Which hole to use on control surface horns
 

Trying to set up proper control surface travel on my Multiplex Easy glider, and having problems. Question is this: Can someone explain to me which holes on the servo arm and which holes on the control surface horn are used to get the longes travel and shortest travel. Basically not sure which holes to install the pushrods in. Thanks Nikolai

RE: Which hole to use on control surface horns
 

See attached diagrams

Ken

RE: Which hole to use on control surface horns
 

Ken, thanks alot for the info its great, Nikolai

RE: Which hole to use on control surface horns
 

It's worthwhile to throw in some advice on this issue.

When you're setting up any connection, it's a decent idea to start out with the pushrod connected to the servo arm in the innermost hole, and the horn connection in the outermost hole. And then check for your recommended throw on the surface.

In any case, you want the lever arm of the servo to be shorter than the lever arm of the surface. If you can. And the above starting point helps insure that result.

RE: Which hole to use on control surface horns
 

I believe this is what was meant.

RE: Which hole to use on control surface horns
 

I'm not into the diagrams but it looks to me like the first and last ones are the same on both answers and need to be rechecked . Correct me if I'm wrong but I really think a closer look is in order. ENJOY !!! RED

RE: Which hole to use on control surface horns
 

Why?

RE: Which hole to use on control surface horns
 

it has to do with tourque. the farther from the center of the servo you make the connection the less tourque that the servo can put out to the control surface. it is like using a wrench to remove a bolt the farther you grab out on the wrench you convert the extra distance moved in to torque but when you have the bolt spinning the wrench the torque is converted into movement. using holes closer to the center also provides finer degrees of control. I usually like to start in the middle of both though and start moving holes one at a time. If shorter throw I start moving first the hole on the servo one then the hole on the control arm one hole next and reverse the order if I have to lengthen the throw

RE: Which hole to use on control surface horns
 

Why?

You're working with levers at both ends of the connecting rod.
If they're the same length, there is no multiplication of load seen by the servo. Whatever load hits the flying surface, that's the load the servo will see. And we size the servos to the model, so they're guaranteed strong enough for that load. So we give the servo the chance to do the job. And it won't take excess battery doing it. Nor will it be "blown back" or fail to give full deflection in a dive when you ask for full deflection. Those problems are silent killers that we might encounter if the surface is rigged to have more leverage than the servo.

There is an excellent chart that was published in a recent issue of one of our RC magazines. It shows the relationships and the gains/losses from different lever arms. I gotta find out which magazine and give credit because it's due.

This chart is from
MODEL AIRPLANE NEWS
July 2007 issue
"Maximum Control, Inflight Servo Tests"

Beginners should find that magazine and buy it. The article should be part of Learning to Fly RC 101.

RE: Which hole to use on control surface horns
 

BTW, when we get that relationship screwed up, we not only make the servo work harder, but we also lose the feel for that surface. The movement of our stick is so degraded that the airplane feels mushy and the precision of the servo is lost.

In other words, there is more than one reason to rig each surface with some thought. Experienced modelers try to get it across to newbies, but it's evidently just a bit too technical and it's almost always ignored.

RE: Which hole to use on control surface horns
 

Look at the amount of force that finally gets out to the surface in that chart.

With the low ratio, a 40oz/in servo can work like a servo that's twice as powerful. With speed models this is a definite positive. And look at the servo arm movement. With that much movement, your TX stick is going to feel VERY precise. That accuracy is what pattern flyies really like.

Look at the high ratio. You bought 40oz/in servos, and you're realizing 20oz out of them. Dumb move. Our servos will still move the surfaces, but they'll suck the battery flat doing it. And look at the servo arm movement. You TX stick is going to feel like it ain't connected. It's going to feel like the pushrod is rubber, or the servo is dying. And it is.

RE: Which hole to use on control surface horns
 

Right, if you have a 40 oz servo that rating is 1 inch from center of servo; at 1/2 inch the servo should pull 80 oz. The rating is actually inch ounces. OK?

RE: Which hole to use on control surface horns
 

Actually, that isn't how it works out.

The servo delivers whatever it can depending on the load first of all, and the available juice secondly. The electric motors in the servos don't simply slam out full power. They respond to the load, and give accordingly. But yes, the rating is in inch ounces to provide a basis for comparison.

The magazine article actually takes a stab at explaining that but only in passing. But the magazine article is very worthwhile reading and the magazine deserves credit for an excellent article and we should give the credit for that chart.

MODEL AIRPLANE NEWS
July 2007 issue
"Maximum Control, Inflight Servo Tests"

RE: Which hole to use on control surface horns
 

The geartrain and gear leverages all have an affect. If the servo was actually loaded to 80ounces, the gears just might not stand the load. Parts are usually designed to the specifications chosen. No guarantees over specs. And the way the power train works in a servo, there is every reason to believe that the servo would give that twice-the-expected load a shot. The way it'd do it is what electric motors do so well and what burns out so many of E-powered model motors. The motor simply sucks whatever battery power is available and strains to give the designed rpm no matter what the load. And if it gets the juice it wants, it gives twice the power it was designed to. And happily melts. That is, unless the gear train pops before the meltdown.

The reason we want to start off with the safe rigging (servo hole inside--horn hole outside) and work from there is a couple of reasons. It's the safe way to start AND it actually lessens the load on the servo. And in that case, the electric motor draws LESS juice than design. And there is less stress on the gears. And all of the servo rotation is more apt to be needed so we get maximum resolution at the TX sticks, so we have more accurate control.

So next time the field expert tells you that you ought to fix a throw with your computer radio, think about it first. If you can change the servo-to-horn connection points to do the same, and doing so will give the better ratio (as shown in the chart), DO IT. It's a win win win situation.

RE: Which hole to use on control surface horns
 

Do you do these diagrams all yourself?

You always seem to have the right one on hand.

RE: Which hole to use on control surface horns
 

Thank you for clearing that up for me Rock. I actually have and read that article in MAN when the issue came out. I knew I saw it somewhere, just couldn't remember where. Now that I've re-read it I understand it better. Sometimes I need to read things several times before it penetrates the dense outer skull. :) Thanks again.

RE: Which hole to use on control surface horns
 

BTW, what scratchonly said about the oz/in deal is correct. If you look closely at the chart you can see the effect he's talking about. The .4" arm location on the servo could provide 100oz max push/pull compared to the .5" arm providing 80oz. But that'd be in a controlled test of the max torque available. Which is certainly of value to know. But our working environment pretty much makes that moot.

For example, if your RX pack is fresh, it'll probably be providing over 5V. And our servos put out more power with higher voltage. And of course, they actually put out whatever is asked for within their ability to do so. So in practice, the rating is just a comparative bit of info.

But we can use that info off the chart to get an idea of how "easy" it is for the servo to work depending on the arm length. The farther from the center for the servo arm, the less max force that servo can provide.

RE: Which hole to use on control surface horns
 

You need to remember that the manufacturers test in laboratories under controlled conditions. They also don't use battery packs during the tests but laboratory power supplies capable of maintaining exact voltages at all times AND delivering as much current as needed.

RE: Which hole to use on control surface horns
 

Very informative thread!

Thanks daRock & Ken!

somegeek

RE: Which hole to use on control surface horns
 

Hi!
There is some basic rules you must follow when you instal the controll linkage. Those are:

Always strive to have as short servo arm as possible and always strive to use as long controll arm as possible. This should be written in stone!!!
Why ? Becuse then you will have a stiff set up!! Which is very important in an airplane.

RE: Which hole to use on control surface horns
 

Thanks to all who gave me info on my control service throw, shall go out and test tomorrow. Nikolai

RE: Which hole to use on control surface horns
 

Da Rock said it in his post, but I believe there is one point that needs some consideration here.

All initial settings should be set to the center of the surface throw. Let's use the Elevator as an object point.

Center the elevator and immobilize it somehow. I use two lengths of 1/8th ply about four inches long by two inches wide. I 'sandwich' the elevator between the ply, securing the ply with two lite clamps (See picture). This secures or immobilizes the elevator to the horizontal stab. This assumes that the horizontal stab is NOT an airfoil shape. Many, if not most, of our trainers or second planes use a flat surfact on the top and bottom of the horizontal and vertical stabs, so this will work. This also centers the elevator to the center of the airflow over and under the horizontal stab.

The next thing I do is center the servo and make sure the trim tabs on the transmitter are in the center. Keep the transmitter and receiver on. This will insure that the servo does not ove during this adjustment. This is easy with the DX7 because the display screen is easy to see and shows where the trims are. Enough commercialization here.. ;) If you use a fresh, reset model on your computerized radio you should be ready to go.

The next thing to do is to connect the control rods to the servos but not to the control surface horns.

Next is to adjust the clevis to the center of the hole you are going to use. (See Ken's and Da Rock's drawings).

Then connect the clevis to the horn, making sure that the connection does not try to move the elevator. If it does, then make the necessary turn adjustments on the clevis until it does not move the surface, and connect the clevis to the horn.

Then carefully and slowly remove the clamp(s if you used two or more). The elevator SHOULD NOT MOVE. If it does move, then investigate why and fix that problem. Remember, you secured the elevator with the clamps and adjusted the clevis to the center of the desired hole on the horn. If the elevator moved, then it is an adjustment issue.

Once this is done correctly, the surface is centered. This will work to some extent with all of the other surfaces. The aileron on flat bottom wings is easy to do, just remember that you are making the alignment to the flat, or bottom surface. On symetrical wings, well, use your imagination and ingenuity, but by the time you are ready to fly a fully symetrical airfoil, you should be well beyond this basic adjustment procedure.

If done correctly, and if you built the aircraft correctly AND straight, it should require a minimum of trim to fly straight and level.

RE: Which hole to use on control surface horns
 

CGRetired,
You might also suggest that as you begin this, you should do any TX centering on any servo that needed it. Subtrim is a very useful tool now that it's available.