Principles of Flight

Question 1

ISA
- Stands for
- Sea level

Answer 1

International Standard Atmosphere
15 deg C
1013.25 hPa
1.1225 kg/m(3) density

Question 2

ISA changes at altitude

Answer 2

2 degrees C lost per 1000ft up to 36,000ft
From which constant -56.5 deg C

Question 3

Dynamic pressure formula

Answer 3

Q = 1/2 x rho x v(2)

Question 4

Calibrated air speed (CAS)

Answer 4

IAS adjusted for instrument and pressure errors (pressure error is due to position of the pitot tube, aircraft configuration etc.)

Question 5

Equivalent air speed (EAS)

Answer 5

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)

Question 6

Mach number

Answer 6

M = TAS / a
Where a = local speed of sound

Question 7

LSS forumla

Answer 7

LSS = 38.95 * sqrt(T)
[T = temperature in Kelvin]

Question 8

Mnemonic for relationship between airspeeds and altitude

Answer 8

ECTM
EAS, CAS, TAS, MN

Question 9

Critical Mach Number

Answer 9

M(crit) is the mach number at which airflow around some part of aircraft will reach local speed of sound

Question 10

When is an airspeed measure a speed, and when is it a pressure?

Answer 10

TAS is speed, all other measures are in fact pressures.
Thus IAS indicator is in fact a pressure gauge, not a speed gauge

Question 11

Bernoulli’s theorum

Answer 11

In the steady flow of an IDEAL fluid, the sum or pressure energy and kinetic energy remains constant.
Directly linked to LIFT.

Question 12

Upwash and downwash

Answer 12

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.

Question 13

Pressure diagram at low AoA

Answer 13

Question 14

Pressure diagram at high AoA

Answer 14

Question 15

Pressure diagram at critical AoA

Answer 15

Question 16

Formula for lift

Answer 16

Lift = C(L) x 1/2 x rho x V^2 x S

Question 17

Talk about lift

Answer 17

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.

Question 18

Movement of centre of pressure

Answer 18

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]

Question 19

Centre of pressure for cambered vs symmetrical aerofoils

Answer 19

Centre of pressure for symmetrical aerofoils does NOT move with AoA.

Question 20

Impact of icing on C(L) vs AoA profile

Answer 20

Question 21

Impact of flaps on C(L) vs AoA profile

Answer 21

Question 22

Aspect ratio, 2 calculations

Answer 22

Wingspan (b) / Average Chord (c)
or Wingspan (b) ^ 2 / Wing Area (S)

Question 23

Induced downwash and effect

Answer 23

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.

Question 24

Effective angle of attack
Induced angle of attack

Answer 24

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.

Question 25

Diagram of alpha (induced & effective)

Answer 25

Question 26

Impact of ground effect on vortices

Answer 26

Ground effect reduces downwash therefore minimising vortex generation. This is why induced drag is lower when in ground effect.

Question 27

Ground effect limitations

Answer 27

Within about half a wing span

Question 28

3 types of parasite drag

Answer 28

Form (aka Pressure drag) Friction Interference

Question 29

Transition point

Answer 29

Point at which boundary layer becomes turbulent

Question 30

Separation point

Answer 30

Further back than the transition point, this is where the turbulent boundary layer separates. Beyond this we get zero lift and high drag.

Question 31

Effect of increasing aspect ratio (AR) on AoA

Answer 31

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.

Question 32

Drag vs speed chart

Answer 32

Question 33

C(L) vs C(D) graph and L/D(max) point

Answer 33

Question 34

3 stall sensors

Answer 34

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]

Question 35

Rectangular wing in stall

Answer 35

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

Question 36

Tapered wing in stall

Answer 36

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

Question 37

Solution to tapered wing stall issues (5)

Answer 37

- Geometric washout - Variance of aerofoil section along wing (greater thickness & amber at root) - Leading edge slots towards tip - Stall strips near root - Vortex generators

Question 38

Swept back wing in stall - 2 effects and the result

Answer 38

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.

Question 39

Solution to swept back wing stall issues

Answer 39

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.

Question 40

C(L) vs AoA chart for swept wing vs normal

Answer 40

Question 41

Forward CoG - Fuel efficiency - Range - Stall speed - Stability - Control

Answer 41

Forward CoG Bigger moment, more downforce from stabiliser. Fuel efficiency & range DOWN Stall speed UP (bad) More stability, less control

Question 42

Definition of a spin

Answer 42

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.

Question 43

Airspeed high or low in a spin?

Answer 43

LOW! As we are stalled. Vertical speed could be high.

Question 44

Accelerated stall

Answer 44

Stall at load factor greater than 1g, so at higher speed than usual. More violent than 1g stall.

Question 45

Secondary stall

Answer 45

A second stall during recovery due to failure to decrease AoA sufficiently, or trying to regain altitude too quickly.

Question 46

Shock stall (high speed buffet)

Answer 46

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.

Question 47

Purpose of leading edge flap

Answer 47

To increase camber on high speed aerofoils

Question 48

Effect of leading edge flaps on lift

Answer 48

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.

Question 49

Effect of slot/slat on C(L)

Answer 49

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.

Question 50

Effect of slats and flaps on C(L) vs AoA

Answer 50

Question 51

How longitudinal stability is achieved

Answer 51

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.

Question 52

Neutral point in longitudinal stability

Answer 52

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.

Question 53

Phugoid

Answer 53

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).

Question 54

Directional Stability definition

Answer 54

The initial tendency of the aircraft to return to its equilibrium angle of sideslip (typically zero). Thus strong directional stability implies difficulty maintaining sideslip.

Question 55

Geometric dihedral

Answer 55

Creates high AoA of wing heading into sideslip and therefore a lifting moment on that side. Positive lateral stability.

Question 56

Spiral Divergence - Cause - Description

Answer 56

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.

Question 57

Dutch Roll - Cause - Description

Answer 57

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.

Question 58

Dutch roll recovery

Answer 58

Risk of pilot induced oscillation (PIO) if using the rudder to correct. Use ailerons instead. Aircraft will have yaw damping to reduce the effect.

Question 59

Mach tuck

Answer 59

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.

Question 60

Supercritical wing profile

Answer 60

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)!

Question 61

Wing thickness in transonic flight

Answer 61

Thin wings have problems with strength and fuel capacity, but reduce the C(L) and drag profiles to stabilise flight in transonic region.

Question 62

Swept wings impact on transonic flight

Answer 62

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.

Question 63

Vortex generators

Answer 63

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).

Question 64

Pitch Angle Flight Path Angle

Answer 64

Flight Path Angle + AoA = Pitch Angle

Question 65

AoB for rate 1 turn

Answer 65

AoB = 7 deg + TAS / 10 [Note: Bigger TAS => slower rate of turn @ given AoB]

Question 66

Secondary effect of rudder

Answer 66

Roll in direction of yaw

Question 67

Secondary effect of ailerons (2)

Answer 67

Adverse yaw Later sideslip in direction of bank

Question 68

Adverse yaw - Description - 2 solutions

Answer 68

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

Question 69

Hard vs soft control protections (fly by wire)

Answer 69

Hard prevent control movements beyond limits. Soft might display a warning or an aural alert, but not physically prevent the control input.

Question 70

Propeller effects - Asymmetric blade effect

Answer 70

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).

Question 71

When is the yaw effect of asymmetric flight (engine failure) strongest?

Answer 71

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.

Question 72

V(MCG) - Description

Answer 72

Minimum control speed on the ground. Requires control of aircraft (rudder only, not nosewheel steering) within 30ft (9.1m) laterally at most.

Question 73

V(MC) - Definition

Answer 73

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.

Question 74

Recovering if speed reduced below V(MC) with engine out

Answer 74

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.

Question 75

Aerodynamic ceiling chart

Answer 75

Question 76

Contra vs counter rotating propellers

Answer 76

Contra: Weird ones on the same shaft Counter: Opposite direction normal props

Question 77

Why does light aircraft descend faster than heavy one?

Answer 77

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.