Principles of Flight Flashcards
ISA
- Stands for
- Sea level
International Standard Atmosphere
15 deg C
1013.25 hPa
1.1225 kg/m(3) density
ISA changes at altitude
2 degrees C lost per 1000ft up to 36,000ft
From which constant -56.5 deg C
Dynamic pressure formula
Q = 1/2 x rho x v(2)
Calibrated air speed (CAS)
IAS adjusted for instrument and pressure errors (pressure error is due to position of the pitot tube, aircraft configuration etc.)
Equivalent air speed (EAS)
IAS corrected for both position (as in CAS) and also compressibility of air, which is a factor at high speeds (i.e. air compresses within the pitot tube)
Mach number
M = TAS / a
Where a = local speed of sound
LSS forumla
LSS = 38.95 * sqrt(T)
[T = temperature in Kelvin]
Mnemonic for relationship between airspeeds and altitude
ECTM
EAS, CAS, TAS, MN
Critical Mach Number
M(crit) is the mach number at which airflow around some part of aircraft will reach local speed of sound
When is an airspeed measure a speed, and when is it a pressure?
TAS is speed, all other measures are in fact pressures.
Thus IAS indicator is in fact a pressure gauge, not a speed gauge
Bernoulli’s theorum
In the steady flow of an IDEAL fluid, the sum or pressure energy and kinetic energy remains constant.
Directly linked to LIFT.
Upwash and downwash
Upwash is the flow of air upwards, towards the low pressure area above the wing, at the front of the wing.
Downwash is the downwards flow of air at the back of the wing.
Pressure diagram at low AoA
Pressure diagram at high AoA
Pressure diagram at critical AoA
Formula for lift
Lift = C(L) x 1/2 x rho x V^2 x S
Talk about lift
Lift utilises bernoulli principle to create an upward force on a wing, by adjusting the velocity of air above and below it and thus the pressure above and below it.
Formula is …
Major components of lift are therefore wing area (s), air density (rho), velocity (v^2) and coefficient of lift.
Coefficient of lift is a constant that can vary in reality based on various characteristics like surface texture, AoA.
Movement of centre of pressure
CoP moves forwards as AoA increases and strong sucking force is created towards leading edge.
It is furthest forward at peak C(Lmax) and then moves backwards after the stall.
[Note: CoP for symmetrical aerofoil is static]
Centre of pressure for cambered vs symmetrical aerofoils
Centre of pressure for symmetrical aerofoils does NOT move with AoA.
Impact of icing on C(L) vs AoA profile
Impact of flaps on C(L) vs AoA profile
Aspect ratio, 2 calculations
Wingspan (b) / Average Chord (c)
or Wingspan (b) ^ 2 / Wing Area (S)
Induced downwash and effect
Trailing vortices create a downwash in the airflow under and behind the wing, which causes effective airflow to be at a higher angle than relative airflow. This requires an increased angle of attack to achieve the same amount of lift, compared to if there were no vortices.
Effective angle of attack
Induced angle of attack
The effective angle of attack is the angle between the chord and the effective air flow.
The difference between total AoA and effective AoA (i.e. chord to relative air flow) is the induced angle of attack, in other words the amount of additional AoA required to maintain lift as a result of induced downwash.
Diagram of alpha (induced & effective)
Impact of ground effect on vortices
Ground effect reduces downwash therefore minimising vortex generation. This is why induced drag is lower when in ground effect.
Ground effect limitations
Within about half a wing span
3 types of parasite drag
Form (aka Pressure drag)
Friction
Interference
Transition point
Point at which boundary layer becomes turbulent
Separation point
Further back than the transition point, this is where the turbulent boundary layer separates.
Beyond this we get zero lift and high drag.
Effect of increasing aspect ratio (AR) on AoA
Higher aspect ratio requires less AoA to generate the same C(L) due to decreased induced drag.
The higher downwash of low aspect ratio decreases effective AoA, reducing C(L). However this also increases the critical AoA. Max C(L) is lower but max AoA is higher.
Drag vs speed chart
C(L) vs C(D) graph and L/D(max) point
3 stall sensors
Flapper switch - activated when stagnation point moves past it
Angle of attack vane - Attached to side of fuselage, vane sits in streamline of relative airflow, detecting AoA
Angle of attack probe - Attached to side of fuselage, probe with slots sensitive to changes in relative airflow
[Wing mounted vain positioned on lower surface of leading edge]
Rectangular wing in stall
Large wing tips create strong, supported tip vortices, reducing effective AoA @ tips.
Thus root stalls first, CoP moves rearward therefore:
- Aileron’s remain effective
- Nose drops
- Aerodynamic buffet (separated air over roots hits the tailplane)
- No violent wing drop
Tapered wing in stall
Smaller wing tips mean relatively more vortices @ root and tips stall first.
Smaller rearward movement of CoP.
Problems therefore:
- Ailerons ineffective
- Limited pitch down
- Limited tailplane buffet
- Increased chance of wing drop
Solution to tapered wing stall issues (5)
- Geometric washout
- Variance of aerofoil section along wing (greater thickness & amber at root)
- Leading edge slots towards tip
- Stall strips near root
- Vortex generators
Swept back wing in stall
- 2 effects and the result
Flow of air over wing from root to tip collides with vortex direction at tip, creating slow moving air and stall at the TIP.
Sweep back also means tips are aft of the root, so tip stall makes CoP move FORWARD.
This creates a dangerous pitch UP effect.
Solution to swept back wing stall issues
Need to prevent spanwise (root to tip) flow of boundary layer over the wing, keep it straight in line with relative airflow.
- Wing fences (boundary layer fences) sit on upper surface.
- Vortilons do a similar job on underside of wing (engine nacelles help)
- Saw tooth leading edge can create vortex over upper surface at high AoA that has the same effect.
C(L) vs AoA chart for swept wing vs normal
Forward CoG
- Fuel efficiency
- Range
- Stall speed
- Stability
- Control
Forward CoG
Bigger moment, more downforce from stabiliser.
Fuel efficiency & range DOWN
Stall speed UP (bad)
More stability, less control
Definition of a spin
A stall must occur before a spin can take place.
A spin will happen if one wing is more stalled than the other - leading it to drop and the aircraft to yaw in that direction, eventually an autorotation maintains the spin.
Airspeed high or low in a spin?
LOW!
As we are stalled. Vertical speed could be high.
Accelerated stall
Stall at load factor greater than 1g, so at higher speed than usual.
More violent than 1g stall.
Secondary stall
A second stall during recovery due to failure to decrease AoA sufficiently, or trying to regain altitude too quickly.
Shock stall (high speed buffet)
Shockwave at high speed (above critical mach) causes boundary layer separation, turbulent air which hits the tailplane and causes vibration that can damage the airplane.
Also C(L) reduces at this point.
Purpose of leading edge flap
To increase camber on high speed aerofoils
Effect of leading edge flaps on lift
Increase angle of attack due to shape of wing around the chord (opposite to trailing edge flaps).
Thus C(L) is increased at all AoA, max AoA is increased and so C(Lmax) is significantly increased at the higher max AoA.
Increase critical angle unlike most types of flap.
Effect of slot/slat on C(L)
Allowing higher kinetic energy air over the wing delays separation of boundary layer, increasing critical AoA.
C(L) vs AoA is mostly unchanged but carries on to a higher C(L) peak.
Effect of slats and flaps on C(L) vs AoA
How longitudinal stability is achieved
Tailplane generates positive stability. Gust will increase tailplane AoA and as tailplane AC is aft of CoG, a pitch down moment force is created.
This is designed to be greater than the pitch up moment of main wing. Force will be lower, but distance from CoG greater, so moment can be greater.
Neutral point in longitudinal stability
If CoG is gradually moved backwards, there is a point where the moment force from the tailplane balances the destabilising forces (main wing, fuselage etc.).
This CoG position is the neutral point, where the aircraft will have neutral stability.
Phugoid
Long Period Oscillation
Oscillations over a period of around 1-2 mins in pitch attitude, altitude and airspeed (with constant AoA).
DO reduce to zero though so reflect positive static and positive dynamic stability.
However because of longer periods do get significant variance in altitude (unlike short period oscillation).
Directional Stability definition
The initial tendency of the aircraft to return to its equilibrium angle of sideslip (typically zero).
Thus strong directional stability implies difficulty maintaining sideslip.
Geometric dihedral
Creates high AoA of wing heading into sideslip and therefore a lifting moment on that side.
Positive lateral stability.
Spiral Divergence
- Cause
- Description
Caused by high directional stability relative to dihedral effect.
Gentle effect where sideslip causes a yaw into the turn, limited correction of the roll and gentle spiral starts. Eventually leads to a spiral dive.
This is easy to identify and resolve.
Dutch Roll
- Cause
- Description
Caused by large dihedral effect relative to directional stability.
After yaw is introduced, aircraft will roll in same direction due to dihedral effect. Increased induced drag on the rising wing causes reverse yaw and thus oscillations between yawing and rolling.
Dutch roll recovery
Risk of pilot induced oscillation (PIO) if using the rudder to correct.
Use ailerons instead.
Aircraft will have yaw damping to reduce the effect.
Mach tuck
Or high speed tuck, or tuck under.
Shock wave on swept back wing reduces lift forward of CoG and reduces downwash over tailplane.
This creates a pitch down moment and aircraft becomes unstable in terms of speed.
Supercritical wing profile
Flatter upper surface delays shockwave, reducing drag and moving it more quickly to the trailing edge.
Larger leading edge radius.
Enables flight in transonic region.
INCREASES M(CRIT)!
Wing thickness in transonic flight
Thin wings have problems with strength and fuel capacity, but reduce the C(L) and drag profiles to stabilise flight in transonic region.
Swept wings impact on transonic flight
Path of the airflow over the wing is stretched out, so acceleration is reduced and C(L) and drag transonic profiles are smoothed. Drag in transonic range is reduced significantly. M(CRIT) increases.
Downside to this is reduced lift so higher required take off speeds, higher drag at supersonic speed and other issues.
Vortex generators
Sit on the upper wing surface just ahead of where significant shockwaves will start. They make the airflow behind turbulent, high energy, delaying the stall and pushing an upper shockwave to the back of the wing.
This can increase critical AoA, but will also increase drag and gives a high attitude on approach (unlike flaps/slats).
Pitch Angle
Flight Path Angle
Flight Path Angle + AoA = Pitch Angle
AoB for rate 1 turn
AoB = 7 deg + TAS / 10
[Note: Bigger TAS => slower rate of turn @ given AoB]
Secondary effect of rudder
Roll in direction of yaw
Secondary effect of ailerons (2)
Adverse yaw
Later sideslip in direction of bank
Adverse yaw
- Description
- 2 solutions
When rolling with aileron, have higher lift on upgoing wing, less on downgoing. Thus higher induced drag on outside of turn than inside, creating a yaw in opposite direction.
Can resolve with:
- Differential aileron (more deflection on upgoing aileron = downgoing wing)
- Frise type ailerons
Hard vs soft control protections (fly by wire)
Hard prevent control movements beyond limits.
Soft might display a warning or an aural alert, but not physically prevent the control input.
Propeller effects
- Asymmetric blade effect
Typically the prop circle faces upwards relative to direction of travel, so blade travels further when going down than when going up.
Thus on right hand propeller, the right side of prop direction will produce more thrust than left side, creating asymmetrical thrust (P-factor).
When is the yaw effect of asymmetric flight (engine failure) strongest?
When at high alpha.
This is when air flow over the rudder and other surfaces is lowest so less torque force around to counter the yaw effect.
V(MCG)
- Description
Minimum control speed on the ground.
Requires control of aircraft (rudder only, not nosewheel steering) within 30ft (9.1m) laterally at most.
V(MC)
- Definition
CAS at which aircraft can be controlled in S&L flight with AoB no more than 5 degrees.
Limited to 1.13 x V(SR) based on a set of assumptions.
Recovering if speed reduced below V(MC) with engine out
Only recovery is to reduce thrust on operating engine(s) and recover speed by pitching down.
Obviously this is not a good position to be in, so great care must be taken to avoid speed reducing to this point.
Aerodynamic ceiling chart
Contra vs counter rotating propellers
Contra: Weird ones on the same shaft
Counter: Opposite direction normal props
Why does light aircraft descend faster than heavy one?
Both descend at same rate if at their optimal glide speed, but this speed is higher for a heavier aircraft. So at a given ATC speed (e.g. 300kt) heavy aircraft are closer to their optimal glide speed and will lose less altitude.
