Helicopter Aerodynamics Flashcards
Define Newton’s first law
The law of inertia. An object in motion remains in motion until acted upon by an outside force. The same occurs with an object at rest
Define Newton’s second law
Law of acceleration, an object acted upon by a force will continue to accelerate in proportion to the magnitude of the force and the mass of the object
Define Newton’s third law Newton’s third law
Equal and opposite reaction. When an object receives a force it pushes back with an equal opposite force.
How does Newton’s first law act upon a helicopter?
Helicopter will continue to move unless acted upon by an outside force such as weight or drag.
How does Newton’s second law impact a helicopter?
The amount of lift or thrust required is proportional to the weight of the aircraft
How does Newton’s third law act upon an aircraft?
Tail rotor counteracts the main rotor torque effect
What is the definition of a scalar?
A scalar is a value with a magnitude only (Speed)
What is the definition of a vector?
Vectors have both magnitude and direction (Velocity)
What is the vector known as if it is the result of two separate vectors?
The resultant vector
The anti torque pedals control what in a helicopter?
The tail rotor which impacts yaw
The cyclic controls what two axes of motion in a helicopter?
Pitch and roll
What is known as the venturi effect?
Fluid velocity increases in direct proportion to the reduction of area of flow
What is Bernoulli’s principle?
As fluid velocity increases, the dynamic pressure also increases, so to keep total pressure constant static pressure must decrease
How does bernoulli’s principle act upon an airfoil?
As the air moves around the airfoil the dynamic pressure increases due to the increase in velocity above the airfoil resulting in a decrease in static pressure generating lift
The symbol CL corresponds to what in the lift equation?
A combination of angle of attack and wing shape
The symbol P corresponds to what in the lift equation?
Air density
The symbol V corresponds to what in the lift equation?
Airfoil speed
Symbol S corresponds to what in the left equation?
Wing surface area
Temperature lapse rate is defined as what?
2 degrees per every thousand feet
As humidity increases, what happens to overall air density?
Overall air density decreases resulting in a increase in density altitude
An airfoil is defined as what?
A surface body or structure designed to produce a lift or thrust force when subjected to an airflow
The front of the airfoil is known as what?
A leading edge
The rear of the airfoil is known as a what?
Trailing edge
A line from the leading edge to the trailing edge is known as what?
The chord
What line is the average between the upper surface and lower surface of the airfoil?
The camber line
What are the characteristics of a symmetrical airfoil?
Equal camber on each side of the cord, each half is a mirror image of the other.
What are some advantages of a symmetric airfoil?
Constant center of pressure, ease of construction, lower cost.
What are some disadvantages of a symmetric airfoil?
Less efficient at a given angle of attack, undesirable style characteristics
What is the description of a non symmetric airfoil?
A cambered or curved area on the upper surface of the airfoil
What are the advantages of a non symmetrical airfoil?
Increase lift drag ratios and better stall characteristics
The center of pressure is defined as what?
The point at which the lifting force from the airfoil is created
Centrifugal force is defined as what?
An outward force produced whenever a body moves in a curved path
Centrifugal force provides what for a helicopter rotor system?
Rigidity
Centripetal force is defined as what?
The normal force of the centrifugal force pointing towards the center of the rotation
What is rotor blade coning?
Upward flexing of the rotor blades which is a compromise between lift and centrifugal force, when lift is stronger than the centrifugal force of blade will cone upward
Excessive coning results in what?
A loss of disk area with a resultant loss of total lift as well as increased stress on the blades
Why are rotor blades twisted?
To even the amount of lifting force along the length of the blade
A lower pitch angle at the tip of the blade results in what?
A lower overall angle of attack due to the higher relative winds that the airfoil experiences
Profile drag is defined as what?
The frictional resistance of the surface of the airfoil
Induced drag is defined as what?
Rotational eddies of air which are formed when the airfoil produces lift which resulted in drag on the airfoil
Parasitic drag is defined as what?
Drag from non lifting surfaces the aircraft moving through the air
The airspeed at which induced drag and parasite drag are at a minimum is defined as what?
Max endurance air speed or the Max l / d
Airfoil stall is defined as what?
An exceedance of the critical angle of attack of the airfoil where the laminar flow separates from the airfoil next slide
During a hover, weight and lift are in a state of what?
A state of balance
What causes the high power requirement for hovering?
The continuous creation of new vortices and ingestion of existing vortices is a primary cause of high power requirement for hover
What is the definition of hovering in ground effect?
Hovering above the ground within one disc diameter where the rotational vortices or mitigated by the ground
What is the definition of out of ground effect hover?
Hovering above one rotor disk where the helicopter is subjected to the full force of the induced drag of the lift
hover power requirement is typically how much less than out of ground effect?
30% less
If hovering over tall grass, rough terrain, water, or other rough surface what may occur with in ground effect hover?
In ground effect hover may have increased power requirements
What is translating tendency?
It is a right lateral tail rotor thrust that results in a right lateral drift on the helicopter
Compensation for translating tendency is typically what?
An application is cyclic to create a main rotor force to the left
How else is translating tendency overcome?
Helicopter design usually has a preference for left role built into the airframe to compensate
What is the transverse flow effect?
Downward air flow at the rear of the rotor disk resulting in a condition of increased drag with reduced angle of attack meaning that the rear of the rotor produces less lift. This overall results in a right rolling motion at low aircraft speed
Dissymmetry of lift is defined as what?
Differential lift produced by the differences in relative wind between the rotating helicopter blades
What 2 design considerations prevent adverse effects of dissymmetry of lift?
Blade flapping and cyclic feathering
what occurs at the retreating blade exceeds the angle of attack?
A retreating blade stall
What is cyclic feathering?
The increase of the pitch of 1 rotor blade while simultaneously decreasing the pitch of the other
What is the definition of effective translational lift?
Point of which the aircraft out runs its own down wash resulting in both blades experiencing smooth oh unaltered error.
At what speed does effective translational lift occur?
16 to 24 knots
As the helicopter speed increases in translational lift becomes more effective what characteristic is seen?
The nose will pitch up which is a tendency called blowback.
Blowback is caused by what?
The combined effects a dissymmetry lift and transverse flow
If the aircraft is allowed to pitch up during blowback while transiting ETL the aircraft may also experience a role to which direction?
A roll to the right
Auto rotation is defined as what?
Rotor spinning through the action of air moving through the rotor disk rather than the engine turning the rotor via the transmission
What are some reasons to auto rotate?
Engine failure, engine under speed, main drive shaft failure, clutch fails to reengage, complete loss of tail rotor thrust that is uncontrollable
Auto rotations are defined by three phases which are known as what?
Entry, steady state descent, deceleration and touchdown
When entering an autorotation what are the four steps?
Collective- adjust, pedals- adjust, throttle- adjust, ear speed- adjust
What is the minimum rate of descent airspeed at 94% Nr?
58 knots
What is the maximum glide distance to the helicopter 94% Nr?
106 knots
What are the three auto rotation regions of the blade during an autorotation?
Driven, driving, Stall
Which region produces lift during auto rotation?
The driving region
The driven region of the blade results in what aerodynamic force?
Drag, which slows down the blade
Reducing the collective during autorotation results in what change of the driving region?
The total size of the driving region increases and results in an increased speed on the blade
