Hey guys,
I've set up my Wildhare Sukhoi (50cc) rudder pull-pull using some 20lb steel leader. Do you think 20lb will hold up ok for 3d? Thanks.

doesn,t this equal 9.1 kgs ? if so what size servo are you running ? lets say you have a 12 kg servo and the wire is 9 kg breaking strain, whats the use of the 12 kg servo ? i normally double the sizes like a 12 kg servo should have a 24 kg wire set.

Say you used a 7955 Hitec servo at 333 ounce-inches. If your servo arm were 3, you'd have 333/1.5 = 222 ounces of cable tension at max rated servo torque. 222/16= 13.9 pounds. So I'd say 20 pound would work. I might go with 30-40 pound for the added safety factor if it's not much heavier. I think I remember 40 pounds being the cable most people use up to 40% aerobats.

PS: freeair, the largest servos are only about 9.5 kg, not 12. And nobody uses a 1" rudder servo arm or horn so the servo torque and cable tension are not 1:1. You need to divide your servo kg by 1.5-2 inches depending on the rudder servo arm/horn length.

I've been using 90# knottable on 50cc planes and it works great. It's about the size of normal pull/pull cable.

Dave Patrick told me that the more strands the stronger, and lighter. That's why he supplied it with his kits. He was really obsessed with less weight on his kits and the builds. Can you blame him?

[quote]ORIGINAL: JoeAirPort

Say you used a 7955 Hitec servo at 333 ounce-inches. If your servo arm were 3, you'd have 333/3 = 111 ounces of cable tension at max rated servo torque. 111/16= 6.9 pounds. So I'd say 20 pound would work. I



Not to nit-pick you math.....but, if the arm is 3 (inches) from pull cable to pull cable, it would be 1.5 inches from the center of the servo shaft to either cable. Since Torque=Force x Radius, you would divide 333 by the radius of 1.5 inch, giving you 222 ounces, or 13.9 pounds on the cable. Either way, it would probably be okay. Especially since, in my opinion, a 50 cc planes rudder servo would never see 333 ounce inches of torque. I think I use 26 pound cable.

Mike

[quote]ORIGINAL: huggins


You're totally right. And no problem, I'm the kind of person who likes to know what I did wrong so that's totally cool. Thanks for correcting that.

[quote]ORIGINAL: JoeAirPort

Say you used a 7955 Hitec servo at 333 ounce-inches. If your servo arm were 3, you'd have 333/3 = 111 ounces of cable tension at max rated servo torque. 111/16= 6.9 pounds. So I'd say 20 pound would work. I might go with 30-40 pound for the added safety factor if it's not much heavier. I think I remember 40 pounds being the cable most people use up to 40% aerobats.

PS: freeair, the largest servos are only about 9.5 kg, not 12. And nobody uses a 1
Joe, since when are the largest servos only rated at 9.5 kg ? i have a few servos which are the HD-9150 MG and these are 15.8 kg , a friend is using 23 kg servos in his 3 meter model.

Oh come on, this is a 50cc plane for God's sake. It doesn't need a 12 kg or 23 kg servo.

Hold your horses guys, i think your all on the wrong track with this servo to rudder gearing so to speak. to get the most torque power from a servo you need to use a small servo arm on the servo and a larger arm on the rudder, the only benefits of having a large servo arm is for more travell. this works the same as say a motor bike where the engine has the small cog and the wheel has the larger cog, what happens if this is swapped around the other way ? a big loss in engine torque. think about this.

Pull-pull are 1:1. You don't do the mechanical advantage trick on these setups. You're thinking push-pull. With push-pull what you're saying is true.

no i am talking about the pull - pull set up, lets say the rudder servo has a 3 inch arm fitted , to make this ratio 1 : 1 you would have to also have a 3 inch control horn on the rudder. as i mentioned if you want more torque you go for a ratio of say 1/2 to 1 meaning a smaller servo arm and a larger rudder control arm.

Freeair,

For the cables to keep the proper tension through the range of motion it needs to be 1:1. If you go to like .5 to 1 your geometry is not going to work out right for your cables.

Yep, that's the reason. That's why rudders need such huge servos, they don't get any help from mechanical advantage. 1:1 with minimal to no cable slop is the goal. Also you don't want the cables to get tighter as the rudder moves from center.

Thanks for your responses guys, I appreciate your insight. I'm using a [link=http://servocity.com/html/hs-5955tg.html]HiTec 5955TG[/link]

I always use the pull-pull kit from Don's Hobby Shop.

They use a kevlar coated cable that is easy to use and extremely resistant. The parts that are all heavy duty and good quality.

Just did the pull-pull setup last night on this new AeroWorks Extra I am working on.

The arms are from Secraft (www.secraftusa.com), which makes a whole list of cool and hard to find accessories for giant scale planes. I think their arms work better than anybody else's...