Rudder[PDF]

Introduction

The rudder is a primary flight control surface which controls rotation about the vertical axis of an aircraft. This motion is called yaw. Like the other primary control surfaces, the rudder is a movable surface hinged to a fixed surface in this case, to the vertical stabilizer or fin. The rudder is controlled by the left and right rudder pedals.

Unlike a boat, the rudder is not used to steer the aircraft. Rather, it is used to overcome adverse yaw induced by turning or, in the case of a multi-engine aircraft, by engine failure and also allows the aircraft to be intentionally slipped when required.

Function

The rudder works by changing the effective shape of the airfoil of the vertical stabilizer. Changing the angle of deflection at the rear of an airfoil will change the amount of lift generated by the foil. With increased deflection, the lift will increase in the opposite direction. The rudder and vertical stabilizer are mounted so that they will produce forces from side to side, not up and down. The side force is applied through the center of pressure of the vertical stabilizer which is some distance from the aircraft center of gravity. This creates a torque on the aircraft and the aircraft rotates about its center of gravity.

Rudder.jpg

Effectiveness

The effective functionality of the rudder is directly proportional to its immediate airflow. At slow speed, large rudder input may be required to achieve the desired results and respectively smaller rudder movement is required at higher speeds.

In many more sophisticated aircraft, rudder travel is automatically limited when the aircraft is flown above VA (manoeuvring speed) to prevent deflection angles that could potentially result in structural damage to the aircraft.

Yaw Damper

Yaw damper is a device used on many aircraft to reduce the rolling and yawing oscillations known as the Dutch roll. It also makes flying more easier for pilots.

Dutch roll

Dutch roll is a type of aircraft motion related to a series of out-of-phase turns, when the aircraft rolls in one direction and yaws in the other. A Dutch roll is usually dynamically stable but it is an objectionable characteristic in an airplane because of its oscillatory nature.

In most modern aircraft, except high-speed swept wing designs, these free directional oscillations usually die out automatically in very few cycles unless the air continues to be gusty or turbulent.

Principle of work

The yaw damper is a servo that moves the rudder in response to inputs from a gyroscope or accelerometer that detects yaw rate. A series of accelerometers or rate sensors (gyros) in the tail constantly communicate yaw trends with the rudder servo system to provide adequate damping information. The rudder is smoothly adjusted in either direction to maintain smooth, coordinated flight.

When active, the yaw damper allows the pilot to get his feet off the pedals while flying. Even when in turn, the yaw damper controls yawing while the slid/skid ball remains centered.
In most airplanes the yaw damper is prohibited from being engaged during takeoff or landing. It could be really hard to counteract it during crosswind corrections. Check the operational handbook of your airplane to become familiar with the appropriate procedure.

In most cases the yaw damper system is separated from the autopilot and can be engaged while on manual flying.

Ydmode.jpg

Rudder for crosswind

Rudder becomes a significant helper during takeoff and landing, especially in a crosswind. Ailerons and rudder are two inputs allowing to properly align an airplane with the runway.

  • Rudder is used during the takeoff roll. If the winds are strong enough, the pilot keeps the airplane pointed down the runway by adding enough rudder. It is important to remember that rudder effectiveness increases with speed. At low speed the airplane is held on a centerline with the nose wheel steering (the same pedals, right!).
  • In both major landing technics (crab method and wing-low method) rudder is involved. When flying the crab method, rudder is used to align the airplane with the runway centerline just before a touchdown (mostly applied to light piston aircraft). With the wind-low method, rudder is applied on final to align the nose with the runway, and ailerons are used to prevent left/right drift.


Multi engine flying

When you fly twin engine aircraft and one engine unexpectedly fails, rudder becomes especially important. Two things happen when an engine fails:

  1. Asymmetrical thrust will cause a yawing motion around the center of gravity toward the inoperative engine
  2. The yawing moment from the asymmetric thrust will cause the operating engine to move faster through the air as the aircraft yaws. This causes faster velocity air to flow over the operating engines wing causing more lift on that wing, thus roll towards the inoperative engine.
Oeirudder.png
To counteract this roll and yaw, rudder pressure must be applied to the side of the operational engine to oppose these forces.
During multi engine training pilots are taught to use the mnemonic rule about engine failure: dead foot, dead engine. For example, when the right engine fails, the power from the left engine will turn the aircraft to the right, requiring left rudder to keep the airplane straight. So, the rule simply requires pilots to take their foot, corresponding with the side of the failed engine, off the rudder pedal. It helps twice: to remember which engine failed and to prevent using the wrong pedal.

VMCA/VMCG

Rudder is applied to counteract yaw and roll from an inoperative engine in a twin aircraft. As airspeed decreases the rudder becomes less effective, eventually an airspeed will be reached where full rudder deflection is required to maintain directional control. At this point, any further airspeed reduction will result in a loss of directional control. While in the air, this speed is called VMCA (minimum control speed(air)). While on the ground, it is VMCG (minimum control speed (ground)).

VMCA is the calibrated speed at which the rudder no longer has the authority to overcome the yaw and roll caused by the engine being inoperative.

Flying at VMCA guarantees 2 things:

  1. Maintaining control of the airplane with the engine still inoperative
  2. Maintaining straight flight at the same speed with an angle of bank not more than 5 degrees (depends on the aviation authority that certified the aircraft).
Generally speaking, the main objective for pilots who lost one engine on a twin aircraft is to fly on speeds above VMCA, recognize and recover from VMCA to stay in safe condition.

To maintain directional control on the ground with an inoperative engine, the rudder must be deflected to counteract the adverse yaw. The force that can be generated by the rudder is dependent upon the size of the rudder, the extent that the rudder can be deflected and the speed of the airflow across the rudder surface. This speed is called VMCG.

VMCG must always be less than V1. So, reduction in VMCG creates potential for a reduction in V1 as well. It usually happens during takeoff with derated thrust.

See also

Reference

  • Pilot’s Handbook of Aeronautical Knowledge by FAA - Chapter 6 Flight controls
  • SKYbrary library - Rudder
  • boldmethod.com
  • THEBACKSEATPILOT.COM - Critical engine

Author

  • 531824 - Creation

DATE OF SUBMISSION

  • 08:12, 6 May 2020

COPYRIGHT

  • This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.

DISCLAIMER

  • The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.