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
How does the intensity of vortices change the amount of induced drag?
STRONGER VORTICES cause a STRONGER CIRCULATION which causes a GREATER amount of DOWNWASH to be INDUCED
What is the equation for CDin according to Kutta-Joukowski theorem?
What does this equation assume?
F(w) = ρwΓ = πcρ(w^2);
The FORCE is in the DIRECTION of the RELATIVE AIRFLOW, OPPOSITE to the MOVEMENT of the AIRCRAFT so is a DRAG;
The EQUATION was DERIVED under 2D conditions and INDUCED DRAG is a 3D situation, so it is NOT EXACT but displays the NATURE of INDUCED DRAG as it is CAUSED by INDUCED DOWNWASH
What is the derived equations for CDin?
CDin = k(CL^2)/πAR OR CDin = CL^2/eπAR;
Where k and e are CONSTANTS related to TYPE of AEROFOIL, for SYMMETRICAL AEROFOIL k and e = 1
How does angle of attack affect CDin?
Give an example of when this is practically applied?
An INCREASE in AOA will INCREASE the CDin;
During TAKEOFF the AIRCRAFT requires MORE THRUST because a HIGH AOA/CL is required which PRODUCES a HIGH INDUCED DRAG
What is the total drag coefficient?
What does the total drag graph tell us?
The SUM of all DRAGS: CD = CDin + CDskin +CDform + CDinterference;
In LEVEL FLIGHT, since WEIGHT is CONSTANT, when AIRSPEED is INCREASED, AOA will DECREASE for a LOWER CL, when CL is REDUCED, CDin will also SIGNIFICANTLY DECREASE;
Since PARASITE DRAG is related to REYNOLDS NUMBER which INCREASES with INCREASED AIRSPEED the DRAG will also INCREASE to v^2
What happens to the aerodynamic forces when an aircraft is fitted with a thin aerofoil?
PRESSURE DIFFERENTIAL between upper and lower surface is RELATIVELY SMALL CAUSING the VORTICES to be WEAKER;
DOWNWASH is SMALLER therefore there is a LOWER CIRCULATION resulting in a LOW CDin and CL;
With a LOW CL, the STALL AOA will also be LOWER;
Condition for ADVERSE PRESSURE GRADIENT occurs EARLIER;
LESS SURFACE AREA produces LESS SKIN DRAG;
What determine the categories of thickness of an aerofoil?
THIN aerofoil: THICKNESS to CHORD RATIO < 7%;
GENERAL PURPOSE aerofoil: THICKNESS to CHORD RATIO ≈ 10%;
THICK aerofoil: THICKNESS to CHORD RATIO > 15%
What is aspect ratio?
What happens to the aerodynamic forces when an aircraft is fitted with a low aspect ratio aerofoil?
The RATIO of WING SPAN (b) to CHORD (c);
The AIRFLOW REPRESENTS a 3D FLOW causing STRONGER SPAN WISE FLOW;
Span wise flow causes VORTICES behind the wing PRODUCING MORE DOWNWASH;
EFFECTIVE AOA is DECREASED and LOFT VECTOR TILTS BACKWARD;
DECREASING COEFFICIENT of LIFT and INCREASING INDUCED DRAG;
STALL AOA will INCREASE
What happens to the aerodynamic forces when an aircraft is fitted with a cambered aerofoil?
CIRCULATION is GREATER at SAME AOA causing a HIGHER LIFT COEFFICIENT;
Since LIFT COEFFICIENT INCREASES, the AIRSPEED to PRODUCE SAME LIFT at SAME AOA is LOWER L = CL x (ρv^2)/2 x S;
Therefore STALL SPEED and STALL AOA DECREASE particularly when MAXIMUM CAMBER is CLOSE to LEADING EDGE which causes a FORWRD ADVERSE PRESSURE GRADIENT which will cause SEPARATION
What are the typical features of a laminar aerofoil?
Why does it have low drag?
Has a THIN LEADING EDGE that allows LAMINAR AIRFLOW to flow OVER the SURFACE as long as possible;
The LAMINAR SPEED PROFILE graph is much STEEPER than the TURBULENT over the THICKNESS of the BOUNDARY LAYER;
This means the AVERAGE SPEED of the LAMINAR layer is LOWER and therefore the PARTICLES have LESS KINETIC ENERGY;
Because of the LOWER CHANGE in VELOCITY in the Y DIRECTIONS, SHEAR STRESS is LESS in LAMINAR layer τ = μ(∂u/∂y) meaning the SKIN DRAG will also be LOWER;
The POINT of MAXIMUM CAMBER much FURTHER AFT which DELAYS BOUNDARY LAYER SEPARATION which REDUCES FORM DRAG as that is caused by FLOW SEPARATION;
The STREAMLINE SHAPE also creates a more EVEN PRESSURE DISTRIBUTION which emphasises the REDUCED FORM DRAG
What is washout?
What does it do?
The ANGLE of INCIDENCE DECREASES toward the WINGTIP;
This will DECREASE the PRESSURE DIFFERENTIAL above and below the WINGTIP to REDUCE the INTENSITY of the WINGTIP VORTEX which will REDUCE INDUCED DRAG;
DELAYS the SEPARATION at the WINGTIP compared to the WING ROOT which REDUCES the tendency to WING DROP STALL
What is the function of shaped wingtips?
What does a winglet do?
What are some other types of wing tip devices?
REDUCE the INTENSITY of WINGTIP VORTICES;
CHANGES the PRESSURE DISTRIBUTION around WINGTIP, and REFORMS the VORTEX into SMALLER VORTICIES behind wingtip;
WING FENCE;
WINGTIP TANK;
MODIFIED WINGTIP
What are the features which delay/prevent boundary layer separation?
How do they work?
SUCTION: Uses a VACUUM pump to REMOVE STAGNANT PARTICLES from the boundary layer however CONSUMES POWER and ADDS WEIGHT;
BLOWING: Uses BLEED AIR or MEMS (Micro-electro mechanical system) to PUSH STAGNANT PARTICLES to TRAILING EDGE;
VORTEX GENERATOR: Produces MICRO VORTICES to TRANSFER KINETIC ENERGY from MAINSTREAM to BOUNDARY layer;
DROOPING LEADING EDGE SLAT: INCREASES AREA to produce MORE LIFT and forms a SLOT that allows AIRFLOW from BENEATH to ADD KINETIC ENERGY to the BOUNDARY layer;
TRAILING EDGE FLAP: INCREASES EFFECTIVE AOA and wing AREA, which INCREASES CL and LIFT
