90 deg. CCPM

Thursday, 26 August 2004 21:35

90 deg. CCPM by CK_

When in MMS mode, move the link from the blue ball to the red ball. Do this on both side of the swashplate.

All credit to CK_

"As for the 90 deg. CCPM, here's why I did it. I started with the stock 120 deg. CCPM with 3 9202s. If I banged the cyclic around quickly there was a lot of collective interaction. Try it for yourself. Wiggle the cyclic and watch the antirotation pin and jump up and down 1/8". The interaction happens because the servos are slow and the front swash input has to move twice as fast as the two in the rear for a fore/aft cyclic movement because the front ball is twice the distance from the center as the rear balls. If you move the cyclic faster than the servos can move then all three servos will move the same speed (the max speed of the servo) and collective interaction will happen because the front swash input is not moving at twice the speed of the other two. Curtis' 140 deg. or whatever it is on the Vigor CS puts the front ball exactly the same fore/aft distance from the center as the two rear balls. This means that all three servos move the same speed for a fore/aft cyclic input. No interaction.

If you use 90 deg. then fore/aft cyclic only moves one servo. There can never be interaction if only one servo is moving. Lateral cyclic will move two servos at equal and opposite speeds. No chance for servos to "run out of speed" like with 120 deg. I still get a little bit of interaction but it's nothing like I had with the stock setup.

Like I said before, I still don't know why 120 deg. is pretty much the standard and 90 deg. is rarely used when 120 deg. has much more collective interaction. Anyone got any ideas?"

Great response by Dr.Ben

"Your info about the 140 degree CCPM is entirely valid. One drawback of having the single ele input up from is that the swash is less stable. 120/140 d. CCPM surrounds the entire swash with support. There is also a control power component here. One reason the big gun CCPM models such as the Fury and Vigor CS can pull such abrupt maneuvers is the combined input of the servos - two for any roll command, THREE for pitch command, and three for a collective command. 90 degree CCPM takes two servos out of the power equation on the pitch axis. I realize the control power issue is of less consequence in a 30 sized bird, but you asked why manufacturers don't employ 90 CCPM more commonly.

Much of the interaction you saw is a direct result of the somewhat slow analog 9202's (still a hell of a good servo) and not the 120 d. CCPM per se. If you go to a upper or lower collective command and input a hard over roll input, you will note a pretty good collective interaction because the servo arm on one of the two roll servos is approaching centerline and thus moving its pushrod further, while the other servo arm is retreating from centerline and moving the pushrod less. This differential phenomena is present in all CCPM models with rotary servos and is somewhat minimized by using servo wheels large enough to avoid movement a large number of degrees off centerline. A linear output servo is the only way to avoid the problem completely; we ain't there, yet <g>. The Caliber 30 system might work a bit better better if the bellcranks were designed to create less swash travel per unit of servo movement (meaning equal ball input points on both arms of the bellcrank), and then larger servo wheels were used to get the collective/cyclic range needed (less differential effect).

Ben Minor"