Control And Forces In Forward Flight Flashcards
(13 cards)
What is flapping to equality?
Created by the problem of symmetry of lift as he advancing blade advances the increased airflow over the brick blade causes the blade to flap up around the flapping hinge.
As the blade flaps upwards, the angle of attack is reduced until the rate of flapping up decreases again.
The reverse is true as the blade then retreats. The blade is flapping to equality.
What is the consequence of flapping to equality and how is it overcome?
Flapback - forward cyclic.
What is inflow roll?
The transient feature that occurs just as the helicopter moves off the hover on a nil wind day. It is caused by inequality in induced flow and results in a role towards the advancing side.
Air entering the front of the desk strikes the disk to shallow angle with a reduced vertical component. This reduces the induced flow and increases the angle of attack.
This causes the blade Highpoint to occur as the blade retreats.
Describe translational lift
As the helicopter transitions forward increased horizontal airflow reduces the induced flow. This increases the angle of attack and therefore the lift the required is reduced.
Why does translational lift cease to assist and the power required increase?
As the disk is tilted further forwards, the airflow has less horizontal component and more vertical effectively increasing the induced flow. This reduces the angle of attack and the lift and therefore the power requirement increases.
Describe tuck under.
Took under occurs at around 30 kn airspeed when the main rotator downwash slips off the horizontal stabiliser and causes the nurse to drop by a few degrees.
Power curve: why does the power required to balance the drag increase as speed increases?
What three types of power form the power curve?
Rotor profile power, induced power and parasite power.
Define the three powers.
Rotor profile power: the power required to drive the rotor at minimum pitch at a constant NR plus Taylor and ancillary equipment.
Induced power: extra power required to overcome the rising drag when the blades are inducing air to flow down through the rotor.
Parasite power: fuselage drag increases as speed does. The rotor must be tilted to increase the horizontal component of total Rotor thrust. This increased induced airflow and rotor drag.
What are the limits to forward speed?
What is airflow reversal?
The speed of rotation of the retreat blade is Hyatt the tip and low at the root.
The airflow however from forward flight has an equal value across the whole length of the blade.
Where the airflow from flight is greater than the blade rotation of velocity, e.g. at the root end the airflow will be from the trailing edge to the leading edge causing a loss of rotor thrust.
The reduction of rotor thrust on the retreating blade by airflow reversal is counted by greater cyclic control and hence the retreating blade operates at an increasingly higher pitch angle and angle of attack.
What is retreating blade stall?
Due to airflow reversal, the angle of attack of the retreating blade is steadily increased with increasing forward air speed.
There will be a speed at which the airflow breaks away and the blade stalls.
The large loss of rotor thrust causes the blade to flap down but instead of flapping to equality the effect will simply be to install the blade even further.
The stall starts at the tip first and spread in board.
What are the causes and characteristics of retreating blade stall?
How to recover?
Causes: high forward speed, turbulent air; rough control movements, high density and AUM.
Characteristics: rotor roughness, erratic stick forces, stick shake. Pitch up and roll to retreating side.
Recover: lower collective, reduce speed
