Transcript PowerPoint
Control on the ground ATC Chapter 2 & 3 Aim To review principals of aerodynamic forces during taxi, take-off and landing Objectives 1. State the differences between tail wheel and nosewheel aircraft 2. Describe slipstream and state how it affects aircraft performance 3. Explain torque effect and how it affects aircraft performance 4. Describe gyroscopic affects which are caused by the propeller on take-off 5. Explain asymmetric blade effect 6. State how ground effect affects the aircraft 1. Nose wheel vs. tail wheel Introduction All propeller driven aircraft have directional problems due to an imbalance of forces caused by the propeller We need to consider two types of aeroplanes when investigating aerodynamic effects on the ground, these are: • Nose wheel aeroplanes • Tail wheel aeroplanes 1. Nose wheel vs. tail wheel Nose wheel vs. tail wheel If an aircraft is to remain stable on the ground the centre of gravity must remain within an area bound by the wheels The further the centre of gravity is from any of these boundaries the more stable it is In both tail wheel and tricycle configurations the large surface area behind the centre of gravity will try to weathercock the aircraft In a tricycle aircraft the CoG is ahead of the main wheels, this design is inherently stable, should a gust of wind try to weather cock the aircraft combination of the main and nose wheel will prevent it doing so 1. Nose wheel vs. tail wheel Nose wheel vs. tail wheel A tail wheel aircraft has the CoG behind the main wheels, this design is inherently unstable If a gust of wind try's to weathercock the aircraft the centrifugal reaction force acting through the CoG acts in the same direction as the gust tightening the turn If control is lost the aircraft will ground loop 2. Slipstream Slipstream Effect The air that is accelerated rearward by the propeller is called slipstream A clockwise spinning propeller will create a clockwise rotation of the slipstream This creates an asymmetric flow over the fin and the rudder The slipstream strikes the left hand side of the fin creating an angle of attack This created angle of attack creates a force on the fin, pushing it right Creating a yawing moment to the left 2. Slipstream Slipstream Effect The magnitude of the slipstream is directly proportional to the magnitude of the thrust being produced • If thrust is slipstream is • If thrust is slipstream is Remember thrust velocity change, as velocity increases thrust decreases Slipstream will be highest at low airspeed, high power settings such as during takeoff For a pilot to counter the aerodynamic force of slipstream • Right rudder must be applied • If the aircraft is equipped, rudder trim can be used to assist 2. Slipstream Design Features There are a number of design features used to minimise the effect of slipstream, the first we will discuss is an offset fin A fin that is not offset has the same amount of free stream air passing each side When the fin is offset, an angle of attack is created between the fin and the relative airflow on the right side of the fin (clockwise spinning propeller) The angle of attack creates an aerodynamic force which opposes the effect of slipstream 2. Slipstream Design Features The second design feature is an offset thrust line To offset the thrust line produced by the propeller the engine must be mounted at an angle to the right By offsetting the thrust line the nose of the aeroplane is pulled to the right, creating a yawing moment to the right The yawing moment created opposes the yawing moment produced by slipstream Offsetting the fin and thrust line is done by the manufacturers during the building and testing phases of an aeroplane, the pilot has no control over these forces 3. Torque Effect Torque Reaction Remember Newtons third law of motion: “For every action there is an equal and opposite reaction” The rotation created by the torque reaction attempts to roll the aeroplane in the opposite direction When the aeroplane is on the ground, the landing gear stops the aircraft from rolling The rolling moment to the left creates an increased force on the left main landing gear Yaw This force increases the amount of friction experienced on the left main landing gear compared to the right This differential in friction force creates a yawing moment on the aircraft to Friction the left 4. Gyroscopic Effect Gyroscopic Effect The gyroscopic effect is most prominent on a tail wheel aircraft, when the tail is lifted off the ground As the tail is lifted, the propeller plane of rotation (PPOR) is changed The force to change the PPOR is applied to the top of the propeller, the resultant force due to procession will be experienced on the right side of the Yaw propeller (clockwise spinning propeller) The force on the right side of the propeller creates a yawing moment to the left The magnitude of the gyroscopic effect is directly proportional the PPOR change force, the greater the rate of PPOR change the greater the gyroscopic effect 5. Asymmetric Blade Effect Asymmetric Blade Effect When a propeller moves through the air, with the propeller at right angles to the relative airflow: • The up and down going blades travel the same distance in the same time • The angle of attack on both the up and down going blades are the same This creates an even amount of thrust being created by both the up and down going blades If the propellers plane of rotation is inclined the down going blade must travel a greater distance than the up going blade through one half revolution This causes more thrust to be produced by the down going blade Additionally the incline angle creates a Up going blade greater angle of attack on the down going blade in comparison to the up going blade further increasing the amount of thrust produced Down going blade 5. Asymmetric Blade Effect Asymmetric Blade Effect The propeller is now generating more thrust on the down going blade side, compared with the up going blade side When looking at a clockwise spinning propeller, more thrust is being produced on the right hand side Yaw of the propeller This imbalance of thrust creates a yawing moment, with a clockwise spinning propeller the moment is to the left Down going blade Up going blade To counter the asymmetric blade effect’s yawing moment to the left the pilot must apply right rudder Asymmetric blade effect is experienced during the take off roll on tail when aeroplanes as the propeller is not at right angles to the relative airflow, once the tail is lifted and the propeller becomes perpendicular to the relative airflow, asymmetric blade effect ceases 5. Ground Effect Ground Effect To explain ground effect we must first start by talking about induced drag Induced drag is created from the pressure differential between the upper and lower surface of the wing and creates wing tip vortices In flight these vortices travel from the bottom surface of the wing to the top 5. Ground Effect Ground Effect If the wing is close to the ground, approximately half of the wing span, the wing tip vortices are deflected This deflection reduces the amount of induced drag Total reaction is the vector addition of lift and induced drag Total reaction Lift + Induced Drag During take off as the aircraft leaves the ground it experiences ground effect allowing the aircraft to be accelerated to the correct climb speed However if the aircraft is heavy, rotated to a steep climb, flap retracted too early or is rotated at a low airspeed once out of ground effect the aircraft will descend and settle back on the ground 5. Ground Effect Ground Effect During landing ground effect is most significant and unavoidable Ground effect is the main reason why the aeroplane floats for an extended distance during the flare, any excess speed during landing will aggravate the float To avoid any negative effects of ground effect the pilot must: For take off: • Rotate at the recommended speed • Do not retract flaps prematurely • On hot days at high elevation, lift off at a higher then normal speed and do not rotate to the normal lift off attitude until the correct speed is achieved For landing: • The negative effect is float • Therefore is lightly loaded or too fast during the flare the float will be extended • This increase in float reduces the available stopping distance Questions?