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  #51  
Old 12-20-2020, 05:29 AM
plehrke's Avatar
plehrke plehrke is offline
 
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Thanks Dan for the data.
I was trying to indirectly indicate to those building that shortening the stick does have some consequences. The RV stick forces are very light in trimmed flight so a shorter stick seems like a good idea. Like everything, when cutting down the stick length there needs to be a conscious decision on what you maybe trading off. One possible thing traded away is landing with runaway trim.
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  #52  
Old 12-20-2020, 07:36 AM
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BCP Boys BCP Boys is offline
 
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Default Good data, best advice

Reading through this on a cold morning while drinking coffee and learning my daily lessons from folks that are way more knowledgable than I am about airplanes.... The one take away for me on most safety items is what Dan H just stated. Go fly your plane, try it on your plane, and know what it does so you know what to expect. Thank you to the OP for bringing this concerning issue back up for discussion so that we all can learn our planes better, even the ones who have built their own time machines and have been flying them for years and years. You can always learn something new.

Also, congratulations on the way you handled the emergency.

Happy Holidays everyone. Fly Safe.
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  #53  
Old 12-20-2020, 08:20 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by plehrke View Post
Like everything, when cutting down the stick length there needs to be a conscious decision on what you maybe trading off. One possible thing traded away is landing with runaway trim.
Change in required force is proportional to change in length. I've attached a quick cheat sheet for those contemplating shortness

Quote:
Originally Posted by BCP Boys View Post
Go fly your plane, try it on your plane, and know what it does so you know what to expect.
Yes, running the exercise is eye-opening. Although the forces are manageable in controlled test, the surprise factor would be huge given an autopilot disconnect. Shoot a landing or two also, as the unusual stick force is distracting. My thanks to Gary for prodding me to go do it...and Mr. Weyant, well done sir.
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  #54  
Old 12-22-2020, 12:35 PM
gereed75 gereed75 is offline
 
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Thanks Dan. Interesting experience huh? Not sure how any individual six/seven, ten or whatever with whatever various trim systems as installed would compare to your airplane, but at least we have one good data point.

From memory and as I imagine it now looking back, the 26 pounds pull is in line with what I experienced. For those who haven’t done it, go grab a Nearly full five gallon gas can and pull it to your waist for comparison. Dealing with that kind of stick pressure is not trivial. I was not too comfortable subjecting my control system to that pressure.

With the way I have my trim servo biased, and the addition of the TCW controller (at low speed and short duration activation), I am comfortable with the measures I have taken to mitigate this risk.

My point was that I hope others have given this some thought and are comfortable with whatever measures they have taken. The consideration is warranted

Happy Holidays and fly safe.
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Last edited by gereed75 : 12-22-2020 at 12:48 PM.
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  #55  
Old 12-22-2020, 01:59 PM
AlpineYoda AlpineYoda is offline
 
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Quote:
Originally Posted by rapid_ascent View Post
I've seen posts in this thread and others about having circuit breakers on the trim servos so that you can pull them in the event such as this. I'm actually at the point in my panel layout were I'm locating some of the breakers and in fact on this iteration I have breakers for the pitch and roll trim to the GAD27 power inputs. My question is how does this solve the problem if the servo has run to an extreme limit?
It depends what is making the trim run. There have been several posts in this thread about a bad hat switch causing the motor to run. A kill switch on the panel would let you disconnect all functionality on the grips, letting you run the trim back to the right position using the panel-mounted switch. Or, if you had a kill switch for each grip, one grip could undo the damage caused by the disabled grip.

If it is a wiring problem, motor problem, or the panel switch/relay fails, then yes, pulling the breaker wouldn't help much once it was at full stop, except it would make things stop shorting, if that is the cause of this all. Wires shorting, or a motor running non-stop and overheating, might end up causing other really bad things, so the ability to stop that is invaluable.

Either way, I've drawn two conclusions from this thread --
1, the original poster did a fantastic job getting down in IMC with control problems.
2, I will be mounting a kill switch for my grips (Kevin at Tosten said a single switch on the common would do it all) when I get to that stage.
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  #56  
Old 12-22-2020, 02:47 PM
Weefle Weefle is offline
 
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I’m not flying an airplane with electric trim just yet but my question for the wise is....if you’ve had runaway trim and it’s a full deflection and you pull the CB or somehow kill power to the servo, how do you get trim back to neutral if there is no manual way to do it in an RV?

Please educate me.

Thanks
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  #57  
Old 12-22-2020, 02:49 PM
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Mike S Mike S is offline
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You could have a reversing switch with its own power source.

Also, if using the Safety Trim, you will not get to full deflection unless you repeatedly try to run the trim. It limits the trim speed in cruise, and the length of time the now slower trim is moving.
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Last edited by Mike S : 12-22-2020 at 02:59 PM.
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  #58  
Old 12-22-2020, 05:51 PM
dweyant dweyant is offline
 
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Quote:
Originally Posted by Mike S View Post
You could have a reversing switch with its own power source.

Also, if using the Safety Trim, you will not get to full deflection unless you repeatedly try to run the trim. It limits the trim speed in cruise, and the length of time the now slower trim is moving.
Not necessarily. The VPX can be set to only allow a run time of three seconds then it will cut off. Due to the nature of my failure, a short in a wire, that was intermittently shorting it managed to run all the way to the stop.

The VPX did eventually, correctly sense an issue and disable the trim. However, and this is what I find most concerning, the trim was mechanically stuck at full deflection.

No amount of switches, or other “interesting” schemes that folks posted on this thread would have prevented this situation.

-Dan
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  #59  
Old 12-22-2020, 06:08 PM
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Steve Melton Steve Melton is offline
 
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electronics run amuck. talk to Boeing about it.
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  #60  
Old 12-22-2020, 06:29 PM
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Ironflight Ironflight is offline
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I’ve been following this thread for a few days, and the thing I haven’t yet seen mentioned is “EAA Flight test Manual, Test Card 18 - Runaway Electric Pitch Trim”.

Yup - if you have thoroughly tested your airplane, then this is the kind of thing that you should have tested. Granted, the FTM is only a few years old now, and many folks did their phase one in the years before that. But....many probably never considered that this is a test that should be performed.

Remember - Phase 1 is not just “flying off 40 hours” - it is testing the airplane - including failure and off-nominal conditions. And there’s nothing that says you can’t do this test on your next flight - regardless of what phase you’re in!

BTW - I have flown all of our RV’s at full trim extremes, and they all survived. All flyable - just not terribly comfortably at all edges of the envelope. Doing it unexpectedly on an IFR flight plan - good job OP!

Paul
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