Principles of Flight Flashcards
What is Newtons first law?
A body will continue in a state of rest or uniform motion unless acted upon by an external force
What is Newtons second law?
The rate of change of momentum of a body is proportional to the applied force (F=ma)
What is Newtons third law?
To every action there is an equal and opposite reaction
What is the formula for Density?
Pressure (P) / Temperature (T) x Constant (R)
What are the four forces that act on an aeroplane?
Lift
Weight
Thrust
Drag
If a plane is in straight and level flight at constant airspeed, how are the forces balanced?
Thrust = Drag
Lift = Weight
Where does lift act?
Through the centre of pressure at 90 degrees to the local airflow over the wing
Where does weight act?
Through the centre of gravity
How does increasing weight affect wing loading?
For a given area, increasing weight increases wing loading
What is the Lift equation?
Lift = CL 1/2 p V^2 S
What is the formula for Aspect Ratio?
Aspect Ratio = Wingspan/Mean Chord
How does the centre of pressure move with increasing angle of attack?
The centre of pressure moves towards the leading edge of the wing as AoA increases, until stall is reached at which point it moves quickly rearwards.
What are the two types of drag?
Parasite Drag and Induced Drag
What are the three types of parasite drag?
Form Drag - Drag created by object shape
Friction Drag - Viscosity of air against the surface
Interference Drag - Spanwise flow disturbing the smooth airflow over the aerofoil
What is Parasite Drag?
Zero Lift Drag
The drag caused by the non-aerodynamic shape of the aircraft. In other words, the resistance to our efforts to push the aeroplane through the air at high speed.
What is Induced Drag?
Aerodynamic Drag
Induced drag is the by-product of creating lift. The amount of induced drag is dependent on the amount of lift being generated. ie Wake Vortices.
What is the Zero Lift (Parasite) Drag equation?
Zero Lift Drag = Cd 1/2 p V^2 S
With respect to drag, when will the aircraft achieve maximum range?
When flown at the airspeed that creates minimum drag. Ie the bottom of the total drag curve.
What happens to the composition of gases as altitude increases and pressure decreases?
The composition (%) of gases remains constant irrespective of pressure or altitude.
As a rule of thumb what is the temperature reduction per every 1000ft?
2 Degrees
Why is an International Standard Atmosphere required?
For aircraft performance calculations.
For standard separation of commercial traffic at altitude
For expected temperatures at altitudes
As a base line to calculate forecasts from
What is Boyles Law?
At a constant temperature, the volume of gas is inversely proportional to its pressure
What is Charles Law?
The volume of a gas at constant temperature is directly proportional to its absolute temperature.
What is Henrys Law?
The amount of gas dissolved in a liquid is proportional to the gas pressure outside of the liquid.
Does air density increase or decrease with altitude?
Decrease
Does air density increase or decrease with increasing temperature?
Air Density decreases with increasing temperature
Is humid air more or less dense than dry air?
Humid air is less dense than dry air. This is because water vapour is less dense than air
Can air of a higher temperature contain more or less water vapour?
Air of a higher temperature can contain more water vapour
Why is Air Density important to flight?
Aircraft Design
Engines are power rated, and Lift capabilties are calculated according to ISA. Any difference from ISA will affect aircraft performance.
Engine Power
Depends on the volume of oxygen available to mix with fuel. Less dense air means less oxygen and hence less power.
Aerodynamic Lift
Depends on air density. Less dense air means less lift. In less dense air, TAS will need to be higher for lift at same IAS.
How to calculate Air Density?
Correct current altitude for ISA pressure differences, attaining Pressure Altitude.
Correct Pressure Altitude for ISA temperature differences, attaining Density Altitude. This is the equivalent ISA altitude that has the same density as our current conditions.
Decribe the Bernoulli Principle
When air is forced through a restriction, it speeds up. Energy is required to speed up the air, which is taken from the pressure and temperature. Pressure and Temperature through the restriction are lower. Known as the Venturi effect. Bernoullis principle relies on Streamline Flow.
What is Static Pressure?
The pressure of the atmosphere caused by the weight of the air. (p)
What is Dynamic Pressure?
Pressure caused by the energy from the motion of the air, as a result of the air density and velocity. (q)
What is Total Pressure?
The total of Static Pressure and Dynamic Pressure.
Total Pressure = p + 1/2 rho V^2
For the same total pressure, if Static Pressure increases, Dynamic Pressure must decrease
What are Streamline Flow and Turbulent Flow?
Streamline Flow - Successive air molecules follow the same steady path
Turbulent Flow - Air molecules do NOT follow a steady path.
Describe how an aerofoil produces lift
- Air at the leading edge splits over the top and bottom
- As the upper wing is half a venturi, it must accelerate
- Static Pressure above the wing is converted to Dynamic Pressure to accelerate the air. (Total Pressure remains the same)
- Static Pressure above the wing therefore reduces
- Static Pressure is higher below the wing than above, forcing the wing upwards, creating lift.
- More significantly, the airflow is directed downwards at the trailing edge of the wing, creating downwash, which produces lift. This is because of Newtons Third Law of opposite and equal reaction.
Describe Three Dimensional Airflow.
- Air flows from leading edge to trailing edge creating lift
- Air flows around the wingtips due to the pressure differential between upper and lower surfaces
- The air on the lower surface is directed outwards, towards the tips, while air on the upper surface is deflected inwards towards the fuselage
- This creates vortex wake and further induced drag
Describe the Boundary Layer
- At the aerofoil surface, air molecules are stationary
- As the distance from aerofoil skin increases, the molecule speed increases until reaching the relative airflow speed
- This layer of decreasing airspeed due to skin friction is known as the Boundary Layer
Describe the Laminar Boundary Layer
A thin layer of air that flows above a smooth surface in streamline flow, that can only be found in a region of decreasing pressure. This occurs from leading edge up to a point of lowest pressure, which is where the maximum point of relative airflow deflection occurs, known as the Transition Point. The laminar boundary layer is very unstable, but has very low skin friction drag.
Describe the Transition Point
The point of lowest pressure where the maximum point of relative airflow deflection occurs.
The Transition Point varies with speed. As the speed increases the transition point moves towards the leading edge resulting in more turbulent flow, and increased drag.
Describe the Turbulent Boundary Layer
Occurs after the Transition Point and made up of Turbulent airflow. The Turbulent Boundary Layer is thicker than the Laminar and has significantly more skin friction drag, however it is more resistant to flow separation.
What is the chord line?
An imaginary straight line joining the leading and trailing edge.
What is the Mean Camber line?
A curved line joining the leading and trailing edge equidistant from the upper and lower surfaces.
What is Camber?
The curvature of the Camber Line. Generally, the greater the camber, the greater the lift.
What is the Angle of Attack?
The angle between the chord line and the relative airflow.
What is the Thickness/Chord Ratio?
Maximum thickness divided by the Chord length
What is the angle of incidence?
The angle between the chord line and the longitudinal centre of the line of the fuselage
What is the amount of lift produced by a wing dependent on?
Air Speed
Air Density
Angle of Attack
Aerofoil Shape (Camber)
Size of the Wing
What is the Coefficient of Lift made up of?
The coefficient of lift is decided by the Aeorofoil Shape (Camber), Planform, Aspect Ratio and Angle of Attack
Describe the Lift Equation
Lift = CL 1/2 p V^2 S
CL = Coefficient of Lift
1/2 p V^2 = Dynamic Air Pressure
S = Wing Surface Area
How does AoA affect lift?
The lift produced by a wing can be increased by altering the angle at which the air is presented to the wing. Higher AoA usually lead to more lift. Highest lift typically occurs at 12 degrees. Additional lift is produced by air hitting the lower surface of the wing as a result of Newtons 3rd Law.
At what angle is lift typically highest?
12 Degrees
What is the centre of pressure?
The Centre of Pressure is the average location of all of the pressure acting upon a body moving through a fluid.
ie the location that all the aerodynamic forces are said to act.
How does the location of the centre of pressure depend on AoA?
The centre of pressure moves forward with increasing AoA
Describe the stagnation point
The stagnation point is the point on the leading edge at which the air splits to go above and below the wing. Stagnation pressure is higher than the air pressure both above and below the wing.
As AoA increases, the stagnation point moves down the leading edge towards the underside of the wing. This is how a leading edge stall warner operates.
How does airspeed affect lift?
Higher airspeed generally means higher lift. However, at speeds over 260kts other factors affect lift and drag, such as compressibility.
In basic terms, what is drag?
Drag is the resistance the air places on the wing, opposing the wings movement through the air. ie Air Resistance.
For a conventional aerofoil what AoA provides best Lift/Drag ratio and why is this important?
Approximately 4 degrees.
This AoA can be translated into an airspeed for a given aircraft to create the ‘Best Glide Speed’
If a constant speed is maintained in a descent, what will a reduction in drag to to rate of descent?
A decrease in drag in a descent will decrease the rate of descent. (Better glide)
How does speed affect drag?
Since Zero Lift Drag = Cd 1/2 p V^2 S
Zero Lift drag increases with the square of the speed of the aircraft. Therefore doubling the airspeed quadruples the drag.
Describe induced drag
- Lift is created via downwash
- Downwash alters the direction of the relative free stream air, creating an average relative airflow over the wings.
- Lift is always produced perpendicular to this relative airflow
- Since the relative airflow is angled, so is the lift vector
- The lift vector can be split into effective lift, which opposes the weight, and induced drag, which opposes thrust.
- Tip vortices are also created, and the spiraling vortex increases the downwash and therefore increases the relative airflow angle, moving the lift vector rearwards and creating more induced drag.
How does induced drag vary with speed?
Induced drag increases inversely with the square of the speed. (Induced Drag increases with lower speed) Therefore induced drag quadruples when airspeed is halved. Therefore, induced drag also increases when AoA increases.
This is why wake turbulence is more significant when aircraft are departing/arriving.
Which factors affect induced drag?
Highest induced drag occurs at minimum clean speed. (ie In a clean configuration at high AoA)
High altitude flight implies higher induced drag, at the same TAS, since AoA will need to be higher to account for reduced air density
Higher weight implies higher induced drag for constant altitude and airspeed, since AoA will need to be higher
Aspect Ratio
Washout (How AoA changes from wing root to tip)
Fitting of Winglets
What techniques should a pilot use to avoid wake turbulence, if they believe they are following a heavier aircraft?
If landing, touch down after the nose wheel touch down point of the heavier aircraft.
If departing, ensure you are airborne before the rotation point of the heavier aircraft
What is the wing planform?
The shape of the wing. Examples are elliptical, rectangular, delta, highly swept etc.
Describe Ground Effect
The ground modifies the downwash, upwash and interferes with wingtip vortices, changing the Total Lift Vector and reducing induced drag.
The wing therefore produces more lift, which can cause a “float” when landing.
Low wing aircraft are more prone to ground effect. It is typically noticeable when within one wing-span above the ground.
What is IAS?
Indicated Airspeed, corrected for Pressure, Instrument and Density errors.
This is the speed shown to the pilot and used to control the aircraft.
If a constant airspeed is maintained in a descent, how will an increase in drag change the rate of descent?
The rate of descent will have to increase to overcome the increased drag.
Is more or less lift required in a turn to maintain level flight?
Since the vertical component of lift must equal weight to maintain vertical flight, we must increase total lift during a turn. This can be achieved by increasing the angle of attack.
In a steep turn, the large increase in lift required also means an increase in drag, so more power is needed for a constant airspeed.
If an aircraft is climbing straight and level, how will a turn affect the rate of climb?
A turn will reduce the rate of climb.
What is the effect of bank angle on Load?
Since we are turning there is an equal and opposite force resisting the turn, known as the centrifugal force. This results in a larger resultant load (increased G). This load increases exponentially for increasing bank.
How does a stall occur?
As the AoA increases, the centre of pressure gradually moves forwards. Airflow separation begins to occur at the upper surface trailing edge and moves forwards.
At the CLmax, airflow separation occurs over the whole wing, Lift reduces drastically and increases turbulence increases drag considerably.
How do weight and speed affect the stalling AoA?
They don’t. The stall for a given clean aerofoil always occurs at the same AoA irrespective of weight/speed.
What factors can affect a stall?
Aerofoil Shape
Such as changes in shape due to ice/contamination
Propeller Slipstream
High power can result in lower stalling speed
Ground Effect
Stall occurs at lower AoA
Centre of Gravity
Stall speed increases if CoG is moved forwards. Increased downforce from tailplane to compensate from forward CoG adds to weight. Stall speed increases as weight increases.
Wing Loading
Stall Speed increases as wing loading increases, either through increased weight or a banked turn.
Altitude
As altitude increases, with no power available to increase speed, lift must be produced by a higher AoA. Stall occurs at a higher TAS, but the same IAS.
Load Factor
Stall speed increases by the square root of the load factor. Increased bank angle result in higher stall speeds
Roughly how much does the stall speed increase for an increasing load factor?
Stall speed increases by roughly half the load factor
Which wing stalls first in climbing and descending turns?
Climbing Turn - Outer Wing stalls first
Descending Turn - Inner Wing stalls first
What types of Stall Warning Devices are there?
Mechanical Stall Warner
Slot Stall Warner
Pre-stall buffet
What must be present for an aircraft to spin?
Stall + Yaw
What is the typical scenario for a spin?
Aircraft turns base onto final, but flies through the final approach. The pilot tightens the turn by increasing the bank, pitching up and applying rudder. This leads to a stall with rudder, meaning a spin.
What is the standard spin recovery?
Close the Throttle
Centralise Ailerons
Identify Spin Direction
Apply and hold full opposite rudder
Ease the control column forward
When the spin has stopped centralise all controls
Level the wings
Ease out of the dive
What is the formula for stall speed in a turn?
Vs(turn) = Vs(normal) * SQRT(1/cos(angle))
What is the purpose of flaps?
To provide increased lift at slower speeds
To provide increased drag for better control at landing speeds
To provide a lower nose attitude during the landing phase
Why is an aircraft more prone to wing drop when flaps are extended?
The spanwise lift distribution becomes more concentrated to the central wing areas
What are the types of flap?
Simple
Split
Slotted
Fowler
What is the purpose of leading edge devices?
Provide a means of delaying airflow separation at high AoA, meaning stall occurs at a higher AoA.
Also to increase lift by altering the camber and surface area of the wing.
Define static and dynamic stability
Static Stability
The ability of the aircraft to self-correct for upsets
Dynamic Stability
The behaviour of the aircraft during the response from static stability correction. Do the corrections reduce, maintain or increase divergence?
Define Positive, Neutral and Static Stability
Positive Static Stability
The tendency to return to an undisturbed state after a disturbance
Neutral Static Stability
The tendency to remain in a disturbed position
Negative Static Stability
The tendency to continue to diverge from its undisturbed state
Match the aircraft axis with the movement
Lateral Axis - Pitching
Longitudinal Axis - Roll
Normal (Vertical) Axis - Yaw
Where is the CoG generally located with reference to the CoP?
The CoG is generally designed to be forward of the CoP, resulting in a pitch down tendency.
How does a high-wing aircraft maintain stability in roll?
Pendulum Effect. The weight of the fuselage will level the wings.
How does dihedral maintain stability in roll?
The dihedral means the upper wings lift vector has a lower vertical component than the lower wing. This means the lower wing has a higher vertical component of lift, tending to raise the lower wing back to equilibrium.
When does spiral instability occur?
When directional stability is too great compared to lateral stability.
Disturbance causes wing drop. Roll causes sideslip. Aircraft yaws into the turn, increasing the roll, snowballing.
How does washout affect the stall?
Washout is a gradual reduction in AoA from wing root to tip. This means the inner wing stalls before the outer section, preventing wing drop.
Why are Trim Tabs necessary?
To relieve the control column forces required by the pilot to maintain attitude
What effect does propeller slipstream have on elevator?
Increased power increases the airflow which equates to better elevator responsiveness
What effect does CoG have on the Elevator?
Forward CoG - Elevator has to work harder to alter the pitch, elevator becomes less sensitive.
Rearward CoG - Elevator finds it easier to alter the pitch, elevator becomes more sensitive.
What is the primary and secondary effect of Aileron?
Primary - Roll
Secondary - Yaw
What is adverse yaw?
The down-going aileron makes the wing produce more lift and hence more drag. The up-going aileron makes the wing produce less lift, and hence less drag. The result is the aircraft yaws towards the up-going wing resulting in adverse yaw.
What are the primary and secondary effects of Rudder?
Primary - Yaw
Secondary - Roll
How is a propeller designed with regards to AoA?
The AoA of a propeller blade is designed to be less at the tip than the hub. This is because the tip will travel faster than the hub when rotating. Therefore the twisted design of reducing angle of attack ensures the same amount of lift is produced across the whole blade.
What is the AoA of a propeller blade dependent on?
Propeller Pitch (Blade Angle)
Forward Velocity
What is the effect on the AoA of a fixed pitch propeller blade as an aircraft accelerates and decelerates?
Accelerates
The AoA will decrease as the relative wind will have more of a component from the aircraft velocity. This means the propeller has less work to do and RPM will increase.
Decelerates
The AoA will increase as the relative wind will have less of a component from the aircraft velocity. The propeller rotation will be more relevant. This means the propeller has more work to do and RPM will decrease.
What is propeller ‘slip’?
The difference between the geometric pitch, aka the design forward movement in one rotation, and the effective pitch, the actual forward movement in one rotation.
How does the propeller affect yaw?
Gyroscopic Forces - Yaw to the left on clockwise prop
Torque - Yaw to the left on clockwise prop
Slipstream - Yaw to the left on clockwise prop
What load must an aircraft in the ‘Utility’ category be capable of handling?
4.4G
What are some causes of load factor increase?
Sharp Bank
Turbulent Air
Wind Gusts
Rough Handling by the Pilot
What is the ASI White Arc?
Flap Operating Range
What is the ASI Green Arc?
Normal Operating Range
What is the ASI Yellow Arc?
Caution Range
What is the ASI Red Line?
Vne = Never Exceed Speed
What are Vs0 and Vs1?
Vs0 = Stall Speed Full Flap Power Off
Vs1 = Stall Speed No Flap Power Off
What is Vfe?
Vfe = Max speed for full flap
What is Vno?
Vno = Maximum structural smooth air cruising speed
What are Vlo and Vle?
Vlo = Max Speed for Gear Operation
Vle = Max Speed for Gear Extension
What are Vx and Vy?
Vx = Best Angle of climb speed
Vy = Best Rate of climb speed
What is Vmc?
Minimum speed to retain directional control on twin engine aircraft
What is Vref?
Landing reference speed at 50ft
What is Va?
Va = Max speed that abrupt control inputs can be made without damaging/over-stressing the airframe
