Principles of Flight Flashcards
Air as a fluid
- fluids = take the shape of their container and do not resist deformation
- ability to flow
Viscosity
- property of a fluid that causes it to resist flowing
- high = thick/higher resistance (oil)
- low = thin/lower resistance (water)
- air has very little viscosity
Friction
- resistance that one object encounters when moving over another
- surface of wing is rough on the microscopic level, causing friction with air
Boundary layer
- air molecules stick to surface of the wing due to friction
- air molecules near the surface of the wing resist motion and have a relative velocity of zero
- layer of air that sticks to the wing is called the boundary layer
- after boundary layer is formed, friction further increases due to air’s tendency to stick to itself
- when these 2 forces meet = drag
Pressure
- force applied in a perpendicular direction to the surface of an object
- moves from high to low
Atmospheric Pressure
- responsible for weather, lift, flight instruments
- air is very light, but still affected by gravity which means it has a weight (force)
- because air is fluid, force acts equally in all directions
- at sea level = 14.70 PSI
- air pressure decreases with altitude
Standard Pressure
- created because air pressure changes with time and location
- 29.92 inches of mercury and 15° F
Standard Temperature Lapse Rate
- decrease of 2° C every 1,000 feet up to 36,000 feet
- above 36,000 = constant
Standard Pressure Lapse Rate
- decrease of 1 “Hg every 1,000 feet up to 10,000 feet
- referred to a ISA
- aircraft instruments are calibrated for standard atmosphere
Pressure Altitude
- height above the standard datum plane, theoretical level where weight of the atmosphere is 29.92
- achieved by setting altimeter to 29.92
- important for performance calculations
- all aircraft above 18,000 feet use pressure altitude
Density Altitude
- pressure altitude corrected for non standard temperature
- less dense air causes decreased lift (thinner air exerts less force on wings), decreased power (less air molecules in the engine, decreased thrust (propellers are less efficient)
- hot day = high DA/worse performance, aircraft “feels” like flying at higher altitude, high elevation, high temperature, low atmospheric pressure, high humidity
- cold day - low DA/better performance, aircraft “feels” like flying at lower altitude, low elevation, low temperature, high atmospheric pressure, low humidity
Effect of Pressure on Density
- density is directly proportional to pressure
- if pressure is doubled = density is doubled
Effects of Temperature on Density
- temperature is inversely proportional to pressure
- decreasing temperature increased pressure
Effects of Humidity on Density
- humidity = amount of water vapor in the air
- water vapor is lighter than air = moist air is lighter than dry air = moist air is less dense than dry air
- high humidity = decreased performance
Most dense air type vs. least dense air type
- most dense = cold dry
- least dense = warm and moist
Newton’s 3 Laws of Motions
- an object in motion will stay in motion unless acted on by an outside force, an object at rest will stay at rest unless acted on by an outside force
- F = MA
- for every action there is an equal and opposite reaction
Bernoulli’s Principle
- as the velocity of a fluid increases, its pressure decreases
Airfoil Design
- airfoil = structure that creates an aerodynamic force when interacting with moving air
- designed to take advantage of Newton’s laws as well as Bernoulli’s principle
Camber
- curvature of the wing
- large camber = large curve
- small camber = little to no curve
Chord Line
- imaginary line passing through the leading edge and trailing edge
- distance between chord line and upper or lower surface = magnitude
How does an airfoil take advantage of air response to these physical laws?
- creates positive pressure lifting force from increased pressure below the wing
- creates negative pressure lifting force from decreased pressure above the wing
- when air strikes flat lower surface = air is forced to rebound downard creating opposite upward force (lift)
Low pressure above
- curved upper surface causes air to move at increased velocity causing decreased pressure
High pressure below
- flat lower surface causes air to move at slower velocity causing increased pressure
Center of Pressure
- point where aerodynamic forces (lift) act through
- high AOA = forward CP
- low AOA - aft CP
- important because it affects position of air loads and balance/controllability
Planform
- what the wing looks like from above
Taper
- ratio of root to tip
- tapered wings increase aspect ratio = increased lift
- smaller wing tip decreased induced drag/wing tip vortices (more complex and expensive)
Aspect Ratio
- divide wing span by the average chord
- larger aspect ratio = less induced drag and more lift
- found on aerobatic planes and fighters that require extreme maneuverability
Sweep
- delays onset of wave drag
- optimised for high speed flight
What is stability?
- the inherent quality of an aircraft to return to its original position after being disturbed
What is maneuverability?
- the quality of an aircraft that allows it to be maneuvered easily and withstand the stressed imposed in during maneuvers
- affected by weight, size, location of the flight controls, and structural strength
What is controllability?
- the capability of the aircraft to respond to pilots flight control inputs
Longitudinal Stability
- stability about the lateral axis, (pitching)
- tendency for the aircraft to return to reduce its pitching and return to SL
What 3 factors is longitudinal stability dependent on?
- location of wing with respect to CG
- location of horizontal tail surface with respect to CG
- area or size of tail surface
How would a longitudinally unstable aircraft react?
- dive or climb progressively steeper
How can a plane achieve longitudinal stability?
- if the plane is suddenly nosed up, the wing and tail moments must change so that their forces provide a restoring moment and bring the nose back down
Explain why the location of the wing in relation to the CG is important for longitudinal stability?
- CG is usually in front of CL causing nose down pitch
- heavy nose countered by downward force created by horizontal tail (negative AOA) lifting nose
- CG and Tail down are downward forces while CL is un upward force, balancing each other
Explain why the location of the horizontal tail surface in relation to the CG is important for longitudinal stability?
- if plane is loaded for CG further forward = requires more tail down force
- nose heavy is countered by difficulty lowering nose from tail
Why is the plane less stable when loaded aft?
- when CG is behind CL, tail must create upward force so nose doesn’t pitch up
- if gust pitches nose up = less airflow over tail causing nose to pitch further
Why is the area or size of the tail surface important for longitudinal stability?
- larger area/size = larger force
What is lateral stability?
- stability about the longitudinal axis, tendency for aircraft to reduce its rolling and return upright
What 4 things affect lateral stability?
- dihedral
- sweepback wings
- keel effect
- weight distribution
What is dihedral?
- upward angle of the wings (tip higher than root)
- when banked, the low wing produces more lift creating rolling moment in opposite direction of turn
- cost = increased drag and decreased roll rate
What is sweep back wings?
- angle at which the wings are slanted rearward
- when disturbed, the lower wing’s leading edge is at an angle to is perpendicular to the relative wind causing more lift in lowered wing
What is the keel effect?
- action of the relative wind on side area of fuselage
- in high winged airplane, fuselage acts as pendulum returning aircraft horizontal
Why does weight distribution increase lateral stability?
- if more weight is located on one side, aircraft will have tendency to bank in that direction
What is directional stability?
- stability about the vertical axis, tendency for the aircraft to yaw back to original position
- affected by are of vertical fin and sides of fuselage aft of CG
- acts like weathervane pointing nose into relative wind
What is load factor?
- ratio of total air load acting on the airplane to the gross weight of the airplane
- measured in G’s
Why are load factors important to pilots?
- pilot can overload the aircraft
- increased LF increases stall speed making stalls occur at seemingly safe airspeeds
