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Propellors (Propellor Effects (Slipstream effect (Propwash wraps in…
Propellors
Propellor Effects
Torque ReactionEffect
Due to every action has equal and opposite - RH Clockwise prop rotating will be countered by AC anti-clockwse
RH prop in ground will yaw left. In air will roll left
Amount of torque depends on:
Mass of rotating prop
RPM
Thrust of prop
Slipstream effect
Propwash wraps in clockwise direction
Creates a negative Beta angle so AC yaws left
As TAS increases yawing moment decreases
Greatest at high power and at slow speed
P Factor - Asymmetric blade effect
( YAW LEFT )
Increased Alpha on the Down-going Blade
At high AoA (slow speed)
The down going blade sees a higher AoA and travels faster, up going is going slower
Increase RAF on Down going Blade
The down-going blade travels a further distance through the air during one rotation than the up-going blade. Consequently the down-going blade experiences a faster relative air flow, and thus produces greater lift.
Asymmetric blade effect is greatest at high power settings and at high aircraft angles of attack, both of which occur when climbing at slow speed after take-off.
Occur when climbing at slow speed after take-off.
Gyroscopic effect
Rotating propellor subject to gyroscopic effects of RIGIDITY and PRECESSION
Precession forces generated by aircraft PITCH and YAW
Pitch up = Yaw Right, Yaw Right = Pitch Down, etc….
Propellor Tip Speed
Prop designed so tips NEVER go transonic
0.85M drag increases rapidly due to shockwaves
Noise due to shocks an issue too
Reducing Propellor Effects
Prop effects are unavoidable in light single engine designs
Multi engine AC can use CONTRA-ROTATING props and COUNTER-ROTATING props
But if one engine fails the remaining engine will cause noticeable effects aka CRITICAL ENGINE
Propellor Icing
EXAM: Icing will accrete ALWAYS on the root - first third.
End section going too fast and sheds off the water
EXAM: In general efficiency losses are <= 10%
EXAM: Worst case 15-20%
Reduction in AC performance beyond this is likely to be due to accretions on other airplane parts
Definitions
Blade
Spinner
Hub
Propellor Disc
Helix Angle = Plane of rotation to RAF
AoA = RAF to chord line
Blade Angle (Pitch) = Helix Angle + AoA
Blade Angle (blade pitch) decreases from
hub to tip due to blade twist
So 75% distance from hub used
Direction of RAF determined by:
rotational speed
forward speed of AC
Geometric Pitch = distance if blade was cutting into air
Geometric Pitch = Effective Pitch + Slip
Coarse Pitch = Hi speed
Fine Pitch = Low Speed
Types
Fixed Pitch
2 Pitch
Constant Speed
Aerodynamics
Torque (aka Torque Drag) = the resistance to rotation
Shaft Torque = Torque force provided by engine to over come torque drag
At constant RPM: Shaft torque = Torque Drag
Blade AoA changes with direction of RAF. Direction of RAF depends on 2 speeds
Rotational speed (RPM)
TAS (~inflow into the prop)
Blade AoA decreases as TAS increases
Decreasing towards the optimal 4deg AoA
Blade AoA increases as RPM increases
Choice of Fixed Blade Pitch Angle
Climb prop (optimum AoA for climb) - training AC that are in circuits and slow speed a lot
Cruise prop (optimum AoA for cruise) - more powerful engines to overcome slow speed inefficiency. Best in cruise
Reverse Thrust @ Low RPM with High TAS
Total reaction incline backwards. This means drag produced not thrust
Total reaction incline backwards. This means increase in torque drag
Blade Twist
Needed as the inside of the Blade travels slower than the tip
Since we know that as RPM increases AoA increase
So tip AoA will be greater than root.
So we reduce Blade angle from hub to tip to try and maintain constant AoA along the blade
Variable Pitch Prop
Maintains optimal 4deg AoA for all speeds
2 Types
manually adjustable pitch propellor
The constant speed propeller
You set the throttle.
Then set the rpm as you want and the governor will adjust the pitch to find that RPM
It does its best to rotate at the speed you tell it to
Power Absorption
Increase blade length
Issue, tips will go faster….and could go supersonic.
Issue - need big undercarriage so AC is heavier etc
Reduce the efficiency of the prop ( but this greatly increase the noise generated)
Increase propellor solidity
Increase no. of blades
(max 6)
Propellor Solidity = Propellor blade frontal area
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Propellor disk area
Issue is more turbulent wake
Limited to 6 Blades
preceding blade disturbs the air of the following blades
hubs design limits the number of blades can be attached.
Increasing the blade chord
Increase Blade Chord
A propeller with wide-chord blades absorbs more power because the blade section generates greater aerodynamic force. However, the reduced aspect ratio increases tip vortices, making it a less efficient design.
Contra rotating blades
Increases solidity
The propellers rotate in opposite directions thus minimising inflow effects. They also eliminate undesirable gyroscopic effects.
Propellor Efficiency (~80%)
Always < 100%
Propellor Efficiency = Output propulsive power generated by Propellor
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Input Shaft Power
Propellor Efficiency = Thrust Power
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Input Shaft Power
EXAM WILL TRY CATCH YOU OUT USING DIFFERENT WORDING OR CHANGE EQUATION. YOU NEED TO ENSURE IT’S ALWAYS OUTPUT / INPUT AND WILL ALWAYS BE LESS THAN 100%
Engine Failure
Windmilling
The torque is in the opposite direction - as prop driven by RAF
The relative airflow approaches the propeller from the blade back.
The total reaction slants backwards slightly with respect to the RAF.
The total reaction creates a large amount of drag and a small torque force rather than torque drag.
High drag = reduce glide & asymmetry on a twin
High drag only useful for engine restart
Feathered
Zero lift AoA
No torque or Aero forces
Reduced drag (parasite) cf Windmilling
Reverse Thrust
Some AC on landing have the ability to go from coarse to fine and past fine into negative AoA. This is powered rearward thrust.
This AoA is know as Beta range
Requires a lot of engine power to counter torque drag
