Vector Diagram, Ground Effect And Recirculation Flashcards
(11 cards)
Draw the vector diagram
Define the 3 airflows.
Rotational airflow: the horizontal airflow blades are subject to as they rotate.
Induced flow: the deflection of air downwards by the rotating blades.
Relative airflow: the direction of the airflow relative to the blade as a result of the blades horizontal travel through the air and the induced flow.
What is the angle of attack?
The angle between the relative airflow and the chord line.
What is the total reaction?
The vector resultant of lift, which is produced by the relative airflow passing over the blade at an angle of attack, and drag, which is perpendicular to the lift.
The total reaction may be split into two components. What are they?
The rotor thrust acting along the axis of rotation.
The rotor drag acting parallel to the plane of rotation
What is total rotor thrust?
The total of the rotor thrust of each blade when added together. The sum of all the blade rotor thrusts act along the axis of rotation to the rotorhead.
Describe ground effect.
The downwash from the helicopter meets the ground is opposed and escapes horizontally. Divergent duct is produced causing an increase in pressure.
The increased pressure of the air beneath the helicopter opposes and reduces the induced flow so that the angle of attack and hence total rotor thrust are increased for a given pitch setting.
The collective is lowered to maintain heights thus TQ reduces.
At what height does ground effect become negligible?
2/3 of the rotor diameter
What 4 factors affect ground effect?
Height - reduces until 2/3 rotor diameter then is negligible.
Slope - on sloping ground much of the air flows downhill reducing the ground effect because there is no development of a divergent duct.
Nature of the ground - rough ground disrupt the airflow preventing a divergent duct from being formed.
Wind - ground effect is displaced down wind reducing the ground effect.
What is recirculation?
As air passes through the disc, some of it is recirculated and appears to increase speed as it passes through the disc a second time.
What is the effect of re circulation?
Induced flow is increased. Total rotor thrust is decreased and the aircraft would descend therefore the collective is raised and more power is required.
