Aerodynamic Forces Flashcards
What is the NACA 4 digit system?
What do each of the digits represent?
A system used to DESCRIBE the GEOMETRIC FEATURES of an AEROFOIL;
FIRST digit: MAXIMUM CAMBER as a PERCENTAGE of the CHORD;
SECOND digit: LOCATION of the MAXIMUM CAMBER from the LEADING EDGE in TENTHS of the CHORD;
THIRD/FOURTH digit: MAXIMUM THICKNESS as a PERCENTAGE of the CHORD
What is the pressure coefficient?
What is the equation?
The RATIO of PRESSURE DIFFERENCE between LOCAL PRESSURE and FREE STREAM PRESSURE to DYNAMIC PRESSURE;
Cp = (p - pfs)/q
What is the line of zero lift?
Is zero lift pitching moment the pitching moment when angle of attack = 0?
The EDGEWISE STRAIGHT LINE THROUGH the AEROFOIL, which is PARALLEL to AIRFLOW, when the AEROFOIL POINTS DOWNWARDS and PRODUCES NO LIFT;
If the AEROFOIL is SYMMETRICAL the ZERO LIFT PITCHING MOMENT will happen when AoA is 0;
If the AEROFOIL is CAMBERED the ZERO LIFT PITCHING MOMENT will occur at whatever AoA that DOES NOT PRODUCE LIFT, for CONVENTIONAL SUBSONIC usually around -4
What is an infinite wing?
An IDEALISED WING with an INFINITE WING SPAN or WITHOUT a WINGTIP over which the AIR FLOW is 2D ie: VERTICAL and CHORD WISE;
A WING with a very LARGE ASPECT RATIO can be REGARDED as INFINITE WING and one WITHOUT a WINGTIP can be REGARDED as ELLIPTICAL WING
What is a finite wing?
Any REAL WING with a LIMITED WING SPAN with a WINGTIP over which the AIRFLOW is 3D ie: VERTICAL, CHORD WISE and SPAN WISE
What is Bernoulli’s theory of lift?
What are the limitations?
P = p + (ρv^2)/2
The TOTAL PRESSURE for every STREAMLINE is CONSTANT in FREE STREAM before LEADING EDGE;
OVER the aerofoil the FLOW PATH between streamlines NARROW and the AIRSPEED INCREASES;
UNDER the aerofoil the FLOW PATH between streamlines WIDENS and the AIRSPEED DECREASES;
This means the PRESSURE OVER is LOWER than the PRESSURE UNDERNEATH so the aerofoil will produce LIFT;
1D STREAMLINE, LEVEL fluid flow applying to INFINITE WINGS (NO SPAN WISE or VERTICAL);
STEADY STATE fluid flow;
INCOMPRESSIBLE fluid (NO FASTER than HIGH SUBSONIC, TRANSONIC or SUPERSONIC);
NON-VISCOUS/NO FRICTION
NO BOUNDARY LAYER SEPARATION;
CANNOT EXPLAIN UPWASH and DOWNWASH
What is Newton’s theory of lift?
What are the limitations?
FΔt = (-mw - mw) = -2mw;
Describes the RELATIONSHIP between MOTION CHANGE and FORCE on the system due to the VERTICAL CHANGE in AIR FLOW when experiencing UPWASH and DOWNWASH;
It assumes the MAGNITUDE of UPWASH is EQUAL to DOWNWASH therefore change in momentum is -2mw as the air switches 180°;
The FORCE is APPLIED BY the AEROFOIL on the AIRSTREAM which ACTS DOWNWARD, meaning the REACTION FORCE from the AIRSTREAM to the AEROFOIL ACTS UPWARD with SAME MAGNITUDE providing it with LIFT;
2D fluid flow applying to INFINITE WINGS (NO SPAN WISE);
NO BOUNDARY LAYER SEPARATION;
NON-VISCOUS/NO FRICTION;
CANNOT EXPLAIN why there is UPWASH and DOWNWASH
What is the Euler/Navier-Stokes equation of lift?
What are the limitations?
A series of very ACCURATE PARTIAL DIFFERENTIAL equations including CONSERVATION of MASS, ENERGY and MOMENTUM and IDEAL GAS LAW used to DESCRIBE 1, 2, OR 3 DIMENSIONAL airflow, which is COMPRESSIBLE OR INCOMPRESSIBLE and can be LOCATED ANYWHERE ie: BOUNDARY LAYER, OVER WING, AROUND FUSELAGE;
It is DIFFICULT to SOLVE, requires COMPREHENSIVE UNDERSTANDING and ASSUMPTIONS are used for BOUNDARY CONDITIONS and VISCOSITY
What is the circulation theory?
How is the lift produced?
Assumes FREE STREAM air VELOCITY is HORIZONTAL and UNIFORM;
The UPWASH at the LEADING EDGE is the FREE STREAM air VELOCITY + a SMALL amount of VERTICAL VELOCITY w+ and the DOWNWASH at the TRAILING EDGE is FREE STREAM air VELOCITY - a SMALL amount of VERTICAL VELOCITY w-;
The LOCAL air VELOCITY ABOVE the aerofoil is FREE STREAM air VELOCITY + a SMALL amount of HORIZONTAL VELOCITY w^+ and the LOCAL air VELOCITY BELOW the aerofoil is FREE STREAM air VELOCITY - a SMALL amount of HORIZONTAL VELOCITY w^-;
This means the AIRFLOW around an aerofoil is the SUPERPOSITION of a UNIFORM FREE STREAM AIRFLOW and a CIRCULATION;
A FORCE is GENERATED when a CIRCULATION EXISTS in a 2D AIRSTREAM at a VELOCITY OR a CIRCULATION TRAVELS in air with a VELOCITY;
The FORCE is PERPENDICULAR to the AIRSPEED and DIRECTION of CIRCULATION and in the case of LIFT, ACTS VERTICALLY UPWARD
What are the conditions and limitations of the circulation theory of lift?
2D fluid flow applying to INFINITE WING (NO SPAN WISE flow);
NO BOUNDARY LAYER SEPARATION;
NON-VISCOUS/NO FRICTION;
Must be a THIN SYMMETRICAL AEROFOIL (CL = CL0 + 2kπα);
AIR PARTICLES must LEAVE TRAILING EDGE EVENLY;
OBJECT TRAVELS at a CONSTANT SPEED
What is the Kutta-Joukowski theorem?
What is the equation?
What does each component represent?
A FORCE per UNIT LENGTH acting at RIGHT ANGLES to the AIRSTREAM EQUALS the PRODUCT of air DENSITY, the VELOCITY of the air and the CIRCULATION; F = ρvΓ; ρ: DENSITY of air (kg/m^3) v: VELOCITY of MAIN STREAM air (m/s) Γ: CIRCULATION (m^2/s)
What is circulation?
The INTEGRAL of the PRODUCT of LOCAL VELOCITY and FLOW PATH;
Γ = ∮vds
What does each finger represent in the right hand law?
X/INDEX FINGER: DIRECTION of VELOCITY
Y/MIDDLE FINGER: DIRECTION of CIRCULATION
Z/THUMB: DIRECTION of FORCE
Explain how angle of attack is related to coefficient of lift?
.
What is drag?
What are the 2 main categories of drag?
A COMPONENT of AERODYNAMIC FORCE that acts in the DIRECTION of RELATIVE AIRFLOW;
PARASITE drag: Due to the AIRFLOW AROUND an AIRCRAFT and impacted by SHAPE and QUALITY of the SURFACE;
INDUCED drag: PRODUCED by AEROFOILS and DOWN WASH
What is skin drag?
What factors affect it?
The VISCOUS FRICTION in the BOUNDARY LAYER due to a CHANGE in AIR PARTICLE VELOCITY in the Y DIRECTION (∂v/∂y > 0);
SKIN DRAG COEFFICIENT is related to air FLOW REGIME, air PROPERTIES and QUALITY of a SURFACE and the MAGNITUDE can be related to REYNOLDS NUMBER;
An UNSMOOTH SURFACE can TRIGGER an EARLIER TURBULENT BOUNDARY layer which draws MORE ENERGY from the MAINSTREAM airflow to INCREASE PARTICLE SPEED therefore COEFFICIENT is HIGHER;
An INCREASE in AIRSPEED will INCREASE REYNOLDS NUMBER which will CHANGE the FLOW REGIME causing a DECREASE in COEFFICIENT of SKIN DRAG but as the DECREASE is PROPORTIONAL to Re^(0.2) which is RELATIVELY SMALL compared to v^2 in D = CD + (ρv^2)/2 therefore an INCREASE in AIRSPEED will INCREASE SKIN DRAG
What is form drag?
What does the intensity depend on?
Caused by the flow SEPARATION of a STREAMLINE flow;
The SEPARATION causes VORTICES to FORM and DETACH from the SURFACE FORMING a VORTEX WAKE;
The VORTICES cause a LOCAL LOW PRESSURE, where the FRONT PRESSURE of the object is HIGHER than the BACK PRESSURE, this PRESSURE DIFFERENTIAL is DRAG to the moving object;
The SEPARATION VARIES with SHAPE and REYNOLDS NUMBER of fluid FLOW;
A STRONGER FLOW SEPARATION results in a HIGHER COEFFICIENT of FORM DRAG which can be caused by a NON-STREAMLINE shape which is RELATED to REYNOLDS NUMBER;
DIRECTLY AFFECTED by TAS as REYNOLDS NUMBER CHANGES with TAS;
INDIRECTLY AFFECTED by AOA as it may CHANGE when SEPARATION OCCURS CHANGING the amount of FORM DRAG
What does vortex wake depend on?
REYNOLDS NUMBER: When this is LARGE, the VORTEX WAKE is STRONG. Since the VORTEX WAKE draws ENERGY from the MAIN AIRFLOW, the MAIN AIRFLOW will WEAKEN when the VORTEX is FORMED;
SHAPE of the OBJECT
How are the components of profile drag related?
PROFILE DRAG is made up of FORM and SKIN DRAG which make up different PROPORTIONS DEPENDING on the PHYSICAL APPEARANCE of the OBJECT;
If FLOW SEPARATION is SEVERE ie: VERTICAL DISK, FORM DRAG is DOMINANT ie: 90% FORM 10% SKIN;
If FLOW SEPARATION is GRADUAL ie: STREAMLINE AEROFOIL SKIN DRAG is DOMINANT ie: 90% SKIM 10% SKIN
What is interference drag?
How is it formed?
What factors affect coefficient of form drag?
A DRAG caused by the INTERACTION of AIRFLOW and WAKES at the JOINTS of an AIRCRAFT where the AIRFLOWS CANNOT PROCEED in their ORIGINAL PATTERN so will INTRUDE into each OTHERS PATH CAUSING COMPRESSION that causes LOCAL PRESSURE CHANGE, TURBULENCE and FLOW SEPARATION;
WAKES FORM and DETACH from bodies and PROPAGATE in air;
Some WAKES will ENCOUNTER another part of the SOLID BODY and REFLECT, producing NOISE;
Some WAKES will INTERCEPT each OTHER and be DEFLECTED/REFLECTED but if the CREST of 2 WAKES MEET, CONSTRUCTIVE INTERFERENCE may occur which CAUSES STRONG OSCILLATION and DISSIPATES significant KINETIC ENERGY from MAIN STREAM
What factors affect coefficient of interference drag?
What can be done to decrease interference drag?
SHAPE of BODY;
AIRSPEED/REYNOLDS NUMBER;
MANOEUVRE of FLIGHT;
DESIGN a SMOOTH PRESSURE DISTRIBUTION at JOINT ZONES to AVOID CONSTRUCTIVE INTERFERENCE of PRESSURE WAVES
How do trailing edge vortices form?
TRAILING EDGE VORTICES only form on FINITE WINGS that have SPAN WISE FLOW;
ABOVE the aerofoil the SPAN WISE airflow travels TOWARDS the FUSELAGE due to the lower pressure compared to the FREE STREAM;
BELOW the aerofoil the SPAN WISE airflow travels TOWARDS the WINGTIP due to the HIGHER PRESSURE compared to FREE STREAM;
The 2 SPAN WISE air flows MEET at the TRAILING EDGE of the wing and since air has VISCOUS fluid PROPERTIES there is a TENDENCY for air PARTICLES to INTERACT and MOVE TOGETHER;
This MOVEMENT causes a ROTATIONAL MOTION at the TRAILING EDGE, PRODUCING trailing edge VORTICES
How do wingtip vortices form?**
TRAILING EDGE VORTICES only form on FINITE WINGS that have SPAN WISE FLOW;
ABOVE the aerofoil the SPAN WISE airflow travels TOWARDS the FUSELAGE due to the LOWER PRESSURE compared to the FREE STREAM;
BELOW the aerofoil the SPAN WISE airflow travels TOWARDS the WINGTIP due to the HIGHER PRESSURE compared to FREE STREAM;
The DIFFERENCES in PRESSURE at the WINGTIP drive the AIRFLOW into a ROTATING MOTION providing the WINGTIP AIRFLOW with a TORQUE;
This ROTATION results in the FORMATION of WINGTIP VORTICES
How is induced drag formed?
When TRAILING EDGE and WINGTIP VORTICES are FORMED the PRESSURE in the CENTRE of the VORTEX is LOWER than the OUTSIDE PRESSURE;
The DIFFERENCE in PRESSURE the RELATIVE AIRFLOW will CHANGE DIRECTION, INDUCING DOWNWASH;
The CHANGE in DIRECTION REDUCES the EFFECTIVE AOA causing the LIFT VECTOR to TILT BACKWARD as it ACTS PERPENDICULAR to RELATIVE AIRFLOW;
The HORIZONTAL COMPONENT of the LIFT VECTOR is the INDUCED DRAG