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Building Tips and technical articles. => Building techniques => Topic started by: Jerry Bohn on February 02, 2007, 11:35:07 AM
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On the plans for the Brodak Ares they show where to locate the bellcank support rod for either a tarditional or reverse bellcrank. What is the advantage to a reverse bellcrank?
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Gerry,
Somewhere in here is much more dealing with this. Basically, it comes down to getting the UP line forward of the DOWN line.
The prop and spinner act as a gyroscope, pretty powerfully, in fact. When you tilt the "axle" of a gyroscope, 'precession' tries to move it as if you pushed it 90° 'later' in the direction the flywheel is rotating.
On a 'standard' stunter - counterclockwise prop rotation seen from the front, flown counterclockwise in upright flight, an inside turn tilts the gyroscope axis up and the precession tries to turn the nose out from the center. Same for inverted and outside turns. It is as if the axis were tilted down, and precession tries to push the nose in.
At the same time, when we put in a control change to turn "inside" , the load on the lines shifts toward the end of the bellcrank pulling with the force to move the flaps and elevators out into the airstream. The other line loses pull by that amount, since pull is pretty well set by the model's weight, line length and flight speed.
So, if the UP line is forward, that pull shift moves forward on up, or inside, control. That can try to pull the nose back in toward the center, while the precession is pushing it out. Vice versa on down and outside control inputs - the load shift tries to oppose the efect of the precession.
(I think - personal opinion - that the values can be calculated for each individual model, and the leadout guide spacing can be found that would do a good job of canceling the precession effects. I've tried that on several models, and they don't hinge, which is related to the yaw that precession can cause... IMHO, anyway.)
Actually you can get the same effect with the bellcrank NOT reversed, but the flap horn has to be under the wing and the elevator horn above the tail chordline... Or you could fly clockwise, or with a reverse rotation engine. ...Simpler to flip the bellcrank over, for most of us.
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Lou, Thanks for the explanation. I also read your responce on the post about "mounting bellcrank on Tempest II". Guess I should have searched first, but appreciate your replying after taking the time on that post. After some study it made sense.
I don't know if it would help me at my beginner level of flying, I will definitely try it in the future.
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With all respects to those more learned, I have a question regarding this theory. My buddy Pat Johnston subscribes to this reversed bellcrank idea, and I use it if for no other reason than he recomends it. HOwever, with regards to the force being loaded on one line or the other, It seems to me that there is only really two times that one line can have more force on it than the other, 1# when you are at max deflection on the surfaces, ie, everything is against the stops and you try to increase the input. 2# during the actual movement of the handle, ie when you are inputing control deflections, as soon as you stabilize the handle, it would seem to me that the system will balance out and both lines carry the same force.and this differential is only a reality if you act to input force faster than the system can accomidate it, as in yanking a hard corner, or having drag in the control system. Oh I guess there is one more time you can have more force on one leadout, if a line breaks :P
If I am way off base, someone please explain to me why? I wanna learn and am open to ideas other than my own but this is my observation,,,,,
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Mark, consider this example. In order to start an inside loop for instance, it requires applying more tension on the "up line"than the down line. The moment the control surfaces are deflected they become loaded by the air rushing over them. The down line becomes unloaded at that point and as long as the control surface is seeing uneven air pressure the up line will have more tension on it than the down line. The control surface load (reaction) must be countered by a line tension input (action).