How many oz of torque are recomended for control surfaces? a rudder that is 10 square inches requires a servo that has *blank* oz of torque? that is assuming that the control horn is mounted as close to the hinge as possible? there are so many variables but im just looking for a guideline.

The ammount of torque for any control surface typically is determined by the size of the surface and the type of plane. The giant scale planes use hi torgue as the control surfaces are large and the planes are heavy. The 3D stuff has really large control surfaces and will use hi speed and Hi torque servos. The majority of the trainer planes use standard servo's. Hope this helps or if you have a specific plane that you are getting ready, let us know what you are building. Good Luck, Dave

I flew an Ultra Sport 60 that I really put through the paces (no 3D though), and I do mean a lot of aerobatics and it had nothing but standard servos in it. A lot of people will dismiss standard servos as not being up to the task, but for a lot of people out there they will rarely need more servos than the standard servos. But..... The best way to determine is check with the aircraft manufacturer. They will normally indicate what size servos to use on any given plane.

Hope this helps

Ken

Here is what you need to know.

What size engine?
What Airplane are you flying. Wingspan?
What type of flying are you doing?


Generally, a 40 size sport or trainer plane needs only a standard Ball Bearing servo. 50oz of torque or so.

a 90 size plane you might go up to 75-100oz of torque. If its an aerobatic plane you might use a metal geared servo on the rudder.

on a 1.20-140 size plane you might go with 125oz. IF you are doing significant aerobatics like 3D you might consider metal or carbonite gears.

In general, if you are flying an aerobatic plane performing agressive aerrobatics you might go one size too big just to be sure you have enough torque.

I am generalizing quite a bit. You can make it more complicated but its not rocket science.

If you have a control (rudder) that takes 40 oz of torque to move to full deflection at 70 miles/hr it will require a lot more to move the same rudder at 120 MPH. Airplane speed, amount of deflection, surface area all come into play when deciding on the power required. And as Mike said, you can also strip plastic gears after a limit is reached. At that point flutter sets in and you no longer have to worry about it because you may no longer have a plane. [:@]

I have wondered this also MMMmmm... My formula is up to .60 sized, I used standard-size 40 +_ servos. After that whatever the MFG recommends, I can afford, and a simple length X width = total area method. On my P-Wag Extra 300, I could've gotten away w/ 70 or 80's, but I used 100 oz's as a measure of safety. Now if you are into all this new exotic flying going on nowadays, you might consider .60-sized ones in a .40-sized plane and so on. Step it up one notch so to speak..... Rob


P.S., I think we all have seen or experienced situations when the servo wasn't up to par___ won't happen again I bet...

I have some planes that weigh up to 10 lb. planes. 1 is a pattern, a couple of sport/warbirds, and a couple of general sport planes. All are speced to use standard servos with 40 oz. torque. The Futabas I use are all rated at 42 oz. torque and I have never had any problems with them. Best to check the requirements listed for any plane that you are considering just to be safe though. [8D]

I have heard from other spaders that 1 oz. of torque per square inch of control surface. Would anyone say this is a good guideline for a trainer to a medium sport plane? how many oz. of torque to inches of control area do your planes have?

type of plane?
max speed of plane?
weight?
size?
control surfaces in inches squared with the output of the servo?
distance of control rod from servo center?
position of control horn and control rod on control surface?

i know im probably disecting this too much but being a mechanic i dwell too much on getting everything right. i just want to avoid any nasty accidents that i can.

The answer to your question is fairly simple, but difficult to explain.

If you are using massive deflections on a .90 or larger size model, you will need 200+ oz/in of torque per surface. If you are flying that same .90 airplane at low speeds with no more than 20 degree deflections, you could get by with 40-70 oz/in. Rudder requires a bit more because it is a larger surface.

An example: I just built a .90 size plane, (ARF), which called for a 1.0 size motor (go figure). The mfg recommended JR 9411 digitals on everything but rudder, and 8411 on rudder. (I think 8411's are 90 oz/in and 9411's are 70...not sure). Because I intend to use this plane for 3D, I installed a 1.25 engine, 120 oz/in servos all around and a 300 oz/in servo on rudder....all regulated at 6V. So it REALLY depends on what you are going to do with it. This same airframe would fly just fine on standard servos if I only flew it flat and slow. (I hope I am making sense with this example...or do I just sound like I want to over-power everything....?).

Anyway....when you blow-back your first servo, you will learn very quickly where the line is.

Good Luck!!

Mike East's post covers the range.

He kept it simple and it is.

There are two things to keep in mind along with that advice. If you fly a very fast airplane or very heavy airplane (they wind up going fast) look for servos on the strong side of Mike's ranges. If you fly an airplane with bigger than normal surfaces (the movable parts are often called surfaces) go for the stronger servos. 3D airplanes have "larger than normal" surfaces, btw.

BTW, no matter what strength servo you wind up putting in the airplane, don't defeat it with your computer radio.

Hooking up your servo/connector/horn can blow a lot of the servo strength you paid good money for. A lot of newbies today think that the computer radio was made so they can snap the connections in any hole on the servo arm and any hole on the horn and they're good to go. Oh yeah, they might have to dink the TX settings to make the max throw 150% or such and change the center, but what the heck, dinking with the radio only takes a couple of minutes. And that saves them from having to take no extra time when they snapped the connectors into those holes....... duh...... wait a minute...... it don't take no extra time to put them snaping thangs..... duhhhh......

It takes not one second more time to rig your airplane one way or the other. So rig it first to give the servo mechanical advantage. Then check the throws for what the plans call for and go from there.

If you plug the connection into the inside hole on the servo arm, you're starting out with that servo having it's most effective output. It will draw the least battery (so you get more flights per charge, no big deal, but you do) and when you're flying the heck out of the airplane and all servos are sucking away, they suck less total amps (so you have less chance of starving the RX or *****g the battery pack's ability to provide amps). And the servo is going to have a chance to push it's max strength down that connecting rod.

Visualize this.......
You're moving an extension ladder. You're only as strong as you are. You're sorta like a servo that's rated at 40oz/in. You can give your rated output, but that's about it.

The ladder is collapsed. It ain't easy to move, but if you put all you got into it, you don't drop the sucker on the car that's parked in the driveway right beside you. (why didn't you move the car, you'd moved your truck didn't you )

OK, you got it moved and extended. Now you're back down on the ground and gotta move it back where it was before you moved it and discovered it was in the right place the first time. So you are ticked, and decide to move it without collapsing it. OH JEEEZZZZ...... HONEY, YOUR CAR WAS PARKED IN THE ............ etc etc

The weight of the ladder didn't change but force needed to move it did. Your strength couldn't change to match. Same thing happens when you hook up your connections to the servo arm. Short reach on the arm (ladder collapsed) and your strength is sufficient. Long reach on the servo arm (ladder extended) and you now need two guys to move the ladder.... uh, the servo does what all electric motors do when loaded, they suck more amps and burn out or strip gears or stall and suck burn up everything that'll burn.

So when you are building that new magic airplane and it comes time to connect the rods to the servos, start out with them plugged into the holes closest to the servo. And go from there.

You basically wind up making the horn connection for the surface wherever needed to get the desired surface movement. That's outside the plane and easier to do anyway.

I do the same thing. I use standard servos' except for the elevator, I use a bit more torque inthat one.

by the way for those who didn't know servos are rated with 1 inch servo arms
and as the arm is shortened the power goes up

Thanks for the info guys! Ihave been scratch building my trainers and teaching myself to fly. i dont recoment it to anyone.. but now i know what servos to put into my planes!

Rob

There are calculators out there like this: [link]http://www.csd.net/~cgadd/eflight/calcs_servo.htm[/link]
It claims to be conservative, but I'd go for a safety factor of 1.5 or even 2 to be sure.
Andy