HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Feb 20, 2013, 09:07 PM
Registered User
hog2soar's Avatar
Central California
Joined Jan 2010
965 Posts
Discussion
Pull Spring vs Pull Pull vs Push Rod for rudder elevator

What's your preference for rudder elevator control?

Do you prefer a Pull Spring setup or Pull Pull or Push Rod?

Is any one of these better than the others? If so, why?

Inquireing minds want to know.
hog2soar is offline Find More Posts by hog2soar
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 20, 2013, 09:11 PM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
USA, OH, Worthington
Joined May 2002
6,739 Posts
I like pull spring because of the simplicity, total lack of double centering, the ability to hide a servo's output shaft or bearing slop, and the light weight.

Drawbacks are the possibility of warping control surfaces and the possibility of a string failure.
tom43004 is offline Find More Posts by tom43004
Reply With Quote
Old Feb 20, 2013, 09:20 PM
Registered User
Joined Jan 2008
4,533 Posts
I prefer push rods, either steel or carbon. It's just a preference - the advantages are largely hidden in other factors, I believe. I usually am lucky at sizing holes in servo and control horns, so slop has not been a factor for me.

I've tried both pull-pull and pull-spring, and while both worked, I had issues with strings slipping their knots. Had I been a better Boy Scout, I'm sure it would have turned out different.

Yours, Greg
glidermang is offline Find More Posts by glidermang
Reply With Quote
Old Feb 20, 2013, 10:52 PM
Registered User
NCDLG's Avatar
United States, NC, Raleigh
Joined Sep 2010
764 Posts
Poll.... pushrods baby!!!!!
NCDLG is offline Find More Posts by NCDLG
Reply With Quote
Old Feb 20, 2013, 10:59 PM
Time for me to Fly...
Mr. Wiz's Avatar
United States, MI, Fenton
Joined Jan 2000
8,640 Posts
You can download videos showing how to tie all sorts of knots. I had to do just that to put a proper loop in the end of the string on my three pull string setups. but honestly next plane is probably going to get pushrods. I don't think the string spring is any more simple than a pushrod to install but it does have a higher degree of failure, IMHO and I don't think it's worth it.
Mr. Wiz is offline Find More Posts by Mr. Wiz
Reply With Quote
Old Feb 20, 2013, 11:57 PM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
USA, OH, Worthington
Joined May 2002
6,739 Posts
How many strings have you had fail?

Just curious.

Tie a knot and hit it with thin CA. Problem solved.

This is a funny debate that is near to my heart. Some people swear by carbon pushrods and I've personally lost two models to cracked elevator rods. I swore by steel pushrods for years but always fought double centering unless I used a huge (heavy) pushrod. I've also had string failures but they were never the system's fault. They were my fault for assembling my model too quickly and not having the loop secured in the horn.

Many pull springs are pull-up and spring-down. Mine are opposite. A string or horn or knot failure will result in a tighly looping model that quickly burns off all speed and goes into a gentle stall - fall pattern. Pull-up systems are lawn darts if you have a failure on launch, which is the most likely scenario. I've seen that happen a few times as well. I've never had a model damaged with a control failure in about four years of pull string.

All of the systems work. You'll find the reasons to like one or another after trying them all. The grass will always be greener as well.
tom43004 is offline Find More Posts by tom43004
Reply With Quote
Old Feb 21, 2013, 12:09 AM
Registered User
Canada, BC
Joined Jan 2010
315 Posts
String Spring

I have tried them all. String spring is less weight, easily adjusted in the field, looks better when run inside the boom, spring tension can quickly be changed by adding more springs or stronger springs.
Did I mention no slop. But it is the reduction of weight in the tail that counts the most.
Airbud is online now Find More Posts by Airbud
Reply With Quote
Old Feb 21, 2013, 06:09 AM
flying is not a dream
lebenj's Avatar
Spain, CT, Cardedeu
Joined Jan 2008
991 Posts
in my XXlite, i have a pull string with a small piano cord provided by Kristof.
it's about 0,25mm and the weight is really not much more a dyneema string

i have it mounted since 1 year and no problem, flying almost 2hours a week

it can be an interested solution for people afraid of knots!!!
i also use dyneema strings in other plane wich works without problem
lebenj is offline Find More Posts by lebenj
Reply With Quote
Old Feb 21, 2013, 07:05 AM
Mickey from Orlando. Really.
Joined Nov 2004
3,521 Posts
What's the preferred spring material, length, placement, and number of springs?
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Feb 21, 2013, 07:12 AM
Registered User
Chris-DC5's Avatar
United Kingdom, England, Bristol
Joined Nov 2009
131 Posts
I am 100% for the use for pushrods.

I am however going to use pull spring just to see how i get on.
Chris-DC5 is offline Find More Posts by Chris-DC5
Reply With Quote
Old Feb 21, 2013, 07:45 AM
Team Horizon Pilot :)
O H I O
Joined Jul 2007
1,500 Posts
Funny.
Tom and have argued the spring thing til we were blue in the face. I will not use strings ever never anymore. The only time it has worked for me was on the Polaris with the under elevator , that gave a pull up with servo and I could not beat it. I still changed it to pushrod when the string started to fray from jamming ballast next to it. .22 304 s's pushrods and forget about failure. As for the double centering that's what my trim buttons are for.
And to all you guys using string and spring , it's going to be a horrible ending to your beloved glider sooner or later.
Just my 2 cents worth
Gavin
Gavin Trussell is offline Find More Posts by Gavin Trussell
Reply With Quote
Old Feb 21, 2013, 08:06 AM
Time for me to Fly...
Mr. Wiz's Avatar
United States, MI, Fenton
Joined Jan 2000
8,640 Posts
Quote:
Originally Posted by tom43004 View Post
How many strings have you had fail?

Just curious.
Zero but I have a Polaris I bought from NCDLG that has a bunch of chunks out of the LE because he did have a string failure. I've never had a pushrod failure on anything, ever. Oh I take that back. once I was stupid enough to use metal threaded clevis on both ends of a 4-40 pushrod and the big 4 stroke vibrated enough to cause the rod to back out of one clevis.

Here is my main issue with it...The loop falls off the hook horn if the elevator is accidentally flexed. I tie a large Perfection loop in the end and I wrap the loop itself around the hook and then I pass the line through the loop an pull it tight. this doesn't come off. It also is nearly impossible to remove making taking off the stab very difficult, at best.

Also, I have a bit of mistrust of the springs ability to hold position. Now again, I haven't experienced a failure and I intend to fly these all season. Maybe by the end of the season ill have a different opinion of them. Right now, I'm keeping a cautious eye on them.

One more thing. The Polaris has the stab on the bottom. If the string fails, the elevator is going down. Will it lawn dart? If its too low when it fails, yes. Otherwise it should loop just like an elevator up failure except ill have it stalled out in the inverted position. Of course, most failures probably happen right alert release which makes the Polaris setup less than ideal in that situation.
Mr. Wiz is offline Find More Posts by Mr. Wiz
Reply With Quote
Old Feb 21, 2013, 08:08 AM
Time for me to Fly...
Mr. Wiz's Avatar
United States, MI, Fenton
Joined Jan 2000
8,640 Posts
One more thing. This is the best place to learn how to tie knots of all kinds.

http://www.animatedknots.com/ipaddetails.php
Mr. Wiz is offline Find More Posts by Mr. Wiz
Reply With Quote
Old Feb 21, 2013, 08:14 AM
Time for me to Fly...
Mr. Wiz's Avatar
United States, MI, Fenton
Joined Jan 2000
8,640 Posts
Quote:
Originally Posted by Gavin Trussell View Post
Funny.
Tom and have argued the spring thing til we were blue in the face. I will not use strings ever never anymore. The only time it has worked for me was on the Polaris with the under elevator , that gave a pull up with servo and I could not beat it. I still changed it to pushrod when the string started to fray from jamming ballast next to it. .22 304 s's pushrods and forget about failure. As for the double centering that's what my trim buttons are for.
And to all you guys using string and spring , it's going to be a horrible ending to your beloved glider sooner or later.
Just my 2 cents worth
Gavin
You must have a different fuse than I do on my Polari. They both have a molded cup right under the wing saddle where the ballast sits, isolated from everything else.

As for double centering of the pushrod. I've never noticed much of an issue with that except my top drive on the ailerons and that's because the rods wore out larger holes in the horns. I just CA them tight again every so often.
Mr. Wiz is offline Find More Posts by Mr. Wiz
Reply With Quote
Old Feb 21, 2013, 08:22 AM
Registered User
United States, FL, Gainesville
Joined Nov 2004
215 Posts
I have tried both and am currently installing pull spring on my Twister 3. I tried stainless wire but the twisted ends were too close at servo ends and kept catching on each other so I will change back to Dynema. I saw a Blackjack at San Felasco contest built by Mickey Nowell of Orlando with pull-pull rudder which worked very well, So I may try that sometime. I have a Zone with pushrods and have struggled with binding and double centering on elevator which can be frustrating. I have also had the pushrod ends fail at the servo side because the glued-on L bend broke loose and crashed on full-launch, so I do not think pushrods are less failure prone.
Pick your poison! Try both and do whatever method you choose with careful attention to all possible failure points and I believe both can be reliable and slop-free. Of course pull-spring is better with a sloppy servo, but we are all using digitals now so this is usually a non-issue.

-Robin
rhoon is online now Find More Posts by rhoon
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Elevator servo torque vs rudder servo torque wollins Modeling Science 9 Jan 11, 2013 05:00 AM
Discussion Push-pull rods gimbal to belt driven. DYORD Multirotor Electronics 0 Jan 08, 2013 01:17 AM
Discussion Detachable Elevator or Rudder methods for Pull-Spring Setups Thermaln2 Hand Launch 13 Feb 20, 2012 12:33 AM
Discussion elevator and rudder servos for pull spring system? Austis Hand Launch 6 May 01, 2010 01:01 AM