RMIT飞行训练课件－Propellers Mass Brief

航空

游客，如果您要查看本帖隐藏内容请回复

航空

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?