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Subtrim versus Trim

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Old 08-23-2005, 05:46 PM
  #1  
Blue_Moon_
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Default Subtrim versus Trim

Can someone explain what subtrim is and why i should use it ?
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Old 08-23-2005, 06:17 PM
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David Cutler
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Default RE: Subtrim versus Trim

Subtrim should be used if you can't mechanically get all the surfaces trimmed to zero. It can't be changed in flight (well, not easily!) but the normal trim can.

When setting up the controls, make sure the normal trims are set to the centre of their travel, and adjust the subtrim so that the surfaces are neutral. That means that you have the most possible trim available to you in both directions in flight to trim out any warps etc.

-David C.
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Old 08-23-2005, 06:21 PM
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michpittsman
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Default RE: Subtrim versus Trim

It comes in very handy with several models in one transmitter; when changing models, the trim levers on the Tx remain centered but the subtrim changes to match the model....Jim
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Old 08-23-2005, 09:10 PM
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Default RE: Subtrim versus Trim

The subtrim has nothing to do with the trim levers, at least on the 9c.. As above, the sub trim is used to center the servo arm on the server. It is set by going to the subtrim window and manualy setting it using the rotary knob... These settings are not affected by the "normal" trim levers on the front of the radio. According to the manul setting the subtrim does affect the end to end movement of the servo arm...
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Old 08-24-2005, 03:38 AM
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Default RE: Subtrim versus Trim

Aha, so the subtrim is used for easily changing the end points? In other words, it is used instead of moving the servo arm on the servo?[&:]
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Old 08-24-2005, 04:22 AM
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Default RE: Subtrim versus Trim

Actually No, there is a mechanical stop within the servo that will affect the end points. Sub Trim is simply moving the center point slightly. Clear as Mud?
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Old 08-24-2005, 06:19 AM
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Default RE: Subtrim versus Trim


ORIGINAL: SoonerAce

Actually No, there is a mechanical stop within the servo that will affect the end points. Sub Trim is simply moving the center point slightly. Clear as Mud?
Yes!

Why is it preferd to use subtrim to "moving the center point slightly" instead of normal trim? I cant get that. Moving the centerpoint is what the normal trim is for, right?
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Old 08-24-2005, 07:35 AM
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Default RE: Subtrim versus Trim

Sub trim should not be used to make up for sloppy mechanical setup. If the servo arm isn't 90 degrees, rotate it on the spine until one of the arms lines up or you can get it as close as possible. Then if you have to use sub trim to neutralize it. You really should use as little sub trim and trim as you can.

As for the mechanical trim levers, some radios like the Futaba Super 7 were computer radios with mechanical trim levers so you could get everything neutral with sub trim so that your trim levers were centered from one model to the other. You also didn't have to worry about bumping trim levers during storage. If you used your subtrim and got your levers centered, no matter where they got bumped, you could recenter them during preflight and know it would be right.

With computer radios with digital trims, I don't think it makes that big a deal with one exception. If you are far off (say almost full Left) on your trim you will only have so much movement left. If you then center the surface and center the trim by adjusting the sub trim, you will then have full left or right trim available during flight again.

Also on radios such as the 9C you can adjust the trim step. That means you can crank it up for test flights so that 1 bump of the digital trim makes a large change, then reduce it on subseqent flights for progressivly finer adjustments.
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Old 08-24-2005, 07:49 AM
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Geistware
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Default RE: Subtrim versus Trim

Ok, Let me take a shot at this answer.
To center a servo you need a 1500us pulse width signal from the transmitter.
With this signal sent, the position of the control surface may not be centered with the airfoil.
Also, if you have multiple servos on the same surface but different channels, they could be fighting.

There are two ways to adjust this, the tried and true way is to adjust the clevis to the point where the control surface is centered and any binding is eliminated. Computer radios allow you to adjust the sub-trims to get this same result by making a position other than 1500us as the center of the servo. Now, because the servo can only handly 900us to 2100us and adjustment from 1500us will make the end of travel shorter for the side the center position moved to.

I hope this clears it all up.
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Old 08-24-2005, 08:09 AM
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Default RE: Subtrim versus Trim

Simply put the sub trim is used to line the servo arem up with the servo generaly before any control linkage is installed.. Generaly you want the arm 90 degrees to the servo so the arm moves equaly in both directions.. On some servoes this cannot be achieved without using sub trim... Generaly speaking for sport flying close counts on the movement and subtrim is not used... Once the servo arm is lined up sub trim is not touched again... The control rod etc installation is set up mechanicaly and the trim tabs on the radio are used for that last little touch..
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Old 08-24-2005, 03:31 PM
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daven
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Default RE: Subtrim versus Trim

Sub trim is for people to lazy to adjust the clevis manually

My planes use very little movement, so I guess I'm one of the Lazy ones. It is very easy to use sub trim to adjust a surface.

For planes that require full movement on the servo, you should fix it mechanically for maximum throw.
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Old 08-24-2005, 04:10 PM
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Default RE: Subtrim versus Trim


ORIGINAL: daven

Sub trim is for people to lazy to adjust the clevis manually

My planes use very little movement, so I guess I'm one of the Lazy ones. It is very easy to use sub trim to adjust a surface.

For planes that require full movement on the servo, you should fix it mechanically for maximum throw.

Read carefuly... Sub Trim is for centering the arm on a servo as some servos are not quite centered from the factory... This is normaly done before any control surfaces are connected to the servo... It is not intended for trim purposes... As a last resort you might try reading the manual... Sub trim has nothing to do with adjusting the clevis manually
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Old 08-24-2005, 05:30 PM
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Default RE: Subtrim versus Trim

Sub trim is misnamed. It sould be called "Center Adjust," in my opinion. Your computer radio has end point adjust and center adjust, only they were called "travel volume," and other names, and "sub trim" by orientals with very little knowledge of the English language. The name sub trim has messed up people new to computer radios for 20 years.

As has been mentioned, it has nothing to do with your trim levers and it is misused a lot.

When you program a new airplane into your radio, the first thing you do without fail is RESET. Clear out the old airplane settings. Do this even for a new radio. It may have been used to demo at your local hobby shop. (And don't tell me this can't happen.) Next, put on the servo arms. If they are not square, that is 90 degrees to the axis of the servo, use sub trim to center the arm. Don't touch your sub trim again.
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Old 08-24-2005, 06:06 PM
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Default RE: Subtrim versus Trim

While I certainly defer to Ed on subjects such as this let me add again that you usually dont need any sub trim even if the arms don't line up if your using a multi armed servo horn rotate it 90 degrees at a time until you have the one closest to 90 degrees. Then if you must, make minor adjustments with sub trim. Most servo splines have an odd number of teeth so rotating the horn will give you different alingments.

Geistware, would you not agree that unless you have your atv's maxed out, you won't see differential throw even with sub trim untill you start to get into large numbers?
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Old 08-24-2005, 06:23 PM
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Default RE: Subtrim versus Trim

very intresting. it gets more clear in my head now. but i still dont understand why i couldent use normal trim instead of sub-trim. []
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Old 08-24-2005, 06:30 PM
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Default RE: Subtrim versus Trim

Not all servoes will line up, even by rotating the arms.. Thus sub trim (center trim is a much better term) .. I doubt most fllyers ever use it and don't even know it is there... However when you start getting into pattern flying etc it becomes more important... Myself I used it to add small amounts of differential on ARF ailerons once in awhile... But that is another discussion..
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Old 08-24-2005, 06:35 PM
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Default RE: Subtrim versus Trim


ORIGINAL: Blue_Moon_

very intresting. it gets more clear in my head now. but i still dont understand why i couldent use normal trim instead of sub-trim. []
You could. Suppose you always brought your Four-Star 40 and your Contender to the field, to fly with the same radio, and you always alternate them. First you fly the Four-star, which has a bit of a warped wing, and you trim the aileron to compensate. Then you fly the Contender, which is straight as an arrow - and you must trim the aileron back.

Or, you could set them each up on a different model entry in your radio, and save the Center Trim / Sub trim for each. When you select the correct plane, you never have to trim your aileron. There's the use for your Center Trim/Sub Trim.

Good luck,
Dave Olson
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Old 08-24-2005, 06:39 PM
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Default RE: Subtrim versus Trim

Most radios that have subtrim, allow you to save separate subtrim settings for each model. If you use a trim setting on one model, it will very likely be different on another. Generally, once subtrim is set for a particular model, you do not need to reset the subtrim unless you make some adjustments to your control surfaces, servos, or linkages.

The trims on the radio can now be used like trims in a full scale aircraft, where the trims are set (and re-set) during flightfor changing flight conditions and maneuvers - such as adding elevator trim on landing approach, or rudder trim for crosswinds.

Scott
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Old 08-24-2005, 06:42 PM
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Default RE: Subtrim versus Trim


ORIGINAL: Blue_Moon_

very intresting. it gets more clear in my head now. but i still dont understand why i couldent use normal trim instead of sub-trim. []
You can... Look at it this way.. The goal is to build and set the plane up so no normal trim is needed... A lofty goal but some guys get pretty good at it with experiance... To do this you start by centering the servo arm on the servo and work out installing the push rods and clevis etc... If it is all done right you won't need normal trim...
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Old 08-24-2005, 10:09 PM
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Default RE: Subtrim versus Trim

Do you guys know why you’re placing the servo arm 90 degrees and or perpendicular to the long side of servo case? If you think equal travel either side of neutral is the answer think again…

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.

Most TX's do not transmit the defacto industry standard control pulse signal for neutral @ 1500us, nor do servos center on this same defacto standard. Enter sub-trim or even trim. Since not all servos have uneven output shaft spines the ability to rotate the servo arm until such time that your close isn't always an option.
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Old 08-25-2005, 05:47 AM
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Default RE: Subtrim versus Trim


ORIGINAL: Ed_Moorman

Sub trim is misnamed. It sould be called "Center Adjust," in my opinion. Your computer radio has end point adjust and center adjust, only they were called "travel volume," and other names, and "sub trim" by orientals with very little knowledge of the English language. The name sub trim has messed up people new to computer radios for 20 years.

As has been mentioned, it has nothing to do with your trim levers and it is misused a lot.

When you program a new airplane into your radio, the first thing you do without fail is RESET. Clear out the old airplane settings. Do this even for a new radio. It may have been used to demo at your local hobby shop. (And don't tell me this can't happen.) Next, put on the servo arms. If they are not square, that is 90 degrees to the axis of the servo, use sub trim to center the arm. Don't touch your sub trim again.
best answer yet! AFAIK the only manufacturer to get this right was ace with their micropro8000. in one simple function you set up each end point and center. and because this was done by specifying the actual pulse width of each point you weren't limited to a few % of movement either. so in one simple step you could set up direction, differential and overall travel. you could even set the "center" and one end point to the same value and therefore make the servo respond only when the stick is moved in one direction and not the other.



dave
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Old 08-25-2005, 11:03 AM
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Default RE: Subtrim versus Trim

Perhaps it is possible to reprogram the transmitter (9c) so that "sub trim" is named "cntr adj" ?
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Old 08-25-2005, 11:54 AM
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Default RE: Subtrim versus Trim

Thanks for the education guys.

I was always content getting it on the spline closest to center, get my pushrods close manually, and use the sub-trim to center the surface.

Honestly, I can't imagine 95+% of the everyday sportfliers noticing a difference using it to perfectly center the servo.
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Old 08-25-2005, 12:16 PM
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Default RE: Subtrim versus Trim

Cuda,
For most planes I would agree.
When you get to 3D flyers or planes with large control surfaces, then we need to look at it differently.
To ensure linear movement, you should have the servo arm 90 degrees to the control surface. This has nothing to do with the servo or it's case. This movement will not be linear but the amount of error can be greatly reduced using ATV and Sub-trims.

When you look at trim and sub trim, they respond the same for single servo/axis applications. I disagree and say they are not the same because no one uses sub trim while flying and no one sets the trim while in the pits (throttle is exception).
To me, this shows two seperate purposes.


ORIGINAL: barracudahockey

While I certainly defer to Ed on subjects such as this let me add again that you usually dont need any sub trim even if the arms don't line up if your using a multi armed servo horn rotate it 90 degrees at a time until you have the one closest to 90 degrees. Then if you must, make minor adjustments with sub trim. Most servo splines have an odd number of teeth so rotating the horn will give you different alingments.

Geistware, would you not agree that unless you have your atv's maxed out, you won't see differential throw even with sub trim untill you start to get into large numbers?
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Old 08-25-2005, 02:37 PM
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Default RE: Subtrim versus Trim

One thing not brought up so far is the effect on sub trim on a buddy box.

Had a computer savy student who used the sub trim exclusively to fix his very poor mechanical connections. We plugged in the the buddy box and the flight controls went so far out of whack that no amount of trimming could fix it. Started from scratch and zeroed the sub-trims, all worked out fine.
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