Flightpath Control 1

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Flightpath Control 1
2013-02-25 03:10:20

Flightpath Control Exam 1
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  1. List the layers of the atmosphere important to aerodynamics.
    • Troposphere
    • Stratosphere
  2. Why is there a standard atmosphere?
    To allow for accurate comparisons of aircraft performance and pressure, temperature and density variations around the globe.
  3. Describe an ideal fluid.
    One that is inviscid and incompressible.
  4. What is the mathematical relationship between P, T and ρ?

    • From this it is clear that:
    • ρ is proportional to P
    • ρ is inversely proportional to T
  5. Define IAS, CAS, EAS, TAS and G/S. State the relationship between them.
    • IAS: Indicated airspeed as read on the ASI.
    • CAS: Calibrated airspeed. IAS corrected for instrument and aircraft setup issues. 
    • EAS: Equivalent airspeed. CAS corrected for compressibility.
    • TAS: True Airspeed. EAS corrected for density. 
    • G/S: Ground Speed. TAS corrected for wind.
  6. State the equation of continuity.
    Mass can be neither created nor destroyed therefore air mass flow is constant.
  7. Describe how the equation of continuity is relevant to a venturi.
    Essentially, air mass flow is the volume of air passing a point over one unit of time. Regardless of the size of tube the air is flowing through, the air mass flow must remain constant. Therefore with the decrease in area that the air can pass through, velocity must increase.
  8. State Bernoulli's Theorem.
    In an ideal fluid with steady streamline flow, the sum of the energies present remains constant.
  9. What are the limitations of Bernoulli's Theorem when considering air as a fluid?
    Above Mach 0.4, changes in heat energy become significant, as does the compressibility of air. Viscosity is also a limitation, as the theorem assumes fluid to be inviscid. 

    (Heat energy, compressibility, viscosity)
  10. Why is the pressure over a wing lower over the upper surface than the lower surface?
    As airflow travels over the upper surface of the wing, velocity increases due to the further path the air must travel. Velocity increases, and hence pressure decreases, creating lower pressure above the wing than below. This difference in pressure creates lift.
  11. On a diagram, label the following:
    Free Stream Flow/Relative Airflow
    Total Reaction
    Chord line
    Angle of Attack
    Centre of Pressure
    ***Q10 FPC#1 Diagram***
  12. Define aspect ratio.
    • The ratio of \frac{Span}{Chord}=\frac{b}{c} 
    • or
    • \frac{span²}{wing area} = \frac{b²}{S}
  13. Define load factor.
    Load factor is equal to total lift to weight. Measured in 'g' or 'n'

    n = \frac{Total Lift}{Weight}
  14. How does centre of pressure move with angle of attack?
    With the increase of angle of attack and hence increased Total Reaction, the Centre of Pressure moves forward. When the wing stalls, the TR reduces abruptly and CP moves rapidly aft.
  15. How does Total Reaction change with increasing angle of attack?
    As angle of attack increases, more lift is produced, and also more drag is produced. These vectors increase in size and hence Total Reaction will increase.
  16. Define the boundary layer.
    The layer of air extending from the surface of the wing to the point where no dragging effect is discernible. It is normally defined as the region of flow in which the speed is less than 99% of the relative airflow.
  17. Draw the boundary layer over an aerofoil section, and identify the following; laminar flow, transition point, turbulent flow and separation point.
    ***Diagram for Q14 FPC1***
  18. Describe adverse pressure gradient.
    Over the surface of the wing, pressure reaches a minimum about the point of maximum thickness, and then increases towards the trailing edge. This increase means there is a pressure gradient opposing the airflow, which is called the adverse pressure gradient.
  19. What is the stagnation point?
    The point on the wing where the velocity of the air particles is said to be zero, i.e. the flow is brought to rest. This will lie somewhere on the leading edge of the wing where the flow divides to go over the upper and lower surfaces.
  20. Why are nosewheel aircraft inherently more stable than tail draggers on the takeoff roll?
    In a tail dragger, the C of G is aft of the main wheels. When the aircraft is not aligned with the direction of travel for any reason, the couple formed by the C of G and the wheel drag will tend to increase misalignment. In a nose wheel aircraft, the couple formed between the C of G and the wheel drag tends to align the aircraft with the direction of travel.
  21. What 5 factors swing the aircraft on the takeoff roll?
    • Slipstream: Clockwise rotation of the prop imparts asymmetric flow over the fin inducing yaw to the left. 
    • Prop & Eng torque: With the prop and engine turning in the same direction, a reaction is created in the opposite direction (Newtons 3rd Law). This reaction applies more apparent weight on one wheel than the other, yawing the aircraft away from the direction of prop rotation.
    • Asymmetric Blade Effect: Tail-down attitude results in higher angle of attack on the downgoing blade than upgoing blade. Produces for thrust and yaws aircraft away from direction of rotation.
    • Gyroscopic: Pitching action of lifting the tailwheel. Rotating prop acts as a gyro, applying a force 90° in the direction of rotation.
    • Wind: Crosswind weather-cocking.
  22. Define lift.
    The component of the total reaction that is perpendicular to the Relative Airflow.
  23. What factors affect Lift?
    • Free Stream Velocity (v2)
    • Air Density (ρ)
    • Wing Area (S)
    • CL factors
  24. State the Lift Formula
    L=CL \frac{1}{2} ρ v S
  25. What factors affect CL(ASCRS)
    • Angle of Attack
    • Shape of the wing section and planform
    • Condition of the wing surface
    • Reynolds Number
    • Speed of Sound
  26. What is the trade off with a symmetrical aerofoil vs cambered aerofoil?
    A cambered aerofoil produces lift even at zero angle of attack, although it will stall at a lower angle of attack than a symmetrical aerofoil. A symmetrical aerofoil loses out as it produces zero lift at zero angle of attack.
  27. How does aspect ratio affect Cvs Angle of Attack?
    Aspect ratio affects the degree to which induced downwash influences the wings characteristics. A high aspect ratio wing will not disturb as much of the wing relative to a low aspect ratio wing. Low aspect ratio suffers increased downwash, reducing the slope of the CL curve.
  28. How does sweepback affect CL?
    Lift is only considered to act when airflow passes over the wing at 90° to the leading edge. On a swept wing, the airflow is less than the RAF, producing less lift than a straight wing which receives all of the RAF as effective airflow.
  29. How does Reynolds Number effect CL when comparing two identical aircraft travelling at the same altitude, with one travelling faster than the other?
    Assuming density and viscosity is constant, velocity is the only different factor between the two aircraft. The faster airflow over the wings of the faster aircraft produces an earlier transition to turbulence, hence an increase in the Kinetic Energy of the boundary layer. This results in delayed separation, and therefore an increased critical angle and CL max than the slower aircraft.
  30. List the three general classes of aerofoil, and the type employed on the Airtrainer.
    • High Lift
    • General Purpose - Used on Airtrainer
    • High Speed
  31. Define Drag.
    Drag is the sum of all of the components of the aerodynamic forces, which act parallel and opposite to the RAF.
  32. List the Drag family tree.
    • Total Drag 
    • Zero Lift Drag and Lift Dependant Drag

    • ZLD
    • Surface Friction, Form Drag, Interference Drag

    • LDD
    • Induced Drag, Increments of ZLD
  33. Describe Zero Lift Drag
    When an aircraft is flying at zero lift angle of attack, the resultant of all the aerodynamic forces acts parallel and opposite to the direction of flight, as there is no lift. This force is Zero Lift Drag, and is comprised of Surface Friction Drag, Form Drag, and Interference Drag.
  34. Describe Surface Friction Drag
    Drag caused by the boundary layer created as objects move through the air. It is essentially caused by the difficulty air faces in travelling over an object due to friction.
  35. Describe Form Drag
    Form drag is the difference in pressure between the front and rear of a moving object. This is caused by the ramming air on the front of the object generating a high pressure, trying to fight the direction of movement as it wants to move towards the low pressure behind the object.
  36. Describe Interference Drag
    Interference Drag is due to the interference of the boundary layers of different planes creating turbulence and energy loss.
  37. Describe Lift Dependant Drag
    In producing lift the aircraft will produce additional drag known as LDD, which is composed of Induced Drag and Increments of ZLD
  38. Describe induced drag
    • Pressure difference upper and lower
    • Airflow spills over wingtips
    • Creates vortices as air meets

    The pressure difference between the upper and lower surfaces of the wing causes air to spill around the wing tips, deflecting the airflow over the upper surface of the wing towards the fuselage, and on the underside towards the tip. When the two flows meet at the trailing edge, a sheet of vortices is formed which tends to drift towards the tip, forming a large vortex. Under conditions of high lift, i.e. manouvres, the pressure difference is increased and hence vortices are stronger.
  39. What 3 factors are part of Increments of ZLD?
    Effect of Lift: Increased lift → Earlier Transition point → Increased APG → Earlier Separation → increased form drag and surface friction drag

    Form Drag: Increased AoA → Frontal area increased → Increased form drag

    Interference Drag: More lift → thicker boundary layers → more turbulent → more energy loss.
  40. Is ZLD affected by weight?
    No, only LDD
  41. On a graph of total drag vs EAS, identify speeds for:
    Max L/D ratio
    Min Power
    Max EAS/Drag
    ***Diagram Q11 FPC2***
  42. Why do some aircraft approach to land with air brakes/spoilers out?
    On approach, decreasing VIMD is sometimes necessary for handling considerations. By increasing ZLD through the use of airbrakes, VIMD is reduced. This allows slower speed without getting on the back of the drag curve.
  43. What is the v in the Lift and Drag formulas?
  44. What is the drag formula?
    D=CD \frac{1}{2} ρ v2 S
  45. What factors affect Drag?
    • Free Stream Velocity
    • Air Density 
    • Wing Area
    • CD factors
  46. Why would putting dimples in golf balls decrease form drag?
    The dimples create an earlier transition point in the boundary layer around the golf ball. The kinetic energy of the turbulence moves the separation point aft, decreasing the pressure difference between front and rear, and hence decreasing total drag. Surface friction is increased, although it is not the dominant drag change.
  47. Draw a diagram of the four forces acting on an aircraft in flight, and show the points through which the forces act.
    ***Diagram 1 FPC3***
  48. What is the purpose of the tailplane, and why is the force it provides so much less than those on the main plane?
    The function of the tail plane is to supply any force necessary to counter residual pitching moments arising from inequalities of the two main couples. It requires only a small force due to the large moment arm the tailplane has.
  49. Which way does the nose pitch when power is increased in the Airtrainer? Why?
    The nose pitches up when power is applied. Although this is contradictory to what would be expected of an aircraft with CG above CP, the increased airflow over the tailplane generates more downward force, overcoming the nose down force and pitching the nose up.
  50. How does power required for flight at constant IAS change when weight is increased? Why?
    Power required will increase. 

    Weight increase → lift required increased → increase angle of attack to do so → increased LDD → more power required.
  51. How does power required for flight at constant IAS change when altitude is increased? Why?
    Power required will increase.

    Power required is a function of drag x TAS. With an increase in altitude, TAS increases, hence more drag is produced. More power required to overcome this and remain at constant IAS.
  52. On a graph of power available and power required vs EAS, identify the points of:
    Min S&L speed
    Min power speed
    Max S&L speed
    ***Diagram 6 FPC 3***
  53. State the three factors that determine the power required in a climb
    • Overcome drag in level flight
    • Lift the weight at a vertical speed known as ROC
    • Accelerate the aircraft slowly as the TAS slowly increases with altitude.
  54. What is the formula for best angle of climb?
    sin⊖ = \frac{T-D}{W}
  55. What is the formula for best Rate of Climb?
    ROC = \frac{v(T-D)}{W}
  56. State the requirements for best endurance
    Rate of descent will be least when the aircraft is gliding at the velocity where minimum power is required.
  57. State the requirements for range
    Best glide angle is obtained when angle of attack is set at the max Lift/Drag ratio, which is minimum drag or VIMD.
  58. How does wind affect endurance and range?
    Wind does not have an effect on endurance, as the aim is max time in the air. 

    In a head wind, distance covered in the glide will decrease. If airspeed is increased slightly, the ground distance covered may be improved as the wind has less time to act on the aircraft. In a tail wind,the ground distance will increase. Airspeed can be reduced towards VMP, increasing time for the tailwind to act as ROD is decreased at VMP
  59. What must be done to maintain straight and level flight when bank is applied? Why?
    The angle of attack must be progressively increased to increase the lift. Power must also be increased. This is because the lift is now in two components, horizontal and vertical. Vertical lift has decreased with increasing angle of bank. Increasing lift results in increased LDD requiring more power.
  60. What is another name for the horizontal component of lift?
    Centripetal Force
  61. Would lowering flap increase turning performance?
    Yes, as it would increase CL and hence total lift, enabling greater FC or bank angle at a given speed. This would enable greater turning performance, although lowering of flap normally imposes limiting speeds or 'g' which outweigh any benefit.
  62. How does drag affect a turn at constant speed?
    With an increase in lift, LDD is also increased. Airspeed will therefore decrease unless more power is used. Most aircraft are limited by the power available to offset the increased drag and sustain a stable level turn at higher angles of bank.
  63. How does power required change at greater angles of bank?
    With an increase in lift required, and subsequent drag rise, more power is required in a turn to maintain constant airspeed.
  64. How does increasing IAS turn performance?
    Increasing IAS enables greater load factor and thus increased turn performance. However, any increase past what is required to reach limiting 'g' will be counter productive, as increased momentum will result in a decreased rate of turn and hence increased radius of turn.
  65. How does increased TAS affect turning performance?
    For the same EAS, TAS is higher at altitude. At higher TAS, momentum is increased and the rate of turn decreases, resulting in greater turn radius.