Aerodynamics 2.3 p32-80: Mechanical Flight Control Systems

Mechanical Flight Control Systems

• These notes cover mechanical flight control systems' fundamentals and inspection.

Cable Tension Regulators

• Steel cables connect to an aluminum alloy airframe.
• Temperature differences cause unequal expansion of the steel and aluminum and should be accounted for
• Excessive cable tension or slack can damage pulleys and cables so should be checked and maintained
• Automatic cable tension regulators mitigate issues from expansion differences.

Cable Quadrant with Tensioner

• Tensioners may be used in Cable Quadrants.

Torque Tubes

• Torque tubes actuate flight controls, gear extension, and retraction mechanisms.
• Direct method provides minimal backlash.
• Suited for installations without excessive air loads.
• Used to change linear to rotary action and vice versa.
• Found as flap levers on early Piper aircraft and landing gear retracts on older Mooney aircraft.

Push/Pull Rods

• Control rods transmit control forces in push and pull directions.
• Aluminum alloy tubes contain threaded inserts at both ends.
• External threaded end fittings connect to the tubes
• Length can be adjusted by turning the rod.
• Commonly found in both airplanes and helicopters.
• Typically made of seamless, heat-treated aluminum alloy tubing or solid steel rod.

Push-Pull Rod System Maintenance

• Check rods and tubes for deformation like stretching, bulging, or bending.
• Feel along tubes to identify hard-to-see discrepancies.
• Closely inspect rod ends for cracks.
• Reject rods with knicks and gouges.
• Inspect internally and externally for corrosion.
• Ensure check nuts prevent rod ends from rotating.
• When reinstalling, measure to maintain the original tube length. Count turns when replacing a rod end, or complete an overall dimension.
• Proper thread engagement is verified by attempting to pass lockwire through the "Witness Hole" after adjustments.
• Never use aluminum rivets in place of steel types.

Fairleads and Rub-Strips

• Fairleads and rub-strips are constructed from Phenolic, Phormica, or plastic
• These are used where cables may contact the structure, hence preventing wear.
• They guide cables, thus preventing cable vibration
• They must not alter cable direction.
• Check for wear, breakage, alignment, and cleanliness and confirm security
• Rub-strips should also be secured to ensure longevity

Pressure Seals

• Seals appear where cables pass through a pressure vessel.
• Seals are filled with grease to improve longevity and lubrication.
• Seals prevent cabin pressurization loss.
• Conduct regular inspections for material wear and/or deterioration.

Control Chains

• Chains are for short distances where control velocity is not high.
• Manual trim drives and interconnection of control wheels in light aircraft provide examples.
• Aircraft chains usually use a simple roller type.
• Roller chains include outer and inner plates, rollers, bearing pins, and bushings.
• Three Key dimensions when measuring: pitch, width between inner plates and roller diameter.

Aileron Control Wheel Assembly

• Aileron control systems feature combined elements.
• The control wheel turns a sprocket, driving a chain.
• The chain links to a cable, changing direction around pulleys.
• Cables route from the fuselage into the wings, connecting to bell cranks.
• Bellcranks (Quadrants) convert cable movement into linear movement of push/pull rods attached to the aileron

Control System Stops.

• Control stops limit overall flight control surface travel or deflection.
• Primary stops are located near the control surface, limiting travel at the output end of the system.
• Secondary stops are near the control column or input, hence limiting the travel of the input device.

Aileron Control - General Aviation

• Ailerons install at the outboard trailing edge of each wing of which there should be two present.
• Manual operation via dual control wheels.
• Linkage includes pulleys, cables, quadrants, push-pull rods, levers, and bell cranks.
• The three-cable system features two direct cables and one "Carry-thru" (or balance) cable.
• Cables drive bell cranks, which then drive a push-pull tube connected to the aileron, causing deflection.

Aileron Control – DHC 8-100

• A single aileron installs along each wing's outboard trailing edge, positioned using servo tabs.
• Dual Control wheels allow manual operation, with independent cable systems that go to a quadrant set to convert to linear movement of push-pull tubes
• Cable system automatically tensions with disconnects between pilot and co-pilot cable systems.

Aileron Control – DHC 8

• Control quadrants, torque tubes, and push-pull tubes extend into the wings.
• Linear movement transfers through the wing, altering direction at two bell cranks.
• Paired push-pull rods connect to the servo tab from the bell cranks to move the freely pivoting aileron.

Rudder Control System

• A Rudder system controls aircraft yaw axis.
• Linkage consists of pulleys, cables, quadrants, push-pull rods, levers, bell cranks, and springs connects pedal assemblies to a rudder.

Elevator Control System

• Cables connect to bell cranks to the control in the cockpit in manually operated cable systems and is connected to the control surface.
• Movement of cockpit controls transfers force through the cable to a bell crank, moving the control surface.
• Push-pull tube systems use metal push-pull tubes (or rods) in place of cables named after direction of force
• In turbulence, a downspring system maintains a nose-down attitude.

Small Aircraft Flap Control System

• Electric drive motor, screw-jack assembly, cables, push-pull tubes, and tracks with rollers drive flaps up and down.
• This is controlled by a selector switch on instrument panel, and series of micro switches provides motor operation position information.
• If the control shut off switches become unrigged, audible cues are generated
• Once at full travel, the motor should automatically power off to prevent damage

Large Aircraft Flap Control System

• Electric and hydraulic drive motors, gearboxes, and torque tubes with ball-screw assemblies inside fairings drive flaps up and down tracks.
• Micro switch series and the instrument panel selector switch controlls the position information controlling motor operation

Trim Control System – Cessna Type

• Various trim systems controlled through trim wheel movement.
• Cable winds a barrel or spool to transfer linear movement to the rim actuator device.
• The cable is wound around another barrel on a screw jack or have a chain
• A safety measure is to check the proper rigging of cables to minimize flight risk

Stabilizer Trim Mechanism

• Stabilizer trim changes the angle of incidence of the horizontal stabilizer
• This unloads control input requirements to maintain a pitch attitude without pilot force on the wheel.
• Manually, electrically, or hydraulically operated
• The stabilizer is hinged at its trailing attach point with some type of actuation device

Flight Control System Rigging Basics

• Follow most current publications.
• Ensure measuring equipment is correctly calibrated and is up to date.
• Use correct techniques and methods for safety.
• Installers should ensure all hardware locking devices work; replace if needed.
• Maintenance manuals explain how to use rigging tools and parts-locking pins.
• Rigging requires parts of parts of the control systems to be locked into a specific location

Control System Sections

• Three parts: an input (handling), transmission system, and control surface.
• Rigging commonly starts with the input (or handling) device, and concludes at the control surface

Generic Rigging Process

• Follow outlined maintenance tool guidelines to do the procedure
• Neutralize and synchronize input devices
• Assemble and set up transmitting devices, positioning and tensioning all cables correctly. Position the bell cranks and actuation devices into neutral position.
• Fine tune the final actuation device to fit the control surface into the neutral location and ensure correct length
• You can adjust control surface travel by adjusting the primary control

Piper PA-28 Aileron Control Input and Transmitting Systems

• Aileron control systems involve the use of multiple key components:
• Control Wheel: Initiates pilot input which then directs the entire system
• Chain Turnbuckle: Serves to manage the turnbuckle
• Control Column: Transfers input from the control wheel to rest of the system
• Chain Sprockets: Cogs around which the control chains move to allow smoother action
• Aileron Control Chain/Cable connection: Moves ailerons according to input
• Right Aileron Primary/Direct Cable: Directly controls aileron movement.
• Right Aileron Balance/Through Cable: Maintains system balance.

Piper Pa-28 Aileron Control Transmitting System

• The bell crank pivots and adjustable primary stops in come into contact with it to limit the travel of ailerons.
• The assembly which uses a bell crank then changes the direction of linear movement with the cable
• Transmits the movement by bell crank to aileron in a push pull configuration
• The cable or direct fork clevis will carry through and direct an aileron .

Cessna 172 Aileron Control System Rigging

• Check that the primary cable (7) is in groove in drum and wound around drum.
• Ensure that direct cable look (14) is setup at the top.
• For best balance measure it from the center to control the sprockets from with in.

Cessna Locking Tools

• Various locking tools can be used to lock the rig in place

Cessna 172 Aileron Control Transmitting & Surface Rigging

• You can adjust the direct from carry - thru turnbuckles located at Bell Crank which is best for setting carry cable and ailerons
• In the final stage set the push pull rods into the right aileron by moving neutral on the edge of wings and adjust them up accordingly.
• You will also have to do a bar control check.

Cessna 172 Aileron Control System Rigging Final Checks

• If making adjustments to the Cessna it is important to ensure that the bush travel is set properly that way you can conduct checks via wheel.
• If all goes well, after you have completed this test you will have to carry out final safety inspections and remove items you used during the check
• After completion of everything do a functionality test to ensure the Aileron moves correctly in both directions.

AS-70-3 Control Board Stop Installation

• Ensure that the AS-70-3's control stop board has been correctly set up.

AS-70-3 Control Board Locking Tools

• Various tools and lock components will be used to keep the board and system in tact

Rigging Bell 47 Tail Rotor Control System

• Using a generic process you can get tail rotor adjustment with either Cena or piler.
• Set the foot petals to center for adjustments which will connect from the neutral set up position (jackshaft pulley)
• Adjust pitch after drum has been wrapped around (tension cable joint)
• For function check remove petal lock to ensure travels.

Rigging Bell 47 Tail Rotor Rigging Jig

• A jig must be in place for the completion of this component to ensure equal force distribution

Certification of Control System Rigging

• To verify repairs the pilot will need an certification after the engine work has been completed (flight controls check in to see the 2 people )
• Always check maintenance records before completing so you can confirm before a single test release.

Dual or Independent Inspection

• Independent dual inspection after each engine to confirm full motion

Scope of Dual Inspection

• Each part has to be checked for correct tension for ease of motion and best ability.

Aircraft Symmetry or Alignment Inspection

• Due to high levels of force there is room for the Air Craft to shift along with wings depending on force with these in mind follow
• Check that the aircraft has no heavy landing

Aircraft Symmetry or Alignment Inspection

• Some general inspections before hand include an assessment of the horizontal structure.
• For better results always use a "washing test" to accurately measure.