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  #11  
Old 02-02-2016, 08:57 AM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Default Sorry for the thread drift...

Lets define a few terms first....

Trim tab - an adjustable tab (in flight or manually on the ground) tab used to move a control surface in a direction we desire (to induce a trimming force).

Servo tab - a controllable tab used to move a control surface. I a sense a trim tab is also a servo tab, but servo tabs (by name anyway) are typically considered more of an active control function. Some large aircraft built prior to hydraulically boosted controls had servo tabs connected to the control system. The control system moved the servo tab which in turn drove the main control surface to the desired deflection (I think the B-29 was one airplane controlled this way).

Anti-servo tab - A tab whose movement is used to counter the movement of a control surface. This is typically done to artificially induce control force / feed back.

The RV-12 has an Anti-servo tab. It also functions as a pitch trim tab by having an adjustable neutral point, but that is a secondary purpose.

With a horizontal stabilator with the hinge point somewhere close to the center of pressure (as we have on the RV-12), there would be nearly zero control force / feed-back without the anti-servo tab. The anti-servo tab adds artificial feed back force by deflecting in a direction that tries to move the main surface back to neutral when it is moved by the pitch control system. That is why the tab constantly moves whenever the stabilator is moved.

In a nut shell.... if the anti-servo tab was non functional you would have little to no control force in pitch, which would probably make the airplane challenging to control.

I think a future release of the POH is going to cover recommendations for dealing with this. I think the primary one is to induce a different trimming force (deflecting the flapperons) that would then allow you to work the stick against the control force induced by the trimming input. It would be very similar to flying an airplane that is out of trim in pitch.... to change pitch you increase or decrease the pull force on the stick but it never gets to neutral/zero.
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  #12  
Old 02-02-2016, 09:06 AM
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Mike S Mike S is offline
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Location: Dayton Airpark, NV A34
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Scott, if the tab linkage failed in flight, would the tab be apt to flutter?

Not necessarily the high freq flutter we associate with structural failure, maybe just a low freq motion that would drive the stab neutral point around, and shake the stick.
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Last edited by Mike S : 02-02-2016 at 09:24 AM.
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  #13  
Old 02-02-2016, 10:43 AM
rvbuilder2002 rvbuilder2002 is offline
 
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It is possible, but has not been tested so can't say for sure.
There is a lot of variability in how the failure could occur (whether the mass of the controlling system was still connected to the anti-servo tabs or not), so it might be fine in one case but not in another.

There has been no indication that there is any "weakness" in any portion of the anti-servo / trim tab system.
Obey the "NO PUSH" placards and do a thorough pre-flight, and have fun.
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  #14  
Old 02-02-2016, 05:33 PM
todehnal todehnal is offline
 
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Great discussion, and insight!! Thanks Scott...........

Tom
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  #15  
Old 02-02-2016, 09:55 PM
RFSchaller RFSchaller is offline
 
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Scott,

Good explanation. I would think slowing down would help since that would give some pitch down tendency like deploying flaperons. Is the center of lift for the stab right on the hinge line? If it is slightly ahead I would expect a tendency to pitch down as the stab leading edge would want to move up. Similarly if the center of lift was behind the hinge I would expect a tendency to pitch up. Does that make sense?

Rich
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  #16  
Old 02-23-2016, 04:33 AM
Handclutch Handclutch is offline
 
Join Date: Feb 2015
Location: Bendigo, Australia
Posts: 63
Default Disconnected anti-servo tab - playing with fire

The tail feathers on Comanche aircraft are near identical to the 12 - stabilator with anti-servo tab. Checking the integrity of the anti-servo tab actuating rod connections is now a vital part of Comanche pre-flights due to an occurrence in the UK many years ago. The bolt securing the tab to the rod worked loose during flight (the nut had been left off during maintenance). Fortunately, there were two pilots on board. It required the strength of both on the controls to get the aircraft safely onto the ground at a nearby field. The vibration had been such that both had blodshot eyes for several days afterwards.

The 12 may perform differently in similar circumstances, but I don't want to be the one to find out. Scott, I'd recommend against testing for this scenario. Based on the Comanche experience it seems too risky.

The Comanche maintenance manual calls for no more than 0.125" free play at the rear edge of the tab. My 12 has more than that, and I noticed at OSH last year that this is common to the type (with one or two exceptions). On mine the slop is in the bolt connecting the rod to the actuator. Seems like it needs a larger diameter bolt but not sure if the next size up would fit and drilling out the actuator clevis is forbidden by the manufacturer.

Clearly, there have been no problems with the setup on the 12 but I would be more comfortable with a tighter fit and less play, given the Comanche experience.

Jack Moore
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  #17  
Old 03-29-2016, 08:08 AM
rgmwa rgmwa is offline
 
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Quote:
Originally Posted by Handclutch View Post
The Comanche maintenance manual calls for no more than 0.125" free play at the rear edge of the tab. My 12 has more than that, and I noticed at OSH last year that this is common to the type (with one or two exceptions).
My anti-servo tab has about 3/16" free play at the trailing edge (after 85 hours). From the above it sounds as though this might not be unusual, but it would be good to know what the acceptable limit is so that it could be monitored during annuals. I don't recall this being mentioned anywhere in the documentation. The Maintenance Manual just says there should be "very little movement", but that could mean different things to different people.
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  #18  
Old 03-29-2016, 09:51 AM
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Piper J3 Piper J3 is offline
 
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Do you have 3/16" free play at the trailing edge when the stabilator is nearly level and the back edge of stabilator is even with the anti-servo tab?
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Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 400

LSRM-A Certificate 2016
Special Thanks to EJ Trucks
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  #19  
Old 03-29-2016, 03:21 PM
rgmwa rgmwa is offline
 
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Yes. When the trailing edges are in line with the stabilator about level, I can lift the edge of the tab up about 3/16" from the in-line position.
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  #20  
Old 03-31-2016, 04:33 AM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by rgmwa View Post
My anti-servo tab has about 3/16" free play at the trailing edge (after 85 hours). From the above it sounds as though this might not be unusual, but it would be good to know what the acceptable limit is so that it could be monitored during annuals. I don't recall this being mentioned anywhere in the documentation. The Maintenance Manual just says there should be "very little movement", but that could mean different things to different people.
I checked my 12 yesterday and have same condition you describe with about 3/16" play at trailing edge. I see lost motion in the horn / rod end on the anti-servo tab. I will tighten this bolt and I think problem will be resolved.
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Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 400

LSRM-A Certificate 2016
Special Thanks to EJ Trucks
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