slow start belcrank

[sleepy gomez]

Would there be a benefit to having a bellcrank whose motion started slightly slower then got faster with more control input?

[Randy Ryan]

A few years back Fred Bachal (splg?) invented the exponential bellcrank. Not sure how far but I'm guessing 8-10 years. It's in stunt new and may be something on it on Stuka Stunt. exactly what you're asking for.

[bill bischoff]

I think this is from the mid-late 60's. I believe that the idea of the elliptical bellcrank was to provide the equivalent of "exponential" in RC; slower control rate around neutral, increasing in rate with increased deflection. You see how popular it still is...

[Brett Buck]

A few years back Fred Bachal (splg?) invented the exponential bellcrank. Not sure how far but I'm guessing 8-10 years. It's in stunt new and may be something on it on Stuka Stunt. exactly what you're asking for.

  It's a lot longer than that, more like 30-40 years ago. There have been numerous discussion about the Expo-Crank over the years, might want to check some of those out.

    Brett

[Trostle]

I have seen a couple of examples of the so-called expo-crank.  If there is any real advantage to it, my reaction is that whatever advantage it might have is negated by excessive slop in the controls imparted to the flying surfaces.

Maybe there are mechanisms out there that do not have such slop, but I have not seen them.

In addition, I am not sure that there is any advantage to a bellcrank system that intentionally accelerates flying surface movement with a given input from the control handle.  I have not seen any explanation that shows that it is necessarily good nor have I seen any demonstration of those systems I have seen that they provide any advantage.

Keith

[Randy Ryan]

  It's a lot longer than that, more like 30-40 years ago. There have been numerous discussion about the Expo-Crank over the years, might want to check some of those out.

    Brett

Yes you're right, but it was in SN 8-10 years ago when Fred made a brief reappearance.

[De Hill]

Wils Bill Netzeband designed one and tested it in the 1980's and early 90's.

[Randy Ryan]

Well regardless of all this Sleepy, if there was any appreciable advantage this would be the hot setup for stunt.

[Bill Little]

About 1995, Fred B. sent me a plexiglass mock up of his "Expo-Crank".  It does have the effect of small control inputs around neutral, with more throw as the crank is driven towards the limits.  Never used one, but his set up would be a very easy installation in an I-Beam wing.

Bill

[Dave_Trible]

Hi Sleepy.  An interesting topic.  I haven't tried it but I did try something that has the same effect and feel at the end of the lines-shore didn't like it!  I was playing with the flap hinge point behind the flap leading edge creating a 'balance' or area forward of the pivot.  Around neutral the airplane seemed quite normal but at a certain angle of deflection the flap leading edge rose above the wing trailing edge.  When that occurred the boundary layer hit that edge full strength, re-attached to the flap and the feel was instant power steering. Trying to hit accurate bottoms was a total crap shoot.  One might be at 3' and the next 8'.  Tracking also was very difficult since response was non-linear and unpredictable.  No Thanks!  I ripped off those flaps and went conventional.
Many achieve the more desirable state of a slow neutral zone by making the elevator some thinner than the stabilizer it's mounted on.  This yields a little blanket effect of the elevator around neutral in the turbulent wash from the stab.  This still gives a linear feel when more control is applied, but the presumed advantage of an exponential bell rank.

Dave

[De Hill]

There have been a couple of handles designed which mimic the effect of the expo bellcrank. One was the Cam-Action handle, and the other is one of the Renger handles on sale at RSM.

[Larry Renger]

Yup! And now there is a hard point expo handle available.

[Brett Buck]

Well regardless of all this Sleepy, if there was any appreciable advantage this would be the hot setup for stunt.

 
    It's probably not an advantage in a regular stunt plane, several of us have noted that it does pretty much exactly what we have tried to design out of the airplanes. I go to some length to make sure the airplane is very responsive around neutral so it's easy to get the corners (and even the rounds) started and stopped smartly. This does just the opposite. Most of it is done through aerodynamics and trim, but if anything I would be tempted to make it reverse-exponenetial. Most airplanes seem to be soft around neutral already, some very exaggerated, like the Windy/Big Jim "giant flap" types - whenever I fly one of these I have great difficulty getting it out of neutral and cannot get the airplane vertical before it is too high.  Most of them I see had that tendency, the notable exception being Bob Baron. 

   Where it might help is with airplanes that are already too twitchy around neutral. A good application was in several of Wild Bill's designs that were extremely short-coupled and therefore hard to fly level while still maintaining enough maneuverability. In this case you are using the mechanical system to make the system more linear overall instead of trying to alter the response intentionally soft around neutral.

     Brett

[Larry Renger]

It would be easy to make my new handle "reverse expo" by putting the stops on the outside of the arms instead of the inside.  Whatever floats your boa...umm...flies your plane.

[Brett Buck]

There have been a couple of handles designed which mimic the effect of the expo bellcrank. One was the Cam-Action handle, and the other is one of the Renger handles on sale at RSM.

   Those are a lot better solution because it's so easy to try variations, you aren't stuck with anything. There have been a few "rolling contact" handles made over the years, and shaping the surface like a cam can make the response anything you want. Another feature is that the cable is not nearly as compliant as it is in a Baron-style handle, since it's held in tension all the time and any bending of the cable is very gentle. Since it's not going around any corners, it lasts a lot longer, too.

  Brett