turnnburn

My Feedback: (49)

In order to maximize servo tourqe from a mechanical linkage standpoint refresh me a bit please. I know I want the shortest servo arm I can. what about my the control horn ? should it be shorter or longer for best setup ? To be clear, Im concerned that my rudder servos are marginal power wise and Im afraid they will be over powered ( and therefore stalled out and unable to move) at high speeds.

madmodelman

Best advantage is horn longest but of course you will lose travel.

husafreak

My Feedback: (3)

Shorter servo arm to longer surface horn gives the best mechanical advantage. Shoot for maximum travel in your Tx setup and adjust the linkage to get the rudder throw you need. Also beware high voltages, an overstressed servo will fail (burn up) much quicker at 6v then at 4.8. If you want to run 6v the heli guys have servo extensions with a built in circuit to reduce the voltage to the tail servos which typically are limited to 4.8 (or used to be).

turnnburn

My Feedback: (49)

Thats what I thought, just needed to make sure I was thinking correctly.

The linkage Im concerned about is the rudders on a Hotpsot with JR 168 digital servos. I got this from a fellow modeller and it has not been flown ever. It currently has short servo arms on the servo but also short arms on the rudder (control surface) as well. I am lso concerned a little bit about flutter at high speeds.

What is the best linakge configuration to help prevent flutter ? I believe it would be the same short arm on servo and longer arm on control surface ?

I am not worried about amount of travel, it has two rudders which are primarily only used and low to medium airpseeds.

ByLoudDesign

When you try to pry some thing you use as long a lever as you can! In the case of a rudder try and set it up so the throw arms are perpendicular to the rudder when centered. this gives you maximum torque with the equal throw left and right. 1:1 is always best, if it gives you the throw you need!

turnnburn

My Feedback: (49)

What do you mean by 1to1 ? I assume you mean equal length arms on both ends ?

ByLoudDesign

Correct!! If you have say 46in.oz. for a servo output it is that at the end of a 1" arm. if the throw arm on the control surface is 1" the the full 46 in,oz. less friction losses is there. If your 1" servo arm is driving a 1/2" throw arm you pretty much double the output. Keeping the drive line in line with the control surface and the arm perpendicular to the surface give you maximum torque transfer.

siclick33

I'm not sure I get what you're saying but I don't think that's right.

You get the best mechanical advantage by using the shortest possible arm on the servo and the longest arm on the surface.

turnnburn

My Feedback: (49)

I can see where an equal llength or as close to equal as possible is the best (I think). If the control arm on the surface is twice the length of the servo arm this would great for s static application but since we have airloads pushing back against the servo the longer the arm the harder it pushes back against the servo, (more leverage). am i on the right track here ?

ByLoudDesign

turnnburn, you are correct! If your arm on the control surface is twice as long as the servo arm you have cut the torque in half and it would require more throw at the servo to get a given throw at the control arm. If you use a shorter arm at the control surface you have reverse that situation.

siclick33

http://downloads.hobbico.com/misc/gp...rod-hookup.pdf

http://www.horizonhobby.com/article/...ical-advantage

Ad Clark

My Feedback: (4)

siclick is 100% "spot on". You will maximize your servo's torque transfer to the control surface by using a shorter arm on the servo and a longer arm on horn of the control surface. It is also important to keep you servo travel at close to it's max (with in reason) and you need to be able to achieve the proper amount of throw. I would strive for at least 100% servo travel in your situation .More would probably be better if your geometry allows w/o binding. My opinion ...your mileage/travel may vary.

It seems the "lever thing" is causing some confusion..remember that the lever also works in reverse. If I was drilling a large hole through a wall log I would prefer a long "cheater bar" pipe handle to keep the 1/2 inch drill from winding my arms up like a pretzel. The long cheater bar does not give the drill the advantage, it gives my arm the advantage to keep the drill from spinning. The longer control horn gives your servo the advantage to "hold" the rudder and prevent flutter as it transfers more torque to the control surface.

If you have a 46 oz. servo and use a 1/2" servo arm instead of a 1" servo arm you are roughly doubling the output torque of the servo-at the sacrifice of travel. You may be able to compensate by running your travel adjust up to something over 100 % , you may have to use a servo arm in between 1" and 1/2" or some combination of all of the above.

Please check out the 2 downloads that siclick has listed in his post

Sorry I got a little long winded here,

best regards,

turnnburn

My Feedback: (49)

Im still trying to get my head wrapped around this and fully understand it. I think I have it striaght in my head now and I think Ad and Siclick have it right.

I/m still trying to figure out if there is ever a time where a long horn or arm on the control surface will work as a lever against the servo and not for the servo. It seems to me for this to be the case the airload force would have to exceed the tourqe of the servo plus the added mechanical advantage of the lever arm, however much that is. Im thinking if the servo tourqe plus mechanical advantge equals 100 ounces and the airload force on a fully deflected control suface at top speed is 50 ounces, then the servo plus mechanical advantage in reality equals 50 ounces under the worst case scenario and we are good to go. This is true for most model airplane control surfaces where we DON"T use a counter balance weight, unlike full scale aircraft where they almost always have a counter balnce weight.

Am I on the right track here with the above paragraph ?

Ron S

My Feedback: (2)

To maximize your servo torque onto the control surface, go with either a shorter servo arm, or a longer control surface arm, or both. As mentioned earlier, what you lose with this is control surface throw. At that point, you can go into your radio and bump up your servo throw (if needed). [8D]

And careful not to mix or confuse servo torque (ounce-inches, for example) with a control surface load or force (ounces, for example). One is force times distance, the other is just force.

turnnburn

My Feedback: (49)

so how is the force of the servo (tourqe time distance) differnt from the airload force (torque (airload X distance (length of the control horn ) ??? I'm trying be a jerk or anything Im simply trying to understand everything that is in play here.

Ad Clark

My Feedback: (4)

Hi turnnburn,

All considered you will probably be better off to upgrade the 46oz. servos on the rudders to something in the 90 oz. or so range. That should give you a good safety margin and a longer servo life imo.

Re: does the lever thing ever work against the servo? Hey I'm just a little country mouse not an aerospace engineer but here is my take on your question.

The relationship of the servo arm vs the length of the control horn establishes a fixed ratio between the two.

servo arm1/2 "

control horn 1 "

This creates a 2:1 ratio with two different size arms, horns or(levers) Once you set up your servo arm and control arm (long or short arms) with servo 1/2' and control horn 1" the ratio and mechanical advantage will remain the same and will be in favor of the servo for torque as long as the servo has the shorter control or moment arm.

Another way to think about it is:

-the distance from the hinge line back to the trailing edge of the rudder is let's say 6". That is a 6" lever, so to speak, with our one-inch control horn mounted 90 degrees at the hinge line (pivot point or fulcrum). As you might imagine, side loads on this six-inch-long rudder could transfer sizable loads through the one-inch horn. If we change the one-inch horn to a four-inch horn, for sake of comparison, it would only transfer one fourth the force of the one-inch horn. This is why we always try to keep a longer horn on the rudder and shorter on the servo. This longer horn also helps to minimize excess play in the linkage.

The same thinking would apply to most all of your control set-ups.

Hope I didn't muddy the water! Just trying to help.

Best regards, Ad

guamflyer

the guys are correct but I'd put a higher torque servo, then worry is much less..all my bigger birds are done that way as I use rudder alot..some birds have 60 oz servos for rudder..you'll never regret it.. 90 oz is even better....I also keep my hook-upsas straight as possible


SLOPE FAST - SOAR DEEP

guamflyer

P-40 Bro #5 and 6

turnnburn

My Feedback: (49)

Thanks Ad, I see where my logic was faulty. I got it now. I think all the other responders agree with the possible exception of by Loud design.

dbsonic

My Feedback: (3)

Great topic. 46in.oz is actually a moment. a force x distance. You can also think of the airload as a moment about the control surface hinge. But you have to determine the load distribution and determine whether it can be reduced down to a concentrated point load at a distance from the hinge line. Then you can see if the moment of the servo is sufficent to drive the flight load control surface moment. I am sure on the aerodynamics forum there is some good stuff about this topic.

Ad Clark

My Feedback: (4)

Hey there turnnburn,
You're welcome sir!

PS I like the 90 oz servo upgrade a lot!

best regards,

familiamarez

Please help.....I needing to know what size screws are in the swashplate as far as thread pitch and such .thank you

bcovish

My Feedback: (8)

Maybe you should post this in the heli section

Art ARRO

My Feedback: (5)

turnnburn,
Note that the rudders on the Hot Spot are quite small in area. I did the math on this and concluded that a 40-50 oz/in torque servo would be sufficent. Also, I would not be doing full power/high speed knife edge passes with the HotSpot and this size servo was good enough.
I still recommend input from some actual jet pilots that have flown the Hot Spot extensively- US types preferred where movable rudders are a requirement.

Art ARRO

littlera

I think the most relevant statement in this thread is by Ad Clark - " It is also important to keep you servo travel at close to it's max (with in reason) and you need to be able to achieve the proper amount of throw."

To get the maximum power from a servo, you must operate it at or near its maximum throw. Any combination lengths of servo arm and control arm will work so long as the end result is the correct movement of the surface. If after doing this, there is still marginal power to move the control at its full throw in flight, a more powerful servo will have to be installed. Period.

Also, arm lengths that maximize the linear movement of the control rod setup will minimize the effects of any slop in the system.

turnnburn

My Feedback: (49)

Not sure I agree with everything you said littlera. The reason we want to use max servo travel is to retain as much resolution as is possible and ofcourse to get the required amount of control surface travel. We have established that a short servo arm and a long control horn (short and long being relative terms) is the best setup.

Im not sure if your statement regarding power and servo travel are correct.

And i'm a little unclear what you mean about linear travel. Linear travel is best acheived with reduced servo travel is it not ? The longer the servo travel the more non linear it gets as it rotates (key word) farther and farther away from center or nuetral. Your statements seem to contradict but maybe Im not understanding exactly what you mean by linear.