gblwy

gblwy

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I was going to add this to the "Crunch!" saga, but it's not easy to search for data on trim when you need to know that "crunch" is the keyword ;-)

The Light Aircraft Association in the UK are particularly interested in trim forces, especially in the runaway case. I eventually found the posting I was looking for. Larry conducted a test, posted 26th June 2010, in the Crunch string.

I repeated these tests yesterday, with a spring balance, as part of my UK flight testing.

I trimmed the aircraft straight and level at 60 knots with full flaps, then explored the out-of-trim forces at full up and full down trim settings.

I repeated the tests trimmed at 100 knots, with no flaps.

The pull, or push, required to maintain level flight at the trimmed speed ranged from 20 to 36 pounds. It is manageable with one hand (as Larry said), but I was surprised at quite how hard I had to push/pull.

At least now, if I ever got into a trim runaway condition, I have experienced the effort that is required to maintain control.

The LAA are interested in this scenario as there have been instances of problems with Sportcruisers fitted with autopilots. Apparently a wiring problem on stick-mounted trim buttons can result in a trim runaway. The autopilot tries to compensate until it finally trips out, at which point the aircraft violently pitches - apparently with sufficient force to risk structural damage.

Cheers...Keith
 
Trim runaway

Thanks for that information, Keith. It is good to know that the plane is controllable with trim set at extremes. On the RV-12, the only way for trim to runaway is for the switch on the panel to short out, unlikely but possible. If that happened and pilot reacted quickly enough, he could shut off the master switch or pull the trim fuse.
Joe
 
Runaway Trim

Just to clear the air on the "Crunch" episode...the Ray Allen 'servo' was destroyed but NOT because of a runaway...the sudden aircraft's stop caused the AST, trim tab if you will, to violently 'flip' up and the actuating rod pushed against the Ray Allen unit so strongly that it tore up the gears and destroyed the enclosure box of the unit. A local A&P initially guessed it was a runaway, but that was completely incorrect.

JohnF
 
What happens...

Hi,

This topic is unrelated to the root cause of the "Crunch" episode, it's just a commentary on UK certification requirements.

More fun today!

The LAA wanted to understand how long a trim runaway condition (up or down) could proceed before the plane gets into a potentially dangerous attitude or G condition. Let the testing commence...

Test one was the trim-up case. The plane was trimmed for a 5000 rpm straight-and-level cruise, at around 105 kts. The plane was close to gross weight with two crew, almost full fuel and 50 lbs of sand in the back. I let go of the stick and pressed the up-trim switch. I chickened out after 5 seconds at what felt like a 70 degree attitude, and pushed gently to achieve level flight which resulted in a negative G pushover. Interesting and a little disconcerting.

Test two was the trim-down case, from the same level condition. Pushing the down-trim switch resulted in an almost vertical descent after 4 seconds, my chicken-out point. The plane went negative G and seemed to be tucking under. Again a gentle return to level flight. Interesting and a little more disconcering.

I accept that this is a very unlikely scenario. Who would wait for 5 seconds before grabbing the stick? However, as discussed previously, I think this was motivated by the autopilot scenario. I have no idea when the autopilot would give up, but I hope it is after a couple of seconds!

My trim runs from stop to stop in 25-27 seconds, which was about the slowest speed I could get smooth results. I'm going to try and slow it a little more as I'd like to install an autopilot in due course and need to reassure the powers-that-be that all will be well.

These results appear to bear out what Rob Byers said in the posting entitled "First Demo in N912RV".

Cheers...Keith
 
Dynon Download

I just downloaded the Dynon stats and ran off a couple of graphs - great for keeping you honest ;-)

I exaggerated the pitch angles at which I initiated a recovery. But the G-loads are real. I recorded minus 1.1G in the down-trim condition (so I guess that is a 2.1 variation from the norm) after 4 seconds.

Cheers...Keith
 
Trim speed

As discussed, I readjusted my trim speed to the slowest possible without the trim motor stalling. It made very little difference. The slowest I could get was 30 seconds in one direction and 40 seconds in the other. This worked fine in the air.

When I repeated the trim runaway case, it made a difference of a second or so to the time which elapsed before the plane got into an uncomfortable attitude/G-load.

Can anyone with an autopilot explain what happens in a possible trim runaway case? Is there any indication that the trim is running and the autopilot is compensating? When does the autopilot give up? What happens? And can the sensitivity of the autopilot disconnect be modified?

I'm still keen on getting an autopilot, but the powers-that-be in the UK will be interested in what happens in a runaway case.

Thanks...Keith
 
Tell that to the powers-that-be

Keith,
I am very familiar with the electrical system on the RV-12. There is only one failure mode that will cause runaway trim and that is failure of the trim switch on the instrument panel. No electronics failures, no wires shorting, nothing can cause the trim motor to run except for that switch. Switches seldom fail and when they do, they usually fail open. Switches that fail in the closed position are exceedingly rare. And if a switch were to fail closed, most likely it would happen while you were operating the switch. That switch is not going to close all by itself without you touching it. And you are not going to be operating the trim switch while the autopilot is on. Just because some other airplane with a different electrical trim system crashed because of runaway trim does not mean that the RV-12 could have the same problem. I am not saying that it is impossible to have runaway trim on the RV-12 while the autopilot is on, but it is more likely that the engine will fail or the wings will fall off or that you will have a midair collision halfway across the Atlantic on your way to OSH. Tell that to the powers-that-be in the UK. :D
Joe
 
gblwy said:
Can anyone with an autopilot explain what happens in a possible trim runaway case? Is there any indication that the trim is running and the autopilot is compensating? When does the autopilot give up? What happens? And can the sensitivity of the autopilot disconnect be modified?

I'm still keen on getting an autopilot, but the powers-that-be in the UK will be interested in what happens in a runaway case.

Thanks...Keith
Click to expand...

Hi keith,

I can't speak specifically for the 12 or for other autopilots but I can give some idea of what will happen with the Trio AP.

I had an issue with my Trio EZ III with Auto Trim in my 9A where the auto trim system would continue to command the trim motor even though neutral trim was reached. So, for example, what started off as a slight/moderate nose up trim could end up as a strong nose down trim without any input from the pilot. Apparently this was an issue that happened with some RV9s and RV10s with the Trio. Trio came out with a new code for the control module and it solved the problem in my 9.

The end result would have been similar to a trim runaway situation. The Trio has an adjustable clutch that is designed to slip when the elevator forces get too big or if the pilot wants to over-ride the AP with direct stick input. I recall that the bright red CLUTCH SLIP display illuminated and if I remember correctly, the aircraft had a mild pitch change. I presume the pitch change was not abrupt because the slipping clutch was still providing a strong input to the elevators.

No real drama at this point however the pilots natural reaction is to disconnect the AP and the aircraft will pitch abruptly unless the stick is held firmly while disconnecting the AP.

Even though I no longer have this problem I am now in the habit of occasionally scanning for the CLUTCH SLIP warning and always very firmly holding the stick when I disconnect the AP just in case there is an out of trim condition.

Fin
9A
 
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You do actuate the pitch trim with the autopilot engaged.

Keith,
I am very familiar with the electrical system on the RV-12. There is only one failure mode that will cause runaway trim and that is failure of the trim switch on the instrument panel. No electronics failures, no wires shorting, nothing can cause the trim motor to run except for that switch. Switches seldom fail and when they do, they usually fail open. Switches that fail in the closed position are exceedingly rare. And if a switch were to fail closed, most likely it would happen while you were operating the switch. That switch is not going to close all by itself without you touching it. And you are not going to be operating the trim switch while the autopilot is on. Just because some other airplane with a different electrical trim system crashed because of runaway trim does not mean that the RV-12 could have the same problem. I am not saying that it is impossible to have runaway trim on the RV-12 while the autopilot is on, but it is more likely that the engine will fail or the wings will fall off or that you will have a midair collision halfway across the Atlantic on your way to OSH. Tell that to the powers-that-be in the UK.
Joe
Click to expand...

I disagree with Joe's statement that you won't be trimming with the autopilot engaged. Any change in airspeed, bank angle, etc will result in an amber "trim down or up" indication on the lower left of the Dynon display. You trim in the direction indicated until the amber "trim down or up" display extinguishes. That removes the out-of-trim pressure from the autopilot pitch servo. If you didn't trim out the pressure, the servo would eventually slip in a somewhat jerky fashion that definitely gets your attention.
 
I've mounted...

... a cutout-switch for both trim and flaps and that'll hopefully avoid the scenario you're describing.

I got the idea after thinking about what would happen if a trim or flaps would run away.
We had the same thing on the F-5's and since a RV is a small fighter (almost) :D I figured it made sense to have a cutout switch in the RV too ... :)

As you can see on the pic below, I've put different colours on the switches based on their function. (the right seat is pilot seat)

It's red and labeled TR/FL CUT-O

 
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ao.frog said:
... a cut-out switch for both trim and flaps and that'll hopefully avoid the scenario you're describing.
Click to expand...

Hi Alf,

The cutout switch is a good idea and I have a similar thing with pull-able breakers on both the trim and flap motors. However it may not help much in the situation Keith is describing (AP engaged) as the pilot would most likely not realize the trim has run away until the AP starts complaining.

Fin
9A

P.S.
Alf,
I wouldn't mind being back in Norway for a few days. Very hot weather here in Australia with a monster cyclone (hurricane) about to hit NE Australia.
 
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"I disagree with Joe's statement that you won't be trimming with the autopilot engaged. Any change in airspeed, bank angle, etc will result in an amber "trim down or up" indication on the lower left of the Dynon display. You trim in the direction indicated until the amber "trim down or up" display extinguishes. That removes the out-of-trim pressure from the autopilot pitch servo. If you didn't trim out the pressure, the servo would eventually slip in a somewhat jerky fashion that definitely gets your attention."

If I understand this, when you ask the autopilot to do something that results in a trim change the Dynon displays the out-of-trim condition, that you then correct with the trim button - right?

So if the autopilot is holding straight-and-level and you then move the trim with the button, does the Dynon show a similar out-of-trim condition? If you continue winding in trim, how soon does the servo slip?

Thanks...Keith
 
Dear GBLWY,

If I understand this, when you ask the autopilot to do something that results in a trim change the Dynon displays the out-of-trim condition, that you then correct with the trim button - right?
Click to expand...

Yes.

So if the autopilot is holding straight-and-level and you then move the trim with the button, does the Dynon show a similar out-of-trim condition? If you continue winding in trim, how soon does the servo slip?
Click to expand...

I'm not sure but I'll bet it works that way.

Re "How soon does the servo slip?" Again, I'm not sure but I think the slippage would depend on what amount you have programmed into the autopilot. Sorry I can't remember the exact terminology since my RV-12 and manuals are at the hangar.
 
A reminder

I think it an appropriate time to bring this thread on top of the stack.
I had it in mind before my first flight anyway but some more recent builders might have missed it.
 
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