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– 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?
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 1
CONSTANT SPEED UNIT
Aim: To understand the principles behind the operation and use of a variable-pitch
propeller (CSU).
Application:
- Anytime when flying an aircraft fitted with a CSU
- C182 or PA28R
Revision:
Fixed-pitch Propeller
- Same rotational velocity, low forward velocity = high AOA, and vice versa.
Definitions:
Fine Pitch
- small blade angles, used at low airspeeds.
Coarse Pitch
- large blade angles, used at high airspeeds.
Feather
- when the blade chordline is parallel with the aircraft’s relative airflow,
minimising drag.
Pitch Stops
- variable pitch propellers are typically limited to angles between 15 and
25. Between these stops (fine/coarse pitch stops) the propeller acts as a
CSU. At the stops it becomes a fixed-pitch propeller.
Governor
- once set, the governor maintains a constant propeller speed, therefore
constant RPM. This is done through speeder springs and flywheels.
Principles:
- By employing a CSU, a more efficient blade angle can be obtained during
most phases of flight.
- Low airspeeds, blade angle small, therefore fine pitch.
- High airspeeds, blade angle large, therefore coarse pitch.
Brief 26
RMIT Flight Training Constant Speed Unit
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 2
Change of Power
- pitch lever controls the blade angle.
- throttle controls engine power, measured in inches of Mercury (“Hg).
Termed manifold pressure.
- once RPM is selected, the CSU senses deviation from the set RPM and
adjusts the blade angle.
Constant RPM, open throttle, coarsen
Raise nose, IAS , finer pitch to stop RPM reducing
Dive, IAS , coarser pitch to avoid overspeeding
Pitch control lever is attached to speeder spring. This adjusts tension which
sets RPM. Speeder spring attached to flyweights. These operate with
centrifugal force, reacted by springs. Movement of flyweights adjusts oil
control valve. Oil flows to hub, where its pressure manipulates a piston, which
is mechanically linked to the blade root. Pressure equalises, valve shuts.
Brief 26
RMIT Flight Training Constant Speed Unit
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 3
McCAULEY NON-FEATHERING SYSTEM. (C182)
Brief 26
RMIT Flight Training Constant Speed Unit
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 4
HARTZELL NON-FEATHERING TYPE. (PA28R)
Brief 26
RMIT Flight Training Constant Speed Unit
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 5
Considerations:
Pre-Flight
- check for leaks and secure blades.
Ground Operations
- important to do propeller check to get warm oil into hub to ensure
serviceable system.
Multi-Engines
- following failure align blade with relative airflow to minimise drag.
(Feathered engine).
Failures
- in Single engine, speeder spring fails, flyweights will fly out, making
propeller go to coarse pitch stop.
- oil pressure failure, propeller will go to fine pitch stop.
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