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runaway trim

1

1bigdog

Well Known Member
OK, I was all but sold on the idea of using electric trim since I'll be putting in an autopilot allowing for autotrim. Then I read that some people have encountered runaway that can (potentially) overwhelm manual control.

When I review detail of the failure mode for the RAC servos they appear to lock in place on power loss. I would have expected them to disengage. So effectively you cut power to runaway trim and you are stuck fighting it to the ground.

When I look at the 9a preview plans I see the servo/cable placement makes it such that it appears to be one or the other. Does anyone make a servo that allows manual control pass-through so that when you disengage the electric trim the servo releases and allows a cable to takeover. Of course the vernier control must be modified to be unlocked during electric control but this would take away runaway angst and servo locking into position.

Sorry for the mouthful, and hope it makes sense. In GA I believe the AP servo still gives access to the trim wheel. Maybe someone else has already addressed this but I don't see it anywhere.

Thanks.


MIchael B.
 
Michael,

You may consider this product to minimize the risk of runaway trim. It also makes wiring the trim much easier in the first place. See

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

I am not flying with it yet but really like the features and ease of install considering I didn't have to mount relays.

Bevan
 
I always worried

about the electric trim in our -10 and -8A --- all of the certified aircraft I ever flew had manual trim wheels, or cranks to "reset" runaway trim (which I had a couple of times) ---- my -6A has manual pitch and aileron trim and I am quite happy with them.
 
Runaway Trim

Don't know about the -9, but for my -8A full nose up/ full nose down trim can be overridden with reasonable muscle power at normal cruise speeds. Not very comfortable - but doable. Also, the runaway will not occur instantaneously - you'll have a short time to react. I've included both elevator and aileron trim cutoff switches in my electrical design.
 
I installed the TCW Safety-Trim controller in my -10 but I also have an ON/OFF toggle switch for power to the trim servo's just to be safe. I very rarely change the elevator trim and have left the aileron trim in its neutral position since day one. I find my elevator trim very sensitive so if it needs a little change up or down, I flip the trim power toggle switch ON, trim the plane, then turn it back off.
 
Bevan said:
Michael,

You may consider this product to minimize the risk of runaway trim. It also makes wiring the trim much easier in the first place. See

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

I am not flying with it yet but really like the features and ease of install considering I didn't have to mount relays.

Bevan
Click to expand...

Here's what I think is a better approach for trim control.

The A or AT version of this device minimizes the electomechanical failure points inherent in other designs and provides stuck trim switch detection and correction. Can use a Garmin or Dynon serial stream for speed scheduling and the AT version provides SkyView autotrim.

Of course, no trim controller can correct a failed trim motor. Fortunately, that's rare.
 
My Garmin G3X has a setting that allows only 2 (setup allows this number to be changed as desired) seconds of trim operation in case of a stuck switch.
I think the possibility of runaway trim is a risk I will take.
I love the auto trim function on my servos and the trim speed control through the G3X.
 
MarkW said:
My Garmin G3X has a setting that allows only 2 (setup allows this number to be changed as desired) seconds of trim operation in case of a stuck switch.
I think the possibility of runaway trim is a risk I will take.
I love the auto trim function on my servos and the trim speed control through the G3X.
Click to expand...

I think it's all starting to sound like a development opportunity for a fail safe trim mechanism. I didn't realize the G3X was now also offering a safety cut out. I was familiar with the others. Making me lean toward manual still. I'm very comfortable with manual and like the feel. Not to happy with the idea of being locked into a wrong trim for a long trip down from altitude.

Thanks.
 
The trim can't run away without electrical power, so the only way to get that is for the electrical side of the trim controller to fail and run away. This can be due to broken switches, shorts in wires, or failures in the controller itself. As mentioned here, lots of the electronic trim controllers work around a lot of those failures. The trim controller in the Dynon system will time out a single stuck switch and still allow you to use the opposite direction switch to return the trim to neutral after a failure, or still use the buttons on the other stick.

However, if this really worries you, install a secondary switch. Normal position is for your normal trim controller, but it also has an up and a down position. This would allow you to re-position the trim after your controller fails for any reason.

Thankfully the trim control motors actually don't go free when they have no electricity. This would be a heck of a recipe for flutter!
 
Search AO Frog's posts. I seem to remember he designed a dual trim system by putting an electric motor that can turn the manual trim knob.
 
N941WR said:
Search AO Frog's posts. I seem to remember he designed a dual trim system by putting an electric motor that can turn the manual trim knob.
Click to expand...

Bill

I've looked for lots of possible variations of. Frog and trim but nothing like what you suggest. I can guess how it work. You stick a reversing axial motor on the cable and the knob on the other side and it would rotate the knob instead. Of using a servo.

If you or someone knows a link because I can't seem to find it and would prefer to see something. That works before I experiment.

I'll keep looking otherwise.

Thanks
 
I had thought about designing some kind of a safety override for the trim. However, it occurred to me that any electronic safety mechanism would have at least as many failure modes as the trim system, possibly more. Don't get me wrong, I love electronics and have been making my living in that sphere for a few decades now. I've just seen enough stuff fail to know that no matter how nice and how clever it is, it can fail in ways you never expected.

I like the idea of electric trim and flaps. Both will have a way to quickly cut power by the time I'm flying.
 
DaleB said:
I had thought about designing some kind of a safety override for the trim. However, it occurred to me that any electronic safety mechanism would have at least as many failure modes as the trim system, possibly more. Don't get me wrong, I love electronics and have been making my living in that sphere for a few decades now. I've just seen enough stuff fail to know that no matter how nice and how clever it is, it can fail in ways you never expected.

I like the idea of electric trim and flaps. Both will have a way to quickly cut power by the time I'm flying.
Click to expand...

Hey Dale

I don't mind that things fail as long as they fail safe so when I started researching trim servos I was caught off guard by how they fail. I like the idea of placing a motor in the middle to slowly turn the cable. If it fails, who cares. You can still turn the knob. In this scenario you can still use a speed controller or a reverser or cut off power but your not stuck at a dangerous position. I like this.

I'm looking forward to seeing how Frog did it but I definitely have an idea now of how to do this.

Thanks.

Michael B
 
1bigdog said:
Hey Dale

I don't mind that things fail as long as they fail safe so when I started researching trim servos I was caught off guard by how they fail. I like the idea of placing a motor in the middle to slowly turn the cable. If it fails, who cares. You can still turn the knob. In this scenario you can still use a speed controller or a reverser or cut off power but your not stuck at a dangerous position. I like this.

I'm looking forward to seeing how Frog did it but I definitely have an idea now of how to do this.

Thanks.

Michael B
Click to expand...

Hilarious. I have what I need in the next room. Its the vernier cable motor drive for my telescope. It even includes a quick disconnect mechanism from the cog on the cable. It has a reversible controller with 2 speeds.

Manual it is.

Cheers
 
I think this is one of those areas where people are worried that just because there is the possibility of failure, then we need to fix it. We really need to focus on the probability, then determine the severity. If the probability and severity are low, press on with current design practices.
 
Not me...

It can't be me mentioned in the post above.
I have wired the power-wires for the trim and and flap motor through a separate switch.
In case of a run-away, I simply turn off the switch.

To have the trim system as simple as possible, I have installed only one trim switch per system: one switch and one indicator for aileron trim, and ditto for the elevator trim.
In other words: I only installed the trim parts which came from Vans.

Having flown with this setup since 2008, I have not yet missed to have the trim on the stick. The panel mounted switches works fine for me.

The power cutoff switch in question is the second one from the right in this pic, is red and it's labeled "TR/FL CUT-O"

1zce3bo.jpg
 
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1bigdog said:
Bill

... I can guess how it work. You stick a reversing axial motor on the cable and the knob on the other side and it would rotate the knob instead. Of using a servo.

If you or someone knows a link because I can't seem to find it and would prefer to see something. That works before I experiment.

I'll keep looking otherwise.

Thanks
Click to expand...
Yes, that is what I recall seeing but it doesn't sound like it was Capt. Frog who made that mod. Keep searching, it is in the archives.

Bevan said:
I beleive the manual trim was heavier.

Bevan
Click to expand...
Yes it is; however, it is very simple to install and doesn't require electricity, breakers, switches, relays, etc. and is very effective. About the only drawback is that it fills one hole through the spars and as i have made changes over the years, it has been challenging running wires aft.
 
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DaleB said:
I had thought about designing some kind of a safety override for the trim. However, it occurred to me that any electronic safety mechanism would have at least as many failure modes as the trim system, possibly more. Don't get me wrong, I love electronics and have been making my living in that sphere for a few decades now. I've just seen enough stuff fail to know that no matter how nice and how clever it is, it can fail in ways you never expected.

I like the idea of electric trim and flaps. Both will have a way to quickly cut power by the time I'm flying.
Click to expand...

+1 Dale

Toobuilder I think this is one of those areas where people are worried that just because there is the possibility of failure, then we need to fix it. We really need to focus on the probability, then determine the severity. If the probability and severity are low, press on with current design practices.
Click to expand...

+1 Michael
 
dynonsupport said:
The trim controller in the Dynon system will time out a single stuck switch and still allow you to use the opposite direction switch to return the trim to neutral after a failure, or still use the buttons on the other stick.
Click to expand...

Here's the problem...
1. As I recall, using a timeout to limit trim operation is patented by TCW.
2. Trim timeout is an unnatural operation that interferes with the desired operation when operating normally. For example, dialing in a lot of trim pre-takeoff or when extending or retracting the flaps requires multiple trim 'blips'. For example, In a go around in a 300 hp Rocket, both hands are very busy and blipping the trim is an unwelcome, possibly unsafe procedure while trying to keep the nose below a 45 degree deck angle with forward stick, retracting the flaps and managing the stuff that makes noise.

The trim switch failure recovery method in the MakerPlane device always recognizes a failed trim switch without interfering with the normal operation of the trim system. It also allows the faulted pole on the trim switch to be bypassed witout the complexity of additional hardware by re-using an existing mode/programming pushbutton. Finally, it indicates a fault with a lamp or EFIS system contact input.

I will admit that a simple trim timeout may be 'good enough' for lower performance aircraft, but how does Dynon stickhandle around the TCW patent on this?
 
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Toobuilder said:
I think this is one of those areas where people are worried that just because there is the possibility of failure, then we need to fix it. We really need to focus on the probability, then determine the severity. If the probability and severity are low, press on with current design practices.
Click to expand...

Hi Michael

I agree with the philosophy.

But to clarify, I'm not at all concerned about failure, I expect it, just the state you will be left in if it fails. Electronics does not fix the problem. It only attempts to limit the impact. The servo locks you in that trim position, whatever that happens to be during worst case scenario.

I'm trying to fix the physical control problem so I'm not locked by the servo. Could be a shear pin, but I'm liking the idea of controlling the knob rotation because it solves the problem and doesn't remove functionality. It just needs a different motor mechanism which I'm happy to pursue. It even puts the mechanism in a better location for maintenance, and has little impact on W&B.

Cheers
 
vlittle said:
Here's the problem...
1. As I recall, using a timeout to limit trim operation is patented by TCW.
2. Trim timeout is an unnatural operation that interferes with the desired operation when operating normally. For example, dialing in a lot of trim pre-takeoff or when extending or retracting the flaps requires multiple trim 'blips'. For example, In a go around in a 300 hp Rocket, both hands are very busy and blipping the trim is an unwelcome, possibly unsafe procedure while trying to keep the nose below a 45 degree deck angle with forward stick, retracting the flaps and managing the stuff that makes noise.

The trim switch failure recovery method in the MakerPlane device always recognizes a failed trim switch without interfering with the normal operation of the trim system. It also allows the faulted pole on the trim switch to be bypassed witout the complexity of additional hardware by re-using an existing mode/programming pushbutton. Finally, it indicates a fault with a lamp or EFIS system contact input.

I will admit that a simple trim timeout may be 'good enough' for lower performance aircraft, but how does Dynon stickhandle around the TCW patent on this?
Click to expand...

How does this MakerPlane device recognize a failed trim switch without using a timeout - i.e. how does it distinguish between an actual failure (shorted switch) and a commanded input (intentionally pressing )?
 
pvalovich said:
Don't know about the -9, but for my -8A full nose up/ full nose down trim can be overridden with reasonable muscle power at normal cruise speeds. Not very comfortable - but doable. Also, the runaway will not occur instantaneously - you'll have a short time to react. I've included both elevator and aileron trim cutoff switches in my electrical design.
Click to expand...

+1 same for my 7A in regards to ability to override.

In regards to the trim controllers........

I have not found the need for the complexity of 2 speed trim control. During construction, I put in a manual pushbutton to slow the trim at cruise, but I rarely use it. The normal full 12v servo speed is slow enough to do what I need. This may be due to the fact I generally don't fly hands off without the A/P turned on. I can get the trim load off the stick and hand fly comfortably or turn on the A/P. This may not be close enough to do hands free long cruise altitude hold without the A/P.
 
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1bigdog said:
...But to clarify, I'm not at all concerned about failure, I expect it, just the state you will be left in if it fails...
Click to expand...

...and that's where "severity" comes in. In the unlikely extent of a hard failure... So what? The airplane is ultimately controllable, if uncomfortable.

A buddy got a ferry permit for an old airplane that had been sitting for a decade in a hangar. The mechanical pitch trim system was frozen solid. He ferried the airplane for 2 hours with the trim set for the landing speed of 70 MPH. His arms were tired when he landed, but that's about it.

A manual/electric trim system certainly has merit at face value, however, one has to honestly evaluate the risk posture of the existing system to ensure the replacement system is a true improvement. Many times a new system, despite apparent redundancies, is actually more failure prone than the system it is attempting to replace.

I'm all for innovation and out of the box thinking, just don't get tunnel vision.
 
bsvantho said:
How does this MakerPlane device recognize a failed trim switch without using a timeout - i.e. how does it distinguish between an actual failure (shorted switch) and a commanded input (intentionally pressing )?
Click to expand...

Sorry I missed this question earlier on.

The MakerPlane device infers a runaway trim when both poles of a trim switch are active at the same time. When this condition is recognized, the motor is stopped and the faulted pole is bypassed with the external pushbutton/programming switch.

The faulted pole is easy to determine from the history of switch operation, which is monitored by the internal processor. It auto clears when the switch becomes unstuck.

This elegantly bypasses the TCW patent with what I think is a simpler and more reliable method that does not interfere with normal operation. There are no unexpected side effects like a time-out.
 
Has anyone ever actually experienced a runaway trim situation (with an RV or similar using a MAC/RAC servo)? I haven't personally in 14 1/2 years of electric RV trim ownership and operation, and have never in my 20 years of RV consciousness heard of one. It's (apparently) such an infrequent condition, that I doubt I'd recognize it until the servo was well deflected. With this much concern of what I believe to be a minuscule probability event, manual trim needs to be the solution. (OMG!)
 
Manual trim is not convenient for formation flying, and it won't support auto-trim.

Right now, the MakerPlane M-PPWR-2AT is the only way to get auto-trim from a Dynon SkyView system.

Admittedly, the stuck-trim recovery function is kind of secondary.
 
Simple on off reverse switch.

I had a runaway aileron trim in my rv-7 because of a short and could 't care less. Easy.
The elevetaor trim on the 10 can be quite a suprise if the passenger hit the switch. I made the choice to put the on off reverse switch so i can reverse it if there is a short and put it to off when in neutral position. Cost only few dollars for the switch. I feel safer. Low rate of failure but cheap fix , really low time mod.
 
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