Aerodynamics Basic Flashcards
Relative Wind is…
airflow relative to an airfoil. Movement of an airfoil through the air creates relative wind.
Rotational Relative Wind is…
produced from the rotation of rotor blades as they turn about the mast. Is highest at blade tips and decreases uniformly to zero at axis of rotation
Induced Flow (Downwash) is…
a downward flow of air caused by rotor blades turning. Most pronounced at hover under no-wind conditions
resultant relative wind is…
rotational relative wind modified by induced flow
Up Flow is…
airflow approaching the rotor disk from below as the result of some rate of descent
Angle of Incidence is…
angle between the chord line of a main or tail rotor blade and the rotational relative wind of the rotor system. It is a mechanical angle. AKA blade pitch angle.
Angle of Attack
is an aerodynamic angle. between airfoil chord line and resultant relative wind. It can change with no change to angle of incidence.
Feathering is…
rotation of the blade about its span wise axis by collective/cyclic inputs causing changes in blade pitch angle
Collective feathering …
changes angle of incidence equally and in the same direction on all rotor blades simultaneously
Cyclic feathering…
changes angle of incidence differentially around the rotor system
Flapping is…
the up and down movement of rotor blades about a hinge. primary means of compensating for dissymmetry of lift.
Lead and Lag (Hunting) …
fore and aft movement of the blade in the plane of rotation in response to changes in angler velocity.
Drag is…
Types of Drag
force opposing the motion of an airfoil through the air
Parasite - non lifting components
Induced - a result of production of lift
Profile - frictional resistance of blades passing through the air.
Airflow in Hovering Flight
for helicopter to hover, lift produced but the rotor system must equal the total weight of the helicopter.
At hover, rotor tip vortex reduces effectiveness of the outer blade portions.
High power requirements for hovering caused by vortices of preceding blade.
In Ground effect
permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced. Left induced flow, increased AOA
OGE
induced flow increase, decreased AOA. Higher blade pitch required. Increased pitch creates more drag. More power required due to the increased pitch angle and drag
Translating Tendency
Tendency of helicopter to drift in direction of tail rotor thrust
Features that help compensate for translating tendency
flight control rigging
transmission mounted
collective pitch control system
programmed mechanical inputs/AFCS/stabilization augmentation systems
Dissymmetry of Lift
differential lift between the advancing and retreating halves of the rotor disk
What compensates for dissymmetry of lift
cyclic feathering, blade flapping
Translational Lift
improved rotor efficiency resulting from directional flight
As incoming wind enters rotor system, turbulence and vortexes are left behind and the flow of air becomes more horizontal
Transverse Flow
Forward air more horizontal Aft air more vertical causes a pitch up. GP causes right roll. Occurs 10 to 20 knots
Most noticeable during takeoff and deceleration for landing
Effective Translational Lift
occurs at 16 to 24 knots. Completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air.
Autotrotation
To sustain rotor RPM when engine fails so continued flight can be conducted to the ground.
Airflow during helicopter descent provides energy ti overcome blade drag and turn the rotor
Settling with power
helicopter settles in its own downwash. AKA vortex ring state
conditions for settling with power
a vertical or near vertical descent of at least 300 FPM
slow forward airspeed (less than ETL)
Rotor system must be using 20 to 100 percent of available engine power with insufficient power remaining to arrest the descent
Conditions conducive to settling with power
Steep approach at high rate of descent
downwind approach
formation flight approach
hovering above the maximum hover ceiling
not maintaining constant altitude control during an OGE hover
during masking/unmasking
Dynamic Rollover
is the lateral-rolling tendency
Occurs: level ground, slope, crosswind landing, and takeoff
Conditions: pivot point, rolling motion, and exceeding critical angle
Dynamic Rollover Human Factors
Inattention Inexperience Failure to take timely corrective action Inappropriate control input Loss of visual reference
Retreating Blade Stall
decreasing velocity of airflow on the retreating blade demands a higher AOA to generate the same lift as the advancing blade. Left roll
Retreating Blade Stall Conditions
high blade loading (high gross weight) low rotor RPM High-density altitude High G-maneuvers Turbulent air
Recover from blade stall
reduce collective reduce airspeed descend to a lower altitude increase rotor RPM to normal limits reduce the severity of the maneuver
