I realize that there are other forums for radios and servos, but think maybe this belongs here as it relates to servos as recommended for ARF’s. I have always used the so-called standard Futaba servos, the S148, S3003 and S3004. I just fly the 40-46 and 60 two cycle powered aircraft, and have never even questioned whether the servos were o.k. Have used them in 60 powered planes such as the Goldberg Tiger, and 46 powered planes such as the GP Revolution and the TH Kaos. I noticed recently that more powerful servos were mentioned in the forum in discussing the Revolution, and then saw an advertisement for the Revolution that I think said a minimum of 54 oz-in was required. Also am interested in the Skylark 70, and a review indicated a minimum of 54 oz-in. Wow! This is why I avoided bigger planes, so I could use the standard servos, and knock on wood, have not had any failures (that I know of) from inadequate servos. Some of my friends assure me that the standard servos are fine (I don’t do any 3D) but am nervous going against either a manufacturer’s or reviewer’s recommendation. Any thoughts or information or ideas greatly appreciated.

If you set your servos up for maximum mechanical advantage, you will not have any problems.

The requirements are simple and you probably already do it. 1) Never turn the EPA/ATV on your transmitter below 100%. 2) Attach the pushrod in the outer hole of the control horn. 3) Attach the pushrod in the servo arm hole, closest to the center of the servo, that will still give you the travel you need. 4) Don't use Expo. If the airplane is too sensitive, reduce the travel.

Doing it this way, you can easily get a mechanical advantage of 3 : 1, giving your 42 oz standard servo the equivalent of 126 oz. servo, at the control surface.

For 3D, you need monster servos. For normal sport flying you don't.

Jim

Sherman,

As you already have alluded too, the type of flying is important among other things to consider. The manufacturer has a minimum requirement that will all but make sure that no flutter from too weak of a servo, the "normal" weight of the plane when equipped as recommended will not over work the servo and a small host of other things.

What you need to remember is even though you may be sport flying you are still putting an air load on the surface, hence loading the servo. Let's use an example using the rudder as the primary control and usually the rudder needs more servo than all the other surfaces. OK, so you now have rolled into a Knfie Edge, the plane is at full power and you have all the throw in that the surface can give and your control rod/horns geometery is good, but the plane will bring the nose up momentarily then drop the nose a bit, what you are seeing is blowback, caused by either not a powerful enough servo, bad geometery, not enough engine power or a combination of all three.

Basically if you are flying along in a certain maneuver and you notice it just will not hold it for very long, it's possible the servo is blowing back because it's not strong enough taking all the other factors into account. Now most, but not all servos, can handle 6 Volts so that is one way to increase servo power without buying new servos.

the other thing is you always want to buy the best servos your budget can afford that match the requirements that are recommended. You can easily see the difference between a plane that has just the bare minimums to one that has a bit too much servo power. More servo power equals a better handling plane.

For me, I tend to use servos with at least 15% more power then needed, I like a safety margin but then I fly 33% or larger planes. I've seen too many planes get torn apart by flutter because of too weak of a servo or bad geometery or a combination of both

for best mechanical geometery, do not set your end points/ATV on the radio below 100% and it will not hurt to set them to 125%

To JRF and Bubba. Thanks for the information. It was extremely helpful. I blush to think that I had not even considered the mechanical advantage in hooking up the controls. I have tended to hook them up any old way, and then use the computer radio to give the proper movement, which often resulte in way less than the 100 % movement. I'll stop this practice at once.
I am also interested in your comments on exponential. Why is this disadvantageous to use? Thanks again, happy landings.

Hi Bubbagates, also do you make it a point to position the (aileron forexample) control horns on the elevator, so that the pushrod is at an angle when at neutral, but a straight shot to the servo horn at full deflection?

I think that helps to maximize the holding power and minimize any pushrod flex. (because at neutral there is not nearly as much load on the system as at full deflection.

Thanks,

Ernie

Typically, those who use expo, use it to reduce the travel that they actually use for flying. That has the same disadvantages as using ATV/EPA. If you change the linkage instead, you will increase the mechanical advantage.

Dual rates have the same problem. If you never use high rates, change the linkages to match your low rates. You will increase the power available at the control surface by 30 to 50%.

On the other hand, if your type of flying uses high travel rates, even occasionally, you will not be able to set up these high mechanical advantage linkages and you will need to go to high powered servos. The best a 3D flyer can usually do is 1 : 1, which is no mechanical advantage at all. A 120 oz servo set up at 1:1 (for 45 degrees of control surface travel), gives less power at the control surface than a 42 oz servo set up at 3 : 1 (and 15 degrees of control surface travel).

Most sport planes will easily do any non-3D aerobatics with 15 degrees of elevator travel. Although, as Bubbagates said above, you may need to go to a higher powered rudder servo if you do much knive-edge flying.

I didn't mention 6 volts. That is also a good way to increase your servo power. In fact, most standard servos are rated at more than 54 oz. when running on 6 volts. Just remember that the servos will draw more power at 6 volts, so you will need a larger onboard battery.

Jim

I have been doing the same thing since I started flying; that's the way I was taught. Connect the surface to the servo, as close to center as you can get. Then adjust the end points in the radio to get the throw you want.

I didn't want to jump your thread but since you brought it up. Expo; I just don't get it. I flew one of my planes and used the rate switch to try expo. Same amount of throw on upper and lower, but one had around 50-60% of expo. I didn't see a difference in how the plane flew or landed.

Maybe I just have a sensitive touch. LOL

Seriousely, if someone can explain what it is you're suppose to see or feel when using expo that would be great for me.

Thanx

p.s. I did that lil experiment on my H9 CAP and a Paramount 52.

I[ve thought about this problem and the answers, and have somewhat changed my mind on the results. First of all, keep in mind that I am severely mechanically challenged, so wrong conclusions should be forgiven!

In using leverage, it seems to me that the larger the arc at the servo end, the more pressure can be exerted at the control end, or put another way, the less force is required by the servo to get the work done. A longer lever exerts more force. By moving the control arm closer to the servo pole, it is the equivalent of moving the fulcrum point closer to the servo, and thus requiring more force by the servo to get the benefits of the leverage (have I now gone completely off the cliff here). I do agree that the servo arm whould move the entire distance for the mechanical advantage. I’m not taking exception as much as just trying to understand the principles involved. Will enjoy your comments and observations.

With a bunch of expo dialed in, I feel the controls are less twitchy or smoother around the neutral position of the stick. If you go all the way with the stick, you still get max deflection though. Makes for smoother landings for me.

Ernie

No, sorry Sherman. You are not correct. The force exerted by the servo is measured in in/oz. A servo that is rated at 42 in/oz will exert 42 oz of force on the pushrod if the pushrod is mounted 1" from the center of the servo arm. It will exert 84 oz of force 1/2" from the center but only 21 oz of force 2" from the center. Try it if you like. Fasten a 6" stick to the servo arm and see how much force it takes at the end of the stick to stall the servo. (7 ounces, actually. If your analysis were correct it should take almost 16 pounds to stall it.)

Math wise, the ounces times the inches will always equal 42 for the servo in this example. (42*1=42, 21*2=42, 84*.5=42)

So basically, at the servo, it is the opposite of what you said. Your analysis describes the control horn end of the pushrod, where the force is coming from the pushrod rather than being put into it. That is why you want the pushrod as close to the center of the servo arm as possible and as far away from the center of the control surface as possible.

Jim

Actually Sherman your thinking of the fulcrum is backwards on this. I've thought about it and it does make sense.

Think of it this way. If you put weight on the end of a yard stick it will bend if you hold up the opposite end; put that same amount of weight closer to where you hold it and the stick won't bend as much or not at all. The same as if you want to push something heavy; you put your hand close to your body to push, not fully extended.

I ran across an intresting experiment, these guys built a test platform that they mounted on the mirror bracket of a pickup to test how much force was required to move a flight control
it had a deflection gauge and a scale hooked to the control horn they drove down a runway at speed up to 80mph I was suprised at how little force was reqired to hold it at 45 degrees I wish I had saved it
can't recall the numbers bit it was way less then what you hear people say you need

The holding force is always less than the moving force required to deflect the control surface from neutral to 45* against a strong airflow resisting the control surface movement. Especially as the servo is attempting to do this deflection as quickly as possible in obedience to a sudden control stick command for full 45* deflection. At full deflection command with the control stick rapidly moved to max deflection command, the servo is probably experiencing a very high moving torque load. Much higher than a holding torque load.

Just get some JR DS821 servos. You can find them on RCU or Ebay for $20 ea. You get more power, better response, better holding power, better centering, and better feel of the plane.

Sherman, your "longer lever exerts more force" is correct in one sense - if you increase the lever arm at the servo, you are increasing the force of the control surface on the servo, so you get half credit

Just thought I'd through this out here. Expo has nothing to do with control throw. It just reduces sensitivity around the neutral stick point.

This is correct. I believe the above reference to expo reducing travel is referring to instances where people are not using the whole range. I made it through a whole first season of flying without knowing I should be using full range of my sticks. With small stick inputs, expo actually reduces the amount of travel of the surface. It catches up with larger stick inputs to equal full travel for full stick input.

The idea is to set up your radio so you are using full range on low and on high rates. You will get more consistent results (roll rates, etc.) and have more room for correction and better control of stopping points. It also would mean full servo travel for desired deflection, and thus reduce strain on the servos.

There has been alot of debate and articles about servo torque requirements over the years. My engineer son tells me it involves the aera of the control surface, the degree of deflection of the surface, and the speed of the plane. Something about fluid dynamics? There have even been programs in which you can plug in the specfics as to surface size and deflection and speeds and get recomended ranges for servo torque. Don't make it harder than it is, the mechanical advantage stuff is right on info and standard size planes are generally ok with standard size servos. If you go bigger or faster or 3D then a little more $ is a good idea.

Now that is alot of great information from
you all. Should be a sticky ?

Thanks to the op for a great question,

Bob