I've flown R/C aerobatics for almost 20 years. There was a custom to set the transmitter controls for an exponential responce on the elevator. This made the elevator less sensitive around neutral, i.e. a small elevator deflection for a given amount of control around neutral and a maximum responce for a simular control near the end of travel. Has anyone come up with a scheme to duplicate this exponential responce on a C/L airplane? It sure would work great when trying to fly very level (inverted or upright). I have moved back to C/L flying just a few years ago (after a 40 year absence) and I am working on practicing the routines.

 

I like simple and tradition dies hard.  If the elevator portion has a trailing edge as thick as the hinge edge then the initial control movement will be instantly effective rather than having a slow reaction.  I proved this out on the rudder/fin when I was competing flying A2 towline gliders.  Also this thick trailing edge is not speed sensitive.  All of my CL planes are built this way, mostly stabilator designs where the stabilator is flat with a half round LE.  If any one is interested just ask and I will post a diagram orf how this works.  SLEEPY

Here is a drawing of an exponential crank taken from Bob Baron's Humbug article, with a longer pushrod arm, so you can see the exponential better.  A pin in the bellcrank drives the pushrod arm via the slot.  The depending on the exact location of the pivots, the pushrod hole moves about 20% further than if the arm was fixed on the bellcrank, and the movement increases(pushrod force goes down) as the mechanism moves.  This means you have to allow for the forces on the elevator, and they have to be lower than on a standard plane, or you will may run into the "Netzeband" wall going into the wind.

Fred Bachle moved the expo arm pivot to the other side of the bellcrank pivot, allowing somewhat better leverage and a faster expo rate.  It necessitated a fairly complicated expo arm to get around the bellcrank pivot.

Any setup where the connections between the lines and the pivot(both at the handle or at the bellcrank) are not parallel and straight lines require you to carefully fine tune the neutral adjustments through out the control system so everything (plane, controls, handle) are at neutral at the same time so you don't introduce control feel problems.  You can see if the handle neutral, the bellcrank neutral, and the controls are not all exactly neutral a the same time the control feel will change as the controls are moved.  You get the similar problems if the handle overhang and neutral hand axis are not all squared up too.  See the thread on how a bad neutral can cause a crash.

The ExpoCrank was covered in PAMPA's Stunt news some years back. I don't recall exactly the year. If I were you, I would PM Larry Cunningham (he posts on this board)--I know he wrote an article or two on it in Stunt News too

So the next logical question is:  how can I buy an exponential bellcrank?  I've made my own control horns, but never a bellcrank.  I'd like to give it a try.

Floyd

You can use a Fox 3 in. bellcrank, add a drive pin, per the diagram, and trim a 2 in. bellcrank down for the elevator drive.  The only tricky part is carving and filing the a 3/32 in slot for the drive pin.  The slot needs to be a close, smooth fit on the drive pin.  Best done on a mill, but you can do it by hand.