How does a Anti-Servo tab provide positive static stability?

[UNDPilot91]

This is a Commercial Oral Prep question and we havent quite gone over it in class.

I may be thinking too far into this question, but the Anti-Servo tab on our C172S is the "trim tab" on the stabilizer. (correct?) Positive static stability is the "skateboard in a pipe tube" example... oscilations will decrease until the plane returns to its original state.

How does an Anti-Servo tab provide the positive static stability?

 

[Roger Roger]

All an anti servo tab does is add resistance to keep the pilot from over controlling. It has little (nothing?) to do with stability. Oh also I'm pretty sure the 172 trim tab acts as a servo tab, not an anti servo tab. The Cherokee family has anti servo tabs.

 

[rframe]

Have someone sit in a Cherokee and move the stabilator and you will see the antiservo tab move automatically in the same direction as the tail is moving. Contrast that with the elevator on your 172 where the trim tab only moves if you adjust the trim wheel.

The difference is because when an elevator gets hit with oncoming air, it naturally wants to get deflected back to neutral, however as a stabilator gets hit with oncoming air (depending upon its balance), it might naturally carve into the oncoming air a bit resulting in very light forces and no natural force moving it back to neutral.

The stability issue comes from making control deflections more and more difficult the further they are deployed... resulting in pitch stability.

 

[gotWXdagain]

C172 tab is a trim tab, not servo or anti-servo. Basically, it rides on the back of the elevator but when the elevator changes position the angle of the tab relative to the elevator doesn't change.

 

[🦈💜]

C172 tab is a trim tab, not servo or anti-servo. Basically, it rides on the back of the elevator but when the elevator changes position the angle of the tab relative to the elevator doesn't change.

Yesyesyesyesyesyesyesyesyes.

I got asked that stupid question "Is the trim tab a servo tab or an anti-servo tab?" and I had to dig around for a while before I realized it's not either. It's a TRIM tab.

"Servo tab" and "Anti-servo tab" actually have engineering meaning. They're not just made-up words.

-Fox

 

[TwoTwoLeft]

Just remember, in the world of engineering, said "doodad" is named after the affect, not what it physically does.

 

[Roger Roger]

C172 tab is a trim tab, not servo or anti-servo. Basically, it rides on the back of the elevator but when the elevator changes position the angle of the tab relative to the elevator doesn't change.

have you looked at it closely? It's been a while but I remember it moving as a servo tab when you move the elevator.

 

[gotWXdagain]

have you looked at it closely? It's been a while but I remember it moving as a servo tab when you move the elevator.

I have looked at it closely, like right up against it to specifically see how it moved closely, if its angle changed I couldnt see it

 

[thevideographer]

When you do the preflight and move the elevator back and forth does the trim tab move? (no)

 

[Roger Roger]

I have looked at it closely, like right up against it to specifically see how it moved closely, if its angle changed I couldnt see it

Like I said, it's been a while but I could swear that the one on the 172 does. If you've got one close at hand though I guess you'd know better than my foggy memories of 4+ years ago.

 

[eightballfreight]

C172 tab is a trim tab, not servo or anti-servo. Basically, it rides on the back of the elevator but when the elevator changes position the angle of the tab relative to the elevator doesn't change.

2:30

 

[gotWXdagain]

Huh, that one appears to be servo-ing, guess I'll take a look again next preflight.

 

[shdw]

I have looked at it closely, like right up against it to specifically see how it moved closely, if its angle changed I couldnt see it

Set the trim to the stop, either direction, then observe again. At a takeoff/neutral setting the movement is very minimal and only really noticeable right around the +/- 5 degree points. At the stops it becomes more noticeable.


@UNDPilot91 The anti servo tab provides stability because it's deflection opposes the movements made by the pilot. In other words, you pitch up, it moves up. If you let go the anti servo tab being deflected upward would push the control surface back down to a neutral position.

If, for instance, it operated as a servo tab and deflected downward when you pull up; it's deflection would hold the control surface up. You'd push really hard to get the nose back down, too hard for sure, and the tab would flip up, shove the control surface to the bottom stop and you'd now have the control stick pinned nose down. In an extreme case such as described here this epic battle would continue till you hit the ground or ripped apart your airplane.

Stability is relayed to the pilot via control pressure. The harder you have to pull the more stable the airplane is. Think about how hard you pull at cruise speed vs slow flight; which condition is more stable?

 

[Airfoil]

This is a Commercial Oral Prep question and we havent quite gone over it in class.

I may be thinking too far into this question, but the Anti-Servo tab on our C172S is the "trim tab" on the stabilizer. (correct?) Positive static stability is the "skateboard in a pipe tube" example... oscilations will decrease until the plane returns to its original state.

How does an Anti-Servo tab provide the positive static stability?

Anti-Servo tab is to prevent over-controlling the aircraft.

On your C172 you have an Horizontal Stabilizer with an elevator.
On the PA28 the entire horizontal stabilizer moves (Stabilator) so the Anti-Servo tab moves in the same direction to create greater pressure on the controls to prevent the pilot from over controlling the aircrafts pitch.