Subsonic_aerodynamics_Definition_and_aerodynamic_laws_principles

1.

SUBSONIC AERODYNAMICS:
Definitions and aerodynamics law
principles
Prepared by: Kaipbek Gulsanat
senior lecturer at “Flight operation
of aircraft” dept

2.

Plan
• Definitions
• Airflow definition
• Two dimensional airfoil
• Three dimensional airfoil

3.

ISA
• The temperature lapse rate is assumed to be uniform at a rate on 2°C per 1000 f (1.98°C)
from mean sea level up to a height of 36 090 f (11 000 m) above which the lapse rate
becomes zero and the temperature remains constant at -56.5°C.

4.

ISA
• Static pressure (at rest)
• Dynamic pressure (in motion) dependent on ρ
T =ρ
Psr = ρ
Humidity = ρ
Increasing altitude will decrease air density because the effect of decreasing
static pressure is more dominant than decreasing temperature
• Ideal gas equation Pv=NRT;
• International Standard Atmosphere (ISA)
The ICAO standard atmosphere
assumes the following mean sea
level values:

5.

Airfoil Terminology
• Movement of the diaphragm moves a pointer over a scale so that changes in dynamic
pressure can be observed by the flight crew. But the instrument is calibrated at ISA sea level
density, so the instrument will only give a ‘true’ indication of the speed of the aircraft through
the air when the air density is 1.225 kg/m3 .

6.

Relative Airspeed
• IAS(Indicated Airspeed) corrected for position error = CAS (Calibrated
Airspeed)
• CAS corrected for compressibility = EAS (Equivalent Airspeed), at CAS >>
300
• EAS corrected for density = TAS (true airspeed)
• TAS corrected for wind velocity = GS(groundspeed)

7.

Airfoil Terminology
• Mean Camber Line
• Chord Line
• Thickness/Chord
Ratio
• Symmetric Airfoil
• Non symmetric
Airfoil

8.

Airfoil Terminology
• Symmetric Airfoil
• Non symmetric Airfoil

9.

Two Dimensional Airflow
• Streamline
• Stagnation point
• Upwash
• Downwash

10.

Airfoil Terminology
• Angle of incidence (angle
BTW Long.axis and chord
line) cannot be
changed/fixed from
Manufacturer
• Angle of attack
• Centre of Pressure where L
generated (moves back at
AoA hits 16 degree)
• Aerodynamic Centre (fixed
point on chord, defined as
point where all the CHGs in
L magnitude take place)doesn’t change during flight

11.

Influence of Angle of attack
• Negative/Low pressure
• Positive/High pressure
• Angle of attack

12.

Lift
• Basic Lift equation
• Application 1:

13.

Lift
• Application 1 (Climb):
For a constant lift force as altitude is increased, a constant mass flow
must be maintained. As air density decreases with altitude, the speed
of the wing through the air (the true airspeed (TAS) must be increased.

14.

Lift
• Application 2 (Level flight):
The relationship between speed and angle of attack at a constant
altitude (air density). As speed is changed, angle of attack must be
adjusted to keep lift constant.

15.

Lift
• Application 3:
If altitude increases, and TAS is kept constant, the amount of
airflow flying over the wing decreases, hence Lift decreases

16.

Lift
• Application 4:
If IAS & AoA are kept const, aircraft even flying in different level
produces the same lift.
IAS remains const, TAS increases, but density decreases (compensated),
hence overall Lift is const.
• High lift devices:
Reducing TO and Landing distance 2
To increase CL at a reduced speed

17.

Lift

18.

Wing shape, wing area and surface of Airfoil
• Rectangular Airfoil (to ease
understanding) –top view S=a*b
• Usual airfoil (top view) with
similar dimensions:
wing area=a*b= wingspan*chord
line
• Thickness of the airfoil T
(25%from the root)
• T/C ratio *100 (%)
• Finness Ratio= T/C
Root chord Cr
• Aspect Ratio = span/chord (at
class room to be discussed) it
plays crucial part in L/D ratio
a
b
a (25% from the root)
b
Tip chord Ct

19.

Lift/Drag ratio
• Drag – aerodynamic force,
parallel to RA and opposite in
direction flightpath
D=CD Q S
• CD - at lower AoA increase
slowly; at higher – rapidly;
beyond AoA critical exponentially
• L/D ratio(also called Glide ratio) –
gives aerodynamic efficiency
• Approx 4 degree AoA – gives max
L/D ratio; L/D = 15nm
15 nm
1nm

20.

Lift/Drag ratio
• Key points:
CL - increases from 0-16 degrees of AoA
L/D – increases from 0-4 degrees; from 4-16 degrees – decreases;
L/D max – at 4 degrees of AoA
CD -increases from 0-16 degrees of AoA; beyond AoA critical increases exponentially

21.

Lift/Drag ratio
• Key points:
CL - increases from 0-16 degrees of AoA
L/D – increases from 0-4 degrees; from 4-16 degrees – decreases;
L/D max – at 4 degrees of AoA
CD -increases from 0-16 degrees of AoA; beyond AoA critical increases exponentially

22.

Three Dimensional Airflow: Wing Tip Vortices
• Three Dimensional Airflow - actual pattern of airflow over an aircraft, it considers the
behavior of airflow over the whole wing (turbulence, vortices and so on)
• Wing tip vortices are generated when there is an interaction between the upper &
lower surfaces of a wing. This occurs at the trailing edge and wing tips.
• At higher angles of attack (lower IAS) the decreased chordwise vector will increase the
effect of the resultant spanwise flow, making the vortices stronger.
Clockwise(left wing)
Anticlockwise
(right wing)

23.

Three Dimensional Airflow: Induced downwash
• Induced downwash- trailing vortices create certain vertical velocity
components in the airflow in the vicinity of the wing. These vertical
velocities cause a downwash over the wing resulting in a reduction in the
effective AoA. The stronger the vortices, the greater the reduction in
effective AoA
• Induced drag - the aircraft must be flown at a higher AoA. This increases
drag.
• Wing tip vortices, in particular their influence on upwash and downwash,
have a significant effect on several important areas of aircraf
aerodynamics, stability and control. Some of these effects will be
examined now and throughout the remaining chapters

24.

Three Dimensional Airflow: Induced downwash

25.

Three Dimensional Airflow: Wake Turbulence
• Wake Turbulence – TE and wing tip vortices which are extended behind
the AC for considerable distance; it cannot be detected and it is
extended throughout the flight.
• Vortices present the greatest danger during the take-off, initial climb,
final approach and landing phases of flight - in other words, at low
altitude where large numbers of aircraft congregate. A wake turbulence
encounter is a hazard due to potential loss of control and possible
structural damage

26.

Three Dimensional Airflow: Distribution of Trailing Vortices

27.

Three Dimensional Airflow: Vortices near the ground
Cross wind 5 kt

28.

Three Dimensional Airflow: Vortices
• Atmospheric turbulence has the greatest influence on the decay o wake
vortices, the stronger the wind, the quicker the decay
• To avoid probability of Wake Turbulence Encounter certain separation
minima are applied by Air Traffic Control (ATC), but this does not full
guarantee avoidance
• Wake Turbulence Avoidance – maintaining proper separation remains
the best advice for avoiding a wake turbulence encounter.

29.

Three Dimensional Airflow: Ground Effect
• The closeness of the wing to the ground prevents full development of the
trailing vortices, making them much weaker. Upwash and downwash are
reduced, causing the effective AoA o the wing to increase.
• An aircraft is “in ground effect” when lift is increased and induced drag is
decreased.
• The reduced downwash will affect both longitudinal stability because of CP
movement, and the pitching moment because of changes to the effective
AoA of the tailplane.

30.

List of References
• National Aeronautics and Space Administration (NASA)
https://www.grc.nasa.gov/
• EASA Pro https://www.easa.europa.eu/
• Hyperphysics http://hyperphysics.phyastr.gsu.edu/hbase/pber.html
• Khan Academy
https://www.khanacademy.org/science/physics/fluids/fluiddynamics/a/what-is-bernoullis-equation

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• https://meet.google.com/wwm-jigu-rar