One of the most confusing concepts for young aerodynamicists is the relative velocity between objects. Aerodynamic forces are generated by an object moving through the air, but the air itself can move also. Aerodynamic forces depends on the square of the velocity between the object and the air. To properly define the velocity, it is necessary to pick a fixed reference point and measure velocities relative to the fixed point. In this slide, the reference point is fixed on the ground
The air in which the aircraft flies can move in all three directions. In this figure, we are only considering air movement which is perpendicular to the ground. The effect of air motion along the flight path is described on another slide.
On this slide, we are considering a glider which moves through the air along a flight path at some velocity called the airspeed. The flight path makes an angle with the horizontal which is called the glide angle. The airspeed can then be resolved into a horizontal component and a vertical component. The aerodynamic forces acting on the glider can also be resolved into the drag along the flight path and the lift perpendicular to the flight path. The aerodynamic lift force depends only on the airspeed along the flight path and is not related to the glide angle.
The air moves at some constant velocity called the wind speed. The wind speed is a vector quantity and if it's direction is positive, we call this motion an updraft . Likewise, if the motion is negative, we call it a downdraft. The vertical velocity of the aircraft, relative to the ground, will be the vector sum of the vertical airspeed and the wind speed. If the magnitude of an updraft is greater than the magnitude of the vertical airspeed, a glider can gain altitude even though it is always falling through the surrounding air !! (The air rises faster than the glider falls.) Similarly, a strong downdraft can cause an aircraft to lose altitude even though it may be climbing through the surrounding air.
Updrafts are found when a wind blowing at a hill or mountain has to rise to climb over the hill/mountain. (But be careful--there may be a downdraft on the other side!) Updrafts can also be caused by the sun heating the ground. The heat from the ground heats the surrounding air, which causes the air to rise. The rising pockets of hot air are called thermals.
(The description given for this slide concerns static performance only. This means that the wind is steady and the aircraft is aligned along it's flight path. Unsteady updrafts or downdrafts will introduce additional forces on the aircraft due to instantaneous changes in the angle of attack. Additional forces can also be generated by maneuvering, the aircraft along the flight path by using the elevator. These effects are not discussed in this slide.)
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