Franco2fly

I didn't want to steal the thread that was talking to travel adjust, so I thought a new thread might be in order.
Here's the basic question. I've seen a screen on the Dx8 (cannot remember where) that deals with sub-trim.

How is this different from travel adjust? Where do I find it and what does it really do?

I think I could benefit from sub-trim use on my flaps where the servo seems to chatter a bit when I
pull the flaps to full retract position. If I move the servo arm one spline then theres a bit of a gap
as the flap is not fully retracted. I'm thinking that trim, or sub trim don't know which might be the
answer.


Thanks.



KKKKFL

JIMF14D

Sub trim allows you to make major changes to the position of the servo, not the end points. Trim makes very fine adjustment to the servo position. Think of sub trim as the same as removing a clevis and adjusting the push rod length to get a surface close to the neutral position.

Normall you start with sub trim set to zero, adjust things mechanically for neutral and then use the trims for minor settings. If you set sub trim way off before flight you might not able to trim your aircraft in flight with the normal trim.

If you have servo buzz (loading) at full flap up or down you probably have the end point (travel) set too large.

Have fun

BuschBarber

My Feedback: (2)

When I set up an aircraft, the first thing I do is set all the parameters in the Model Memory to Default. It is usually that way in an Unused Model Memory, but sometimes I forget if I have used a particular Model Memory for testing. Next would be setting the Servo Direction for each channel. With all the Trims and Sticks at Neutral, I would next install the Servo Arms, on each servo, as close to 90 deg as possible. Due to the nature of the Spline on the Servo Output Shaft, the Servo Arm cannot always be installed at 90 deg to the Servo Case. This is where Sub Trim comes in. It is used to Tweak the Neutral setting for each channel until the Servo Arm is perfectly at 90 deg. You want to use as little SubTrim as possible since it takes away from the Total Travel of the servo (if you have 60 deg of travel on either side of Neutral, SubTrim would take some travel away from either the Right or the Left). Use SubTrim sparingly.

SubTrim is in the Function Menu.

Trim is what you change, while Flying, to get your aircraft to fly Straight and Level with no Stick Inputs. If I have to use more than 1/3 of the Trim Adjustment, I adjust the Pushrod on that control surface and set the Trim back to Neutral. With Analog Trims, the Trim Levers move Right/Left, or Up/Down, and you can visually see where they are relative to Neutral. With Digital Trims, the Trims are Momentary Switches. You view how much Trim you have added by looking at the LCD Display on your Tx.

In the System Menu of the DX8 (the menu that comes up when you Hold the Roller and turn on the Tx power), there is a menu choice for Trim Step. This setting determines how far the Digital Trims will move with each Click of the Trim Lever. You can set it to large or small increments.

I do not use Trim Step very often.

I use SubTrim when initially setting up a servo and sometimes to make minute adjustments such as when Dual Flaps or Dual Elevators are slightly out of parallel.

rmh

I once had a bipe setup by a friend-who used subtrim (a LOTof it) to set up and match four ailerons
the result was simply unusable - tho things centered, the relative throws were all over the place .
Needless to say , I cancelled everything and started over -
Time spent mechanically matching -servo arm and horn positions- really reduces problems in final setup.

Franco2fly

Thanks guys,
Mister Buschbaby...  I thought you did an exceptional job of differentiating the various "Trim/SubTrim/TrimStep" features.  I would like to cut&paste onto my PowerPoint Architecture diagram.  Now I only have SeventyEleven more features to describe.

KKKKFL

chuckk2

What I usually do to minimize subtrim use.
Set the servos up at midpoint with a reference pulse width from a generator. 1.00 low 1.50 center 2.00 high
Get the servo arms as close as I can to a right angle with the servo body. Set the control surface to center.
Adjust the linkage accordingly. Check that control surfaces, such as ailerons or dual flaps track properly.
Most Transmitters with default 0 trim settings will have 1.50 pulse width at the 50% or stick centered position.
Travel and subtrim may interact on some transmitters. Flaps can be setup differently, in that flaps up may be set at
almost full servo travel, or 50%. It depends on how the transmitter flap programming works out.
Mine are set at about 95% or 5% of servo travel, with transmitter travel used to prevent a servo stall.
The powerup sequence starts with the flaps moving to about half deflection, then as things initialize, moving to the close position.
With my transmitter and receiver, this causes the flaps to go to the closed position if the data link is lost. (Transmitter turn off to simulate)
I use sub trim only as a last resort, when I cannot get any closer with mechanical trim.