Trim Disconnect... Do I need it?

[idleup]

I currently am drawing my wiring diagram for the electric aileron and elevator trim using the hat switch on the Tosten CS-8 stick. I am using a fuse rather than a breaker for the Tosten Trim Relay Deck. Given that the fuse is not accessible in flight, should I have a trim disconnect somewhere?

[SportAvServ]

I have had a trim motor stop in an unfavorable position in a 6A, which wasn't bad, but I did have one run away in an Archer, I was very glad it had a disconnect button. I like it!!

RT

[idleup]

So if it stopped in an unfavorable position and you disconnected it does that cause it to return to a favorable position or are you really just making sure it does not unpredictably move into another unfavorable position at an inopportune time?

[rzbill]

Assuming you are speaking of the Ray Allen actuators supplied by Vans, then no, disconnection will not cause them to move. They are a packaged motor, screwjack and variable resistor for position feedback. The screwjack has a high enough gear ratio that it will hold position with no power applied to the motor.

PITCH
For my RV7A, I only have electric elevator trim. I have flown it with the trim at limits. It is not comfortable due to stick load but it is flyable. The motor moves slowly enough that sudden application is not really possible electrically although a mechanical failure that disconnects tab from actuator is certainly possible.

However, there is a series of events that CAN, from the pilots point a view, present a craft suddenly out of trim. It has happened to me once.
It requires an autopilot with elevator control such as alt hold or better. I turned my autopilot on in a climb (DIGI II VS) without diligently triming first. Then leveled off (with the DIGI II) and began gaining speed. Shortly afterwards, the autopilot tripped out due to excessive electrical loading trying to compensate for out of trim. When it tripped, the aircraft did a sudden sharp pullup in response to the trim setting. I grabbed the stick and restored normal flight. It taught me a lesson to be more dilligent about trim setup prior to autopilot use (and I also now watch for an indication on the AP that says how hard it is working)

ROLL
I can only comment on spring based aileron trim (since the Vans stock electric system uses the same spring style interconnection as my manual system) and it is again easily flyable even at full trim deflection.

**AND NO I AM NOT IN MY JAMMIES AT THE COMPUTER AT 2:41 IN THE MORNING. I AM IN GERMANY ON BUSINESS WHERE ITS NEARLY 9AM **

[flyinga]

I put a "pullable" CB in a handy location for my trim. The RV is flyable with trim at the extremes (assuming it is set to spec) without too much difficulty; unlike some other aircraft.

[pierre smith]

Originally Posted by flyinga

I put a "pullable" CB in a handy location for my trim. The RV is flyable with trim at the extremes (assuming it is set to spec) without too much difficulty; unlike some other aircraft.

...Except the -10...it becomes a bear!

Best,

[idleup]

Okay, so I am leaning towards not putting in a trim disconnect. In the event something failed having the trim disconnect would not change the position that the trim was stuck in. The only advantage sounds like it would be to prevent any erratic movement that could occur involuntarily. However, it sounds like the trim moves slow enough that it would not be a problem.

I wonder what percentage of people put in a trim disconnect. I am pretty sure many of the panels I have seen posted do not have a breaker or switch for the trim disconnect.

[BobTurner]

The point of a disconnect button is that if your hat switch sticks in an 'on' position, or shorts out in an 'on' way, and you detect it, you can stop it before it runs to its full limit. As others have said, if the stick force is not excessive with the trim full up or full down, at cruise and at landing speeds, then you really don't need a disconnect.
In my -10 I have well marked pullable breakers directly in front of my right hand for the trim and autopilot, since my Trio autopilot can also run the trim.

[gereed75]

I have a very strong opinion based on testing that I did during Phase 1.

I simulated runaway trim conditions in flight in my -6 with the "stock" Ray Allen trim servo (elevator) just by depressing the trim and holding it, up and down, at different air speeds.

A nose up runaway gives you several options and is inherently easier to deal with especially at lower speeds (nose up trim authority goes down as air speed reduces and you have a chance to get to an airspeed by reducing power where the stick force to over ride and prevent a stall becomes manageable).

Nose down is another story. At cruise speeds, it only takes 3 or so seconds of nose down trim to build over-ride stick pressures that are most certainly not comfortable (sorry RZBill) and probably dangerous to control system integrity (IMHO).

I have a trim power switch on my panel. I strongly feel that in the event of a nose down trim run-away at cruise speed in a "stock" -6 that you would not recognize, react and operate a disconnect fast enough to avert disaster.

I have installed a TCW trim control module set to a slow speed to mitigate that risk. There are some other ways to mitigate the risk (other wise reducing servo speed, biasing/reducing the trim actuator travel to limit nose down trim travel etc etc) but I just don't believe that a disconnect is a real effective means to do it in a worst case scenario.

Put a switch in by all means as a way to secure a defective trim system, but don't count on it and you to save the day in a run away nose down trim situation at speed.

My opinion only. TIFWIW

Happy to hear otherwise from someone brave enough to punch nose down trim and hold it for 5 seconds in a "stock" -6, 7 or 9 at 155 KIAS.

[ArVeeNiner]

Have you looked at installing a Safety Trim system? I have one in my plane.

http://www.tcwtech.com/Safety-Trim-Page.htm