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MEPT Groundschool > Principles of Flight > Flashcards

Principles of Flight Flashcards

1
Q

Why does the A400M have scimitar shaped prop blades?

A

The new broader, paddle type blades are swept at the tips, which delay compressibility wave drag and increase propeller thrust.

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2
Q

Why is compressibility a problem for propeller aircraft?

A

Compressibility effects decrease thrust and increase the rotational drag. This imposes a limit on the propeller diameter, its RPM and the TAS at which it can be used.

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3
Q

What is the formula for propeller solidity?

A
  • Nb is the number of blades;
  • C is the chord length;
  • rm is the mean radius.
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4
Q

What is asymmetric blade effect?

A

Asymmetric blade effect increases the thrust produced by the down-going propeller blade when the aircraft is at a higher angle of attack.

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5
Q

Why can asymmetric blade effect be problematic for multi-engine aircraft?

A

It will produce a significant difference in the thrust moment between engines, especially at low speeds. Which can cause large asymmetric forces in the event of an engine failure.

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6
Q

During a single engine failure on a twin-engine aircraft, which way will the slip indicator move?

A

Toward the live engine.

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7
Q

What phrase can be used to intuitively diagnose which engine has failed (in a twin-engined aircraft)?

A

“Dead leg, dead engine”

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8
Q

What is the preferred method of controlling asymmetric forces?

A

Wings level, then rudder.

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9
Q

Why is asymmetry worse in a propeller powered aircraft, than in a jet aircraft?

A

Loss of prop wash over the control surfaces reduces the control effectiveness on the dead engine side (which you would also roll towards if left yaw unchecked).

NB. A ‘thrust vectoring’ C-17 would also experience a similar issue.

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10
Q

What is the formula for jet engine propulsive power?

A
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11
Q

What is the formula for jet engine thrust?

A

The difference in gas velocity between the intake and jet exhaust, multiplied by mass, determines thrust.

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12
Q

Why does a turbofan engine generate more thrust at lower speeds, than a turbojet?

A

Turbofan has a larger fan which generates a larger mass flow at low speeds.

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13
Q

What is the formula for jet engine efficiency?

A

The ratio between Energy Out and Energy In. Multiplied by 100 gives a percentage.

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14
Q

What is VMCA in the Phenom?

A

97 KIAS

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15
Q

How can the critical engine be defined for propeller aircraft?

A

The engine which produces the greatest overall asymmetric effect should it fail.

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16
Q

Before V1, how should asymmetric forces be controlled?

A

Reducing the thrust from the good engine to idle.

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17
Q

Define VMCA, and list the conditions required to achieve it.

A

The minimum speed, whilst in the air, that directional control can be maintained with one engine inoperative under the following conditions:

  • Most aft C of G;
  • Full Rudder Applied;
  • 5º of bank applied towards live engine;
  • Gear up;
  • Flap 1;
  • Live engine at TOGA thrust.
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18
Q

List three functions of spoilers.

A
  • Speed Brakes
  • Lift Dump
  • Roll Control

Damo’s bonus answer: Spoilers.

🙄

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19
Q

Why do some aircraft, like the A400M, have counter rotating propellers?

A

To cancel out the asymmetric blade effect.

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20
Q

What’s the difference between counter and contra-rotating propellers?

A

Counter rotating propellers are mounted on opposite wings and rotating in opposite directions to oppose asymmetric forces. eg. A400M

Contra-rotating propellers also rotate in opposite directions but are mounted on the same shaft. eg. TU-95

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21
Q

Define Mach number.

A

The ratio between TAS and Local Speed of Sound.

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22
Q

What is the only thing that causes the local speed of sound in air to vary at altitude?

A

Temperature (i.e density).

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23
Q

What is aerodynamic heating?

A

When the air temperature increases due to compression at high speeds.

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24
Q

Define MCRIT.

A

Critical Mach Number (MCRIT)

Mach Free Stream (MFS) at which the Local Mach Number (ML) of flow about an aircraft first reaches Mach 1.0. Shockwaves begin to form at this speed.

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25
Q

Define ML.

A

Local Mach Number (ML)

Mach number of the airflow over the aircraft as measured by a localised point.

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26
Q

Define MFS.

A

Free Stream Mach Number (MFS)

Mach number of the airflow past an aircraft that is unaffected by its passage.

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27
Q

Define MDET.

A

Detachment Mach Number (MDET)

MFS at which the bow shockwave, first formed at MFS Mach 1.0 just ahead of the leading edge, attaches to the leading edge and varies with a further increase in speed. MDET marks end of transonic flight and beginning of true supersonic flight.

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28
Q

Why are thin wings advantageous for transonic flight?

A

A low thickness/chord profile reduces transonic drag.

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29
Q

What are the disadvantages of thin wings?

A
  • Poor lift at low speeds;
  • Small volume makes it difficult to incorporate flight controls, flaps and slats, landing gear and integral fuel tanks;
  • Difficult to ensure structural integrity and stiffness.
30
Q

Why are swept wings advantageous to transonic flight?

A

Wing sweep delays the formation of shock waves as the flow normal to the leading edge is slower than the span-wise flow. Wing sweep effectively reduces aspect ratio and thickness/chord ratio, thus mitigating shock wave formation and the sharp rise in transonic drag.

31
Q

What are the disadvantages of swept wings?

A
  • Difficult to construct a swept wing with sufficient structural stiffness;
  • Reduced CLMAX leads to high approach and landing speed at high AoA;
  • Formation of ‘ram’s horn’ vortex caused by requisite high AoA.
32
Q

What is area ruling?

A

The smoothing out of an aircraft’s cross-section area in order to look more like the Sears-Haach shape. It reduces shockwave drag profile significantly.

33
Q

What are the characteristics of the super-critical wing?

A
  • Blunt nose;
  • Flat upper surface;
  • ‘S’ shaped Camber.
34
Q

Which is the odd one out and why?

  1. Fowler flap;
  2. Krueger flap;
  3. Plain flap;
  4. Slotted flap.
A

Krueger Flap

It is situated at the leading edge of the wing, akin to a slat.

35
Q

What is the difference between flaps and slats?

A

Flaps increase lift at a given AoA, slats increase the critical AoA.

36
Q

Which should be retracted first, slats or flaps?

A

Always retract flaps first.

37
Q

How does a flap increase lift?

A

Increases CL by:

  • Increasing chord length;
  • Increasing surface area;
  • Increasing camber.
38
Q

Define ‘coffin-corner’.

A

Where aerodynamic stall speed = shock stall speed.

39
Q

What is the Mach Tuck?

A

As MFS increases towards Mach 1.0 an aircraft’s centre of pressure moves rearwards, causing an increasing nose down pitch force known as Mach Tuck.

40
Q

………. directional stability and ………. lateral stability will increase an aircraft’s tendency to Dutch Roll.

A

Weak directional stability and strong lateral stability will increase an aircraft’s tendency to Dutch Roll.

41
Q

………. aspect ratio and a ………. thickness/chord ratio is best suited for transonic flight.

A

Low aspect ratio and low thickness/chord ratio is best suited for transonic flight.

42
Q

Define spiral stability.

A

An aircraft’s tendency to return to laterally level flight when controls are released in a level, balanced turn.

  • Positive - the aircraft will roll itself wings level;
  • Neutral - the aircraft will maintain the set angle of bank;
  • Negative - the aircraft will increase the angle of bank.

Spiral stability is a type of static stability.

43
Q

Will a large fin or small fin have the effect of poor spiral stability?

A

Large fin.

It causes weather-cocking into the roll, causing yaw, before a correcting roll can take place during sideslip.

44
Q

Describe adverse yaw.

A

Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll, especially with swept wings.

45
Q

Describe positive lateral stability.

A

The tendency for an aircraft to return to wings level if it is disturbed in the roll axis.

46
Q

What design features increase lateral stability?

A
  • Dihedral wings;
  • Swept wings;
  • Wings, fuselage, and fins above the CofG.
47
Q

Is Dutch Roll more or less pronounced at altitude?

A

More pronounced. Directional stability is decreased due to the low KIAS.

48
Q

How can Dutch Roll be prevented?

A

By using a Yaw Damper.

49
Q

What is the difference between spiral and lateral stability?

A

Spiral stability: Tendency to return to wings level when [controls released] in a turn.

Lateral stability: Tendency to return to wings level when disturbed in roll during level flight.

50
Q

Is air considered compressible or incompressible at low speeds?

A

Incompressible at low speeds (below 250 - 300 kts).

51
Q

What is the principle effect of engine loss?

A

Yaw.

52
Q

What will happen if no action is taken in the event of engine loss?

A

The aircraft will enter a spiral descent.

53
Q

Why will swept wing aircraft pitch up when approaching the stall?

A

The tips of swept wings will stall first, moving the centre of pressure forward. This causes a pitch up effect, exacerbating the stall.

54
Q

Swept wings stall at a lower/higher* AoA than straight wings.

*Delete as appropriate

A

Swept wings stall at a higher AoA than straight wings.

55
Q

How does a Fowler flap increase lift?

A

It increases the surface area and camber.

(The camber influences a wing’s Coefficient of Lift, CL).

56
Q

What is the Lift Equation?

A
  • CL is the Coefficient of Lift;
  • ρ is the air density;
  • v is velocity;
  • A is wing surface area.
57
Q

Define directional stability.

A

Directional stability is the aircraft tendency to return to the initial directional equilibrium condition, if perturbed.

  • Positive - will tend to return to equilibrium;
  • Neutral - will remain where it is left;
  • Negative - will diverge from equilibrium.

It is a type of static stability.

58
Q

Define Dutch Roll.

A

Dutch Roll is a type of aircraft motion consisting of an out-of-phase combination of ‘tail-wagging’ (yaw) and rocking from side to side (roll).

It occurs with weak directional stability and strong lateral stability.

It is a form of dynamic stability.

59
Q

Define lateral stability.

A

Lateral stability is the aircraft tendency to return to the initial lateral equilibrium condition, if perturbed.

  • Positive - will tend to return to equilibrium;
  • Neutral - will remain where it is left;
  • Negative - will diverge from equilibrium.

It is a type of static stability.

60
Q

Define static stability.

A

An aircraft’s tendency to return to an original attitude, following a disturbance.

61
Q

Define dynamic stability.

A

An aircraft’s tendency to oscillate due to returning forces following a disturbance.

62
Q

What is the simplest way propeller solidity be increased?

A

By increasing the number of blades.

63
Q

Why is tip speed critical for propeller engine efficiency?

A

If the tip exceeds MCRIT then the engine will have to use more power to overcome the resulting drag.

64
Q

What is a secondary effect of losing an engine (asymmetric thrust)?

A

Roll towards the dead engine.

65
Q

What is a major factor in calculating VMCA?

A

Having full rudder applied.

66
Q

When do shockwaves start forming on an aircraft?

A

MCRIT.

67
Q

What effect do shockwaves have on drag?

A

They increase it.

68
Q

Under what circumstances will the greatest yaw effect occur as a result of engine failure?

A

Low speed, high thrust (e.g. TO/GA).

69
Q

What is an advantage of slotted flaps?

A

Slotted flaps re-energise the airflow over the wing to keep the boundary layer attached for longer.

70
Q

Of the following, which will increase the effect of Dutch Roll?

  1. Large fin;
  2. Increasing IAS;
  3. Increasing altitude;
  4. Yaw damper.
A

Altitude. The reduced air density lowers the IAS and decreases the directional stability.

Remember: Dutch Roll is a result of weak directional stability and strong lateral stability.

MEPT Groundschool (6 decks)

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