Subtrim versus Trim
08-25-2005, 09:37 PM
#26
Join Date: Sep 2002
Location: Dearborn, MI,
Posts: 362
Likes: 0
Received 0 Likes
on
0 Posts
RE: Subtrim versus Trim
This topic HAS made things clearer for me. THANKS! I had a friend at the field try to zero my trim by compensating with the subtrim, believing that there was a 1:1 correlation. There wasn't. I now understand that there are two different purposes for each. I am one of those that rotate the arm (of four) closest to 90 degrees and then physically set my control surfaces to neutral (as best as possible), using as little trim as possible (going back and resetting my physical trim as needed). Even with a computer transmitter, I prefer to set my plane up manually as close to perfect as I can. Maybe I'm sidestepping the money I've spent, but as a builder I want to set to true. Thanks again for this thread.
PEACE
PEACE
08-25-2005, 11:08 PM
#27
My Feedback: (31)
RE: Subtrim versus Trim
Your not side stepping anything, you should always do the mechanical setup best case prior to playing with the TX.
However, positioning the servo arm perpendicular to the servo case or the appendage is of little value. Many think it’s an imperative, yet they overlook the simple fact that the control arm is atypically offset from the hinge line. Even when the hinge line and control arm pivot points are inline the secret formula of 90 degrees perpendicular to the case is inaccurate in most cases. The reasoning for the 90 degree magic is to obtain equal travel and or linear throw. In a perfect world and where all things are equal and or geometric this might have some merit until such time that you use sub-trim or trim to center the servo arm at 90 degrees to the case. Once trim is added to the equation linear throw of the surface and travel of the TX is impossible as an offset is now in place. Enter programmable servos…
Accurate results are garnered when one uses a device to note the rotation in degrees of the servo and the deflection of the surface. The goal is to achieve equal deflection with equal amounts of servo arm angle.
I use a photocopied 4” protractor trimmed, centered on the servo output shaft and taped to the wing/stab or whatever. This allows me to note the degrees rotation of the servo arm
To measure the surface deflection you have a few options, a pendulum type throw meter, an angle finder, a tape measure or my favorite the Aero Perfect electronic digital display angle finder (There is a short review by a modeler in the RCU Magazine on this device).
When its all said and done you can achieve very close to linear throw with equal servo angle mechanically by adjusting the pushrod length, adjusting the control arm pivot height and lastly the servo arm angle at neutral. In nearly all cases the servo arm angle at neutral will need to be 1-2-3-4-5 degrees rotated from perpendicular to the servo case.
After you’ve got it right mechanically turn on the TX and check again with the measuring gear in place. You should start with everything zeroed and equal travel volumes. If you have to use sub-trim to position the servo arm as required by the mechanical set-up and subsequently use ATV or end-point percentages to realize equal travel arcs. This is as close as you can get with anything but programmable servos to linear servo throw as related to TX throw.
Attached below is a scale drawing I made to explain the concept as related to throttle linkage on a 40% gasser. The idea is the same; you can change the height of the servo, the pivot height and the offset from perpendicular of the control arm. All of these variables affect the final servo arm angle at TX neutral.
However, positioning the servo arm perpendicular to the servo case or the appendage is of little value. Many think it’s an imperative, yet they overlook the simple fact that the control arm is atypically offset from the hinge line. Even when the hinge line and control arm pivot points are inline the secret formula of 90 degrees perpendicular to the case is inaccurate in most cases. The reasoning for the 90 degree magic is to obtain equal travel and or linear throw. In a perfect world and where all things are equal and or geometric this might have some merit until such time that you use sub-trim or trim to center the servo arm at 90 degrees to the case. Once trim is added to the equation linear throw of the surface and travel of the TX is impossible as an offset is now in place. Enter programmable servos…
Accurate results are garnered when one uses a device to note the rotation in degrees of the servo and the deflection of the surface. The goal is to achieve equal deflection with equal amounts of servo arm angle.
I use a photocopied 4” protractor trimmed, centered on the servo output shaft and taped to the wing/stab or whatever. This allows me to note the degrees rotation of the servo arm
To measure the surface deflection you have a few options, a pendulum type throw meter, an angle finder, a tape measure or my favorite the Aero Perfect electronic digital display angle finder (There is a short review by a modeler in the RCU Magazine on this device).
When its all said and done you can achieve very close to linear throw with equal servo angle mechanically by adjusting the pushrod length, adjusting the control arm pivot height and lastly the servo arm angle at neutral. In nearly all cases the servo arm angle at neutral will need to be 1-2-3-4-5 degrees rotated from perpendicular to the servo case.
After you’ve got it right mechanically turn on the TX and check again with the measuring gear in place. You should start with everything zeroed and equal travel volumes. If you have to use sub-trim to position the servo arm as required by the mechanical set-up and subsequently use ATV or end-point percentages to realize equal travel arcs. This is as close as you can get with anything but programmable servos to linear servo throw as related to TX throw.
Attached below is a scale drawing I made to explain the concept as related to throttle linkage on a 40% gasser. The idea is the same; you can change the height of the servo, the pivot height and the offset from perpendicular of the control arm. All of these variables affect the final servo arm angle at TX neutral.
07-15-2007, 09:25 PM
#28
RE: Subtrim versus Trim
ORIGINAL: mglavin
Anytime sub-trim, trim or mixes with offsets are used differential or NON-linear travel arcs ensue regardless of travel volume or percentages. Programmable servos have this conundrum beat hands down, there is no competition.
Anytime sub-trim, trim or mixes with offsets are used differential or NON-linear travel arcs ensue regardless of travel volume or percentages. Programmable servos have this conundrum beat hands down, there is no competition.
07-16-2007, 12:48 AM
#29
Senior Member
Join Date: Sep 2002
Location: Lewisville,
TX
Posts: 669
Likes: 0
Received 0 Likes
on
0 Posts
RE: Subtrim versus Trim
It depends on the radio, but the issue is that most don't move the endpoints - just the center point with subtrim. So, with zero subtrim, let's say one extreme is 0, the other is 100, and the center is 50, so you move "50" either way. Now let's subtrim the center point to 40, to correct a bad setup or something. Now, full stick deflection still gives you 0 and 100 as endpoints, but the servo is travelling 60 in one direction vs. 40 the other, giving asymmetrical travel, or non-linearity. I know JR is like this, and I think Futaba is too. Notably different is Multiplex, where a "subtrim" (different name there, but same feature) shifts the entire curve (endpoints included) and stays linear . . . kinda handy when the servo arm fits two ways - too far clockwise and too far counter-clockwise . . . .
- Tim
- Tim
07-16-2007, 04:18 PM
#30
RE: Subtrim versus Trim
GW you are 100 percent correct, on 3d planes with exaggerated throws it is all the more important to do a proper setup.
Mike, its simple, if you use too much subtrim then one direction will be up against the mechanical limit of the servo, hence moving farther one way than the other.
The higher the atv's, the lower the sub trim number to produce this effect.
Mike, its simple, if you use too much subtrim then one direction will be up against the mechanical limit of the servo, hence moving farther one way than the other.
The higher the atv's, the lower the sub trim number to produce this effect.
