Birds Aerodynamics 鳥類空氣力學

希望借由人類傳統的航空知識,及近來專家們借由直量測鳥類飛行所得新發現來 介紹鳥類飛行

Wednesday, July 19, 2006

Airfoil crosssection

The flight is the result of lift ,drag and gravity.飛行是升力,阻力及重力的總合現象

The cross-section of bird's wing just like the airfoil of the aircraft so that I will introduce some basic aerodynamics relative items.

1. Airfoil relative items , 翼截面相關項目

CL:Lift coeifficient升力係數

CD:Drag coeifficient阻力係數

Chord line : line between leading and trailing edge 弦線 :前緣與後緣的連線

AOA :α Angle of Attack, the angle between the wind level and the chord line 攻角 :翼弦線與氣流的夾角

S: Area of wing 翼面積

AR:Aspect ratio 展弦比

b: SPAN :翼展

ρ: Density of fluid 流體的密度

2.Lift 升力
No one perfect rule for the secret of lift even we have arrived space.
We calculate the lift of wing base on the following formula.

Lift : L= 0.5* (CL)*ρ* (V^2)* S

Lift occurs when a moving flow of gas is turned by a solid object.

For lift to be generated, the solid body must be in contact with the fluid: no
fluid, no lift.
There must be motion between the object and the fluid: no motion, no lift.
Lift acts perpendicular to the motion.
Drag acts in the direction opposed to the motion

3.Aircraft control surface 飛行器的控制面

Wing 翼

Rudder 舵

Elevator 升降舵

Vertical stabilizer 垂直安定面

horizontal stabilizer 水平安定面




Canard :small front wing locate the front side of the primary wing 前翼 : 在主翼前方的一個小翼

4.Re no. and Vortex 雷諾數及渦流

Re=(ρ•V•b)/μ 雷諾數

V: velocity of the fluid 是流體速度 ,μ黏性係數 ,ρ:Density of fluid 流體的密度, b: SPAN :翼展

Votex 渦流

LEV : Leading edge vortex 前緣渦流

Bird Flight 鳥的飛行

鳥類不是飛行器,它在不同的飛行過程如起飛,轉彎及降落中包含振翼,滑翔及旋停等飛行動作。它們的飛行原理不完全相同於人類用以製造定翼機的傳統空氣動力學知識, 它們飛行在我們非常陌生的低雷諾數空氣中。當在流體中運動的物體尺寸變小,速度減慢時,流體的黏性力的影響也就顯得比較重要,所以鳥對飛行的感受和我們可理解飛機飛行不同,應該比較像在我們在油裡「游泳」時的感覺一樣
The bird is not a airplane and its flight include the following flapping, gliding and hovering during the different flight process like take off ,turning and landing. They employ the different aerodynamics that is not the fully same with what human apply to construct the airplane.
It fly in the low Re air that is really strange for human being. The viscosity effect is going to “important” as the size is become smaller and velocity is low down in the fluid so that the flight feeling for birds is not consistence with what we think of the airplane and it would be more a realize condition as “ people swim in the oil”.

1.Bird Wing Structure 鳥翼結構

Feather 羽毛

Wing 翼

Alula (bastard wing)

2.Bird Flight Action 鳥的飛行動作

2.1 Gluiding 滑翔
Birds do not flap its wing and the wing act as the wing of the air aplane.
At this time ,it need the enough free stream ,soaring or groud effect.
AR(aspect ratio) is the key point, some birds has high AR and it can apply the
guilding skill to start the travel over the sea.
The follow is the ref data for different AR.

2.2 Flapping 拍翼
鳥翼如何拍翼飛行的? 我們將其分為二個行程:上行程及上行程
How dose it Flap its wing ? We separate the flapping process into two strokes
as upstroke and down stroke.

A. 上行程: 翼翅向上向後運動之過程,上行程具有較小之攻角(甚至為負攻角)
The upstroke : The process of the bird’s wing raising up and back .
The upstroke come with the smaller AOA or negative AOA.

B. 下行程: 翼翅向下向前運動之過程,下行程具有較大之攻角
The Down Stroke : The process of the bird’s wing power downward and forward.
The down stroke come with the larger positive AOA .

2.3 懸停 Hovering

3.Bird Flight Aerodymanics 鳥的飛行空氣動力學

What bird need to do is that it manage all force and let it orient to the desired
motion. How can it do force management by its wing and body.

3.1 升力來源 Lift source

3.2 推力來源 Thrust source

3.3 鳥類的飛行控制 Fight control in birds