Aerodynamics (Fundamentals Of Flight) Flashcards
FEATHERING
Feathering is the action that changes the pitch angle of the rotor blades by rotating them around their feathering (spanwise) axis. (FAA-H-8083-21B) (figure 1-18).
Flapping
When blade flapping compensates for dissymmetry of lift, the upward and downward flapping motion changes induced flow velocity. This changes AOA on the advancing and retreating blades
Advancing side - minimum blade pitch angle and AoA (flap up)
Retreating side - high blade pitch angle and AoA and is climbing towards the highest position at the back of the disk. (Flap down)
GYROSCOPIC PRECESSION
The phenomenon of precession occurs in rotating bodies that manifest an applied force 90 degrees after application in the direction of rotation.
Parasite Drag
Parasite drag is incurred from the non-lifting portions of the aircraft.
Profile Drag
Profile drag is incurred from frictional resistance of the blades passing through the air.
Profile of blade
Induced Drag
Induced drag is incurred as a result of production of lift.
IN GROUND EFFECT
Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters. Induced flow reduced this increase in AOA requires a reduced blade pitch angle. This reduces the power required to hover IGE. Less wing tip vortices.
Ground slows down induced flow velocity
Out-of-Ground Effect
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. Large wing tip vortices.
Induced flow velocity is increased
TRANSLATING TENDENCY
During hovering flight, the counterclockwise rotating, single-rotor helicopter has a tendency to drift laterally to the right. The translating tendency (figure 1-52, page 1-36) results from right lateral tail-rotor thrust exerted to compensate for main rotor torque (main rotor turning in a counterclockwise direction).
Dissymmetry of lift
is the unequal lift across the rotor disk resulting from the difference in the velocity of air over the advancing blade half and the velocity of air over the retreating blade half of the rotor disk area.
TRANSLATIONAL LIFT
Improved rotor efficiency resulting from directional flight is translational lift.
In addition, the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight.
TRANSVERSE FLOW EFFECT
In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion.
This effect causes unequal drag in the fore and aft portions of the rotor disk and results in vibration easily recognizable by the aviator. It occurs between 10 and 20 knots.
EFFECTIVE TRANSLATIONAL LIFT
Occurs with the helicopter at about 16 to 24 knots,
when the rotor system completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air.
Manifests as a right roll
BUCKET SPEED
Bucket speed is the airspeed range providing the best power margin for maneuvering flight.
Using the cruise chart for current conditions, enter at 50 percent of maximum torque available, go up to gross weight, over to the lowest and highest airspeed intersecting the aircraft gross weight, and note speeds between which there is the greatest power margin for maneuvering flight.
MUSHING
Mushing is a temporary stall condition occurring in helicopters when rapid aft cyclic is applied at high forward airspeeds. Normally associated with dive recoveries.
High aircraft gross weight and high density altitude are conditions conducive to and can aggravate mushing.