Principles of Flight Flashcards
Convert 1 nm to m
1nm = 1852 m
Convert 1 m to ft
1m = 3.28 ft
Convert 1 kg to lbs
1 kg = 2.205 lbs
Convert m/s to kts
m/s = 0.5144 * knots
Newton’s first law?
Body will remain in rest or uniform motion unless acted on by an external force
Newton’s second law?
The acceleration of a body in a straight line is proportional to the force and inversely proportional to the mass
Newton’s third law?
Every action has an equal and opposite reaction
What is power?
Force * Speed
What happens to streamlines when faster air is flowing?
They move closer together
What is mass flow?
Avrho = AreaVelocityDensity
What is dynamic pressure?
‘Kinetic energy’ of air:q = 1/2 * rho * v^2
Convert C to Kelvin?
K = C + 273
What happens to density with increasing humidity?
Density decreases.
ISA mean sea temp?
15C
ISA mean sea pressure?
1013.25 hPa
ISA Mean sea density?
1.225 kg/m^3
ISA Lapse rate?
1.98 C per 1000 ft
Temp of tropopause?
-56.5 C
Difference between true and indicated airspeeds?
Indicated airspeeds are derived from dynamic pressure, can be affeced by density, measuring the ‘energy of the air’, ruler bend effect.True airspeed is the fixed speed of the aircraft in terms of metres/second instead of molecules it passes affected by density.
How is dynamic pressure related to true airspeed and indicated airspeed?
q = 1/2 * rho * (TAS)^2 ~ (IAS)^2So as density drops with altitude, to maintain IAS for flight TAS must increase!
What are the differences between the different airspeeds?
Remember ICE T IPCD (Ice T is pretty cool drink)Ias Cas Instroument & Pressure error removed you getEas Compressibility error removed you getTas Density error removedRemove instrument error from IAS you get CAS, remove Pressure error from CAS you get EAS, remove Compressibility error from
How is IAS related to TAS and density?
IAS = TAS * sqrt(rho/rho0)As you go higher, hitting less particles (IAS) so pressure ratio must get less.
What remains constant through a venturi?
Total pressure (dynamic + static) because we can’t create or destroy energy.Mass flow remains the same (Avrho).
Why does velocity increase in throat of venturi?
Because mass flow remains the same, area decreases so velocity must increase.As a consequence, dynamic pressure increases, and because total pressure must remain constant, static pressure must drop.
What does Bernoulli’s principle state?
Energy in flow must remain constant
How do we use Bernoulli’s principle to generate lift?
Similar to venturi,Faster moving air on the top surface of the wing increases dynamic pressure, reducing static pressure to conserve total pressure.Higher static pressure below the wing causes net force upwards.
How is air density related to Pressure and temperature?
rho ~ p/T
What is pressure drag (form drag)?
The effect of viscosity on the air causes air to lose energy as it goes over the camber.This means that the airflow breaks away at the rear of the cylinder producing wake turbulence.Air behind has velocity now so there is a static pressure inbalance causing form drag.
What is a stagnation point?
A point where the dynamic pressure is at 0. (Flow stops)Static pressure is now total pressure and is at a maximum.
What is leading edge radius?
The leading edge is usually round, the leading edge radius is the radius of the circle on the leading edge.Larger radius less energy lost as less abrubt change in directionHigh speed - Smaller radiusLow speed - Higher radius
What is the chord line?
An imaginary straight line drawn between the centre of the leading edge and the trailing edge.
What is the chord?
The chord is the distance between the leading edge and trailing edge.
What happens at the point of maximum thickness on a wing?
Airflow accelerates the most, maximum lift due to lowest static pressure
What is angle of attack?
Angle between the chord line and the relative air flow
What is the camber line?
An equidistant line between the upper and lower surfaces of the aerofoil.
What is camber?
Distance between the camber line and the chord line.
Name 2 types of camber wing
Positive camberSymmetrical camber
What is thickness to chord ratio?
Ratio of maximum thickness to length of chord
What thickness to chord ratio do low speed wings have?
Higher because airflow over top acclerates faster than below at slower speeds (more lift at lower speeds)
What is the total reaction?
The sum of all the unbalanced forces over the entire aerofoil and it acts through the centre of pressure.
What is the centre of pressure?
The average point where all the unbalanced forces on the wing act.
Where is the centre of pressure on a symmetrical wing?
25% along the chord line
On a symmetric wing what happens to centre of pressure with increasing angles of attack?
Remains in the same place
On a cambered wing what happens to the centre of pressure with increased angles of attack?
As we have to accelerate more air over front, lower static pressure on front so the centre of pressure moves forward.
How can we increase total reaction?
SpeedAngle of attack
What two forces comprise the total reaction?
Lift (Acting perpendicular to RAF)Drag (Acting paralell to RAF)
How is total reaction different on a cambered wing vs symmetrical at 0 degrees angle of attack?
We still get the generation of lift as there is a total reaction present still.
Why does angle of attack increase total reaction?
Greater proportion of air accelerates over the top of the aerofoil increasing pressure differential.
What happens to stagnation point with angle of attack?
Goes lower down the leading edgeOn a cambered wing it pushes centre of pressure forwardOn symmetric it keeps centre of pressure the same same
Why is the stagnation point lower on a cambered wing?
Because of the effect of upwash
On a cambered aerofoil what happens to centre of pressures with increasing angle of attack?
Centre of pressure moves forwardSuction point (lowest pressure point) moves forward
What happens to the centre of pressure on a cambered wing with decreasing angles of attack?
Centre of Pressure Moves backwards
Which two forces increase with angle of attack?
Total reaction which is comprised of:Lift and drag
What is the equation for aerodynamic force?
F = 1/2 * rho * v^2 * S * C = qSCWhere C is the coeficient of aerodynamic force.
What 3 things effect coefficient of lift?
Angle of attackCamberThickness
What is the coefficient of aerodynamic force as a ratio?
Ratio of surface pressure to dynamic pressure
How can we write the equation of lift in terms of IAS?
L ~ (IAS)^2SC
How does coefficient of life vary with angle of attack?
Increases linearly with angle of attack up to stalling angle at C_lmax (max coefficient of lift) where the lift will no longer support the weight
Which forces act through the centre of pressure?
Lift and drag
Where must the centre of pressure be in relation to the centre of gravity?
Centre of pressure must be behind centre of gravity so the aircraft has a nose down tendency
How does the coefficient of lift change for a thicker aerofoil?
Maximum coefficient of lift increasesStalling angle of attack increases
How does camber effect the coefficient of lift?
Produce lift at 0 alpha (Angle of attack)C_lmax higher - max coefficient of lift occurs at higher angle of attack
What is the centre of pressure range for a cambered wing?
23-27% along the chord line
How do flaps increase the coefficient of lift?
They change camber length by changing the chord line
How do flaps affect the Coefficient of lift vs AoA graph?
Cl line is higher/steeper but the critical Angle of attack is smaller
What is surface friction drag?
To do with the boundary layerDue to the viscosity of air, it decelerates as it gets closer to the aerofoil surface.
What is form drag caused by?
Static pressure to the rear of the aerofoil is lower (turbulent airflow with velocity -> higher q) than at the front stagnation point, causing a net force rearwards.
Where does skin friction occur?
Within the boundary layer
What is the drag equation?
D = qSC = 1/2rhov^2SC ~ (IAS)^2SCWhere D is the drag force and C is the coefficient of drag.
What is Cd as a ratio (Coefficient of drag)?
Ratio of drag per unit area and unit dynamic pressure
What type of force is produced between layers in the boundary layer?
Shear force creating drag
What defines the boundary layer?
Where the layer is 99% of the free stream flow.
What type of forces does the boundary layer contain?
Shear forces felt as drag
Where do all the effects of skin friction drag take place?
Within the boundary layerBoundary layer where 99% of the free stream flow is and below
How many forms does the boundary layer have?
2 forms, laminar and turbulent.Boundary layer starts off as laminar flow with velocity increasing steadily up to free stream flow.At some point along the surface we get a turbulent flow, this is unsteady so the layers become mixed.
What are the characteristics of the laminary boundary layer?
Slippery with low energy and drag
What are the characteristics of the turbulent boundary layer?
Draggy with high energy and dragBecause it’s mixing with faster air above
How deep is the laminar boundary layer?
2mm not thick
How deep is the turbulent boundary layer ?
2 cm, thicker than laminar
Why does a laminar flow separate easily?
Separation occurs easily because near the surface of a laminar flow wing velocity gets very low due to skin drag under the boundary layer, so it has less kinetic energy and separates more easily from the surface of the aerofoil.
What is the velocity gradient in a laminar boundary layer?
More shallow
What is the velocity gradient in a turbulent boundary layer?
More steepGet mixing so all similar speed then drops off very close to surface
Why does a turbulent flow separate less easily?
Due to mixing close to the surface , the velocity is greater and the velocity gradient is much steeper. Therefore the higher energy flow closer to the surface of the wing vs laminar flow doesn’t separate as easily.
Why does a turbulent boundary layer cause more skin friction drag?
Faster airflow near the surface due to mixing where skin friction stops particles at the surface means more drag because of the higher speeds near the surface (greater deceleration of particles).
What is the transition point?
Where the boundary layer changes from laminar to turbulent flow.At some point along a wing laminar flow becomes turbulent flow due to adverse pressure gradient.The mixing of the layers begin to happen and the boundary layer thickens quickly as these layers interact due to the turbulet layer being thicker.
What is the adverse pressure gradient?
High pressure below the wing tries to meet lower pressure above the wing slowing upper surface flow, causing separation from the upper surface.
What is the separation point?
The point the airflow separates from the upper surface of the wing
Where is the transition point in relation to the point of maximum thickness of the wing?
Just ahead of it.
What is Reynold’s number?
Ratio of inertial to viscous forces
What does a high Reynold’s number indicate?
Turbulent airflow as inertial forces are dominanat
What does a low Reynold’s number indicate?
Laminar flow as viscous forces dominate
Why does the placement of the thickest part of the wing aft increase the duration of laminar flow?
Because the point of lowest surface pressure occurs at approximately the point of greatest thickness. The further back this point is the less effect the adverse pressure gradient has on the top surface flow.Also because the speed of flow increases more gently to the point of maximum thickness, because of the greater distance from the leading edge to this point, losing less energy these both factors delay the onset of turbulent flow.
Where is the point of max thickness on a modern airliner?
Between 50-60% of the chord as this increases the transition point for laminar to turbulent flow. We get more laminar flow and less surface drag!
What axis does the aircraft pitch around?
Lateral axis
What axis does the aircraft roll around?
Longitudinal axis
What axis does the aircraft yaw around?
Normal axis
What is wing area?
Span * Chord (Average chord)
What is wing taper?
Shorter chord at the tip than at the span
What is taper ratio?
Tip chord/Root chordAlways less than 1.
What is aspect ratio?
Span/Chord (Straight wing)Span^2/Area (Highly tapered wing)
What is aspect ratio for a highly tapered wing?
Span^2/Area
What is aspect ratio for a straight wing?
Span/Chord
What is sweep angle?
Angle between lateral axis and a line at 25% of the chord.
What is the angle of incidence?
Angle at which the wing is attached to the longitudinal axis.Difference between longitudinal axis and the chord of the wing.
What is it called when we decrease angle of incidence from root to tip?
Washout
What is MAC? (Mean aerodynamic chord)
Chord of an imaginary straight wing with similar longitudinal stability properties as actual wing.
What is dihedral?
High angle between wing root and tipAngle of wing relative to lateral axis
What is anhedral?
Wing tip lower than wing rootAngle of wing relative to lateral axis
What is spanwise flow?
3D lateral flow of air from high to low pression region over wingtip
How is a wingtip vortex formed?
Spanwise flow from high->low pressure above the wing, air flows over but also backwards.Vortex is formed.
Why does a vortex sink down?
The downward component of the vortex is greater due to gravity so vortex sinks down.
Which way does a vortex flow on the left/right wing?
Clockwise/Anti-clockwise
What 2 factors affect Vortex intensity?
Pressure differential between upper and lower surfaces (Angle of attack)Tip vortices decrease with increased speed (less time to form)
What is load factor?
Lift/Weight (1 in straight and level, >1 in turn)
How does length of chord affect vortices?
Length of chord increases vortices because it increases the time for them to build up as the parcel of air is acted on for a longer amount of time
What direction is spanwise flow?
Below wing -> Root to tipAbove wing -> Tip to root
What happens to the intensity of vortices along the wing?
Spanwise flow means there are vortices all along the wing but they are strongest at the wing tips and get weaker towards the wing root.
What aspect ratio causes higher vortices?
Low aspect ratio
What 3 factors REDUCE vortex intensity?
Airspeed increase (less time to form vortex)Wing aspect ratio increaseAmount of lift reduces
How do vortices affect upwash and downwash?
Downwash is increased more as vortices are more affected by gravity.
What is effective airflow?
Relative airflow which has been modified by downwash (due to gravity assisting vortices on their way down) and is now referred to as effective airflow.
On a rectangular wing, where is the greatest downwash?
Due to spanwise flow and pressure differential being greatest the wing tips, strongest vortices form here and hence greatest downwash occurs here.
On a rectangular wing where is effective airflow steepest?
Wing tip
On a rectangular wing where is effective airflow shallowest?
Wing root
In 3D , which airflow affects the wing’s aerodynamic forces?
The effective airflow.
What is effective lift?
The lift caused by the effective airflow caused by downwash from vortices.
What is the induced angle of attack?
Angle between Relative airflow and effective airflow caused by downwash from vortices
How does induced drag vary with induced angle of attack?
Increases induced drag
What is the effective angle of attack?
Between the effective airflow and chord line
When is the induced angle of attack larger?
Aircraft is flying at lower IAS (larger angle of attack)The vortices are stronger producing greater downwash:- High angles of attack- Greater wing camber
What happens to effective and induced angle of attack at high speeds?
Downwash decreases (less time for vortices to form)Effective angle of attack and induced angle of attack both decreaseInduced drag decreases
For a given angle of attack, how do smaller vortices effect effective angle of attack and induced angle of attack?
Effective angle of attack is larger, induced angle of attack is smaller.Because induced angle of attack is smaller, induced drag is less.Weaker vortices improve lift to drag ratio
For a given angle of attack, how do strong vortices effect effective angle and induced angle?
Downwash is stronger so effective angle of attack is smaller, induced angle of attack is larger.Induced drag is more.Strong vortices decrease lift to drag ratio.
What happens to the effective angle of attack with span? (Rectangular wing)
Vortices and downwash are greatest at the tips.Induced angle of attack increases and effective angle of attack is relatively smaller than the wing root.Also induced drag greater at wing tip for this reason
Where do we produce the most coefficient of lift on a wing? (Root or tip)?
Due to decreased effective angle of attack at the wing tip.The coefficient of lift is greatest at the wing root (as it’s affected by angle of attack)So we produce most of Cl at the root.
On a straight wing, which parts stalls first?
The wing root, as the effective angle of attack is greater than the wing tips
What happens to a swept wing’s effective angle of attack spanwise?
Less downwash at wing tipsEffective angle of attack higherInduced angle of attack lowerEffective angle of attack influences a stall so wing tips stall first!
What wing shape creates the required amount of lift for the least drag?
Elliptical wing
Why don’t we use a high aspect ratio wing to reduce induced drag? (3)
In needs a stronger, heavier main sparIt’s prone to tip strikesAt high speed, parasite drag is more significant than induced drag so advantages of high aspect wing not relevant here.
For a given aspect ratio, what is the most effective shape of wing?
Elliptical wing (Elliptical pressure distribution)It lifts lots at the rootLess at the tip, so less vortices
Why do we not see many elliptical wing designs?
Expensive to manufacture
What graph do we plot to look at wing effeciency for induced drag?
Ratio section Cl to Wing Cl vs Wingspan
How does taper ratio effect efficiency of a wing?
Increases it, with a taper ratio we reduce induced drag from wingtip vortices and downwash at the wing tip.Our coefficient of lift at the wingtip is greater (more angle of attack as less effective airflow from downwash)
Why does an elliptical wing stall all at once?
Reduced downwash (pressure differential along wing - less vortices)=> Effective airflow is pretty constant-> Effective Angles of attack all pretty constant-> All stalls at once
What does sweepback do to downwash along the span of a wing?
Makes it greater at wing root (Designed to reduce induced drag at tips)
Why do we use a swept tapered wing even though it’s not an efficient design for Cl ?
We have speed, so this design allows for less drag
How do we minimise the effect of an inefficient swept tapered wing design?
Washout, Camber change, thickness
Why does washout make a wing more efficient?
Want less Cl at the wingtips to reduce vortices/downwash/induced drag, this is the reason for washout.
What is wing loading?
Weight/Wing area
How does camber at the tips reduce induced drag?
Reducing camber at the tips reduces the Cl and hence lift production at the tips. This reduces the pressure gradient and hence vortices/downwash/induced drag at the tips.
Why does a high wing loading produce a greater pressure differential?
Surface area is lower so Cl must be greater to produce the same lift.Cl is greater we get a greater pressure differential, more vortices, more downwash, more induced angle of attack and greater induced drag.
What is interferance drag?
Lateral flow due to pressure differential at wingtips.Spanwise flows cause drag when they have to change direction.
What is parasite drag made up of?
Form dragSkin friction dragInterferance drag
What is total drag made up of?
Parasite drag (Form, skin friction, interferance) andInduced drag
Why is form drag related to frontal area?
Because form drag is to do with the pressure differential in front and behind of the surface.Frontal surface area has a direct affect on this.
What are the profile drags?
Skin friction and form drag (pressure differential front and behind of object)Known as this because they depend on the profile (shape and size) of the object
How do we reduce form drag?
Streamlines
4 Factors affecting skin friction drag?
SpeedSurface areaBoundary layer conditions (Turbulent layer causes more than laminar)Surface roughness
How do we reduce interferance drag?
Fillets and fairings at junctions
How does parasite drag increase with speed?
Parasite drag ~ V^2
What do flaps do to the V^2 relationship of parasite drag and speed?
Pivot it up
What affects the coefficient of parasite drag? (3)
Aircraft’s shape and amount of streamliningSurface roughnessThe presence of fillets/fairings
What are the main influences on induced drag (3)?
SpeedAircraft WeightManoeuvre
What is the relationship between induced drag and speed?
Induced drag ~ 1/(IAS)^2Because induced drag depends on Coefficient of lift! Coefficient of lift also depends on speed.
What does weight do to the induced drag curve?
Pivots it from the other end (higher airspeed) as inverse square relationship
What is the Coefficient of induced drag dependant on?
Cdi = Cl^2/Aspect ratioWhere Cdi is coefficient of drag and Cl is coefficient of lift.
What is speed stability?
On a total drag curve to maintain a certain speed, drag must equal thrust.Where we plot this constant line on the y axis across the x axis for a given thrust / drag, it intercepts the curve at two points. On the right side of the curve, disturbances correct themselvesi.e Speed increase -> Drag increase -> Speed reduceOn the right we get speed stability
What is speed instability?
On the left of the total drag curve we get more drag at reduced speeds due to induced drag, this drag reduces speed further and speed is said to be unstable.Airspeed disturbances are amplified
How does total drag curve change with density?
It doesn’t
How does total drag curve change with density if we plot it again TAS instead of IAS?
Shifts to the right, same drag profile (same dynamic pressure) but occurs at higher TAS speeds.
How does the total drag curve change with increasing weight?
More induced drag is generated with weight (pivots the induced drag curve).
Parasite drag doesn’t change
The curve moves up and right from the induced drag addition.
How does the total drag curve change with changed configuration?
Flaps slats and landing gear increase parasite drag.
Curve moves up and to the left from the pivoting caused by these changes to the parasite drag curve.
How does Coefficient of parasite drag vary with Coefficient of lift?
It doesn’t until we reach Cl max where turbulence from separation causes an increase in form drag
Offset from origin as parasite drag as soon as we’re moving, don’t have to be generating lift
How does coefficient of induced drag vary with coefficient of lift?
Varies with Cl^2 so we get increases of Cdi with Cl^2 exponentially.
Goes through origin as no Cl -> No induced drag
How does polar drag curve go?
Offset from parasite drag, looks like induced drag curve.. (doesn’t go through origin due to offset)
Cl (y) vs Cd (x)
Induced drag curve Cdi ~ Cl^2
What does the drag polar show?
Measure of the efficiency of the wing at different coefficients of lift.
How do we find the best Cl/Cd ratio?
Draw a tangent to the curve (steepest line from origin) of the Cl vs Cd graph.
At any other point, Cl/Cd decreases.
What is a typical angle of attack Cl/Cd occurs at?
4 degrees
What happens in a stall?
Airflow separation from the wing
What happens in a stall in swept back wings?
Tips stall first
Center of pressure moves forward
Potential to pitch nose up if C of P moves ahead of C of G
What is a deep stall?
Occurs in aircraft with sweepback
Forward movement of Centre of Pressure when tips stall first.
Causes nose to pitch up, if horizontal stab has insufficient nose down authority, aircraft will pitch further into stall.
How does mass affect stall speed?
Mass increases weight -> We require more lift.
Clmax is fixed for all angles of attack
We need more speed so stall speed increases!
What happens to stalling speed with gear down?
Increases because gear causes a drag force back (form drag) forming a moment with the c of g causing a nose down moment.
Increases lift force required from the wing as tailplane downforce to balance gear has to be balanced from lift from wing.
How does thrust affect stall speed?
For props it decreases it as extra wash over wing generates lift reducing overall lift required.
More upward component from thrust less lift required so stalling speed decrease.
How does C of G affect stall speed?
Further forward C of G created more moment from lift causing nose down which has to be balanced by downforce from the tail plane, increasing the amount of lift to rebalance it.
Increases the stall speed
How does sweepback affect Clmax?
Swept wings don’t produce lift as effectively because swept wing is not perpendicular to the airflow. -> Smaller value for Clmax.
Smaller value for Clmax means increase in stalling airspeed to balance weight
How does altitude affect stall speed?
Compressibility will increase once TAS reaches above 300 kts, increasing stall speed as energy required for compression.
How does load factor increase in a bank turn?
Load factor = L/W = 1/cos(theta)
Purpose of wing fences on swept back wings?
Improve slow speed handling as spanwise flow makes vortices and downwash smallest at wingtips maximising effective angle of attack and causing the wingtips to stall first.
Wingtips stalling first on swept back wings is bad as the centre of pressure moves forward causing a pitch up moment.
Wing fences prevent this spanwise flow
How do we relate Vs to Vs1g?
Vs = 0.94*Vs1g
In cs23, what is takeoff Vs1 (takeoff) (configuration) margain for stall speed?
1.2
In cs23, what is takeoff Vs0 (landing) (configuration) margain for stall speed?
1.3
In cs25, what is takeoff Vsr1 (takeoff) (configuration) margain for stall speed?
1.13
In cs25, what is takeoff Vsr0 (landing) (configuration) margain for stall speed?
1.23
