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Flashcards in Aerodynamics 2 Deck (104)
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1

Define Boundary Layer

The layer of airflow over the surface of an airfoil, which shows local airflow retardation caused by viscosity. The boundary layer is very thin at the leading edge and grows as it moves over a body. It is composed of laminar flow and turbulent flow.

2

DESCRIBE the different types of flow within the boundary layer

  • Laminar flow: the air moves smoothly along streamlines. It produces very little friction, but is easily seperated from the surface.
  • Turbulent flow: the streamlines break up and the flow is disorganized and irregular. It produces higher friction drag, but adheres better to the upper surface of the airfoil, delaying boundary layer separation.

 

3

DESCRIBE boundary layer separation

If the boundary layer does not have sufficient kinetic energy to overcome the adverse pressure gradient, the lower levels of the boundary layer will stagnate and the boundary layer will separate from the surface. Airflow along the surface aft of the separation point will be reversed resulting in a turbulent wake.

4

DEFINE CL MAX AOA

The angle of attack, beyond which CL begins to decrease is CLMAX AOA.

5

DEFINE stall

A condition of flight in which an increase in AOA results in a decrease in CL

6

EXPLAIN how a stall occurs

The adverse pressure gradient is strongest at high lift conditions and high AOA. The boundary layer will not have sufficient kinetic energy to conform to the surface of the airfoil, and will separate. At high AOA, the separation point moves forward toward the leading edge, resulting in a stall.

7

IDENTIFY the aerodynamic parameters causing a stall

The only cause of a stall is excessive AOA. Regardless of flight conditions or airspeed, the airfoil will stall when the AOA exceeds CLMAX AOA, also known as stalling AOA or critical AOA.

8

COMPARE power-on and power-off stalls

The aircraft will stall at a lower airspeed in power-on stalls because at high pitch attitudes, part of the weight of the airplane is being supported by the vertical component of thrust. Also, in propeller driven aircraft, the portion of the wing immediately behind the propeller continues to produce lift at high angles of attack because the air is being accelerated by the propeller.

9

DESCRIBE the order of losing control effectiveness approaching a stall in the T-6B

Ailerons, elevator, then rudder

10

EXPLAIN the difference between true and indicated stall speed

An airplane will stall at a higher TAS as altitude increases, but will stall at the same indicated airspeed regardless of altitude.

11

EXPLAIN the effects of gross weight, altitude, load factor and maneuvering on stall speed, given the stall speed equation

Increased weight, altitude, and load factor will result in a higher stall speed. You will also experience a higher stall speed in maneuvering flight.

12

STATE the purpose of using high lift devices

The purpose of high lift devices is to reduce takeoff and landing speed by reducing both indicated and true stall speeds.

13

DESCRIBE how different high lift devices affect the values of CL, CL MAX, and CL MAX AOA

Slats and Slots do not change CL at low AOA, but CLMAX and CLMAX AOA are increased

Flaps (both leading edge and trailing edge) increase CL and CLMAX, but CLMAX AOA is actually lower when flaps are deployed.

 

14

DESCRIBE devices used to control boundary layer separation

  • Slots allow high static pressure air beneath the wing to be accelerated through a nozzle and injected into the boundary layer on the upper surface of the airfoil, delaying boundary layer separation at high AOA.
    • Fixed slots are gaps at the leading edge of a wing that allow air to flow from below the wing to the upper surface.
    • Slats are moveable leading edge sections used to form automatic slots. These may be deployed aerodynamically, mechanically, hydraulically, or electrically.
  • Vortex Generators are small vanes installed on the upper surface of an airfoil that disturb the laminar flow and introduce a turbulent flow to the boundary layer, delaying boundary layer separation.

15

DESCRIBE devices used to change the camber of an airfoil

  • Plain Flap: a simple hinged portion of the trailing edge that is forced down into the airstream to increase camber
  • Split Flap: a plate deflected from the lower surface of the airfoil. This creates a lot of drag.
  • Slotted Flap: Is similar to a plain flap, but opens a narrow slot between the flap and wing.
  • Fowler Flap: moves down and aft, increasing camber and significantly increasing wing area as well as opening one or more slots.
  • Leading Edge Flaps: change the wing camber at the leading edge and may also open a slot.

16

DESCRIBE methods of stall warning used in the T-6B

  • AOA indicator: calibrated so that the airplane stalls at 18 units AOA regardless of airspeed, attitude, weight or altitude. It autmotaically accounts for the differences in full-flap and no-flap stall angles.
  • AOA indexer: Receives input from the AOA prope on the left wing.
  • Stick shaker: Also receives input from the AOA probe and is activated at 15.5 units AOA, followed by buffeting

17

DESCRIBE the stall tendency of the general types of wing planforms

  • Rectangular wing: Strong root stall tendancy
  • Highly Tapered wing: Strong tip stall tendancy
  • Swept wings: Strong tip stall tendancy
  • Elliptical Wing: even lift distribution, with all sections stalling at the same AOA
  • Moderate taper wings: similar to eliptical wing. The pilot loses lateral control during a stall. T-6B uses this design.

18

DESCRIBE the various methods of wing tailoring, including geometric twist, aerodynamic twist, stall strips, and stall fences

  • Geometric twist: A decrease in angle of incidence from wing root to wingtip. The root stalls first because of its higher AOA.
  • Aerodynamic twist: also called section variation, is a gradual change in airfoil shape that increases CLMAX AOA to a higher value near the tip than at the root, either by decreasing camber or relative thickness of the wing.
  • Stall strips: sharply angled piece of metal mounted on the leading edge of the wing root, which causes the boundary layer to separate at a lower AOA in that section.
  • Stall Fences: redirect airflow along the chord of the wing thereby delaying tip stall.

The T-6B uses both geometric and aerodynamic twist and has stall strips on the root section of the wing leading edge.

19

DEFINE takeoff and landing airspeed in terms of stall speed

Takeoff Airspeed: 20% above the power off stall speed.

Landing Airspeed: 30% above stall speed

20

STATE the various forces acting on an airplane during the takeoff and landing transition

  • Rolling Friction (FR): friction between the landing gear and the runway.
  • Thrust
  • Drag
  • Net Accelerating Force: Thrust minus Drag and rolling friction.
  • Net Decellerating Force: Drag plus Rolling Friction minus Thrust

 

21

STATE the factors that determine the coefficient of rolling friction

  • Runway surface
  • runway condition
  • tire type
  • degree of brake application

22

DESCRIBE the effects on takeoff and landing performance, given variations in weight, altitude, temperature, humidity, wind, and braking

  • Weight: Increasing weight increases rolling friction, requires greater lift and a higher takeoff velocity. Doubling weight will increase takeoff distance four times.
  • Increasing airfield elevation (altitude), increasing temperature, or increasing humidity will increase Density Altitude (DA). Higher DA requires a higher takeoff velocity and decreases the amount of thrust the engine can provide, thereby increasing takeoff distance.
  • Braking: A decrease in braking effectiveness will incresae landing roll.

Mnemonic: "4-H Club": High, Hot, Heavy and Humid. Whenever three or more of these are present, expect extended takeoff and landing distances.

23

DESCRIBE the effects of outside air temperature (OAT) on airplane performance characteristics

Increasing outside air temperature increases density altitude resulting in less lift. It also decreases thrust available. It will result in a longer takeoff roll, and a lower rate of climb.

24

DEFINE maximum angle of climb and maximum rate of climb profiles

Maximum Angle of Climb (AOC) is a comparison of altitude gained to distance traveled. Maximum vertical velocity for a minimum horizontal velocity.

Maximum Rate of Climb (ROC) is a comparison of altitude gained relative to the time needed to reach that altitude. Results in maximum vertical velocity.

25

EXPLAIN the performance characteristics profiles that yield maximum angle of climb and maximum rate of climb for turboprops.

Maximum AOC performance depends upon thrust excess. Occurs at a velocity less than L/DMAX  and an AOA greater than L/DMAX AOA for a turboprop

Maximum ROC performance depends upon power excess. Occurs at L/DMAX AOA and velocity for a turboprop.

Max AOC and max ROC are not used in the T-6B. Best climb speed of 140 KIAS is used instead.

26

DESCRIBE the effect of changes in weight, altitude, configuration, and wind on maximum angle of climb and maximum rate of climb profiles

An increase in weight, increase in altitude, lowering the landing gear, or lowering flaps will decrease max AOC and max ROC performance.

A headwind will increase AOC performance due to the decrease in groundspeed, while a tailwind will decrease AOC. Wind has no effect on ROC

27

DESCRIBE the performance characteristics and purpose of the best climb profile for the T-6B,

Best climb speed will meet or exceed any obstacle clearance requirements while providing a greater safety margin than slower airspeeds.

28

DEFINE absolute ceiling, service ceiling, cruise ceiling, combat ceiling, and maximum operating ceiling

Combat ceiling: Altitude where max power excess allows only 500 fpm ROC.

Cruise ceiling: altitude at which an airplane can maintain only a 300 fpm ROC.

Service ceiling: altitude at which an airplane can maintain only a 100 fpm ROC.

Absolute ceiling: The altitude at which an airplane can no longer perform a steady climb since maximum thrust excess is zero.

Operational ceiling: 31,000 ft for the T-6B

29

STATE the maximum operating ceiling of the T-6B

31,000 ft

30

STATE the relationship between fuel flow, power available, power required, and velocity for a turboprop airplane in straight and level flight

  • Fuel flow varies directly with the power output of the engine (PA).
  • Minimum fuel flow for equilibrium flight will be found on the power required (PR) curve.
  • The power required curve will tell us the velocity we must fly to acheive equilibrium flight. The pilot must adjust the throttle to eliminate thrust excess.