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.