Aircraft Control Devices

Aim

To introduce the design features used to aid in aircraft control

Objectives1. State the design features used for aircraft control2. State the design features used to aid in control

response

1. Aircraft Control DevicesControl devices

Control devices are used to enhance the control effectivenessControl devices include:• Modified aileron design• Spoilers• Slots• Leading edge strips• Speed brakes• Vortex generators• Strakes

Control devices – Modified aileron designAircraft manufacturers can alter aircraft design to minimise the effects of adverse aileron yawSome techniques employed are:• Differential ailerons• Frise ailerons• Spoilers• Aileron/rudder interconnect

1. Aircraft Control Devices

Control devices – Differential AileronsWith differential ailerons, one aileron is raised a greater distance than the other aileron is lowered for a given movement of the control columnThis produces an increase in drag on the descending wingThe 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 AileronsWith a frise-type aileron, when pressure is applied to the control wheel or stick, the aileron that is being raised pivots on an offset hingeThis 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 – SpoilersFound 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 dragSpoilers are often used for roll control, an advantage being the reduction of adverse aileron yaw

Eg, to turn left, the spoileron 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 – SpoilersSpoilers used at the same time (speedbrakes) allows the aircraft to descend without gaining speedSpoilers 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 rudderCoupled ailerons and rudder are linked controlsThis 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 deflectedEg- 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 – SlotsSlots are high lift devices applied to the leading edge of an aerofoilFixed slots direct airflow to the upper wing surface and delay airflow separation at higher AoA’sThe slot does not increase the Wing camber, but allows a highermaximum coefficient of lift because the stall is delayed until the wing reaches a greater AoAThey are usually incorporated within the wing leading edge ahead of the aileron to retain aileron effectiveness

1. Aircraft Control Devices

Control devices – leading edge stripsLeading edge strips AKA stall strips encourage early separation of airflow at high AoAThey are fitted to the inboard section of the wing to ensure it reaches the critical AoA before the wing tipThis reduces the tendency to wing drop

1. Aircraft Control Devices

Control devices – vortex generatorsVortex generators are used to delay or prevent shock wave induced boundary layer separationThey are small, low aspect ratio aerofoilsplaced at 12 to 15 degrees AoA to the airstreamThey are usually spaced a few inches apart along the wing ahead of the aileronsVortex generators create a vortex which mixes the boundary airflow with the high energy airflow just above the surface,This produces higher surface velocitiesand increases the energy of the boundary layer, thus a stronger shock wave is necessary to produce airflow separation.

1. Aircraft Control Devices

Control devices – strakesStrakes are devices which are fitted to the fuselage to improve stabilityThese 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 spinOn some aircraft, strakes also have the effect of reducing drag in the cruise by guiding airflow over the lower rear area of the fuselage

1. Aircraft Control Devices

2. Aircraft control responsesControl responses

When controls are deflected an aerodynamic force is created opposite to the direction of deflectionIt is this force which we feel through the control columnThis force is dependant on the hinge position for the control surface and how it is linked to the control columnIf 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

Control responsesDesigners incorporate certain features into aircraft design to make them easier to fly, and to give us feel of what the aircraft is doingCertain features such as will be described:• Aerodynamic balancing• Mass balancing• Balance tabs• Anti balance tabs• Servo tabs

2. Aircraft control responses

Control responses – Aerodynamic balanceAerodynamic balancing can be achieved by the following features:• Inset hinge• Horn balance

2. Aircraft control responses

Control responses – Mass balancingMass balancing is designed to prevent flutteron control surfacesFlutter is a vibration of the control surface which is often experienced at high speedsIt 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 pointTo avoid flutter we can employ “mass balancing” to align the CoGcloser to the hinge point

2. Aircraft control responses

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

2. Aircraft control responses

Control responses – anti balance tabAn anti balance tab works on the same principal as the balance tab, however, it moves in the same direction as the control surfaceThis acts to increase the stick force requiredIt is often used on aircraft witha stabilator or an all flying tailplane

2. Aircraft control responses

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

2. Aircraft control responses

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