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发表于 2012-7-12 16:06:41
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本帖最后由 Ragnarok 于 2012-7-14 16:39 编辑
http://en.wikipedia.org/wiki/Angle_of_attack
Angle of attack is the angle between the body's reference line and the oncoming flow.
飞机相对于地球的速度方向并不总与飞机相对于空气的速度方向相同。事实上二者相同是一种理想状况，也许不会发生。因此用飞机相对于地球的速度与reference line之间的夹角来定义AOA是错误的。
另外根据飞机设计以及对reference line的理解不同，reference line并不总是平行于飞机纵轴。In aerodynamics, angle of attack specifies the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing can have twist, a chord line of the whole wing may not be definable, so an alternate reference
line is simply defined. Often, the chord line of the root of the wing is chosen as the reference line. Another alternative is to use a horizontal line on the fuselage as the reference line (and also as the longitudinal axis). Some authors do not use an arbitrary chord line but use the zero lift axis instead - zero angle of attack corresponds to zero coefficient of lift.
所以FPM与一些HUD上代表飞机纵轴指向的W符号之间的夹角只能被认为是攻角的近似值。
HUD符号没有大小，只有视角，即符号上距离最远的两个点与视点（摄像机或者眼睛）连线的夹角。美国人一般用milliradian/MR也就是千分之一弧度来表示这个夹角，这种方式也许起源HUD的祖先------二战时一些战机的瞄准镜的原理，用同心圆视角的大小和已知的飞机的大小来判断飞机的距离，这种情况中弧度比较适合近似计算。
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