标题: RMIT飞行训练课件－Propellers Mass Brief [打印本页] 作者: 航空 时间: 2010-8-16 00:40:19 标题: RMIT飞行训练课件－Propellers Mass Brief
作者: 航空 时间: 2010-8-16 00:40:49
Propellers Mass Brief Aim • To gain an understanding of the principles and forces acting on a propeller in flight. Application • All stages of flight where thrust is required Overview • Definitions • Principles • Considerations • Air Exercise • Airmanship • Review Definitions • Blade – An aerofoil which converts engine power into thrust. • Blade Face – The flat side of the blade (the side the pilot sees). • Blade Back – The cambered side of the blade. • Blade Angle – The angle between the blade chordline and the plane of rotation. Spinner Direction of flight Blade Angle Plane of rotation Propeller Blade Blade back Blade Face Chord Definitions • Angle of Attack – The angle between the chordline of the blade and the relative airflow. • Helix Angle (Angle of advance) – The angle between the resultant velocity of the blade and the plane of rotation. Angle of attack Blade Angle Helix Angle Resultant velocity of propeller blade through the air Velocity due to rotation Velocity due to forward motion Relative Airflow Definitions • Engine Rotation – The rotational force produced by the engine that turns the propeller. • Propeller Torque – Resistance to motion in the plane of rotation. Direction of flight Engine Rotation Propeller Torque Definitions • Fixed pitch propeller – A propeller where the blade angle cannot be varied • Variable pitch propeller – A propeller where the blade angle can be varied Blade Angle Principles • The combination of the forward velocity and rotational velocity creates a helical motion of the propeller through the air. Principles • The tip travels fastest due to greater distance travelled in same time. Root travels slowest. Therefore varying lift (thrust) forces. • To even out forces, blade is twisted to compensate. Blade angle at hub needs to be much greater than blade angle at tip. Principles • Forces on a propeller Propeller Torque Force Thrust Relative Airflow Engine Torque Force Considerations • Geometric pitch is how far the propeller would theoretically advance in one revolution. • Effective pitch is how far the propeller actually advances in one revolution. • The difference between geometric and effective pitch is called slip. Principles • Effect of airspeed – Reduction of thrust with increasing airspeed Low Speed Higher AoA High Speed Lower AoA Principles • Effect of rotational velocity (RPM) – Increasing rotational velocity will lead to an increase in AoA and Thrust High rotational velocity Higher AoA Low rotational velocity Lower AoA Air Exercise • Propeller Damage – Usually occurs first on leading edge, cause it hits the air first. – Avoid high power settings on loose surfaces like gravel. • Propeller Fatigue – Most likely to occur on blade face. • Efficiency – Fixed-pitch propellers are most efficient at only one airspeed and RPM combination. The C172 is fitted with a cruise prop. Airmanship • Always perform thorough pre-flight inspections on the propeller. • Handle the engine smoothly. • Use low power settings on ground to avoid damage. • Warn passengers about staying clear of propellers of aircraft on the tarmac • When conducting run-ups always look behind the aircraft so people and other small objects don’t get blown away Review • Which part of the blade is the blade back? • Which part of the blade is the blade face? • Why is the blade twisted? • What forces act on the blade?