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Computer drawing of a glider showing the airspeed components and vertical wind speed.
 vertical velocity = vector sum of vertical airspeed and wind speed.

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 which is called an updraft, if it is directed away from the ground, or a downdraft if it is directed towards the ground. The effect of air motion along the flight path is described on another slide. The effects of cross winds, which are parallel to the ground and perpendicular to updrafts and downdrafts, are also described on another page.

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 relative to the ground plane. For a descending glider, the vertical airspeed is negative. The aerodynamic forces acting on the glider are defined relative to the flight path. The drag acts along the flight path while the lift acts perpendicular to the flight path.

The air moves at some constant velocity called the wind speed. The wind speed is a vector quantity having both a magnitude and a direction. We define an updraft to be in the positive direction and a downdraft to be in the negative direction. The vertical velocity of the aircraft, relative to the ground, is the vector sum of the vertical airspeed and the wind speed.

Vertical Velocity = Vertical Airspeed + 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. Updrafts can also be caused by the sun heating the ground. The heat from the ground warms the surrounding air, which causes the air to rise. The rising pockets of hot air are called thermals. Downdrafts occur on the downwind side of a hill or mountain. Downdrafts are often found in the vicinity of strong thunderstorms. A micro-burst is a highly localized downdraft of air. Micro-bursts near airports have been blamed for airplane crashes in recent years.

The description given for this slide concerns static performance only. This means that the wind is steady and the aircraft is aligned along its flight path. Unsteady updrafts or downdrafts 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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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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