Aerodynamics Ch 5 PHAK Flashcards
What are the four forces acting on an aircraft
Thrust
Drag
Lift
Weight
What is thrust?
forward force produced by the powerplant/propeller/rotor
What is drag?
a rearward, retarding force cause by disruption of airflow by the wing, rotor, fuselage, and other protruding objects
What is lift?
force produce by dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path through center of lift and perpendicular to the lateral axis
What is weight?
combined load of the aircraft itself, crew, fuel, cargo and baggage.
force that pulls the aircraft downward by gravity
opposes lift and acts vertically downward through aircrafts CG
Newton’s third law?
for every action or force there is an equal, but opposite, reaction or force
In steady flight are all forces equal?
No
The sum of all upward components of forces (not just lift) equals the sum of all downward components of forces (not just weight)
The sum of all forward components of forces (not just thrust) equals the sum of all backward components of forces (not just drag)
When the flightpath of an aircraft is not horizontal, lift, weight, thrust, and drag vectors and how many components?
2 components
What is angle of attack? (AOA)
acute angle between the chord line and direction of relative wind
Describe the relation ship between thrust and drag
When power is increased, thrust is greater than drag causing the aircraft to accelerate until drag equals thrust
When power is decreased, thrust is less than drag causing the aircraft to slow down until drag reduces to equal thrust
In order to sustain straight and level flight at all speed regimes, what 2 things must be coordinated?
Thrust and AOA
What are the three speed regimes of flight?
Low-speed flight
Cruising flight
High-speed flight
Level unaccelerated slow flight requires what kind of AOA?
high AOA
In slow flight, what components of lift exist?
Lift generated by the wings (increased AOA) and lift generated by thrust (nose high attitude)
At very high speeds, what kind of AOA is possible?
slightly negative AOA
What is critical AOA?
The point at which a wing will stall
What control input influences AOA?
Moving the yoke forward and aft
What is stalling AOA?
CL-Max or critical AOA
Point at which lift diminishes quickly
Lift is proportional to?
The square of the aircraft’s velocity
aircraft traveling 200KIAS has 4x the amount of lift an aircraft traveling at 100KIAS does
As an aircraft increases velocity, what changes must be made to the AOA?
the AOA must be decreased to counteract the increase of lift produced as velocity increases
Does an airfoil always stall at the same AOA?
yes
Density is affected by what factors?
pressure, temperature and humidity
At 18000 feet what is the density of air relative to sea level?
one half the density of sea level air; to maintain lift an aircraft must fly at a higher true airspeed
Warm air is less dense than?
Cool air
Moist air is less dense than?
dry air
Lift varies with wing area. How much lift would a 200sqft wing produced compared to a wing with 100sqft
It produces twice as much lift
what is the lift-to-drag ratio? (L/D)
amount of lift generated by a wing or airfoil compared to its drag
What two major aerodynamic factors can be controlled by a pilot’s inputs?
lift and airspeed
Aircraft with higher L/D indicates?
higher airfoil efficiency
Aircraft with lower L/D indicates?
lower airfoil efficiency
How is the coefficient of drag affected when changes in flight attitude are made at low AOA
slight changes in CD occur at regimes requiring low AOA
How is the coefficient of drag affected when changes in flight attitude are made at high AOA
significant changes in CD occur at regimes requiring high AOA
The configuration of an aircraft has a great effect on what?
L/D
Lift-to-drag ratio
What are the two basic types of drag?
Parasite and induced drag
Define/describe parasite drag
Drag that is not associated with the production of lift
examples are displacement of the air by the aircraft, turbulence and hindrance of air moving over the surface of the aircraft
What are the 3 types of parasite drag?
Form, interference and skin friction drag
Define/describe form drag
parasitic drag produced by the aircrafts shape
engine cowling, antennas, aerodynamic shape of other components
Define/describe interference drag
- the intersection of airstreams that create eddy currents, turbulence or restricts smooth airflow
- intersection of the wing and fuselage at the wing root; the two different air currents collide creating a new air current
- most interference drag is observed when two surfaces meet at perpendicular angles
Define/describe skin friction drag
- aerodynamic resistance due to contact of moving air with the surface of an aircraft
What is the free stream velocity?
Separation of the boundary layer results in what?
A larger reduction of lift
How is skin friction drag reduced?
flush mount rivets and a smooth glossy finish
Define/describe induced drag
no system that does mechanical work is 100% efficient
it is the aerodynamic penalty of the production of lift
Bernoulli’s principle?
an increase in the speed of a parcel of fluid occurs simultaneously with a decrease in either the pressure or the height above a datum.
What is the source of induced drag?
Wingtip vortices; the energy it takes to create downwash and vortices creates drag
Describe the upwash and downwash of wingtip vortices
Wingtip vortices create an upwash at the wing tip and a downwash behind the trailing edge of the wing
The greater the size and strength of wingtip vortices and consequent downwash…
the greater the induced drag effect becomes
What is CG considered?
the point at which all the weight of the aircraft is concentrated
Why is CG important?
the position of CG has great bearing upon stability
Center of gravity vs center of pressure
CG - the point at which all the weight of the aircraft is concentrated
CP - point at which aerodynamic forces of lift occur
What happens when the CG is forward of the CP?
pitch down tendency occurs
What happens when the CP is forward of the CG?
Pitch up tendency occurs; the aft limit of the CG is always forward of the CP to retain flight equilibriium
Describe the pressure around an airfoil at a positive AOA
Air above the airfoil is less than atmospheric pressure
Air below the airfoil is equal to or greater than atmospheric pressure
Since pressure flows from high to low, this causes spanwise movement of air from bottom of wing at root to the wingtip
Describe spanwise flow as it relates to wingtip vortices
Since pressure flows from high to low, this causes spanwise movement of air from bottom of wing at root to the wingtip
The air on top of the airfoil has a tendency to travel inward toward the wing root limited by the fuselage (insignificant)
Why does an increase in AOA creates more induced drag?
greater pressure differential between the top and bottom of the wing and a greater lateral flow of air; results in more violent wingtip vortices
When will an aircraft create maximum strength wingtip vortices?
takeoff, climb and landing phases of flight
How to avoid wake turbulence?
- avoid flying through an aircrafts flightpath
- rotate prior to the point of the preceding aircraft rotation
- avoid following another aircraft’s similar flight path when they are within 1000 feet of your altitude
- approach runway above preceding aircraft’s path when landing and touchdown beyond its touchdown point
Distance you should avoid a helicopters rotorwash?
at least 3 disc diameter lengths
Wingtip vortices drift with the wind at what speed?
- The same speed as the wind; consider windspeed and direction when following another aircraft
- 3 minutes provides a margin of safety for wake turbulence dissapation
Distance above ground that an aircraft comes into ground effect?
enters ground effect about one wingspan above the ground, altering the upwash, downwash and wingtip vortices
How does high AOA interact with downwash?
high angles of attack produce strong downwash
What will an aircraft leaving ground effect experience/require?
- require an increase of AOA
- an increase in induced drag and thrust required
- decrease in stability and a nose-up change in moment
- reduction in static source pressure and an increase in indicated airspeed
Where do the axes of an aircraft all converge?
The CG
What is stability?
inherent quality of an aircraft to correct for conditions that may disturb its equilibrium and then return to its original flight path
Limitations on maximum performance and maneuverability of an aircraft?
aerodynamic characteristics
propulsion system
structural strength
What are the 2 types of stability?
Static and dynamic stability
What is positive static stability?
initial tendency of the aircraft to return to the original state of equilibrium after being disturbed
Neutral static stability?
initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed
Negative static stability?
initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed
What is dynamic stability?
refers to the aircraft response over time when disturbed from a given pitch, yaw, or bank
3 types of dynamic stability?
Positive, neutral and negative dynamic stability
Positive dynamic stability?
over time, the motion of the displaced object decreases in amplitude and returns toward equilibrium
Neutral dynamic stability?
once displaced, the object neither decreases nor increases in amplitude.
Negative dynamic stability?
over time the motion of the displaced object increase and becomes more divergent
Stability in an aircraft affects two areas significantly which are?
Maneuverability and controllability
Define maneuverability
quality of an aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers
governed by weight, inertia, size location of flight controls, structural strength and powerplant
Define controllability
capability of an aircraft to respond to the pilot’s control especially with regard to flight path and attitude
quality of the response to control application when maneuvering the aircraft
Define longitudinal stability
- quality that makes an aircraft stable about its lateral axis
- longitudinally unstable aircraft tends to dive and climb progressively into steeper attitudes
Static longitudinal stability is dependent on what three factors?
- location of the wing with respect to the CG
- location of the horizontal tail surfaces with respect to the CG
- area or size of the tail surfaces
How do you obtain static longitudinal stability?
/relation of wing and tail moments must be balanced so that if a nose up or nose down tendency is to occur, the aircraft will correct on its own through an unbalanced and restoring moment
