ACAT.08 Aerodynamics Flashcards
Define tranlating tendency
The tendency of a single rotor helicopter with a counterclockwise rotating rotor system to drift laterally to the right while at a hover
What causes Translating tendency
Thrust produced by the tail rotor to compensate for main rotor torque.
Translating tendency compensation
- Rigging of the flight control system
- Tilting of the mast
- Stabilization augmentation systems
- Left cyclic input by the pilot
Define Dissymmetry of lift
Dissymmetry of lift is the unequal lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half. If the helicopter is at a steady hover there is no dissymmetry of lift (with no wind)
Relative wind velocity of advancing blade is blade speed ____ airspeed
Plus
Relative wind velocity of retreating blade is blade speed ____ airspeed
Minus
Main rotor method to overcome dissymmetry of lift
Flapping
When blade flapping has compensated for dissymmetry of lift, the rotor disk is tilted to the rear is called?
Blowback
What can the pilot do to compensate for dissymmetry of lift?
Cyclic feathering
Explain Effective tranlational lift (ETL)
It is when the A/C completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air which occurs between 16 to 24 knots. As translational lift becomes more effective, the A/C will increase in altitude without any increase of power.
Airspeed for ETL
16-24 kts
ETL causes relative wind to become more horizontal, resulting in:
- More vertical lift component
- Less induced drag
- Increased angle of attack
Define Settling with power
A condition of powered flight in which the helicopter settles in it’s own downwash
What conditions must exist simultaneously for settling with power to occur
- Vertical or near vertical descent rate of at least 300 ft/min depending upon: - Gross weight- Rotor RPM- Density altitude
- 20% - 100% of available engine power with insufficient power remaining to arrest the descent
- Slow forward airspeed, less than ETL
Settling with power recovery
Increase airspeed with forward cyclic. Reduce collective pitch as altitude permits
Define Dynamic rollover
A helicopters susceptibility to a lateral rolling tendency
Dynamic rollover conditions
Pivot point, Rolling motion, Exceed critical angle
Physical factors for dynamic rollover
- Main rotor thrust - CG - Tail rotor thrust - Crosswind component - Ground surface - Sloped landing area - Low fuel condition
Human factors
- Inattention - Inexperience - Inappropriate control input - Failure to take timely corrective action - Loss of visual reference
Recovery of dynamic rollover
A smooth moderate collective reduction
Primary factor for retreating blade stall
Excessive airspeed
Contributing factors for retreating blade stall
- High gross weight
- High DA
- High “G” maneuvers
- Low rotor RPM
- Turbulence
Symptoms of retreating blade stall
- Abnormal vibrations
- Pitch-up of nose
- Tendency to roll toward the stalled (left) side
- Loss of control (if corrective action is not applied)
Corrective actions for retreating blade stall
- Reduce collective pitch
- Regain aircraft control
- Reduce airspeed
- Increase rotor RPM to normal operating range
- Minimize maneuvering
- Descent to a lower altitude.
Explain why hovering OGE requires more power than hovering IGE
Induced flow velocity is increased, causing a decrease in AOA. A higher blade pitch angle is required to maintain the same AOA as in IGE hover. The increased pitch angle also creates more drag. More power to hover OGE than IGE is required by this increased pitch angle and drag
What is translational lift?
The efficiency of the rotor system is improved with each knot of incomming wind gained by movement of the A/C or surface wind. As the incomming wind enters the rotor system, turbulence and vortexes are left behind and the airflow is more horizontal. The improved rotor efficiency resulting from directional flight / wind is translational lift.
What is transverse flow effect?
Occurs between 10 and 20 knots. Airflow over the forward half of the rotor is more horizontal, and more vertical over the aft half resulting in greater lift in the forward area and greater drag in the rearward. This effect causes unequal drag in the fore and aft portions of the rotor disk. Adding gyroscopic precession results in a lateral vibration easily recognisable by the aviator during takeoff, and to a lesser degree during approach. Because of the greater lift in the front and gyroscopic precession this effect results in a right rolling motion.
