Design > Stunt design

Walker Patent: is it better to have the up line to the rear?

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Brett Buck:

--- Quote from: Paul Smith on September 23, 2023, 07:00:57 AM ---I've built many models BOTH ways and it only matters if and when you hang the model on one line.  Otherwise the bellcrank equalizes line pull and it doesn't matter.

--- End quote ---

    It may or not matter, but as noted, your basic premise is certainly not correct. Any time the controls are deflected, the tension in the lines is different. The bellcrank doesn't "equalize" the tension, you have to have some torque to deflect the bellcrank, that is supplied by differential in the torque on the two arms of the bellcrank, and shows more tension on the line pulling the direction of the turn.

   Also as noted, you do not have to "hit the stops" to have the airplane "hang on one line" (that is, the line tension differential reaches the line tension, so it is all on one line). It is relatively easy to run out of torque before you hit the stop (if any), particularly on Nobler-type airplanes with gigantic flaps. Bill Netzeband recognized this a long time ago, it is called the "Netzeband Wall" in his honor. This is also why 4" bellcranks work better than 3" bellcranks, bigger the better.  Most modern stunt designs are intended to make this a non-factor but it was a serious problem in the good old days.

   I also note that I and most others do not build in "stops" on the bellcrank and with a conventional pivot attached to a spar, it can't go over-center.
 
     Brett

Ken Culbertson:

--- Quote from: Brett Buck on October 07, 2023, 04:24:31 PM ---   I also note that I and most others do not build in "stops" on the bellcrank and with a conventional pivot attached to a spar, it can't go over-center.
 
--- End quote ---
Brett:
I am not disagreeing with you for today's normal construction but actually, it can happen and I have had it happen to  me.  Assuming a conventional mount with the upline in the rear and the leadout guide for the upline is in front of the BC center it can over center if your flap has enough range to take the full amount of pushrod movement.  You would probably never notice it since any pulling on the down line would release it immediately.  It happened to me on takeoff when I was in a hurry to get into the air (one last trim) and forgot to center the controls before the motor ramped up.  The plane did a flip and smack before I could process what was happening.  When I picked up the nose-less plane the controls were locked.  I was able to duplicate it in the shop.  I will admit that it is very unlikely with a normal spar since most of us like to but the BC as close to the CG as possible, but this plane had a "Rabe" like airfoil and the spar was a good 4" from the LE at the BC mount and well behind the CG.   And Yes, I had the ribs hollowed out all the way through the wing.  Normally the ribs would prevent over centering until you cut them in half with the leadouts and most flaps don't move enough and become a stop in themselves which is undue stress on the hinges and flap horn mount.

I have put stops at 60 degrees ever since.  Doubt I have ever hit them.  Going to yellow Spectrum lines has given me a field trip classroom demonstration of what you just posted.  You can see the trailing line go slack even in the entry to loops and it is a real heads up as to when you are approaching that very real wall and a bell ringer when you hit it!

Ken

Footnote:  I don't make the stops special either.  I just make the cutout in the spar on the side opposite the pushrod small enough that the bellcrank cannot rotate through the spar before the flaps hit their maximum deflection.  Works for me.

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