Purdue Arrow
Topic Author
Posts: 947
Joined: Tue May 25, 1999 1:49 pm

Anti-Servo Tabs

Sat May 27, 2000 12:50 pm

I am trying to write a lesson plan on trim devices for my CFI class, and I can't find any information on anti-servo tabs, which is the type of trim device found on Piper Cherokees/Cadets/Arrows/probably others. If anyone can help me by providing a description and/or differences between anti-servo tabs and trim tabs, it would be GREATLY appreciated. Thanks in advance!
Posts: 8210
Joined: Mon May 24, 1999 1:12 am

RE: Anti-Servo Tabs

Sat May 27, 2000 7:45 pm

Sorry I can't help you (although I fly Aztecs as a safety pilot), have you tried the Tach / Ops forum? I put an interesting question like yours here on the main forum and it sank unanswered like a stone. Have you tried the forum at www.pprune.org (could be .net, don't remember)? It's the Professional Pilots Rumour Network, and they have a "wannabes" forum as well as the main one. Good luck!
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

RE: Anti-Servo Tabs

Sun May 28, 2000 3:55 am

This is a sore topic with me because I thought I knew what it was for the past year based on what one of my lecturers told me, then for another lecturer to tell me it's wrong.

An anti-servo tab is used to move a stabilator (all moving tailplane). The pilot wants to go into a nose up attitude. The tab moves down, the aerodynamic forces acting on the trailing edge of the tab exert a force on the leading edge of the stabilator and causes it to go down, the aerodynamic forces acting on the stabilator now cause the aircraft itself to pitch nose up. The opposite happens when the pilot wants a nose down attitude. (This was the first explanation).

Apparently, I'm told, when the aircraft is not moving and no aerodynamic forces are acting on the tab/stabilator arrangement then movement of the tab is not accompanied by movement of the stabilator.

Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

RE: Anti-Servo Tabs

Sun May 28, 2000 4:40 am

The 727 rudder incorporates anti-balance tabs. The 727 has an upper and lower rudder, each rudder has a forward and aft section. The aft section moves in the same direction as the forward halve. This increases the effectiveness of the rudder.
You're only as good as your last departure.
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

The Real Difference.

Sun May 28, 2000 5:15 pm

Ok guys this is the definition that I was taught in my aerodynamics class at my A&P school.

There are three types of tabs. Trim, Servo and Anti-servo.

I'll explain the difference between the servo and anti-servo tabs first. They directly deal with the control of the aircraft.

An anti-servo tab is generally used on an all moving stabilizer. It resists the movement of the control. An anti-servo tab will move in the same direction at the control surface. Say you wanted a rudder to swing to the right, the tab would also swing to the right and airflow would try and force the rudder back to center. Piper airplanes have these a lot. Because of the weak "feel" caused by the stabilizer. An anti servo tab is placed on it to artificially provide feedback to the controls...This keeps the pilot from overstressing the control surfaces.

A servo tab is was used on large control surfaces before the advent of hydralic controls. They will move opposite of the control surface. If you look at a rudder from the rear and you wanted it to swing to the right. You would move the servo tab to the left and the airflow would make the servo tab force the rudder to the right. The controls on the Hindenberg where moved with servo tabs. The controls in the comand car where connected to the tabs not the controls themselves.

A trim tab is used to adjust the angle a surface is sitting at inflight. For example...If I set the CG just a little far aft I can feel it in the controls at takeoff because I will need to put a larger nose up input into the controls. Instead of manhandling the controls I can adjust the trim tab so that it will force the surface into that position. The tabs will move just like servo tabs, that is they will move opposite of the desired control movement.
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

RE: Anti-Servo Tabs

Sun May 28, 2000 6:54 pm

L-188 you want to get technical, why not go into ALL of em. I was told there was seven tabs in all.

Fixed tab
Balance tab
Anti-balance tab
Mass-balance tab
Servo tab
Anti-servo tab
Spring tab

Though there is a lot of confusion and some people say that the mass-balance tab cannot really be called a tab; and also the servo/anti-servo, balance/anti-balance tabs have some overlap and can't be really separated.

Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

RE: Anti-Servo Tabs

Sun May 28, 2000 10:13 pm

Points well taken.....Especially about the point about the functions of tabs overlaping in some cases. I should haves stated that but didn't think of it...it was three in the morning here.

I would also remove the fixed tab off the list too since they are just a piece of metal that is rivited on the control surface and bent to achive the desired effects. I just say that because it isn't a surface that can be adjusted from the cockpit. The fixed tab on the tail of most single engine Cessnas comes right to mind...It is there to counter the rolling tendency of the engine and the propeller. When they are adjusted it is usually to counter some undersiable charicteristic that probably could be fixed correctly another way......A wing may be slightly out of square comes right to mind....this would produce a rolling motion and instead of rerigging it the cheapskate pilot...rather then paying a mechanic to do the job. will get out his leatherman and reef on the tab to get it to force the rudder over and count the rolling tendency of the aircraft.

Needless to say I don't think this is the way that correcting that little problem should be done.

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos