Hi all,
I have to accept some degree of blame and/or damnation for the concept of circular bellcranks for stunt ships. Yes, indeed, I published two construction articles, the Imitation and the Excitation, which included circular bellcranks for pretty much the reasons suggested in this thread. In addition, mine were made by a local modeler/machinist, John Shwickrath (sp?) to my linear specifications and, for a short while, were available for purchase.
Unfortunately, although both published airplanes flew extremely well with those 3.5" diameter cranks both also suffered premature degradation of performance due to poor selection of bearing materials. After several hundred flights apiece on these airplanes heart surgery was required due to excessive wear of the bearings resulting in significant "wobble" of the crank on its axis of rotation which resulted in an unrecoverable hunt in level flight and less than ideal response during maneuvers. In both cases the circular cranks were replaced with the then "latest thing" 4" cranks. After surgery, appropriate recovery and rehabilitation both airplanes flew equally well on the larger "conventional" bellcranks and, thus, my experimentation came to a close. When I build nowadays...rarely...I've no intention whatsoever to utilize the three remaining circular cranks on my shelf.
Here are a few thoughts on subjects brought up in this thread.
The very real value of larger diameter leadout arms on a bellcrank is the multiplication factor of that arm times the line tension available to deflect the controls as required by our tricks. An evaluation of control systems has to pretty much work backwards, first determining and recognizing the amount of force required to deflect the elevators (and flaps, if installed) and the line tension available to allow us to obtain that necessary deflection, multiplied by the moment arm of the associated bellcrank arm. Large airplanes with large flaps and elevators require more force to deflect than does a Ringmaster or a Skyray, for instance. In order to deflect the flaps and elevators as required we utilize the line tension available multiplied by the "arm" by which the line tension force is employed. If, for instance, we took a two inch bellcrank like we used in a .15 powered sport ship and installed it in our high zoot Thundergazer or other modern "pro-stunter" The one inch up or down arm of that little bellcrank will be hard pressed to provide the muscle required to drive the large surface area controls against the airloads they produce. Double that to a four inch bellcrank and, all of a sudden, line tension X bellcrank arm is more than adequate to win the Nats.
Realize also, that the bellcrank is only one part of the airborne system and requires a commensurate increase in the arms of the control horns so as not to lose what you've gained; i.e. those left over 1/2 inch max arm Veco horns from that dinosaur kit in your attic can very easily undo all the advantages you gained by using the bigger bellcrank! Howard can do the math but it's pretty simple. You wouldn't try to deflect the ailerons of your B-747 with 1/2" arm horns and one of Paul's Superstunters is a large step in that direction.
Any advantages of circular bellcranks, I believe, quickly fell into the "noise" category when coupled with conventional control horns. As someone suggested, a fully linear system throughout the full range of control motion would require not only a circular bellcrank but also that circular handle someone previously mentioned and a pulley system rather than control horns to drive the flaps and elevators. The only technical advantage of a stand-alone circular bellcrank would be a constant ratio of bellcrank movement per handle movement...only if the Handle was circular as well.
All things considered the effort to construct a satisfactory circular bellcrank seems to me to far outstrip any value received from doing so.
Here's one last thought. There is some question in my mind that a fully linear (a given degree of handle displacement results in identical control surface deflection) would be desirable to win stunt trophies. The only potential advantage would be in cornered maneuvers and that advantage would appear to be solely that the pilot could continuously more aggressively drive the ship in the desired direction required by the maneuver...which sounds good in and of itself. What I fear would be the undoing would be the difficulty of going from "max pitch change per unit of input" to a straight flight segment in the desired angular direction from the previous leg. In our conventional control system the "rate of change" per unit of input degrades as we get deeper into a corner which likely improves our ability to exit the corner more accurately on the angle and in the direction defined in the Rule Book.
Although I've still got three of John's beautifully machined 3.5" circular cranks in their original packages and could probably sell them for megabucks as the Holy Grail of stunt immortality I thought I'd just tell the truth of my experiment instead. Keep an eye out if you remain a believer. When I croak the kids'll probably try to sell them on e-Bay...if they can figure out what the heck they are.
Ted Fancher
Topic: Circular bellcranks- Pro's and Con's (Read 7744 times)