Principles of Flight Flashcards
Wing loading
Aeroplane weight / wing area
in kg/m(2)
Boundary Layer
Layer of air next to aerofoil, few mm thick
Air outside boundary layer (term)
Freeflow airflow
Streamline flow
Each air molecule has the same velocity and static pressure as preceding molecules
Turbulent flow
Each molecule follows a different path to the one preceding it
Transition point
Point where boundary layer thickens and flow starts to become turbulent
Separation point
Point where boundary layer separates from surface of the aerofoil
Compressibility of air
This is a factor above 300kts but ignored for PPL
Total energy equation
Pressure energy + kinetic energy = constant total energy
Effect implied by total energy equation
Venturi effect - an increase in kinetic energy must result in a decrease in pressure energy
Kinetic energy equation
1/2 * Mass * V(2)
Dynamic pressure equation
1/2 * rho * V(2)
where rho is air density
Relationship between kinetic energy and dynamic pressure
Essentially equivalent, rho represents air density which is the equivalent of mass when considering a volume of air
Total pressure equation
Static pressure + dynamic pressure = constant total pressure
Bournoulli’s principal
Increased velocity results in decreased static pressure (and opposite)
Direction of force on aerofoil and how it is represented in components
Camber & mean camber line
Curvature in the shape of the aerofoil.
Mean camber line is half way between top and bottom edge of aerofoil at all points.
Chord line
Straight line joining each end of the camber line (i.e. furthest points of the aerofoil).
Chord
Distance from one end of aerofoil to another (i.e. leading edge to trailing edge)
Purpose of camber
Increases lift by increasing acceleration of airflow over the cambered part of aerofoil.
Higher lift at a given speed and angle of attack (i.e. allows lower AoA or lower speed)
Aerofoil diagram
- Leading edge & LE radius
- Max camber & location
- Max thickness & location
- Chord & mean camber lines
Angle of Attack
Angle between chord line and direction of relative air flow
Angle of Incidence
Angle between chord line of wing and longitudinal axis of aircraft (fixed, unlike AoA)
Point through which total reaction on aerofoil is said to act
Centre of Pressure
Relationship between centre of pressure and angle of attack
At cruise (about 4 deg AoA) CoP is around middle of chord line.
As AoA increases it moves forward to about 20% along chord line.
At critical angle it starts to move backwards.
Equation for Lift
Lift = C(Lift) * 1/2 * rho * V(2) * S
Relationship between C(Lift) and AoA
Vertical moment forces on aeroplane
Effect on moment forces of AoA changes
At cruise a downforce at tail is required to offset moment forces (lift & weight).
As speed reduces and AoA increases, CoP moves forwards which reduces the moment and therefore the downforce required at tail.
Lift from symmetrical aerofoil against AoA (compared to cambered)
Similar relationship but symmetrical has zero lift at zero AoA.
C(Lift) profile (vs AoA) is therefore parallel but slightly lower than cambered profile.
Laminar Flow wing
Lower camber and thickness than typical (cambered) wing.
Better streamline flow over surface therefore less drag, suitable for high speed craft.
Has a higher stall speed however and lower maximum C(Lift).
[note same level of lift force can be generated but requires higher speed]
Induced drag
Drag resulting from the production of lift, i.e. wing vortices at trailing edge and wingtips
Parasite drag (3 types)
All drag other than induced drag
e.g. form drag, skin friction, interference drag
Skin friction
- Definition
- Factors (6)
Friction forces between air and aircraft skin.
- Surface area of aircraft
- If boundary layer is turbulent or laminar (more friction from turbulent)
- Surface roughness
- Airspeed
- Aerofoil thickness
- AoA
Form drag
Drag due to the shape of the object.
Can be combatted with streamlined design (e.g. fairing around landing gear)
Interference Drag
Caused by flow interference around junctions in aircraft shape
Relationship of parasite drag with speed
Parasite drag is zero at zero speed and increases with the square of speed
Wing design features to limit induced drag
- High aspect ratio
- Tapered wings (narrower at tip than at root)
- Geometric Washout
- Wingtip modification
Aspect ratio
Issues around high aspect ratio
= span / chord
High aspect ratio reduces induced drag, however it is structurally challenging and parasite drag is slightly increased.
Washout
Higher AoA at root of wing than at tips, reduces induced drag by generating less drag at tips and more closer to the root.
Induced drag vs airspeed
Total drag vs airspeed
Formula for drag
Drag = C(Drag) * 1/2 * rho * V(2) * S
C(drag) vs angle of attack
How Lift/Drag is optimised
Importance of this
Lift/Drag = C(Lift) / C(Drag)
[other components 1/2 rho v(2) S cancel out]
This varies based on angle of attack.
Highest Lift/Drag ratio gives the most efficient flight configuration.
Lift/Drag ratio plotted against AoA
Formula for IAS
IAS = 1/2 * rho * v(2)
[this is a relationship, not a true equality]
Propeller blade face
The face of the blade faces the airflow. It is the “back” of the blade when looking at the propeller from in front of the aircraft.
Blade angle
Angle between the plane of rotation of the propeller and the chord line of the propeller.
Helix angle
Angle between the propeller plane of rotation and the resultant velocity of air flow experienced - from rotation of propeller and forward movement of the aircraft.
Propeller angle of attack
The difference between helix angle and blade angle
Blade/helical twist
As rotational velocity is different at different points on the blade, helix angle changes along it’s length (higher at root).
Blade angle needs to vary along the length to achieve suitable force along it.
Where along blade is most thrust generated?
Due to interference around the propeller hub and vortices @ tip, area of 60-90% propeller length generate most of the thrust.
Peak is at around 75% so pitch angle is usually quoted at this point.
Optimising angle of attack of propeller blades
For fixed propellers, the angle of attack will be optimised for a certain airspeed, however will become less optimal at different airspeeds.
Variable pitch propellers combat this.
CSU/PCU
- Stands for
- Description
Constant-speed unit or Propeller control unit.
Adjust pitch of blades (fine vs coarse) to maintain selected rpm as engine power is increased or decreased.
CSU during take-off
In take off speed is low => helix angle low => fine pitch angle desired.
Set high rpm so high power setting doesn’t lead to a coarse blade pitch.
As propeller rotation increases to max rpm, blade pitch will coarsen at the same time as airspeed increases, thus maintaining good blade AoA.
CSU in cruise
Increase in power will translate to coarsened blade pitch to maintain rpm, so as forward speed increases the pitch angle increases to maintain AoA of blades.
General method when increasing power with variable pitch propeller
- Increase RPM
- Increase manifold pressure
General method when decreasing power with variable pitch propeller
- Decrease manifold pressure
- Decrease RPM
Slipstream effect
Clockwise (from cockpit) spinning propeller creates spiral airflow, going under fuselage and striking right side of tail as it flows backwards.
Aerodynamic effect pushes tail to the right and yaws nose to the left.
Propeller torque reaction
Clockwise rotating propeller creates an opposite torque reaction making the aircraft roll to the left.
Two moment forces around aircraft
CoG forward of CoP creates a moment that pitches nose downwards.
Thrust created at lower point than drag creates a moment force pitching nose upwards.
Might be other way around, ideally the two should balance.
Advantage to CoG forward of CoP
When thrust is lost (drag also reduces), the pitch down moment takes over, which makes the plane tend towards a glide when power is lost, rather than towards a stall.
How are the two moment forces controlled in flight?
Tailplane is the balancing moment. Set a long distance from the other forces so that minimal force will create a large moment.
Axis names and names of movement around those axes
Longitudinal axis: Roll
Lateral axis: Pitch
Normal axis: Yaw
Stability axis to movement relationship
Longitudinal stability: Pitch
Directional Stability: Yaw
Lateral stability: Roll
How is longitudinal stability achieved?
If wind pitches the nose up, the AoA of the tailplane will increase, generating more lift and therefore a pitch down force.
Impact of CoG position on longitudinal stability
Further forward CoG increases the moment force of the tailplane, increasing stability but reducing controllability
How is directional stability achieved?
Inadvertent yawing presents one side of the tailplane to the airflow which results in a correcting force
How is lateral stability achieved?
In an unintended roll the plane sideslips and airflow over lower wing is at higher AoA, causing higher lift.
In addition, higher wing is shielded from this airflow by fuselage, decreasing its lift relative to lower wing.
Plane designs increasing lateral stability
- Dihedral
- Wing sweepback
- High keel surfaces relative to CoG
- High wings
Dihedral
- Definition
- Effect on lateral stability
This means wings are angled upwards.
Results in the AoA of lower wing being higher to relative airflow than the upper wing, increasing the recovery forces.
Wing sweepback
- Definition
- Effect on lateral stability
Swept back wings.
Lower wing will face resultant airflow “straight on” with a greater effective span and therefore more lift than upper wing which is angled very far away from airflow.
High keel surfaces effect on lateral stability
Sideways airflow from side slipping will create a force on the keel to roll in opposite direction.
High wing effect on lateral stability
Low CoG relative to CoP creates a moment force to roll aircraft back when they are not in line
Roll followed by yaw
The sideslip caused by rolling creates a yaw in the same direction due to airflow over the tail.
Roll followed by yaw design considerations
If the directional stability creating the yaw is greater than the lateral stability correcting the roll, get spiral instability (dutch roll).
Yaw followed by roll
Yaw initially presents outer wing to the airflow which then has more lift and creates roll in the direction of yaw.
Lateral stability characteristics (dihedral, wing sweepback) will exacerbate this effect.
Aircraft design and stability vs control (in pitch)
Larger tailplane gives greater stability
Larger elevator gives more control
Stabilator
A tailplane and elevator in one.
The entire stabilator pivots based on control inputs, no fixed tailplane.
Ruddervator
A v-shaped stabiliser and rudder in one
Adverse aileron yaw
When initiating roll with ailerons, the rising aileron is down and experiences more drag.
This creates a yaw effect away from the direction of the roll.
Differential ailerons
To combat adverse aileron yaw, the ailerons can be designed to deflect more in upwards direction than downwards, creating more parasite drag on the lower wing when rolling.
This only partially offsets the adverse aileron yaw, remainder with rudder.
Frise type ailerons
Create drag on lower wing in roll to offset adverse aileron yaw
Coupled ailerons and rudder
Some linkage built in between ailerons and rudder so that the rudder automatically compensates for adverse aileron yaw
Adverse aileron yaw in practice
This is an initial effect felt on initiating roll, but yaw in the direction of the turn will take over once aerodynamically stable in the turn
Control effectiveness
- factors to be aware of
At low air speed controls will become sloppy.
For control surfaces within propeller slipstream, high power and low airspeed will maintain control (e.g. stabiliser in a climb).
Stick-force
The force required on the controls to maintain control position against airflow which is trying to return the control surface to its faired position
Inset hinge
Hinges set into the control surface so that the pivot point is closer to the centre of pressure on the control surface, reducing required stick-force.
Horn balance
Pivot the control surface somewhere in the middle, so the “horn” (front section) will deflect in the opposite direction to the rear part.
Increases drag due to horn protruding, but pressure on the horn will offset pressure on the surface, reducing stick-force required.
Balance tab
Small section at back of elevator which pivots in opposite direction to the elevator automatically. It creates a small aerodynamic force acting to maintain the elevator position.
Servo tab
Variation of balance tab. Pilots controls connect to the servo tab only, not the control surface directly. Movement of the servo tab creates the aerodynamic force that moves the control surface where it needs to be.
Anti-balance tabs
Work opposite to balance tabs to increase the level of force felt by the pilot through controls.
What is flutter?
Vibration of control surfaces which is exacerbated when mass of the surface is too far away from the hinge and no mass balanced on the other side of hinge.
Methods of reducing flutter
Need to move CoM closer to the hinge point.
For inset hinges or horn balance surfaces can add mass to the smaller side of the surface.
Otherwise a mass balance may help.
Mass-balance
Prevents flutter (vibration) of control surfaces
Control of flaps, slots and slats
Flaps are controlled by the pilot.
Slots and slats could be automatic or controlled by the pilot.
Impact of flap on pitch
Flap will move CoP backwards (due to creating high camber at trailing edge of wing) and also impact thrust-drag balance due to new source of drag.
The net effect on pitch will depend on the moment couples of the aircraft.
Impact of flap on L/D ratio
Flap increases both lift and drag but will reduce the L/D ratio.
Attitude vs angle of attack
Attitude is the angle of the aircraft with respect to the horizontal plane.
AoA is the angle with respect to the relative airflow.
We focus on AoA, although attitude may be more noticeable when flying.
Impact of flaps on AoA (including stall AoA)
Flaps will decrease the angle of attack at which stalling occurs.
However C(Lift) will be greater at a given angle of attack.
Impact of flaps vs clean wing on take-off profile
Clean wing will have a longer ground run but a steeper rate of climb.
Types of flap (4)
Purpose of slotted flap
High energy air from below the wing flowing over the flap delays the stall
Purpose of fowler flap
Increases the wing area as well as the camber, therefore increasing lift
Leading Edge Devices (3)
How slots/slats work
Slats can be moved to create slots.
The slot allows high energy air from below the wing to flow over the upper surface, delaying separation and thus stall.
Drawback to fixed slot
Fixed slots create a high level of drag during normal flight therefore aren’t common in high performance craft
Spoiler
Device which when extended disturbs airflow on upper surface of the wing, reducing lift and increasing drag.
Also forces weight onto wheels, increasing effectiveness of the brakes.
Other uses for spoilers
Can assist with roll or other control required during flight.
Assumed forces in S&L flight
We assume thrust to act in parallel with direction of travel, therefore take drag to act in the opposite direction and lift/weight at 90 degrees.
Requirement for S&L flight in formula
Weight = Lift = C(Lift) * 1/2 * rho * V(2) * S
AoA and speed relationship in S&L flight
As speed (V) decreases, C(Lift) must increases to maintain S&L condition (weight = lift).
AoA must be increased to increase C(Lift).
How do IAS and TAS fit into lift formula?
V is TAS.
rho * V(2) relates to IAS.
Power available vs power required chart
Explanation of power available and power required vs speed
Power available increases as higher airflow increases potential engine output.
Power required relates directly to drag and is high at both low speed and high speed, with a low point in the middle.
Maximum endurance and maximum range speeds
(power vs speed chart)
Maximum range speed in terms of Thrust/Drag
Maximum range speed by calculation
Want to minimise fuel / distance.
= fuel rate / speed
= power / speed (minimise the ratio)
= thrust * speed / speed
= drag * speed / speed
= drag (minimise drag)
Speed stability issue
At airspeeds below max range speed, speed is unstable.
A gust causing speed to decrease will increase drag, decreasing speed further towards stall.
A gust increasing speed will reduce drag causing further acceleration.
Above max range speed, speed is stable and momentary changes will self correct.
Relationship between IAS, TAS and altitude in S&L flight
As altitude increases, rho decreases, so V (TAS) needs to increase to maintain level of lift.
IAS on the other hand relates to rho * V(2) therefore remains stable as altitude changes.
Relationship between altitude and max range/endurance
Max range is better at higher altitudes as TAS increases.
Max endurance is higher at lower altitudes however as the denser air requires less power to offset lift.
Zoom climb
Exchange of speed energy (1/2 * m * V(2)) for height.
Steady climb
Fuel energy in excess of that required for S&L to gain height.
Balance of forces in steady climb (description)
Thrust increases to greater than drag, with the remainder offsetting part of weight.
Lift is therefore less than weight.
Steady climb forces diagram
Impact of weight on climb
Higher weight reduces climb performance as more excess thrust is required to offset the extra weight
Maximum climb gradient
As climb gradient is increased, lift direction turns away from weight direction and more thrust is required to offset weight. Thus the limiting factor is the “excess thrust”, i.e. thrust less drag.
As such clean wings is key to maximising excess thrust and thus climb gradient.
Maximum rate of climb
Maximum rate is at maximum excess power rather than force. This relates to the optimal lift/drag ratio point and will be at a faster speed and lower angle than maximum climb gradient.
Designation for speed of maximum climb gradient and maximum rate of climb
Maximum gradient speed: V(X)
Maximum rate climb: V(Y)
Cruise climb
Lower angle than max angle or max rate climb. Will climb less steeply and less quickly than both, but speed over ground will be greater.
Service ceiling
Altitude at which max climb rate is below 100ft/min
Absolute ceiling
Altitude at which max climb rate is zero
Overshoot causes (3)
i) Increasing headwind
ii) Decreasing tailwind
iii) Updraft (cumulus cloud)
Undershoot causes (3)
i) Decreasing headwind
ii) Increasing tailwind
iii) Downdraft (cumulonimbus cloud)
Balance of forces in glide
Maximum glide distance
Want to maximise L/D ratio as the more excess lift available to offset weight, the better.
This will be AoA about 4 degrees (to relative airflow). As AoA can’t be seen, target best glide speed.
Effect of moving away from best glide speed
Pitching down will increase airspeed but create a steeper profile and less forward speed.
Pitching up (trying to stretch the glide) will increase drag more than lift and also lead to a steeper profile.
Flaps and best glide
As usual, flaps reduce L/D and therefore decrease glide distance
Effect of weight on glide
Glide angle and glide distance will not be affected by weight.
However endurance will be better when the aircraft is lighter.
Effect of weight on optimal glide speed
Optimal glide speed in FM is based on maximum weight.
Technically the optimal angle of attack (and glide angle) will reduce in a lower speed for a lighter plane, but in reality the same speed can be targetted.
Impact of headwind/tailwind on glide performance
Naturally headwind will reduce glide distance and tailwind will increase it
Balance of forces in a turn
Angle of lift turns from vertical.
A component of it contributes to centripetal force.
Amount of lift must be increased to maintain altitude.
Load factor
Measure in # of “g”, this is the lift/weight ratio.
Load factors at various bank angles
30 deg: 1.15g
45 deg: 1.41g
60 deg: 2g
70 deg: 3g
80 deg: 6g
Stall speed at various bank angles
Multiply stall speed by square root of load factor.
e.g. 60 degree bank => 2g => 1.41 * stall speed
Overbanking effect in level turn
Outer wing is travelling faster therefore generates more lift, contributing to overbanking.
Over/under banking in non-level turns
Each wing travels through same vertical distance but different horizontal difference, leading to different relative airflow angles.
Airflow to the outer wing is “flatter” than the inner wing. This leads to higher AoA in climb and lower AoA in descent, therefore overbanking/underbanking respectively.
Balance of over/under banking in descending turn
Overbanking effect from level turn (outer wing having higher airspeed) offsets underbanking from descending turn (outer wing has lower AoA).
Rate 1 turn
180 degrees in 1 minute (3 degrees per second)
How to achieve rate 1 turn
Bank angle = 7 degrees + 10% of IAS (kts)
Critical angle
Angle of attack at which the coefficient of lift starts to fall, i.e. the wing stalls.
In other words, the AoA where C(Lift) is maximised.
Changes in critical angle
Critical angle does not change!
Stall speed changes as AoA isn’t always the same for the same speed (e.g. in a turn) but the wing will always stall at the critical angle.
Relationship between lift and stall speed
L = C(Lift) * 1/2 rho V(2) * S
Lift then is proportional to V(2)
IAS(stall) is proportional to sqrt(C(Lift max))
Thus load factor of 2g (e.g. 60 degree bank) leads to 1.41 increase in stall speed.
Impact of weight on stall speed
As weight increases more lift is required in S&L.
Weight and lift are proportional so stall speed adjusted by sqrt of weight factor.
Impact of forward CoG on stall speed
Forward CoG requires downwards moment force from tailplane to balance.
This extra downforce requires more lift from the wings and therefore increases stall speed.
Impact of wing loading on stall speed
Low wing loading (i.e. large wings creating less lift per m(2)) decreases stall speed.
Impact of altitude on stall speed
Stall speed is in terms of IAS not TAS therefore altitude won’t affect it.
Impact of power on stall speed
Slipstream effect will increase airflow over wings (but not IAS) therefore creating more lift, reducing required AoA and decreasing stall speed.
Impact of ice on stall speed
Ice increases weight and decreases effectiveness of wings, both effects increasing stall speed
Washout
- Description
- Purpose
Wing design such that root stalls before the wingtips.
This maintains aileron control even as wings start to stall
Deep stall
When tailplane stalls due to being in turbulent flow behind wings (possible with swept back wing profile)
6 features of a spin
- Stalled
- Rolling
- Yawing
- Pitching
- Sideslipping and
- Rapidly losing height
How spin is entered
Approaching stall angle either yaw or misuse of aileron.
Outer wing travels faster and generates more lift while inner wing stalls.
Autorotation is entered and the spin accelerates.
Misuse of aileron
Trying to raise a wing using aileron increases AoA of the lower wing. If the wing is close to critical angle it will stall instead of rising and a spin can develop.
Use of instruments during a spin
Only the turn coordinator/indicator will be reliable (not attitude indicator).
Balance ball will be useless.
Slipping vs skidding turn
Slipping means not yawing enough in direction of turn (understeer), skidding means yawing too much
Correct action to take to prevent yawing if a wing drops close to stall
Apply rudder in direction to oppose the yaw experienced
Which way to adjust bendable rudder trim tab
Opposite direction to where you want rudder to go, it works like a balance tab
More or less likely to drop a wing in stall with flaps extended
More likely
What does neutral stability mean?
Aircraft will tend to stay in its new attitude after a disturbance
Is saturated air more or less dense than dry air?
Less dense - thus high humidity reduces performance
What is a control stop?
NOT a control lock.
This limits the movement of control surfaces to an appropriate amount.
Rudder input required to correct for wind from left on take-off
Right rudder required.
Wind from left will push the tail primarily, yawing the aircraft to the left.
Load factor limits for normal and aerobatic light aircraft
Normal: 3.8g (4.4g with reduced mass)
Aerobatic: 6g
Does windmilling propeller generate thrust during engine-off glide?
No, it generates drag.
Feathering would help, but not possible in single piston aircraft.
Stagnation point
Point on the leading edge which moves down with increasing angle of attack.
Used to trigger stall warning.
Are wingtip vortices produced while aircraft is on the ground?
No
What wing shape (cross sectional) has lowest induced drag?
Elliptical
What is pressure equalisation?
At the wing tips pressure differential between upper and lower surfaces “spills out”, creating wingtip vortices.
