DownSideUp

I have always read posts on a certain airplane setup where people have ten different ideas about the torque requirements for all of the surfaces for the airplane

I would like to know how one would come up with the requirements to hold a certain surface for a certain airplane in place during flight. I know that the way you fly and what prop you use will change everything, but it would be a ballpark figure. I don't want to go out and buy a servo that won't do the job, and I don't want to spend way too much money either. Lots of people just say, "Buy $60 digital servos and then you won't have to worry." I don't want to do that. Thanks for all the help in advance.

LuvBipes

My Feedback: (4)

Even on the same plane with the same engine, it depends on the type of FLYING you intend to do. If your setup on an 15 lbs Extra 300 is for precision IMAC pattern (control surfacethrows +/- 25 degrees), then a 60 oz nylon geared servo will do fine. However, if you intend to do 3D w/ the same plane (throws 50 degrees plus) you will need 100 oz metal geared servos.

Think about it, all else being equal, a 40 oz servo using a 1/2" servo arm exerts the same force on a control surface as a 80 oz servo using a 1" servo arm. The 1" servo arm produces much greater throw, of course, but the torque requirement is proportionally greater too.

Sport_Pilot

Yeah, but the 3D plane isn't going very fast. Been wondering about this also. I think a lot of the 3D requirement is that there isn't as much airflow to center the control surfaces, to make matters worse the higher rate makes the error worse. The high torque servos have more torque near neutral as well, thus more predictable performance.

I really don't know just therorizing.

DownSideUp

I have thought about sport_pilot's idea as well. This was kind of what I was digging for, cause I didn't really have an answer and I wanted to see what responses I was going to get. Personally, I would try doing more 3D flying rather that winding out the motor and going as fast as I possibly could.

LuvBipes

My Feedback: (4)

Sport_Pilot, what you say is true, but you also have to take into account a servo's ability to resist (note I didn't say "prevent") control surface flutter. Again, all else being equal, a servo capable of exerting high force on a control surface will better resist the onset a flutter, which is very common in 3D setups because of the extremely large control surfaces.

trailingedge

My Feedback: (1)

Sport Pilot there's a lot more air rushing over those large 3d surfaces in slow manuvers than you would think. Take a hover for instance. The plane isn't moving, but the plane's weight in thrust is being pushed across those control surfaces. Other manuvers like a wall or elevator normally see the controls deflected greatly for brief periods. Okay, here's a simple test. Tie your 3d capable plane down and run the engine up to about half throttle. Then, unhook something like the elevator servo and try moving that surface linkage with your hand. It takes a lot of force to hold those big surfaces at an angle. If you really wanted to get technical, you could hook each control surface to a scale (one at a time) and measure the force needed to bring the surface to full throw at full throttle and consider that a ballpark minimum. I normally take this rule of thumb as my personal minimums, but you'll hear a lot of different opinions from others.

up to 40 sized plane: standards or servos with equivilant torque are just fine (I did go higher torque for my funtana, but I wanted to push it hard).

60 size: Standards would work in a pinch, but I like to stay above or around 65 oz.

giant scale: If I'm going to invest the money into a monster plane, I'm going with the highest torque/best responding servos I can buy.

DownSideUp

So there's no need for all those servos in between 65 oz. and 150 oz. of torque?

mr_matt

My Feedback: (10)

I use a program called linkage design. It is actually a very advance spread sheet.

It take into account the speed of the plane and the size and deflection of the surface. It models all of the linkage parameters as well to take the non linearities into account.

http://members.cox.net/evdesign/page...ge_design.html

JoshFlies3D

It's commonly overlooked when calculating Force and or Torque that the rated TORQUE a given servo can deliver is a constant, regardless of the servo/control arm length.

LuvBipes

My Feedback: (4)

[/quote]

It's commonly overlooked when calculating Force and or Torque that the rated TORQUE a given servo can deliver is a constant, regardless of the servo/control arm length.
[/quote]

Industry standard is to measure rated torque at one inch (that's why it's labelled "ounces/inch". Do not confuce force appliend to a control surface with rated torque. If you double the length of the servo arm, trust me that you will significantly reduce the FORCE applied to the control surface. It's simple leveraging/de-leveraging.

mglavin

My Feedback: (31)

Josh's comment is accurate... There is no confusion. The comment was made to differentiate the two specifications and realized performance thereof.

FORCE and the servo rated TORQUE are one and the same at the 1" industry standard. FxD=T.

A servo rated at 100oz-in Torque is providing 100ozs FORCE with a 1" servo arm. A 2" servo arm will reduce the FORCE to 50ozs.. The same servo with a .5 servo arm will generate 200ozs Force.

So we can visualize how a shorter/longer arm will increase/reduce the SERVOS available Force while maintaining the servos rated TORQUE specification. As mentioned previously the servos TORQUE is constant and FORCE is a variable of the arm length in play. The FORCE introduced at the control arm directly influences the TORQUE a surface will realize. Again FxD=T.

100ozs Force introduced to a 1.5" control arm generates 150oz-in, 100x1.5=150oz-in TORQUE. Torque is defined as a twisting force.

JoeAirPort

My Feedback: (41)

I found out the hard way that what he said is true. 100 oz metal gears for 3D on bigger planes is exactly correct based on my experience.

JoeAirPort

My Feedback: (41)

Yes correct again. I put in a CAP232 120 a week ago due to this phenomenon. If the servo isn't strong enough to hold the surface in place, it will flutter and snap those babies off right at the shaft. I saw it happen before my very eyes. Why? Because I set up my linkages for 3D rates and the surfaces can go unstable on you since the servos can't hold them still. The servos start to fight the surfaces in a flutter since they can't keep up, and they fail. Metal gear and at least 100 oz-inch or you will pay, trust me.

BTW, I used a HT 635HB on my ailerons. 82 oz-inch at 6 volts. I thought they would be enough but they are not. They snapped off at the shaft right where the screw ends.

Sport_Pilot

Oh there is a lot of force, but not as much as say the same plane running at 50 MPH. Maybe the rudder and the elevator will be close because the prop wash is over almost the entire area, but the ailerons will have much more force.

After doing your test, try out a standard $10 ounce servo on your smaller sport or trainer models. See how much force it takes to pull it out of neutral. Don't push the control surface, it will likely break! Much more force is required than to move the 3D elevator.