[NSRCA-discussion] Pull-Pull

Wed Jan 10 08:10:43 AKST 2007

Good summary Wayne. 

This topic is like religion or politics. I know several top level FAI pilots that swear the DEPS (or similar) system is the most accurate for exact elevator matching and they'd not use anything else. I also know some FAI pilots that have made the finals of NATS (even won) with dual elevator servos (of course, these guys could probably kick butt with only one elevator half).

I don't recall many, if any, top guys in the NATS FAI finals flying with pull-pull recently.

My observation is that my rudder pull-pull tension varies over time and I think the same would occur with the elevator. If it did then it seems like an opportunity for the two halves to get out of unison. 

There's also the constant debate about what the best cable is based on weight and how much each stretches. Some will tell you that fishing leader doesn't stretch, others say Kevlar doesn't stretch, and on and on.

Well I've tested multiple types and sizes of cables, fishing leader and Kevlar, under controlled tests with weight hanging on them for a day or so and they all stretch. Maybe the stress we put on them isn't enough to cause stretch, but I think it is. 

I personally prefer either dual elevator servos or DEPS, that's my religion ;-).

Keith

  ----- Original Message ----- 
  From: wgalligan 
  To: NSRCA Mailing List 
  Sent: Wednesday, January 10, 2007 10:17 AM
  Subject: Re: [NSRCA-discussion] Pull-Pull

  I have tried all these methods of elevator setup.   Pushrod with Y at the end, pushrod to MK bellcrank, dual servo's,  pull/pull steel line and Kevlar, and DEPS(dual pushrods.

  FWIW...    All these setups work good and are personal preference based on your building abilities and the type of airframe your using.  One thing I have found is its paramount to have a fuse that does not flex or twist under load.  If it does have any movement I would think the dual servo's or DEPS may have an advantage over the pull/pull. But I have been wrong before.

  1. Pushrod with Y...   worked good only if it had a bearing at the front and tail to support pushrod.
  2. Pushrod to MK bellcrank... was very solid with nary any blow back.  But a little heavier then some setups. Pushrod needs support in middle of rod with foam to prevent oscillation of the rod.  Has to many points of possible failure and requires more hardware, although I put over 800 flights on one setup.
  3. Dual servo's...  Very redundant and with the right transmitter can be setup precisely.  Slightly heavier then pushrod setup with servo's and wire leads.
  4.Pull/pull...  Very light and most adjustable of the setups,  Requires more time to set up properly and is a little less ahhh...  aesthetic with all the lines protruding from fuse.  Wire lines require good crimps and straight exit holes in line with servo and control surface.  Kevlar(my choice) requires Teflon or nylon exits to prevent chafing of the line but does not have to be a perfect line to the control surface when using this type of exit.   I went gun shy on pull/pull my setup(wire) broke on the down elevator on the 10th flight causing a mishap.   One other plane with Kevlar had hundreds of flights before a radio failure retired it and it was still in good condition on inspection after the crash.  BTW  I have found a spool of Kevlar control line works good and will last for many setups and cost about 8 bucks.
  5. DEPs ... dual elevator rods (.07 c.fiber with Teflon sleeves) has been in the last two planes I have built.   Requires a little time setting up in a straight line through exit to assure smooth operation. Very light and positive control to elevator halves.  When exited below stab looks cleaner.

  Wayne Galligan
    ----- Original Message ----- 
    From: Bob Richards 
    To: NSRCA Mailing List 
    Sent: Wednesday, January 10, 2007 7:38 AM
    Subject: Re: [NSRCA-discussion] Pull-Pull

    Not all planes have push-pull elevators. I've had planes with all pull-pull tail surfaces.

    But, there are a few reasons why elevators would be setup with push-pull. Seperate servos (much smaller ones) can easily be mounted in the tail. Having seperate servos allows easy travel adjustment (travel matching) through the radio programming, and also allows redundancy on this critical control.

    The rudder requires more torque -- heavier servo -- and it might be a design consideration (balance) to keep the weight of the heavier servo closer to the CG.

    Personally, I like pull-pull and would prefer it in my models.

    JMHO.

    Bob R.

    Jay Marshall <lightfoot at sc.rr.com> wrote:
      Why are rudder controls pull-pull and elevator controls push-pull?

      Jay Marshall

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