Aircraft Control Devices

Aim To introduce the design features used to aid in aircraft control

Objectives State the design features used for aircraft control State the design features used to aid in control response

1. Aircraft Control Devices Control devices are used to enhance the control effectiveness Control devices include: Modified aileron design Spoilers Slots Leading edge strips Speed brakes Vortex generators Strakes

1. Aircraft Control Devices Control devices – Modified aileron design Aircraft manufacturers can alter aircraft design to minimise the effects of adverse aileron yaw Some techniques employed are: Differential ailerons Frise ailerons Spoilers Aileron/rudder interconnect

1. Aircraft Control Devices Control devices – Differential Ailerons With differential ailerons, one aileron is raised a greater distance than the other aileron is lowered for a given movement of the control column This produces an increase in drag on the descending wing The greater drag results from deflecting the up aileron on the descending wing to a greater angle than the down going aileron on the rising wing

1. Aircraft Control Devices Control devices – Frise Ailerons With a frise-type aileron, when pressure is applied to the control wheel or stick, the aileron that is being raised pivots on an offset hinge This projects the leading edge of the aileron into the airflow and creates drag It helps equalise the drag created by the lowered aileron on the opposite wing and reduces adverse aileron yaw

1. Aircraft Control Devices Control devices – Spoilers Found on many gliders and some aircraft, high drag devices called spoilers are deployed from the wings to spoil the smooth airflow, reducing lift and increasing drag Spoilers are often used for roll control, an advantage being the reduction of adverse aileron yaw Eg, to turn left, the spoiler on the left wing is raised, destroying some of the lift and creating more drag, the left wing drops and the aircraft banks and yaws to the left

1. Aircraft Control Devices Control devices – Spoilers Spoilers used at the same time (speedbrakes) allows the aircraft to descend without gaining speed Spoilers are also used to reduce the ground roll after landing, by destroying lift and transferring weight to the wheels, therefore improving braking effectiveness

1. Aircraft Control Devices Control devices – Coupled aileron and rudder Coupled ailerons and rudder are linked controls This is accomplished with rudder-aileron interconnect springs, which help correct for aileron drag by automatically deflecting the rudder at the same time the ailerons are deflected Eg- when the control wheel is moved to produce a left roll, the interconnect cable and spring pulls forward on the left rudder just enough to prevent the nose from yawing to the right

1. Aircraft Control Devices Control devices – Slots Slots are high lift devices applied to the leading edge of an aerofoil Fixed slots direct airflow to the upper wing surface and delay airflow separation at higher AoA’s The slot does not increase the Wing camber, but allows a higher maximum coefficient of lift because the stall is delayed until the wing reaches a greater AoA They are usually incorporated within the wing leading edge ahead of the aileron to retain aileron effectiveness

1. Aircraft Control Devices Control devices – leading edge strips Leading edge strips AKA stall strips encourage early separation of airflow at high AoA They are fitted to the inboard section of the wing to ensure it reaches the critical AoA before the wing tip This reduces the tendency to wing drop

1. Aircraft Control Devices Control devices – vortex generators Vortex generators are used to delay or prevent shock wave induced boundary layer separation They are small, low aspect ratio aerofoils placed at 12 to 15 degrees AoA to the airstream They are usually spaced a few inches apart along the wing ahead of the ailerons Vortex generators create a vortex which mixes the boundary airflow with the high energy airflow just above the surface, This produces higher surface velocities and increases the energy of the boundary layer, thus a stronger shock wave is necessary to produce airflow separation.

1. Aircraft Control Devices Control devices – strakes Strakes are devices which are fitted to the fuselage to improve stability These devices called strakes are quite common in training aircraft under the fuselage which increase directional stability at high angles of attack and reduce the tendency to diverge or spin On some aircraft, strakes also have the effect of reducing drag in the cruise by guiding airflow over the lower rear area of the fuselage

2. Aircraft control responses When controls are deflected an aerodynamic force is created opposite to the direction of deflection It is this force which we feel through the control column This force is dependant on the hinge position for the control surface and how it is linked to the control column If the hinge is at the leading edge of the control surface it will be more difficult to move than if it is hinged in the centre

2. Aircraft control responses Designers incorporate certain features into aircraft design to make them easier to fly, and to give us feel of what the aircraft is doing Certain features such as will be described: Aerodynamic balancing Mass balancing Balance tabs Anti balance tabs Servo tabs

2. Aircraft control responses Control responses – Aerodynamic balance Aerodynamic balancing can be achieved by the following features: Inset hinge Horn balance

2. Aircraft control responses Control responses – Mass balancing Mass balancing is designed to prevent flutter on control surfaces Flutter is a vibration of the control surface which is often experienced at high speeds It is caused by pressure disturbances over the control surface and can be particularly bad if the control surfaces CoG is not in line with the hinge point To avoid flutter we can employ “mass balancing” to align the CoG closer to the hinge point

2. Aircraft control responses Control responses – balance tab The control forces in some aircraft can be excessively high, and in order to decrease them, the manufacturer may use balance tabs They look like trim tabs and are hinged in approximately the same place The essential difference between the two is that the balancing tab is coupled to the control surface rod so that when the primary control surface is moved in any direction, the tab automatically moves in the opposite direction The airflow striking the tab counterbalances some of the air pressure against the primary control surface, and enables the pilot to move more easily

2. Aircraft control responses Control responses – anti balance tab An anti balance tab works on the same principal as the balance tab, however, it moves in the same direction as the control surface This acts to increase the stick force required It is often used on aircraft with a stabilator or an all flying tailplane

2. Aircraft control responses Control responses – antiservo tab A servo tab is a variation in the balance tab where the pilot control is connected to the tab and not the main control surface As the control input moves the servo tab into the airflow, the aerodynamic forces generated drive the main control surface in the opposite direction

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