Flight control surfaces must be balanced to achieve

desired output.

Two types

◦ Static Balancing

◦ Aerodynamic Balancing

A control surface which is statically balanced,

reduces possible flutter.

A control surface that is aerodynamically

balanced to reduce the force necessary to

displace it by providing some area ahead of

the hinge line.

A condition of balance of an aircraft control

surface in which the surface balances about

its hinge line. Lead weights are often installed

in parts of the surface ahead of the hinge line

to balance the surface. Controls are statically

balanced to prevent control-surface flutter.

Has to be done before painting as the

distribution of paint over the control surface

can seriously change it.

Over-balance

Should the trailing edge of the control surface rise some position above a horizontal plane, an overbalance condition is apparent. Typical overbalance condition essential for good results in high-performance aircraft.

Under-balance

Chord angles down at trailing edge. If the control surface assumes a trailing-edge-low attitude, it is statically underbalanced and a tail-heavy condition exists. The static under balance, or tail heavy condition first described, is the least desirable as it may result in unpredictable flight performance.

Neutral balance

If the control surface remains in a level

(horizontal) attitude, it is said to be statically

100% balanced and its center of gravity (cg) is

co-located with the hinge axis. 100% balanced

to a level attitude should consistently give

good results.

Aerodynamic balance involves using the

aerodynamic forces on the control surface to

reduce the hinge moment / stick force.

The aerodynamic force on the controls will

depend on the area of the control surface, its

angular displacement and the IAS.

There are many ways to aerodynamically

balance the control surfaces.

Inset hinge

Horn Balance

Internal Balance

Trim and Balance Tabs

As the name implies the hinge is set inside.

If distance (d) is reduced the hinge moment

will be reduced. Smaller the hinge moment,

smaller the stick force.

Aerodynamic force should never be forward

of the hinge. If it does then it will be an

"overbalance" condition in which at first there

is a reduction then reversal of force.

Part of the surface is forward of the hinge

line, and forces on this part of the surface

give hinge moments which are in the

opposite direction to the moments on the

main part of the surface. The overall moment

is therefore reduced.

Aerodynamic balance area is inside the wing.

Movement of the control causes pressure

changes on the aerofoil, and these pressure

changes are felt on the balance area.

The pressure difference on the balance therefore

gives a hinge moment which is the opposite to

the hinge moment on the main control surface,

and the overall hinge moment is reduced.

Balance and trim tabs move in the opposite

direction to the main control.

The force generated on the tab then works to

assist the movement of the main control.

The purpose of the trim tab is to adjust the main

control position in flight so that there are no

feedback forces (i.e. trim the aircraft so that it

can fly hands off).

Balanced tab is similar to trim tab but it is coupled to the control surface.

It moves in the opposite direction to the control surface.

This provides a counterbalance and makes it easy for the pilot to move and hold the control surface position.

They work in the opposite way to a balance tab. They deploy in the same direction as the control surface, making the movement of the control surface more difficult and requires more force applied to the controls by the pilot.

It is commonly used on aircraft where the controls are too light or the aircraft requires additional stability in that axis of movement.

The B elevator with D control tab

(moved by the control column) and

C geared tab(mechanically linked to

the A horizontal stabilizer). As the

elevator moves, the geared tabs

move in the same direction as the

control tabs to provide additional

aerodynamic control surface.

The control column is connected to the tab.

Pilot control input deflects the servo tab only,

the aerodynamic force on the tab then moves

the control surface.

The disadvantage of the servo tab is reduced

control effectiveness at low IAS.

The flight control systems need to be rigged from time to time so that they carry out their function correctly.

Flying control system rigging is carried out: 1. After manufacture 2. When stated in the aircraft maintenance

schedule 3. When a component in the system is

changed 4. After a reported flying fault from the pilot 5. Sometimes after a heavy landing or flight

through turbulent air

Rigging procedures vary depending on whether the controls are manually operated or power operated. It will also vary depending on whether the controls are operated by a cable system, push-pull rods or fly-by-wire.

The AMM Chapter 27 gives the proper procedure for rigging the particular control system. It may be necessary to refer to Chapter 20 Standard Practices.

The general procedure for both manually operated and power-operated flying control systems is: 1. Refer to the AMM

2. Set the control system to neutral

3. Check cable tensions

4. Do a sense of movement check

5. Do a freedom of movement or static friction check

6. Do a range of movement check

7. Loose Article Check

8. Duplicate Inspection

Jacked and levelled the aircraft and placed in the rigging position.

Stable aircraft temperature, ±3C along the cable for at least one hour.

Tripped off appropriate electrical circuits and place warning notices on the control surfaces and circuit breakers.

Release hydraulic pressure from the hydraulic system accumulators.

Before disconnecting drive shafts, mark the angular position of the joints to ensure correct alignment on reconnection

Place rigging pins or rigging fixtures on the pilot's controls at the control surface end of the system and intermediate cable-quadrants or linkages in the system.

Ensure that the whole control system is in neutral, including: 1. PFCUs and artificial feel units 2. Cockpit indicators 3. Trimming systems 4. Tabs and trim tab position indicators

EXAMPLE OF A CONTROL COLUMN RIGGING FIXTURE

When new control cables are fitted, tighten to a higher than normal cable tension, then operated through their full travel for a given number of times.

When rigging is complete, make sure that each rigging pin can be removed and replaced easily. This shows that cables are correctly adjusted for length and correctly tensioned.

Remove all rigging pins and fixtures. Recheck cable tensions and control neutral settings.

The cockpit controls are moved and a check made to make sure that the control surfaces move in the correct sense. The control surface position indicators in the cockpit must also move in the correct sense.

On most large aircraft the spoilers move asymmetrically when the ailerons are moved. Correct sense of movement under autopilot command must also be checked.

Pull the control system through its full range of movement using a spring balance attached to the control column and rudder pedals. The force required to operate the controls should not exceed that given in the Table.

Maximum Weight of Aeroplane kg (lb)

Maximum Static Force on Control

N (lbf)

5700 kg

(12 500 lb) or less

Elevator

17.79 N

(4 lbf)

Aileron

8.89 N

(2 lbf)

Rudder

26.68 N

(6 lbf)

22 680 kg

(50 000 lb) or more

44.48 N

(10 lbf)

35.59 N

(8 lbf)

44.48 N

(10 lbf)

Linear variation should be assumed between these weights

Move the pilot's controls in both directions from the neutral position and measure the range of travel using a ruler.

If incorrect, adjust the control surface stops (primary stops). The cockpit control stops (secondary stops) are then adjusted to a specified clearance.

Check the controls in power and manual.

Ensure that cockpit indicators give correct indications

Lock all the system points where previous adjustments have been made.

Ensure that the trim tabs and other tabs operate correctly, in a similar manner to the main control surfaces. Make sure that trim tab position indicators function properly.

Make sure that the control locks engage and disengage properly. Make sure that the associated warning devices are operating correctly.

This should be done after every job has been completed on an aircraft.

Inspect the area to make sure that no tools, spares, locking wire, cleaning cloths etc. have been left behind.

Do a tool check outside the aircraft.

All the parts of a flying control system are generally classed as VITAL POINTS and if disturbed will require a duplicate inspection.

A vital point is any point on an aircraft at which a single incorrect assembly could cause loss of the aircraft and/or fatalities.

A duplicate inspection of a vital point/control system is an inspection which is first made and certified by one qualified person and then made and certified by a second qualified person.

The inspection must be carried out systematically to ensure that each and every part of the system is correctly assembled and is able to operate freely over the specified range of movement without risk of fouling.

Also that it is correctly and adequately locked, clean and correctly lubricated and is working in the correct sense in relation to the movement of the control by the crew.

Aviation Dictionary

http://history.nasa.gov/monograph12/ch6.htm

http://www.theairlinepilots.com/forum/viewtopic.php?p=642&sid=474eb5932661e518b619280685fd5ab9

http://www.pilotfriend.com/training/flight_training/fxd_wing/flutter.htm

http://www.atpforum.eu/showthread.php?t=5869

http://www.hilmerby.com/dc9/flap.html

Air Service Training (Engineering) Ltd, Aeronautical Engineering Training Notes