17.3 - Propeller Pitch Control Flashcards
(18 cards)
Disc braking.
Some props can be moved to a very fine pitch , causing a negative AOA and offers a resistance to rotation causing a braking force.
The braking force is proportional to the speed of the a/c.
Power on braking
Reverse pitch, negative blade angle.
Need to spool up engine.
Feathering.
In the event of an engine failure.
CTMs work to move the props to a fine angle, results in ‘disc braking’ and windmilling and high drag.
Featuring is to reduce drag.
Single acting piston for single engine a/c
The spring returns it to a low pitch
Should the engine failure in flight, it makes it favourable for windmilling, making it easier to restart.
Single acting for multi engine aircraft.
Spring acts to move prop to a high course angle (feathered) to reduce the drag and reduce the yawing action for the other engine.
The other engine is up trimmed meaning it gets an extra 10%
Centrifugal force pitch change moments
CTMs push it to low blade angle.
Counterweights overcome it and force it to a high blade angle.
When is negative pitch used
Power on braking.
Manoeuvring.
Propeller governor
A spur gear type pump to provide the positive pressure.
A pilot valve.
Electrical prop control
Electrical power is supplied through a slip ring and brush assembly.
The governed moves a servo piston connected to a reversible electrical motor, passes the info to the motor by the slip rings and bushes.
Electrical prop control
Has an ECU electric control unit with 2 channels that crosstalk.
ECU recieves pilot initiated speed demands and other inputs. The output is sent to the torque motor on the servo valve that meters oil supply.
Normally accurate to 2 rpms within selected rpm.
Where do you move the controls to feather the aircraft. In manual feathering controls.
Move it to the low speed stop.
Some models have feathering buttons.
Auto feathering.
During take-off and cruise.
In event of engine failure.
When is uptrim triggered
Torque of engine falls by 25%
Rotational speed of prop falls by 80%
10% extra power to the engine that didn’t fail.
Un feathering accumulator
Prop must be un feathered if the pilot wants to attempt to re start a failed engine.
Provide a reserve of pressure.
Achieved by moving the lever to higher rpm position.
Centrifugal latches
Applicable for Single acting pressure on multi engine aircraft.
If there is a leakage in oil in the governor then the blades are forced to a high pitch by the spring pressure.
This would put a load on the starter system
Latches prevent this from happening by locking the piston in. Slow pitch position when the engine is not rotating.
Engaged by spring pressure at low RPM and disengaged by centrifugal force.
Advantages and disadvantages of reverse thrust.
Reduce landing roll
Reduce brake wear
Improve manoeuvrability on the ground
Reversing a/c on ground.
Cons
Reduced engine cooling
Increased blade damage
Complicated control system.
In beta mode what what is blade angle directly changes by
The power lever.
Overspeed protection
Normal governor 102-103%
Overspeed governor 104-106%
106-107% bleed air from fuel flow transmitter pressure - reduce fuel flow to engine. Only on special air craft.
