Rotary Wing aerodynamics Flashcards
Pitch angle
is the angle between the chord line of the rotor blade and the plane of rotation
Tip-path plane
imaginary circular surface formed by a plane passing through the path of the tips of the rotor blades
Rotor disc
is the circle of air swept by the rotor
What axis does feathering occur on ?
The Longitudinal axis
What axis does flapping occur on ?
The normal axis ( vertical )
What axis does lead lag occur on ?
The lateral axis
Induced airflow
air that is sucked in through the top of the rotor blades and blown out the bottom.
Increased induced airflow
reduces angle of attack
Coning
is the upward flexing of the rotor blades
Resulting from
the forces of lift distributed along the blades
Tips of the blades produce
more lifting force than the roots
Rotor disc starts to
take up weight of fuselage , the disc begins to cone
Factors that increase coning and cause a high coning angle
The RPM of motors ( Low rotor RPM ) , The weight of the vehicle ( High weight increases coning ) Any G forces experienced in flight ( High G manoeuvres )
Excessive coning results in
Undesirable stresses on the blades , A decrease in effective disk area and hence lift. Tip path plane decreases therefore an overall decrease in lift
Helicopter has three controls
Collective , cyclic , tail rotor pedals
Collective pitch lever causes
it to rise and descend
Produces simultaneous
increase or decrease in angle of pitch of rotor blades by same amount
Cyclic pitch control
causes it to tilt in the desired direction
Resembles
control stick , alters pitch of rotor blades by different amounts on each side , differential in lift causes disc to tilt
Tail rotor pedals
cause it to yaw
Anti torque pedals are
foot controls which change the pitch of the tail rotor and the thrust which it is producing.
Concentric shafts a pair of
helicopter rotors mounted one above the other on concentric shafts
with the same
axis of rotation but turning in opposite directions
Gyroscopic procession
means a force applied to a gyro (rotor disc ) will produce an effect up to 90 degrees forward of the applied force.
Rotor spinning Anti Clockwise , want to tilt rotor forwards
force on disc needs to be applied up to 90 degs before hand
Tail Rotor Drift also called
Translating Tendency
If a rotor spins ACW, fuselage of helicopter will spin
CW (Newton’s 3rd law. Think of drill bit getting stuck)
This CW motion will also cause a sideways force
to the right, shown at the rotor mast as drift.
Tail rotor provides
anti-torque for fuselage
which also causes a net sideways force
to the right
These two forces provide an overall
net movement of the helicopter to the right.
Pilot actions to counteract: Pilot uses
cyclic movement in opposite direction to TRD
Dissymmetry of Lift , Relative airflow is different
on advancing side and retreating side of rotor disc
Compensated automatically
in rotor system through flapping.
Retreating side flaps
down
advancing side flaps
up
This flapping causes relative airflow
to be different on either side and consequently angle of attacks are different on either side.
Retreating side flaps down. Causes an
increase in AoA, creating more lift, causing blade to flap back up.
Advancing side flaps up. Causes a
decrease in AoA, diminishing lift, causing blade to flap back down.
This is called
flapping to equality.
If flapping wasn’t allowed
DOL would cause FLAPBACK.
Advancing side would produce
more lift
Gyroscopic precession would transfer this increase in lift
to the front of the helicopter (approx. 90 degs forward) causing uncontrollable rise to the nose…. Could be fatal.
Pilot actions : As a helicopter moves forward,
constant force must be applied to keep the disc tilted forward.
As helicopter moves faster,
disc tilts more, causing retreating side to continually experience higher AoA than advancing side
Eventually AoA will
exceed critical angle, causing retreating blade stall
The stall will be experienced at the
rear of helicopter due to GP, causing severe vibration and pitching up!!
Transverse flow effect : In forward flight, the rotor disc is tilted
forwards and the air enters it from above.
The air entering the rear of the disc is acted upon
by the blades for a longer period of time and is accelerated to a higher speed than that coming through the front of the disc.
This results in a
greater downwash from the rear of the disc and a decreased angle of attack, hence decreased lift experienced by the rear of the disc.
Combined with
gyroscopic precession, this difference in lift causes the helicopter to roll in the direction of the advancing blade.
Countered by use of the
cyclic
This effect is most evident at
low airspeeds and exaggerated forward pitch angles of attack.
Effective translational lift : 3 stage of ETL , 1st stage
Stationary hover (0kts forward speed) , Helicopter is operating in its own wingtip vortices, with tips of the blade ineffective at producing lift
2nd stage ,
Translational flight (moving from hover to forward flight)
Rotor tilted
forwards
There is still some
wingtip vortices but they are starting to get pushed away from rotor system by forward airflow
causing rotor disc
to receive greater efficiency.
Bonus in lift from this.
Same lift can be achieved with less engine power
3rd stage :
ETL (12-40 kts airspeed)
Helicopter has moved
out of its wingtip vortices and into ETL
Greater
Lift can be produced with less power until 40 kts
After 40 kts
parasite drag from the fueslage negates ETL
Coriolis effect states that due to
conservation of angular momentum, coning will cause the centre of mass of the blades to come closer to the mast,
increases
rotor rpm
This increase in RPM
if not managed could overspeed the engine and cause damage to rotor and engine.
In semi rigid rotor systems
automatic flapping down of one side and up on the other diminishes the effect of coriolis and avoids over-RPM of engine
In a fully articulated head with more than
two blades
a drag hinge is used
to allow backward and forward movement of blade
Retreating blade
flaps down, moves slower) lags
advancing blade
(flaps up, moves faster) leads
Ground effect becomes non existent once
the helicopter achieves a translational speed of more than 5 knots
When operating away from the ground, helicopter experiences
recirculation of air at the blade tips
Similar to
wingtip vortices
This recirculation makes an area
around the blade tip ineffective at producing lift.
When in ground effect
within a rotor span from ground
recirculation is
disrupted and not allowed to flow back into the rotor system, so less of the blade tip is affected.
This results in bonus
lift production IGE.
Helicopters can seem to carry
heavier loads when IGE and use less power to produce same amount of lift as when OGE.
Know the dangers of ground effect: If helicopter is operating IGE on top of a peak
and accidentally drifts off the peak or ledge and ends up OGE, the helicopter will immediately lose lift.
If lift is not restored by raising
collective, the helicopter will sink and possible collide with terrain.
Autorotation : Is the state of flight where rotor is driven
by relative airflow rather than power of the engine.
All helicopters must have the ability to
disconnecting the main rotor from the engine so that the rotor can turn even if the engine cannot.
tail rotor also needs to be
turning during autorotation to provide some counter to the frictional forces in the gearbox which would otherwise induce the fuselage to turn in the same direction.
1st step
Disengage the rotor system from the engine.
Lower the collective pitch to reduce pitch of blades to a minimum, reducing torque
Nose is pushed down to give an angle of glide and to provide some translational lift
2nd step As the helicopter descends,
the air coming up through the rotor disc causes the blades to rotate, and as they do, progressively store more energy.
3rd step When the ground approaches, the pilot:
flares the helicopter in a similar manner to a normal landing and during this phase,
the rotor will usually increase speed because of the flare.
The final speed increase provides enough extra lift to arrest the helicopter’s descent
4th step : helicopter has lost
its forward kinetic energy and has no air flowing upwards to keep the rotor turning.
the helicopter settles on the ground, with the pilots last action:
As rotor speed decays , Extract the remaining lift from the rotor by increasing the collective pitch to its maximum.
This increases the lift, but also increases the drag and the rotor quickly loses speed as the helicopter settles gently on the ground.
Cyclic acts 90 degrees
before the total effect is achieved by the rotor
