Elevator Trim - Trio Alt Hold

If anyone has wired their elevator trim on the Trio Alt Hold (Gold Servo) to the auto bypass schematic (option 2) could you please let me know?

You probably already know that it requires the special servo for the auto trim feature. I actually installed the auto trim servo but forgot about installing the external diode circuit per the drawing until it was all wired. I wasn't really sure where to mount the external electrical parts and where best to cut into the wiring so I never installed it. I've only been flying the airplane for about 5 weeks so far, but what testing I've done it seems to work fine without auto trim. I trim the airplane up good prior to enabling the autopilot, then later when I release the autopilot I'm not getting any sudden pitch changes. At this point I may never install the parts to use auto trim, but we'll decide after taking a few long trips. The advantage I see is if you take off from an airport and program the autopilot to climb to a preset altitude, level and go to the waypoint. Then you would have too much pitch trim up after level off if you didn't re-trim manually.

mcattell said:

You probably already know that it requires the special servo for the auto trim feature. I actually installed the auto trim servo but forgot about installing the external diode circuit per the drawing until it was all wired. I wasn't really sure where to mount the external electrical parts and where best to cut into the wiring so I never installed it. I've only been flying the airplane for about 5 weeks so far, but what testing I've done it seems to work fine without auto trim. I trim the airplane up good prior to enabling the autopilot, then later when I release the autopilot I'm not getting any sudden pitch changes. At this point I may never install the parts to use auto trim, but we'll decide after taking a few long trips. The advantage I see is if you take off from an airport and program the autopilot to climb to a preset altitude, level and go to the waypoint. Then you would have too much pitch trim up after level off if you didn't re-trim manually.

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Mark, I don't have auto-trim on my Pro-Pilot because my RV-6 Classic has manual elevator trim. The only time I've seen where auto-trim would be useful is when leaving level cruise flight for a high-speed descent where you leave the throttle set at cruise power. As airspeed builds in the descent, if the plane isn't retrimmed the servo clutch slip message can appear and the autopilot will not be able to control the descent until the trim is adjusted. In actual practice I just twist in a little down trim before the speed builds very much and that usually prevents any servo errors.

I did build a trim relay package to allow automatic or manual trim control with an autotrim servo. Not sure what you mean by Option 2, but no matter. What do you need?

Dan, FYI "Option 2" autotrim schematic is on page 78 of the ProPilot manual; pdf here: http://www.trioavionics.com/Pro Pilot 3.2 Monochrome.pdf

On our RV-6, I didn't use either Option 1 or Option 2. Instead, I installed a relay that gives the autopilot control of the elevator trim when the pitch servo is engaged, and lets the flesh-and-blood pilot control it otherwise.

(The schematic for this is basically Option 2, but the + side of the relay coil is wired to pitch servo +12 volts, and the - side is pulled low by pin 35 on the ProPilot head when VNAV is on. No diodes...)

That way of doing it just made more sense to me: the autopilot is controlling elevator trim when it's controlling the elevator, and I'm controlling both when I'm hand-flying, automatically. It works fine, though I haven't been able to convince the Trio guys to include it as Option 3 .

BTW I agree with Sam and mcattell that autotrim isn't really needed on a 2-place RV (maybe more useful in a -10), but if you paid for it, you might as well run it!

--Paul

I'm just now installing the relay and diode pack for the option 2 approach. What I like about it is that the trim switch on the stick will always override the autopilot... just in case. I do like the idea of the automatic switching using the VNAV pin 35 though. Very cool!

All options

Thanks for the replys, Dan, I just wanted to see if people were happy with the Auto Trim and whether or not to include it and if there are any other wiring options, which it appears Garage Man has.

On our RV-6, I didn't use either Option 1 or Option 2. Instead, I installed a relay that gives the autopilot control of the elevator trim when the pitch servo is engaged, and lets the flesh-and-blood pilot control it otherwise.

(The schematic for this is basically Option 2, but the + side of the relay coil is wired to pitch servo +12 volts, and the - side is pulled low by pin 35 on the ProPilot head when VNAV is on. No diodes...)

That way of doing it just made more sense to me: the autopilot is controlling elevator trim when it's controlling the elevator, and I'm controlling both when I'm hand-flying, automatically. It works fine, though I haven't been able to convince the Trio guys to include it as Option 3 .

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Sounds a good idea Garage Guy, could you send me a quick hand drawn schematic or pic just so that I know just how you have wired your "option 3"

Thanks

Martin

Ahhhh, that option 2.

Martin, at 20 hours I'm just into autopilot setup and test, not far enough along to render a big-picture opinion about the autotrim option. Plus my EZ system is an early version and going back for an update, so things may change.

BTW, I'm told spike supression diodes on the relays are a good idea.

shiney said:

Sounds a good idea Garage Guy, could you send me a quick hand drawn schematic or pic just so that I know just how you have wired your "option 3"

Click to expand...

Here you go:



One thing to check first, if you want to go this way. Call Trio to find out how much current pin 35 can sink with your serial number autopilot head, and be sure to get a relay with a coil pull-in current rating that it can handle. They changed the current limit at different points in the production run.

--Paul

I know this is an old thread; I have a question regarding this subject. Namely, how to wire up an auto trim relay for a Trio Pro Pilot.

The VNAV servo may be wired to a trim servo by an auto trim relay as depicted on page 79 of the manual here:
http://www.trioavionics.com/Pro Pilot Manual 3.8.pdf

I could use some dirt simple guidance on how to wire the relay. The diagram provided on the page cited above makes no sense to me. I just installed a Trio Pro Pilot. I have a Ray Allen 8A trim servo. I am lost as to how to connect the Pro Pilot to the trim servo.

1. The relay shown is polarity sensitive (note the + on one side of the coil). The four diodes make sure positive voltage is supplied to the + side of the coil regardless of which incoming line has the positive voltage, and which is ground. In this example the incoming lines will be ground and 12 volts for moving the tab in one direction; the ground and 12 volts are reversed for moving the tab the other way. If you use a relay which is not polarity sensitive (coil doesn't care which end is + or -) then skip the diodes, just hook the incoming wires across the coil as well as to the normally open contacts. Note that the control lines must be 12 volts to run the relay; if you try to use lower voltages, to slow the motor speed, they may be too low to operate the relay. Likewise, if you use pulse width modulated (PWM) circuit for speed control the relay may not work properly.
2. The Trio uses PWM to control the trim speed. It must be "straight thru" to the motor. Do not use the Trio output to control a relay.
3. I have two trim switches in the cockpit, plus the Trio autotrim, so I built a relay logic deck to make sure no one control "fights" another. I use two DPDT relays for each cockpit switch (one for up, one for down). They're all in series, going thru the normally closed contacts. The Trio runs thru all of them. This way, you can apply less than 12 volts if you want the trim motor to run slower.
4. Safety: in case of a runaway trim I have the Trio and trim CB's side by side in front of me, so I can easily pull both of them.
5. As I'm sure you know, the Trio trim speed is pulse width modulated, and can be programmed to run more slowly at cruise speed and to run full speed at low air speeds. It also can be used manually even when the autopilot is not engaged. I have mine programmed to run very slowly at cruise speed, and I use it to fine tune the trim when hand flying the airplane.
6. The auto trim is most useful when changing airspeeds, e.g., letting the autopilot fly the climb, then level out to cruise. Or an approach. It is amazing for me to fly an ILS, starting from cruise, and ending up at 80 knots and partial flaps, and never having to touch the trim controls.

Thanks for the reply Bob.

I have installed a couple nav/coms, 430, GTN650, couple Trio Pro Pilots, ADS-B, etc but all seemed to have more straightforward connect A to B... type instructions.

I was planning to go to radio shack and see if they could shed some light on how to connect a bridge rectifier to a DPDT relay. Perhaps the parts will have a wiring diagram I can understand.

I have the Trio Pro with auto-trim. It works properly, but you have to choose who gets to trim - you, or the Trio. You'll figure out pretty fast that the switch is in the wrong position on the downwind leg

Went to radio shack and showed them this:

They could not tell me how to wire it and they did not have the listed bridge rectifier part number 276-1183.
Still trying to figure it out.

johngoodman said:

I have the Trio Pro with auto-trim. It works properly, but you have to choose who gets to trim - you, or the Trio. You'll figure out pretty fast that the switch is in the wrong position on the downwind leg

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You must be using a switch to choose between them. My relay deck gives first priority to the pilot's top hat, second priority to the right seat trim switch, last priority to the Trio.

Forane: A bridge rectifier looks exactly like the schematic on pg 79. It's just 4 diodes wired as shown. You can build it yourself from four diodes if you want. And if your relay coil is not polarity sensitive, you don't need the diodes at all.
You didn't say if you wanted a second manual switch, but it can be accomodated with a second relay.

The reason I went with two relays per top hat switch was so I could use less than 12 volts and/ or pulsewidth modulation to slow down the trim motor when using the stick switch. However, in practice, I find I only use the stick tophat at low speeds (takeoff and landing) and am happy to use the Trio to trim, whether manually or autopilot engaged, when in cruise.

BobTurner said:

You must be using a switch to choose between them. My relay deck gives first priority to the pilot's top hat, second priority to the right seat trim switch, last priority to the Trio.

Forane: A bridge rectifier looks exactly like the schematic on pg 79. It's just 4 diodes wired as shown. You can build it yourself from four diodes if you want. And if your relay coil is not polarity sensitive, you don't need the diodes at all.
You didn't say if you wanted a second manual switch, but it can be accomodated with a second relay.

The reason I went with two relays per top hat switch was so I could use less than 12 volts and/ or pulsewidth modulation to slow down the trim motor when using the stick switch. However, in practice, I find I only use the stick tophat at low speeds (takeoff and landing) and am happy to use the Trio to trim, whether manually or autopilot engaged, when in cruise.

Click to expand...

Bob, your setup sounds great. This stuff is just Greek to me. I look at that schematic and do not get it. Did fine with making my own harness for the Pro Pilot as all that made sense to me. Just don't understand the schematic...

basic info

1 Check how much current the coil on your selected relay requires.
2. Buy 4 power diodes rated at 5 times that current, or more.
3. Wire as shown. Each diode will have a band on one end. That corresponds to the end with the bar on the schematic. The triangle end has no markings.
4. The wires that say 'manual trim motor drive" come from your stick switch. These wires supply 12 volts and ground for trim in one direction, and the reverse (ground and 12 volts reversed on the two wires) for trim in the other direction. Since most stick switches just close a connection to ground you may need one or more relays to switch the needed 12 volts and ground back and forth between these two wires. Which is why I used several relays that just need closure to ground to turn them on.

more info

Remember that a diode conducts conventional current (+ to -) in the direction of the arrow. A bridge configuration cleverly takes whichever input is at 12 volts and allows it to conduct into the + end of the coil, while blocking it from the - end. And vice versa for whichever wire is at ground, it conducts from the - end of the coil and is blocked from the + end. And it does this regardless of which wire is at ground, and which is at 12 volts. Remember in use the polarity on these wires changes, depending on whether you want trim up or trim down.