Trim tab and stabilator connection on Pipers?

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drotto

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drotto
Recently, somebody posted a video of landing a Cessna without elevator usage. They used the trim tab to basically have some pitch control. For this to work I am assuming that the trim tab must be separate from the elevator control. When I look at the Piper stabilator linkage it always unnerves me that basically one small bolt is all that seems to attach to control rod. That attachment point is than placed on the trim tab and not on the main stabilator, as shown by moving the stabilator on preflight the trim tab will travel in the opposite direction. Is there a second connection I am not seeing inside the empennage? Is trimming for pitch in a stabilator failure situation even possible in a Piper?
 
drotto said:
Recently, somebody posted a video of landing a Cessna without elevator usage. They used the trim tab to basically have some pitch control. For this to work I am assuming that the trim tab must be separate from the elevator control. When I look at the Piper stabilator linkage it always unnerves me that basically one small bolt is all that seems to attach to control rod. That attachment point is than placed on the trim tab and not on the main stabilator, as shown by moving the stabilator on preflight the trim tab will travel in the opposite direction. Is there a second connection I am not seeing inside the empennage? Is trimming for pitch in a stabilator failure situation even possible in a Piper?
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Yes.

The trim surface is controlled separately from the elevator on both aeroplanes. If you think about the aerodynamics for a moment you will see why the trim tab moves opposite the elevator/stabilator.



"...That attachment point is than placed on the trim tab and not on the main stabilator..."

The actuating arm on the bottom side that you are looking at is the trim control surface connection. There is a completely separate system inside the fuselage, connected to the yoke that is moving the stabilator.

Try running the trim control through it's full range on the ground during one of your pre-flights. Don't touch the yoke. See what happens and what actually moves (and don't forget to return the trim to "take-off" position when you are done).

Great question btw. Understanding how each of the systems on the airplane works is, imo, critical to gaining confidence flying the aeroplane as a pilot.
 
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What GRG55 said. The trim system is independant of the stabilator system, yet they both work together to relieve control pressure felt in the yoke. In the (unlikely) event of stabilator control failure it will be possible to control pitch with use of trim. Once you observe how they each work, you will understand how amazingly simple the system actually is. (simple=less room for failure)
 
GlennAB1 said:
Don't Piper's with stabilators have an Anti-servo tab, not a trim tab? Maybe Jim can help out here?
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Yes, but the anti-servo tab is adjustable so it also functions like a trim tab. It works well. -Skip
 
GlennAB1 said:
Don't Piper's with stabilators have an Anti-servo tab, not a trim tab? Maybe Jim can help out here?
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Yes. I tried to make the distinction by using the phrase "trim control surface" in the second part of my post describing the Piper system, but would have been better to make the distinction between a trim (servo) tab and an anti-servo tab clearer.
 
GlennAB1 said:
But it moves the same direction as the stabilator, not opposite, like "trim tabs"?
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Yeah I was doing some research on that and the anti-servo does move the same direction. Now I am curious what the actual stabilator connection looks like. I know Piper likes to claim the stabilator is lighter, simpler, and less prone to failure.
 
drotto said:
Yeah I was doing some research on that and the anti-servo does move the same direction. Now I am curious what the actual stabilator connection looks like. I know Piper likes to claim the stabilator is lighter, simpler, and less prone to failure.
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Do you have a link or reference with respect to the "less prone to failure" claim? I have never heard that before.
 
Ok, now I'm confused..... Can anyone help me get clear on the distinction between "servo"and "anti-servo"?
 
Volitation said:
Ok, now I'm confused..... Can anyone help me get clear on the distinction between "servo"and "anti-servo"?
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A servo or "trim" tab moves in the opposite direction to the control surface it is attached to. So for up (climb) elevator the servo (trim) tab will be deflected downward, thus aerodynamically forcing the back of the elevator to deflect upward compared to the horizontal stabilizer.

An anti-servo is the opposite of a servo tab. It moves in the same direction as the control surface, but normally has a higher deflection from neutral position.
 
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Thank you Glen and Colarado. The wiki reference defines the functions, and I didn't realize servo and trim tab were synominous. That clears it up even for me. Lol.

And GRG!
 
They are just different ways of pitch control, engineering, same as the DC-10 using cables for leading edge slats and L-1011 using torque tubes and 727 using individual hydraulic actuators.
 
coloradobluesky said:
Go to:
http://www.slideshare.net/lccmechanics/assy-rigging
Then go to page 51 of 121, there is a drawing that may help you.
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So there is basically a T type connector on the leading edge of the stabilator inside the empinnage. An up cable and down cable connect to that T connection, and a spring and counter weight make sure everything moves smoothly. Think I got it. I am also glad there is a bit more holding that thing together.
 
Volitation said:
Thank you Glen and Colarado. The wiki reference defines the functions, and I didn't realize servo and trim tab were synominous. That clears it up even for me. Lol.
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The posts you reference are incorrect.

First, the purpose of the anti-servo tab is to INCREASE the control pressure needed to move the elevator/stabilizer, as without it, control pressures would be too light.

Second, the anti-servo tab moves in the SAME DIRECTION as the elevator/stabilator whenever the primary control surface is moved. In other words, when the elevator is moved up, the anti-servo tab moves up more, trying to force the elevator back down, and vice versa – thus reiterating the first point.

Third, the Piper anti-servo tab does also serve as a trim tab, but the trim function is independent of the anti-servo function. When the trim is cranked up or down, the tab moves in the expected direction -- opposite the primary control surface, just like on a Cessna 172.
 
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It is clearly independent based on the trim screw noted on page 2C20 in the previously provided link. The Wiki reference agrees with what you wrote.
 
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GlennAB1 said:
It is clearly independent based on the trim screw noted on page 2C20 in the previously provided link.
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Yes, I agree, and on further examination, I think I accidentally caught you in crossfire, it was some other posts I was arguing against.
 
The PA-28-181 maintenance manual posted above specifies: "Stabilator and Tab" but then goes on to refer to it as Trim Tab, ok. But there are plenty of websites you can find them called anti-servo tab.
 
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If you look in the slot (with a flashlight) where that tab rod comes out, you'll see a jackscrew. The screw has a sheave for the cable that controls it, and the cable is moved by the trim crank up front. The jackscrew changes the position of the tab relative to the stabilator.

Owners need to spend some time exploring their airplanes while they're opened up for annual. They'd learn a lot. I do hope their mechanics actually open this stuff up. We often find things that have obviously been overlooked for a long time.
 
Do that many planes loose elevator control?

The trim to landing exercise always seemed gimmicky to me
 
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