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Run-away trim

G

gereed75

Well Known Member
I recently completed first flight on my very slow build RV-6 - BIG GRINS!!

On Flight 4, I checked for the ability to over ride a fully deflected elevator trim tab (as might occur in a run-away trim scenario). As expected, it was possible to override at moderate A/S. Also as expected, stick forces grew as A/S increased. Somewhere around 150 KIAS, the stick force to override became excessive - I was able to physically overide but became uncomfortable with the force required -fear of damage/failure of control components caused me to desist.

This has caused me to consider shortening the trim tab control arm. I have a "stock" electric trim with a Mac servo. I realize this will slow the trim tab movement (not a concern). The objective will be to reduce the tab total travel. It does not appear that this will limit trim effectiveness as trim tab movement required to trim the elevators through normal speed changes at normal CG appears to need no where near the amount of tab deflection I am getting now.

Any experienced responses??

PS: I have not checked the amount of servo travel, I assume it is the "normal servo" everyone uses. (I did not build the Elevators).
 
Be Careful!

You will need to make sure you have sufficient travel remaining for all corners of the envelope - forward CG, aft CG, heavily loaded, lightly loaded, slow speed, and high speed. I would approach changes to the trim throw very carefully, as this has been engineered by Vans to be sufficient under all published operating conditions.

It would not be good to limit your throw based on a flight test you did with just yourself in the plane, only to find out that with a passenger, you don't have sufficient trim on approach and have to hold in a bunch of aft stick. That could get very dangerous, very quickly! :eek:

Runaway trim concerns are best handled via other means, such as breakers and motor reversing switches / circuits. There are several alternatives out there which are preferable, IMHO.
 
Why couldn't you just slow down.

150 K is cooking right along.

It will fly fine at 80 K or so.


JMHO

cary
 
Slow Down

I agree with making sure you check all weights and cg positions before modifing the trim travel. Also, if the trim runs away I am slowing down to find a place to land, not blasting on at >150kts
 
Gents, There are many flight regimes and circumstances that will preclude you from "just slowing down".

How about full down trim that you can not override?? - no slowing down there.

How about full down trim during IFR cruise or with little terrain clearance?? This is not a question of continuing flight with a run-away trim, it is a question of can you maintain controlled flight in the event of a runaway within the normal flight envelope without damaging a control.

I have a fuse in the circuit along with a separate trim power off switch. Even if you recognize the problem while overriding and secure the power, you are now flying with an extreme trim condition. Can you maintain flight without overstressing controls?

Has anyone done the test (at cruise A/S) and what were your results??
 
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gereed75 said:
This has caused me to consider shortening the trim tab control arm.
Click to expand...

First of all, I don't recommend any modifications to the trim system. It's been proven to be sufficient on thousands of RVs. Secondly, shortening the control horn will increase travel, not reduce it.
 
Thanks Mel, You are absolutely right.

Thanks Greg. looks like a great product.

And yes, Van's trim has certainly proven sufficient. My point of concern is that the tab, driven by the usual MAC servo, seems to be way more than sufficient and from my experience, appears to be capable of driving the trim tab way more than necessary. In this case, more is certainly not better.

Take it for what you think its worth.
 
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Trim disconnect

I have the trim switches on the stick. On my EXP switch panel I have a trim power switch. When I move the trim, I'm ready to kill the power to the trim if I see or feel a runaway situation. In any set up, you can wire in a cutoff switch.

As Mel noted, don't modify something that works fine.
 
gereed75 said:
Gents, There are many flight regimes and circumstances that will preclude you from "just slowing down".

How about full down trim that you can not override?? - no slowing down there.
Click to expand...

Just roll inverted....

:D

Once you have the airspeed under control, then stick forces will not be excessive and you can again roll rightside up.
 
Hmm.. inverted

Yea, Thought of that. Sounds like big fun.

I think I will bias my trim servo actuation rod to limit the nose down trim to only what is needed. A runaway nose up trim is much easier to deal with. In any case, the possibility is slim - circuit protected against shorts by a fuse, Trim Disable switch on the panel.

Fly on !
 
I have the same trim in my kitfox and have never had a trim runaway, but to be safe, I put a fuse right in front of the throttles, just above the trim switch, it's a pull off fuse for such a thing. I went one step further and put both the flaps and the trim on that fuse. Problem, I pull it. I know then it cuts off both flaps and trim. But, a person should be able to land without either flaps or trim. The only problem I have had with the trim is the relays, ray allen relays are junk, they WILL give up the ghost at about 300hrs, that is if you use them a lot, like I do. My answer to this is to use another relay, I use the ones from aircraft extras, http://www.aircraftextras.com/Index.htm

They have an internal diode setup to protect the contacts, I'm using these on both flaps and trim.
 
Put in a nutter switch

If your really worried about a runaway condition. The first thing that comes to mind is a switch is stuck or welded. Take the wires to a switch on the panel is ok, but this just cuts the power. how about if you put a DPDT switch on the panel then you have it normally in the up "forward" position and if the servo runs away switch it to the middle "off" position and you can then switch it to the down "reverse" position and you still have control of your trim.

if your going to add the weight of an extra switch why not get the mose bang for your weight buck.

My .02
 
That thought was in my mind the whole time. I was wondering how many posts it would take before it showed up in print.!!

Ah, the siren song of bigger better faster more dollars. It's a never ending slippery slope.
 
I went with manual as well.

I tried to build my -7A light and as simple as reasonable. A couple months back I posted asking about the rigging of the neutral position. The manual knob will go 10 turns stop-to-stop so I set it for about 25 degrees up and 35 down. (Trim tab, not elevator). I was told that I would never use ONE full turn in normal operation and that has proven to be the case. Like Mel says, it'll never run away on you and it is so intuitive you could actually take off, fly the pattern and land it with just the trim knob.
 
Hawkeye7A said:
I tried to build my -7A light and as simple as reasonable. A couple months back I posted asking about the rigging of the neutral position. The manual knob will go 10 turns stop-to-stop so I set it for about 25 degrees up and 35 down. (Trim tab, not elevator). I was told that I would never use ONE full turn in normal operation and that has proven to be the case. Like Mel says, it'll never run away on you and it is so intuitive you could actually take off, fly the pattern and land it with just the trim knob.
Click to expand...

It's not that way with the tandems. The trim tab will have almost full nose down deflection when the CG is close to the aft limit with my RV-4.
 
Trim Issues

I would like to let you people in on an experience that happened to me recently. Runaway trim is front and foremost in my mind right now because of this experience. It was a lesson well learned and part of my ignorance in a poor walkaround was a good thing because of what I learned.
I was taking off in my RV-9A with a FULLY deflected trim tab in the UP position. This put all of the DOWN trim into the aircraft. As the speed was increasing I could NOT pull back on the control stick to raise the nose wheel. I aborted the take off and as the plane slowed down I got the full movement of the control stick back. The trim indicator wasn't working at the time. Yes a few mistakes were made but a valuable lesson was also learned at the same time. It felt as if the elevator control locks were still on but they weren't. This makes me wonder if the trim ever did runaway on the "9" model in cruise, you better have some strong muscles to over ride it. I will be coming up with some way of controlling the trim in flight. The trim tab on the "9" is longer than the other models.

Learn from others so you don't make the same mistakes!

Peter:rolleyes:
 
Peter,

Not to take a shot at you, but the major mistake I see in your story is not following the pre-takeoff checklist. I'm not sure a non-functional indicator would be a reason for not conducting a flight, but it sure would require a visual inspection that the trim was set in the proper postion prior to takeoff.

Maybe it's because I fly tadem seating aircraft, but the position of the trim is very critical during takeoff and landing because the CG can change dramaticly when fuel burns off and there's a passenger. I won't say that I've never made a mistake setting the trim before takeoff but my fingers were fast on the button when I found it set incorrectly.

As far as a runaway trim "issue", everyone should make it a step in their flight testing to fly the aircraft with fully deflected trim conditions to prove to yourself that the airplane is controllable. I don't believe extra switches are the answer to the problem either. The switch is probably the least reliable mechanical device in your circuit and you are just adding another point of failure. To me, a runaway trim is essentially a non-issue. It is rare, and if it occurs, the airplane is still flyable. Sometime we can overthink these things and turn small problems into major ones by over-engineering the fix.
 
Thanks Randy for adding your comments and to you Hawk. That is exactly the kind of data input I was looking for.

I take from these comments that this is a valid concern and deserves consideration.

The point remains, there is no reason to have more trim authority than is required to maintain trim through the envelop of normal CG and airspeed conditions. Anything else is unnecessary - and an unnecessary risk.

I will continue to work to ascertain exactly what the necessary amount of travel is for the various CG loading, A/S and flap configurations. My initial data points say that required tab movement for an RV-6 is much less than is provided by the "normal" tab and MAC servo arrangement. This much travel may be needed in a tandem RV.

Again, more (or excess) in this case is not better. A simple change to a lesser travel servo would be a prudent course. Not a big deal. Just add it to the Phase 1 list.

I'll post when I have definitive info. Thanks all
 
Cutout switch here...

Hi.

I've wired the trim and flaps power via a separate switch placed close to the throttle and the flap switch so my hand can reach it instantly.

I've labeled it and also put on a red "hat".

That way, if ever either the flap or trim schould run away, I can stop the runaway in less than a second.

I got the idea of having a cutoutswitch from the previous a/c I've flown: F-5, F-16 and B-737.
They all have cutout swiches for elevatortrim at least.

PS: the right side side is pilotseat.

 
The throttle and mixture positions should never be reversed regardless of where the pilot sits. This will cause great problems anytime you fly another aircraft.
 
run-away trim prevention

This is the exact reason we developed our product: Safety-Trim. It limits trim travel to a maximum of 3 seconds per switch actuation. It also has the ability to do 2 speed control of the trim system as the airspeed increases. Slow trim for high airspeed and fast trim for low speed flight.

All the details are available at: www.tcwtech.com


Thanks,
Bob Newman
 
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