[NSRCA-discussion] Pull-Pull

Wed Jan 10 08:49:55 AKST 2007

Keith, have you tested nylon coated SS cable? Do cables continue to stretch
or just initially?

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Keith Black
Sent: Wednesday, January 10, 2007 12:11 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Pull-Pull

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 <mailto:wgalligan at goodsonacura.com>  
To: NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>  
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 <mailto:bob at toprudder.com>  
To: NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>  
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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