Servo Torque and Arm Length....???
02-16-2005, 08:50 PM
#1
Servo Torque and Arm Length....???
I've been told that the torque rating on a servo is measured by the manufacturer on a 1 inch servo arm... a general standard in the industry...
So that a servo rated at 160 oz. has that torque at 1 inch from the center of the output gear shaft...
Thus if one would use a longer arm... maybe 1.25 inch arm, the torque at 1.25 inches is less than the stated rating of 160 oz.... and so on ... it would be less with a 1.5 inch arm .... etc. I've always know this implicitly... as longer arms cause flutter...
How can one compute the torque that is lost by using servo arms which are longer that 1 inch....????
Conversely... if the arm is shorter than 1 inch... then the torque should be more than the stated torque of the servo... How do you compute this torque...??
Thanks
Larry
[email protected]
So that a servo rated at 160 oz. has that torque at 1 inch from the center of the output gear shaft...
Thus if one would use a longer arm... maybe 1.25 inch arm, the torque at 1.25 inches is less than the stated rating of 160 oz.... and so on ... it would be less with a 1.5 inch arm .... etc. I've always know this implicitly... as longer arms cause flutter...
How can one compute the torque that is lost by using servo arms which are longer that 1 inch....????
Conversely... if the arm is shorter than 1 inch... then the torque should be more than the stated torque of the servo... How do you compute this torque...??
Thanks
Larry
[email protected]
02-16-2005, 11:27 PM
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RE: Servo Torque and Arm Length....???
Torque is a twisting force. The torque is not changed by servo arm length. The force applied to the linkage and the distance traveled by the linkage will change.
Torque is basically distance multiplied by force. Draw a line through the axis of the output gear on the servo. Go out one inch and apply a force 90 degrees to that line. In your case the force is 160 ounces.
160 oz-in = 160 ounces X 1 inch
Torque = Force X Distance
If the servo arm was 1/2 inches the force doubles.
160 oz-in = 320 ounces X 1/2 inch
So the force with a 1 inch arm is 160 ounces. The force out 1/2 inches is 320 ounces.
Output force = Torque / Distance
So a 2 inch servo arm.....
80 ounces = 160 oz-in / 2 in
.....will have 80 ounces of force at the link. The servo will have a harder time holding and moving the control surface with a longer servo arm due to the decrease in force applied to the linkage. Sometimes a longer servo arm is needed or more throw is needed, then a torquier servo is required.
Torque is basically distance multiplied by force. Draw a line through the axis of the output gear on the servo. Go out one inch and apply a force 90 degrees to that line. In your case the force is 160 ounces.
160 oz-in = 160 ounces X 1 inch
Torque = Force X Distance
If the servo arm was 1/2 inches the force doubles.
160 oz-in = 320 ounces X 1/2 inch
So the force with a 1 inch arm is 160 ounces. The force out 1/2 inches is 320 ounces.
Output force = Torque / Distance
So a 2 inch servo arm.....
80 ounces = 160 oz-in / 2 in
.....will have 80 ounces of force at the link. The servo will have a harder time holding and moving the control surface with a longer servo arm due to the decrease in force applied to the linkage. Sometimes a longer servo arm is needed or more throw is needed, then a torquier servo is required.
02-17-2005, 06:17 PM
#3
RE: Servo Torque and Arm Length....???
Thanks... and my research into this subject indicates that you are correct...,
HOWEVER.... as I have learned from studying this ... the mechanical advantage of the system is another factor... for instance:
with a one inch servo arm and a two inch control surface horn, there is a 2 to 1 mechanical advantage... therefore, the force applied to the control arm would be doubled..., e.g., a 1 inch servo arm with a 160 oz-in servo torque and a 2 inch control horn would result in a force of 320 oz-in on the control surface....
Granted this 2 inch control horn would reduce the degrees of control surface to half of the servo arm movement... e.g., if the servo arm moves 60 degrees (30 degrees in each direction from center) then the control surface's movement would be limited to 30 degees (15 degrees in either direction from neutral)....
My overall point in my trying to understand the physics of these systems is that ... I think the hobbist buys too much servo power... spurred on by the "merchandising" of the servo manufactures...
For instance... if on a specific plane, the builder wants 160 oz-in torque on an elevator half and that 15 degrees of throw in either direction from neutral is acceptable... then basically a wide variety of servos, servo arms, and control horn lengths are available.
Two choices might be:
Choice A: 160 oz-in servo; 1 inch servo arm; 1 inch control horn ... servo cost (approximately) = $100
Choice B: 80 oz-in servo; 1 inch servo arm; 2 inch control horn ... servo cost (approximately) = $40
I think most giant scale builders/pilots would opt for Choice A... thinking the 160 oz-in servo is the best.... and spend $60 more and not get any more benefit in torque at the control surface.... BOTH CHOICES PRODUCE 160 oz-in of torque on the control surface...
What happens also is that builders/pilots put these big torque servos on and with longer servo arms (1.25 inches) and shorter control horms (or move the control horn linkage closer to the control surface) to get the "big" surface throws.... then use the ATV (or end point adjustment) feature of their transmitters to reduce the surface throw to what is required... essentially spending perhaps hundreds of $$$ for excess servo strength... in an airplane....
.... am I right .... ?
Larry
HOWEVER.... as I have learned from studying this ... the mechanical advantage of the system is another factor... for instance:
with a one inch servo arm and a two inch control surface horn, there is a 2 to 1 mechanical advantage... therefore, the force applied to the control arm would be doubled..., e.g., a 1 inch servo arm with a 160 oz-in servo torque and a 2 inch control horn would result in a force of 320 oz-in on the control surface....
Granted this 2 inch control horn would reduce the degrees of control surface to half of the servo arm movement... e.g., if the servo arm moves 60 degrees (30 degrees in each direction from center) then the control surface's movement would be limited to 30 degees (15 degrees in either direction from neutral)....
My overall point in my trying to understand the physics of these systems is that ... I think the hobbist buys too much servo power... spurred on by the "merchandising" of the servo manufactures...
For instance... if on a specific plane, the builder wants 160 oz-in torque on an elevator half and that 15 degrees of throw in either direction from neutral is acceptable... then basically a wide variety of servos, servo arms, and control horn lengths are available.
Two choices might be:
Choice A: 160 oz-in servo; 1 inch servo arm; 1 inch control horn ... servo cost (approximately) = $100
Choice B: 80 oz-in servo; 1 inch servo arm; 2 inch control horn ... servo cost (approximately) = $40
I think most giant scale builders/pilots would opt for Choice A... thinking the 160 oz-in servo is the best.... and spend $60 more and not get any more benefit in torque at the control surface.... BOTH CHOICES PRODUCE 160 oz-in of torque on the control surface...
What happens also is that builders/pilots put these big torque servos on and with longer servo arms (1.25 inches) and shorter control horms (or move the control horn linkage closer to the control surface) to get the "big" surface throws.... then use the ATV (or end point adjustment) feature of their transmitters to reduce the surface throw to what is required... essentially spending perhaps hundreds of $$$ for excess servo strength... in an airplane....
.... am I right .... ?
Larry
02-17-2005, 09:43 PM
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RE: Servo Torque and Arm Length....???
This thread might get moved to the radio forum, little about aerodynamics.......
The most expensive servos I run are high speed standard digital servos. The largest plane I fly now is powered by a Saito 120. I know the guys that fly the real big stuff might chime in. After holding out for a number of years, this will be my first year flying IMAC.
Say that you want a plane that reacts quickly and predictably, which is a must if you are into any competition. If you take a servo with half the torque (yet same speed) and use half the output arm to apply the same force on the linkage, it will make the control surface move about half as fast to get the same amount of deflection. Sure it still has the same authority to move, but it is much much slower. Torque and force is not the only factor when considering servo set-ups, for many people speed is a huge factor. Also the radios allow the user to program different rates and exponential in the throws. At low rates, the servo might not even come close to it's full potential. But say that it's time for some wild maneuvers and the high rate switch is flipped or the exponential is taken advantage of, then the servos will use more of it's potential.
The newer digitals have more holding power, resolution, and will help larger planes hold trim. Holding trim well is also another characteristic you want if you are going to fly competitively.
I myself like reaction times for the servos around 0.15 seconds to rotate 60 degrees. I love precision flying and a tight feel on the controls. When I hit the ailerons for a point roll, I don't want the plane just to mush around. I want to roll the plane 45 degrees and then stop then roll then stop.....
I have been present when larger aerobatic planes are test flown. I remember one case in which a plane would not hold trim and just would not respond like it should. The plane would wander around and feel mushy with the controls. The owner replaced the servos with high torque digitals and the plane was then responsive and held trim.
Now if you like to lumber around in the skies, you don't need much when it comes to servos. I guess it all depends on your flying style how much you will invest in servos. Also, why not get a little more than you need as an insurance policy, rather than probably barely enough. Buying servos twice for an airplane is pretty expensive.
As an example my small car has 190 HP. I rarely use it all, but when I want to accelerate quickly or drive over a mountain pass (which we have plenty of here) it is nice to have all of that power. Now imagine that my car had half of that power. The transmission could be geared to give me the same torque at the rear wheels, but my quick acceleration and climbing abilities would be a thing of the past. The reason being for that in basic terms is power is equal to torque multiplied by rotating speed. The more power the faster I can turn the wheels with a given torque.
My next car will hopefully have 400 HP. How often will I be able use all of that HP without getting into trouble with the law.....
So basically buy a servo that fits your needs.
The most expensive servos I run are high speed standard digital servos. The largest plane I fly now is powered by a Saito 120. I know the guys that fly the real big stuff might chime in. After holding out for a number of years, this will be my first year flying IMAC.
Say that you want a plane that reacts quickly and predictably, which is a must if you are into any competition. If you take a servo with half the torque (yet same speed) and use half the output arm to apply the same force on the linkage, it will make the control surface move about half as fast to get the same amount of deflection. Sure it still has the same authority to move, but it is much much slower. Torque and force is not the only factor when considering servo set-ups, for many people speed is a huge factor. Also the radios allow the user to program different rates and exponential in the throws. At low rates, the servo might not even come close to it's full potential. But say that it's time for some wild maneuvers and the high rate switch is flipped or the exponential is taken advantage of, then the servos will use more of it's potential.
The newer digitals have more holding power, resolution, and will help larger planes hold trim. Holding trim well is also another characteristic you want if you are going to fly competitively.
I myself like reaction times for the servos around 0.15 seconds to rotate 60 degrees. I love precision flying and a tight feel on the controls. When I hit the ailerons for a point roll, I don't want the plane just to mush around. I want to roll the plane 45 degrees and then stop then roll then stop.....
I have been present when larger aerobatic planes are test flown. I remember one case in which a plane would not hold trim and just would not respond like it should. The plane would wander around and feel mushy with the controls. The owner replaced the servos with high torque digitals and the plane was then responsive and held trim.
Now if you like to lumber around in the skies, you don't need much when it comes to servos. I guess it all depends on your flying style how much you will invest in servos. Also, why not get a little more than you need as an insurance policy, rather than probably barely enough. Buying servos twice for an airplane is pretty expensive.
As an example my small car has 190 HP. I rarely use it all, but when I want to accelerate quickly or drive over a mountain pass (which we have plenty of here) it is nice to have all of that power. Now imagine that my car had half of that power. The transmission could be geared to give me the same torque at the rear wheels, but my quick acceleration and climbing abilities would be a thing of the past. The reason being for that in basic terms is power is equal to torque multiplied by rotating speed. The more power the faster I can turn the wheels with a given torque.
My next car will hopefully have 400 HP. How often will I be able use all of that HP without getting into trouble with the law.....
So basically buy a servo that fits your needs.
02-17-2005, 11:40 PM
#5
RE: Servo Torque and Arm Length....???
Thanks AQ...
You're right about the servo speed also...
It just seems like there are so many fliers who "just putter around the sky" with the big, high-dollar servos... and do loops and rolls and an occasional split-ess ...
I would think that they would benefit $$$-wise from less of a servo...
By the way, do you know any guideline or "rule of thumb" for determing the "required" servo torque for a specific control surface...???
If the manufacturer of a plane doesn't recommend anything ... how would you select a servo other than by gear-train and speed....????
Thanks
Larry
You're right about the servo speed also...
It just seems like there are so many fliers who "just putter around the sky" with the big, high-dollar servos... and do loops and rolls and an occasional split-ess ...
I would think that they would benefit $$$-wise from less of a servo...
By the way, do you know any guideline or "rule of thumb" for determing the "required" servo torque for a specific control surface...???
If the manufacturer of a plane doesn't recommend anything ... how would you select a servo other than by gear-train and speed....????
Thanks
Larry
02-18-2005, 12:06 AM
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RE: Servo Torque and Arm Length....???
Do a search on Yahoo and you will find a bunch of servo torque calculators. I just look at the plane and decide what I want to do with it. I then look for servos that I think will make it respond like I want it to. I don't have much experience on the larger planes. I'm sure that they have their own methods. My next plane is a Funtana. I know that I want a servo that centers well due to the large control surface, which probably means I'll go digital. I'm going to get the 40 size so a huge amount of torque is not needed, but I wouldn't mind a faster servo. I will also ask around and see what others are using for servos in this plane.
I think that the speed of the servo depends more on the taste of the pilot.
I also keep my plane sizes down, which helps control the cost of the servos. One day I might get smart and downsize my fleet for a few large planes.
http://www.coloradogliders.com/servo...calculator.htm
I think that the speed of the servo depends more on the taste of the pilot.
I also keep my plane sizes down, which helps control the cost of the servos. One day I might get smart and downsize my fleet for a few large planes.
http://www.coloradogliders.com/servo...calculator.htm
02-18-2005, 01:09 PM
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RE: Servo Torque and Arm Length....???
The thread started out asking about servo torque------now it has morphed into servo selection which is both mechanical AND subjective.
Torque requirement addresses only the MINIMUM needed -----there is a whole lot more to the choice.
Some (and I mean good flyers) say they see no difference between the base digital and the high dollar variety. I can see AND feel just a whole lot of difference between the two. For my personal use I will pay the price for TIGHT !!
If you don't understand what is meant by 'tight' then you can fly the base line servo----if you do understand 'tight' then high dollar it is.
Frankly, I'll change servos anytime I can get tighter. I'll pay the cost and weight penalty although this isn't too much. Current choice JR 8611-----changed from JR 8411. Yes the 8611 is tighter and smoother but not an enormous amount -----enough though! Torque----well each are overkill @220ozin and 155ozin respectively.
The thread really needs to return to torque measuremant.
Torque requirement addresses only the MINIMUM needed -----there is a whole lot more to the choice.
Some (and I mean good flyers) say they see no difference between the base digital and the high dollar variety. I can see AND feel just a whole lot of difference between the two. For my personal use I will pay the price for TIGHT !!
If you don't understand what is meant by 'tight' then you can fly the base line servo----if you do understand 'tight' then high dollar it is.
Frankly, I'll change servos anytime I can get tighter. I'll pay the cost and weight penalty although this isn't too much. Current choice JR 8611-----changed from JR 8411. Yes the 8611 is tighter and smoother but not an enormous amount -----enough though! Torque----well each are overkill @220ozin and 155ozin respectively.
The thread really needs to return to torque measuremant.
02-19-2005, 09:52 PM
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RE: Servo Torque and Arm Length....???
The reason that the torque question came up was due in fact to servo selection. I see no problem with where this thread has gone. Half of all of the posting going on in RCU is subjective and only scratches the surface of the subject. That is what makes it a forum. If the original poster wants to broaden the scope of the thread to answer what is behind the original question, I see no problem in it happening. That is why RCU is great!
Imagine a forum where the handling of a car was discussed. We wouldn't be able to talk about tires, because selection is subjective and there are many brands and types out there. We would have to start a seperate tire thread to discuss how tires affect handling. Sorry, just another bad example.
The only problem I see is that the thread may be in the wrong forum, but I see a parallel because it is aerodynamic forces that play a roll in what the thread is discussing. It would be great if some of the people flying the bigger birds would chime in and give the original poster some more insight on what was behind his original question.
Imagine a forum where the handling of a car was discussed. We wouldn't be able to talk about tires, because selection is subjective and there are many brands and types out there. We would have to start a seperate tire thread to discuss how tires affect handling. Sorry, just another bad example.
The only problem I see is that the thread may be in the wrong forum, but I see a parallel because it is aerodynamic forces that play a roll in what the thread is discussing. It would be great if some of the people flying the bigger birds would chime in and give the original poster some more insight on what was behind his original question.
02-19-2005, 10:53 PM
#9
RE: Servo Torque and Arm Length....???
Thanks to everyone who has responded... the help was great... and I feel I have a better understanding of the salient issues in servo torque measurement... and servo selection.
I'm a big bird flyer ... but I guess I don't have the experience to understand "tight" ... maybe someday I will... I do recognize performance as measured in servo strength and speed... translated into the control surfaces and finally the airplane in aerobatic manuevers.
As a businessman and I hope an economic man, I recognize the performance per dollar spent on servos and how this translates to one's wallet. I still see those pilots who, in my humble opinion, overspend on servos just to have the latest and greatest... as promoted by pure company marketing.... it's like putting a JR 8611 on a H9 Funtana 40 size...
I think we need to scientifically understand what servo fills our flying style needs and the airplane's needs. To this end... I came across an Excel spreadsheet, http://www.geistware.com/rcmodeling/...calculator.xls, that seems to allow some intelligence to be applied in servo selection. Give it a try.
Thanks again all...!
Larry
I'm a big bird flyer ... but I guess I don't have the experience to understand "tight" ... maybe someday I will... I do recognize performance as measured in servo strength and speed... translated into the control surfaces and finally the airplane in aerobatic manuevers.
As a businessman and I hope an economic man, I recognize the performance per dollar spent on servos and how this translates to one's wallet. I still see those pilots who, in my humble opinion, overspend on servos just to have the latest and greatest... as promoted by pure company marketing.... it's like putting a JR 8611 on a H9 Funtana 40 size...
I think we need to scientifically understand what servo fills our flying style needs and the airplane's needs. To this end... I came across an Excel spreadsheet, http://www.geistware.com/rcmodeling/...calculator.xls, that seems to allow some intelligence to be applied in servo selection. Give it a try.
Thanks again all...!
Larry
