[NSRCA-discussion] Rudder pull-pull

[vicenterc at comcast.net]

I am flying a 35% IMAC plane with a single servo on rudder.  Of course, I am not using 3D setting.  I am saving around 3/4 lb in servos and trays.  

Clearly the ratio between the servo arm and the rudder horn can not be equal if we want to set for precision and gets the mechanical advantage.  Usually I use 35-40 degree rudder throw.  The servo moves around 60 degrees.  The ratio has to be between  0.58 - 0.66  since I want to use the 100% of the servo travel.  Looks like is very difficult to work this out and have a perfect system.  I could be wrong as usual.  I didn't have chance to read all e-mails.  My question is:  What is the best compromise?    

--
Vicente "Vince" Bortone

-------------- Original message -------------- 
From: <glmiller3 at suddenlink.net> 

> Lance, 
> 
> I did this on my first Focus II, but since then have just used the servo arm 
> directly. I think the loads on a pattern plane are significantly less than on 
> the IMAC giants. 
> 
> George 
> ---- Lance Van Nostrand wrote: 
> 
> ============= 
> Jim, 
> I've done this on heavier IMAC style planes but never thought it could be done 
> light enough for pattern. If you have thoughts on this, I am interested. 
> Thanks, 
> --Lance 
> 
> ----- Original Message ----- 
> From: J N Hiller 
> To: General pattern discussion 
> Sent: Sunday, May 11, 2008 8:58 PM 
> Subject: Re: [NSRCA-discussion] Rudder pull-pull 
> 
> 
> Lance I use a bell crank to get equal length arms and control horns. This 
> allows continued use of full servo travel and removes the cable tension from the 
> servo shaft and mounting grommets. The servo is connected with a short link with 
> ball end fittings. It works well especially on elevators. 
> 
> Jim Hiller 
> 
> 
> 
> -----Original Message----- 
> From: nsrca-discussion-bounces at lists.nsrca.org 
> [mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Lance Van Nostrand 
> Sent: Sunday, May 11, 2008 5:38 PM 
> To: General pattern discussion 
> Subject: Re: [NSRCA-discussion] Rudder pull-pull 
> 
> 
> 
> You all are so fast on this stuff that by the time I read the post the thread 
> is old. 
> 
> 
> 
> Ackerman exists if the rudder has control horns (as opposed to a wheel, which 
> is very hard to build into a rudder). The only ideal way to have constant 
> tension is to use a symmetrical servo arm that is offset from its pivot (servo 
> screw) the same amount as the rudder control horn attachment point. This will 
> work every time and is simple, however with a 1" thick rudder post and two 3/4" 
> control horns you will need a 2.5" servo arm (1.25" each side of center) and the 
> servo throw will only be equal to your rudder throw (maybe about 30 degrees) 
> which gives you poor mechanical advantage and poor use of your servo. If you 
> use a shorter servo arm and more servo throw the cables will go slack. The 
> round servo wheel is a compromise by offsetting this slack tendency and letting 
> you get more servo throw. It's not a perfect match but it can be very close. 
> 
> 
> 
> Anyone that can do the math on this and present a complete answer can get a 
> recommendation to advance from a shade tree aerodynamicist to an apprentice. 
> 
> 
> 
> --Lance 
> 
> 
> 
> ----- Original Message ----- 
> 
> 
> From: John Pavlick 
> 
> To: General pattern discussion 
> 
> Sent: Friday, May 09, 2008 9:02 AM 
> 
> Subject: Re: [NSRCA-discussion] Rudder pull-pull 
> 
> 
> 
> Chris, 
> 
> The only benefit that I know of is that you don't have to deal with the 
> Ackerman angle like you do with arms / horns. The cable is always the same 
> distance from the hinge center no matter what the angle is. 
> 
> 
> 
> John Pavlick 
> http://www.idseng.com 
> 
> ----- Original Message ----- 
> 
> 
> From: chris moon 
> 
> To: nsrca-discussion at lists.nsrca.org 
> 
> Sent: Friday, May 09, 2008 9:13 AM 
> 
> Subject: [NSRCA-discussion] Rudder pull-pull 
> 
> 
> 
> Can someone explain the benefits (real or otherwise) of the pull-pull 
> wheel type rudder servo arm vs. a regular servo arm? Is there some real 
> benefit, or is it bling? 
> Thanks 
> 
> Chris 
> 
> 
> 
> 
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