw is changed by resetting the height of the stabilator horn.  The output ball link and the swing arm ball link are mounted on a crosshead laminated from 3 pieces of pc board.  No exotic materials as my sole source of affordable supply is the scrap box.

Last photo.

Tom,

Is that model your design?

I'm working on a twin boom.

Charles

aha  ... I thought I did not get something what I should 

I dunno if it ever had any feedback worth mentioning because the stabilator axle is at 25% MAC.  I'll have to fly it and see.  I could move the axle if it seems to need it, but my guess is that the gain should be optimized first.  Make sense?

     Larry - The static adjust slider horn alone would give about the same effect as a wider handle spacing.  The effect of the 'expo' linkage is to make the entire system less sensitive around neutral, with faster control deflection near the end of travel.  The prior flights on this airplane, and an earlier version flying tail, both resulted in a lack of finding a good compromise between acceptable neutral stability and adequate cornering.  When the CG was moved aft, or the handle spacing increased, or the stabilator horn shortened, it would reach some pretty aggressive turn rates, but at the cost of a 'hunting' level flight.  Hence, some form of rising rate system is an attempt to get both neutral stability and hard turns. 

My Dad befriended Fred Bachl a few years back and bought a few of his plans. We showed them at the cub meeting in Columbus. The bellcrank became the topic of conversation over the stunt ships at the meeting real quick. Within a few weeks Bob Campbell had a working expocrank he made out of lexan for demo purposes. From there Joe Reinahrd  put one in the new club trainer. The club trainer was highly modified flying wing more like the Half Fast than the Stunt wing with an RC motor mount up front and a more stunt friendly airfoil which held a custom tank neatly concealed in the LE.  The thing was pretty rugged and was perfect for putting new people in the air with a hand launch out in the grass anywhere big enough. Capable of doing a recognizable pattern and any other zorping around you might want to do. But, ...I never got a warm fuzzy out of the feeling at the handle based on what the expocrank was bringing to the equation. JMHO



It was stable and easy to fly, but i never got used to the feel

Hi Larry!

I certainly remember your articles on this of course. I suppose you may be right. Perhaps, had I flown an expo/logari crank "exclusively" for a bit, I might have developed a better feel for it. I certainly would not be one to argue that they aren't a big plus in a real stunt ship either. But I did get to fly it a fair amount...and I flew the crap out of it and showed it no mercy just to see if it would put up a dog fight as a slow combat ship. Still a hasty conclusion on my part.