空气动力学讲义

“In the streamline flow of an ideal fluid, the
sum of all energies will remain constant”
An example of Bernoulli’s Theorem can be seen in a Venturi.
The Venturi
Airfoils
• Leading edge: the part of airfoil meets the airflow first.
• Trailing edge: where the airflow over the upper surface rejoins the lower surface airflow.
Section A Lift and Drag
• The First Law of Thermodynamics: Energy can not be created or destroyed, Energy can be converted from one form into
another.
Bernoulli’s Principle
• • Chord line：an imaginary straight line drawn
through an airfoil from the leading edge to the trailing edge.
Airfoils
• Camber: the characteristic curve of its upper and lower surfaces. The upper camber is more pronounced, while the lower camber is comparatively flat. This causes the velocity of the airflow immediately above the wing to be much higher than that below the wing.
• To maintain the same total lift,when an indicated airspeed is decreased,the angle of attack must be increased to compensate补偿 for the loss of lift.
• If the angle of attack doesnot change,the airspeed doubles,the lift will increase by four time.
Skin Friction
• Skin Friction is caused by the roughness of all the airplanes surfaces.
• A layer of air clings (sticks) to these surfaces, creating small eddies which contribute to drag.
Static Pressure + Dynamic Pressure = Total (Constant)
Bernoull’s Equation:
• Bernoull’s Equation:
A1,v1
ቤተ መጻሕፍቲ ባይዱ
A2,v2
Dynamic Pressure Static Pressure Total Pressure
– Angle of Attack – Wing Design
If you increase CL, you increase Lift
Coefficient of Lift - CL
Simplified Lift Formula
Lift
Airspeed Angle of Attack
• Angle of attack and indicated airspeed determine the total lift.
parasite drag.
At lower airspeeds,induced drag increases which increases the
total drag
L/D RATIO
• 升阻比是相同迎角下，升力系数与阻力系 数之比，用K表示。
• 升阻比的大小主要随迎角变化而变化。 • 升阻比越大，飞机的空气动力性能越好。
Thrust
• Thrust is the forward-acting force which opposes drag and propels the airplane. In most general aviation aircraft, this force is provided when the engine turns the propeller
T
Drag
• Drag is a component of the total aerodynamic reaction which acts in the same direction as the relative airflow.
• It is a force in the opposite direction to flight. D
Not be confused with an airplane’s attitude in relation to the earth’s surface, or with “angle of incidence” （安装角）
Four forces
Lift
Thrust Weight
Drag
Weight
• Smoothing of aircraft components, and laminar surfaces help to reduce skin friction drag.
Parasite Drag
As airspeed is increased, parasite drag increases at the square of the speed
垂直的
The Lift Formula
LIFT = CL. ½ V².S
CL is the Coefficient of Lift
½ V²is the Indicated Airspeed
S is the Wing Area
Co-efficient of Lift
• A Component of Total Lift. Depends on:
As airspeed is increased the amount of Induced Drag is decreased
Total Drag Curve
D
CD
1 2
v2
S
At higher airspeeds,total drag increases because of increasing
Induced Drag
• Whenever we produce Lift, we will produce Induced Drag.
Induced Drag
• Because of the pressure differential between the upper and lower surfaces of a wing, there is a tendency for the air to ‘leak’ around the wingtip翼尖 (air wants to move from areas of high pressure to areas of lower pressure).
• As the airplane is travelling forward, this pressure differential creates ‘wing-tip vortices涡流’ which come down behind the wing.
Induced Drag
L
L’
D
Induced Drag
• Relative wind: is parallel to and opposite the flight path of the airplane.
Airfoils - Definitions
Angle of attack
The angle between the chord line of the wing, and the relative wind.
Performance
Airfoils
• An airfoil is any surface, such as a wing, which provides aerodynamic force when it interacts with （相互作用） a moving stream of air.
• Weight is the force of gravity which acts vertically downwards through the centre of gravity (CG) towards the centre of the earth.
W
Lift from an Aerofoil
Chapter 3 Aerodynamics
This chapter establishes the basic knowledge elements of aerodynamics.
• Sections
• A Lift and drag • B Stability • C Flight controls • D High-lift devices • E Turn • F Vmc • G High speed flight
• The air flowing over the upper cambered surface flow faster than the air over the opposite surface to reach the trailing.Thus the pressure on the upper wing surface is lower than that on the lower surface and lift is produced.
• An example of Interference Drag is the mixing of air over landing gear struts（支柱）.
• Fairings（整流罩） and blending of shapes help reduce interference drag.
Form Drag
Interference Drag
Skin Friction
Form Drag压差阻力
Results from the turbulent wake caused by the separation of airflow from the surface of a structure.
Drag can be classified into 2 broad types:
Parasite (寄生虫） Induced
Parasite Drag
• Parasite Drag is caused by any aircraft surface which deflects or interferes with the smooth airflow around the airplane. Three Types
The amount of drag is related to the SIZE and SHAPE of the object.
Form Drag
Form Drag can be decreased by ‘streamlining’
Interference Drag干扰阻力
• Interference Drag occurs when varied currents of air over an airplane meet and interact.