PROPELLERS Aim • To understand the principles and considerations in a propeller producing thrust. Application • All stages of flight where the engine is running Overview • Definitions • Principles – Effect of Airspeed – Effect of Rotational Velocity – Blade Twist – Efficiency • Considerations • Airmanship Definitions Blade • an aerofoil which converts engine power into thrust. Definitions Blade Face • the flat side of the blade • What you see from the cockpit Blade Back • the cambered side of the blade. Definitions Blade Angle • the angle between the blade chordline and the plane of rotation. Angle of Attack • the angle between the chordline of the blade and the relative airflow. Definitions Helix Angle • the angle between the resultant velocity of the blade and the plane of rotation. • Sometimes Referred to as “advance angle” Definitions Fixed Pitch Propeller • A propeller which the blade angle or “pitch” cannot be changed Definitions Torque • resistance to motion in the plane of rotation. • “Rotational Force” Definitions Engine Rotation Force • Force that turns the propeller Propeller Torque • Produced by the drag of the propeller blades • Opposes the Rotation of the Propeller Principles For Constant Propeller RPM • Propeller Torque = Engine Rotation Force • Propeller Torque > Engine Rotation Force – RPM Decreases • Propeller Torque > Engine Rotation Force – RPM Increases Principles • Effect of Airspeed Principles • Effect of Rotational Velocity Principles • The combination of the forward velocity and rotational velocity creates a helical motion of the propeller through the air. Principles Principles • Geometric Pitch is the theoretical forward distance • Effective Pitch is the ACTUAL distance travelled Principles Fixed Pitch Propellers • For a given rotational Velocity there is only one forward velocity in which the propeller operates most efficiently Considerations • Propeller Damage • Propeller Fatigue • Efficiency Airmanship • Always perform thorough pre-flight inspections on the propeller. • Handle the engine smoothly. • Low power settings on ground to avoid damage. Review • Which side of the propeller do we see from the cockpit? • What happens to the angle of attack when we increase RPM? • Why do we have “Blade Twist”?作者: adamhou 时间: 2012-4-8 12:05:57
