Subtrim vs. Programmer for Centering Servos

[econpatric]

So, I just installed a couple of Hitec HS-5245MG's in my 3DHS 59" Slick and have been thinking about hooking them up to a Hitec servo programmer to center the arm vs. using my subtrim feature in the radio. I have always used subtrim and am now wondering if there are any benefits to either option?

I have went through the arms to find the "closest to 90*" setup, and all is well currently using sub trim. Is it worth the extra work? Should a person do both?

Thanks.

djmoose: I seen your response on GiantScaleNews.com about servos centering and am hoping you'll chime in here. I think that your contribution to that discussion was mainly focused on multiple servos, any advice here?

 

[rcbirk21]

i think to a certain point a setup is "perfect enough". Is it better to do the hitec programmer centering? sure. is it gonna work just fine without doing so? of course. i dont think it is a big enough advantage to be worth going through the hassle. i really dont think it will effect flight characteristic enough to notice if at all. the reason it is important in multi-servos per surface setups is just to avoid the servos fighting each other and burning them up

 

[gyro]

I agree, if you have them centered with the subtrim, I think that is probably just fine. I have the programmer, and I haven't centered any servos with it.
What I have done, is make sure all the deadbands are equal. It was something Doc Austin mentioned many months ago when he did it for the first time. When I checked, I was surprised how different the settings were between my servos!

 

[djmoose]

A few things, and I can go on and on and on and on...but what I have learned is that there defintely is a law of dimishing returns when it comes to these things and even moreso as you go down in airframe size.

First of all, the "closest to 90" is a myth. (it's *almost true, actually) Truth is that the servo arm should be parallel to the hingeline. Most of the time, the servo pocket is perpendicular to the hingeline...so it works out....however, many times it's not.

About subtrims, remember (and you hardly ever hear anyone talking about this....or realizing that it's happening) that whenever you use subtrim...you're LOOSING servo movement in that same direction. If you use 3 units of subtrim to the left...your servo will move 3 units LESS when you move your stick all the way to the left.

to answer your question, which is better, subtrim or programming? It's really your decision. (especially on a 59) How much time do you want to spend? Using the hitec programmer is kinda complicated and there really is no easy way to go about it. (main reason is that the hitec programmer programs the center of the servo for 1500us (nanoseconds) while a lot of radios (A9 included) have a center stick position reading of 1517-1524...so if you program the servo for 90 degrees and then plug it into your radio...it's going to be off by a bit...and then people say to just use subtrim to fix that...which defeats the whole purpose of programming.

To summarize, if you want to correctly program hitec servos (and futaba servos) you need to either play the guessing game and un-hook, re-hookup, write your numbers down, check, try again, etc etc to get it exactly right.....OR....get out the non-computer hitec programmer and measure all of your radio's pulse widths and figure out what's really going on.

Is that all worth it for a 59? That's for you to decide. Are you competing at a level where having EXACTLY the same throws matters? That's for you to decide.

I would say it's probably not worth programming on a 59. However, if you want to, I'll help you do it.

 

[rcbirk21]

moose, thats a great point. I think it is also worth mentioning that if you are using double ball links or adjustable turnbuckles, you can get the surface just about perfectly centered without using any subtrim, but the horn may not be parallel to the hinge line. I think it is good enough, and the law of diminishing returns is a perfect analogy

 

[econpatric]

First of all, the "closest to 90" is a myth. (it's *almost true, actually) Truth is that the servo arm should be parallel to the hingeline. Most of the time, the servo pocket is perpendicular to the hingeline...so it works out....however, many times it's not.

Can you elaborate on this. This is similar to the comment you made on another site. I cannot seem to rap my head around it. I think that you are saying that the manufacturers make the servo pockets with a few degrees of twist to them? Please instruct your Padawan :bouncy:.

... subtrims, remember (and you hardly ever hear anyone talking about this....or realizing that it's happening) that whenever you use subtrim...you're LOOSING servo movement in that same direction. If you use 3 units of subtrim to the left...your servo will move 3 units LESS when you move your stick all the way to the left.

This is something that never crossed my mind. So, question is, would using a programmer have the same affect? On one of the servos, I'm using like 65 clicks of sub trim to get it to where I want it, this seems like a lot, but I haven't noticed a diminishing throw. Do you think that this much sub trim will cause problems?

 

[econpatric]

What I have done, is make sure all the deadbands are equal. It was something Doc Austin mentioned many months ago when he did it for the first time. When I checked, I was surprised how different the settings were between my servos!

How do I do this? Would this be beneficial on a 59" airframe?

 

[flylow3d]

For this size of plane. I set the servo up based on getting equal throw in both directions with matching endpoints in the radio. I start with the arm as close to 90 degrees as possible. I then measure the throw for up and down travel. I always get more throw traveling down than I do up. To fix this, you have to move the servo arm on the spline in the direction in with you are receiving too much throw ( which for me has always been moving the servo arm so it points more towards the leading edge of the wing.) The servo arm is hardly ever at 90 degrees when I get finished setting it up.

When you are finished setting up you ailerons, your endpoints in the radio should be very close to maxed and they should be pretty close to being equal. Then when you measure the throw for both up and down travel, they should be equal.

 

[flylow3d]

DJmoose's point is that the servo pocket is not always perpendicular to the hinge line. The servo arm should be placed so that it is parallel with the hinge line regardless of what angle that is compared to the servo. In a perfect world, this would give you matched endpoints in the radio, and equal travel in both directions. Unfortunately, other things can affect the travel. For example, the center of the ball link might not be perfectly centered over the hinge line.

If we had linear servos, this would all be alot easier. lol The problem is that we are turning circular motion into linear motion so moving the servo arm on the spline will cause travel to be gained in one direction and lost in another.

 

[econpatric]

For this size of plane. I set the servo up based on getting equal throw in both directions with matching endpoints in the radio. I start with the arm as close to 90 degrees as possible. I then measure the throw for up and down travel. I always get more throw traveling down than I do up. To fix this, you have to move the servo arm on the spline in the direction in with you are receiving too much throw ( which for me has always been moving the servo arm so it points more towards the leading edge of the wing.) The servo arm is hardly ever at 90 degrees when I get finished setting it up.

When you are finished setting up you ailerons, your endpoints in the radio should be very close to maxed and they should be pretty close to being equal. Then when you measure the throw for both up and down travel, they should be equal.

So Joe if I understand you correctly you don't use any subtrim or programming to achieve proper throws on an airplane of this size? Its all mechanical?

When you get to larger airframes is it the same process?

Is the above true for all control surfaces?

Thanks!