Anti-servo Tab

montana

New Member
I'll be taking a PPL checkride in a few days, and the particular examiner I have likes to ask about anti-servo tabs and how they work. Jeppesen is a little bit thin on the description of how it works only saying that it "provides you with a control 'feel' ( on planes with a stabilator) similar to what you experience with an elevator." Any other insights into this would be helpful!
 

Eagle

New Member
The anti-servo tab moves in the same direction as the primary flight control surface. It is a stabilizing force which increases control force pressures and prevents the control surface from moving to a full-deflection position.
 

A300Capt

Freight Dawg
[ QUOTE ]
Of course, I have a =little= trouble with the concept that making an aircraft less stable decreases workload and that making it more stable increases workload.


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Think of it this way. Power steering vs a non-power steering car. The heavier forces required to move the steering wheel without power steering increases the driver's physical workload but makes the vehicle more stable without the tendency to overcontrol as with "powered" or "boosted" steering.

A friend of mine here at UPS who I fly with is an ex-Blue Angel pilot. After many discussions with him about his years with the team he told me that their aircraft are modified from the stock F18's by adding a 30lb spring to the stick which provides a constant forward pressure. This increases the pilot's physical workload (having to apply aft pressure against it for 45 minutes), but it helps to prevent PIO while in close formation due to the boosted controls, which he said would be the case without the spring. I'll take his word for it. Without the spring, the workload would be less on the pilot but the aircraft would be more unstable since the slightest movement (twitch)of the stick may provide you with an unwanted result.
 

Derg

Cap, Roci
Staff member
Boosted controls? Ha! THey're for sissies!

The MD-88 is pure control tab and no hydraulics!

Heavier control forces than a turboprop!
 

A300Capt

Freight Dawg
[ QUOTE ]
Boosted controls? Ha! THey're for sissies!

The MD-88 is pure control tab and no hydraulics!

Heavier control forces than a turboprop!

[/ QUOTE ]


Ha! When I was instructing on the B757, the ex-DC8 crews transitioning over would just about snap roll the sim on their first steep turn attempt.
They'd grab that yoke with both hands like they were flying the "Diesel 8" and forcibly crank in some aileron. The B757, and especially the B767-300, are finger tip flying airplanes that require a very light touch to fly precisely.
 

Derg

Cap, Roci
Staff member
The -88 is definitely a 'two hander' as well.

If you've ever flown a Boeing in "manual reversion", that's about as close as it gets to flying an -88!

Luckily all of the quirks of the McD here at Delta keep the seniority list pretty junior on that equipment.

Pilot: "Hey Doug, I've been thinking about bidding the 'Mad Dog'"

Me: "You're a 97 hire?"

Pilot: "Yup"

Me: "To be real honest, everything you've heard about the mad dog is true.. and more. Just like standing on a basketball when you're on approach and it's got real heavy controls. You know, the -88 right seat is the busiest seat in the entire passenger airline business!

Pilot: "Yikes, what was I thinking!"

Me: "Knowing is half the battle."
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
Think of it this way. Power steering vs a non-power steering car. The heavier forces required to move the steering wheel without power steering increases the driver's physical workload but makes the vehicle more stable without the tendency to overcontrol as with "powered" or "boosted" steering.

[/ QUOTE ] Ah. It was a semantic thing. When I think of "workload" I tend to think of the big picture (the "overall workload" of controlling an airplane in a phase of flight) rather than the specific amount of force required to perform a discrete task (the "task workload" of pulling back on the yoke once).

In both the servo and anti-servo situation, I understand the goal to be a decrease of "overall workload" and to reach some parameters of consistency with the amount of "task workload". I'm limited in my knowledge of anything other than 4-place singles, but my experience with them is that servos tend to be placed on airplanes that have too-heavy controls to bring the amount of force necessary to move the controls within "normal" parameters. The same with anti-servos and controls that are too "loose" and take too little effort to move.

In other words, servos add a degree of instability to controls that are too stable and anti-servos add a degree of stability to controls that are too unstable. In both cases, at least the intended result, is to reach a "happy medium" — an aircraft with a "proper" balance between stability and controllability.
 

seagull

Well-Known Member
The issue of control heaviness is more complex than what has been stated here, as heavy controls can force a pilot to make large control inputs than would otherwise be required and then a corrosponding larger correction. This is known as "higher gain", and is a known factor in leading to PIO, or, more correctly, aircraft-pilot coupling (APC). Either high or low control forces can lead to APC. One of the main reasons the Wright Bros were successful is that they made their craft very unstable, which, as the time, was more controllable than the too stable aircraft designed by others. Those others were not controllable, although the Wright aircraft were virtually also not controllable, they are more controllable than too stable designs.

In the case of the close formation flying, the tight tracking task of flying in that formation will push a low gain, anticipatory type pilot towards the higher gain/reactionary mode. The constant control pressure was a way of forcing the gain down during the required tight tracking maneuvers they fly.
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
The issue of control heaviness is more complex than what has been stated here,

[/ QUOTE ]That makes sense. In simplest terms, all servos and anti-servos do is, in the former case, lower the amount of effort required to move a control surface and, in the latter, increase the effort, with an affect on stability.

How they are used depends on the aircraft and the aircraft's mission.
 
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