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?
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Trim Disconnect... Do I need it?
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SportAvServ
Well Known Member
Disconnect
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
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
rzbill
Well Known Member
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 **
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 **
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pierre smith
Well Known Member
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.
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.
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.
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.
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.
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.
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ArVeeNiner
Well Known Member
Have you looked at installing a Safety Trim system? I have one in my plane.
http://www.tcwtech.com/Safety-Trim-Page.htm
http://www.tcwtech.com/Safety-Trim-Page.htm
rzbill
Well Known Member
No need to apologize at all, Gary. Maybe I was not clear with my definition of "flyable" in that context. What I meant was it would not put you in the dirt, but was very uncomfortable, still flyable in a manner to safely choose an airport and get on the ground. No way would I continue a flight in that condition. Spring roll trim on the other hand......yeah, probably would finish the mission and fix it at home base.
Not sure I would go as far as saying pitch deviation is dangerous to the control system though. Just my opinion without doing any figurin' or cypherin'. Vans would probably know straight away.
Your post did make me curious about the "excessive nose down" aspect. I remember during phase 1 learning (by pure accidental conversation) about the differential travel specified by Van on the elev trim. I went back to the book, found the values and reset the rigging (from the symetrical travel I set originally to the assymetrical spec). As a result, in cruise, my elev trim is almost all the way nose down. Sure there is a little left, but not much. A sudden drop to 100% down might go unnoticed since it would be a small change, especially if the AP was on. My issue would be a reversal to all nose up trim as I said happened when the AP gave up.
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I put a master switch for trim that is protected on with a guard. If I get a runaway trim I can kill the power to all trims. I have breakers to allow me to pull the breaker of the bad trim, then power the master back on for the trims. (I have 3 trim servos) I find it easier to just kill a switch quickly than finding the right breaker in a "panic" situation.
rzbill
Well Known Member
I simply tell my right hand to stop turning the trim knob.
Touche'! But...do you have to take anything for trim elbow????
