Apparent Wind

The apparent wind is the Vent felt by an observer which is located in a vehicle in displacement (car, bicycle, ship…). He is opposed to the real wind which is the wind felt by the same stopped observer.

The apparent wind can thus differ from the real wind in direction like of speed.

The apparent wind is the vectorial Somme real wind and wind generated by the displacement of the vehicle (which one describes of wind speed or relative wind ). The relative wind associated with a vehicle in displacement has a speed equal to the rate of travel of the vehicle and a direction opposed to that followed by the vehicle.

Illustration:

the passenger of a convertible vehicle smokes a cigarette. The vehicle is with the stop along a long straight line. The wind blows across the road towards the line of the passenger. The smoke of its cigarette under the push of the real wind (we are with the stop) moves towards the right door of the vehicle. The vehicle starts and accelerates gradually. The vehicle, while moving, generates a relative wind which comes to combine with the real wind. Smoke under the push of the apparent wind moves from now on towards the right back of the vehicle. If the driver still accelerates, smoke will take a direction which will approach the back of the vehicle more and more and will be increasingly horizontal: with acceleration the apparent wind changed direction and speed .

Apparent wind and sailing

The apparent wind, in spite of its name, is the wind really undergone by the person who moves in the vehicle. On a sailing ship, it is thus the apparent wind which acts on the aerofoil: the adjustments of the aerofoil must be carried out according to this wind and not of the real wind with sometimes astonishing consequences:

a sailing ship sails with a well regulated aerofoil. Suddenly the wind forcit. Under this additional push, the speed of the sailing ship increases. The direction and the speed of the apparent wind change since wind-speed was modified. The adjustments of the aerofoil are not adapted any more to the new direction and start to beat. These adjustments should be modified. Little time after the wind weakens, the same reasons generate the same consequences.

a multihull of race sends its veils in a breeze moderated with a wind which blows to the 3/4 back. The sailing ship takes speed. The apparent wind changes, it blows from now on by through. Under the push of this wind, the sailing ship still accelerates. Speed increases again; on account of this pace, the apparent wind turns and forcit inducing each time an acceleration of the sailing ship. The phenomenon stops either because the direction of the wind approaches the axis of the sailing ship too much and that it cannot act any more effectively on the veils or because the speed reached by the sailing ship generates a trail which prohibits all new acceleration.

The relative Vent on a ship is generated by the rate of travel on the bottom. It thus results from the combination of a directly observable phenomenon - the speed of the boat on water - and of a more underhand phenomenon - the current velocity. In the zones of high tension current or by weak wind, this last component can modify the felt wind appreciably and require to regularly re-examine the adjustment of the aerofoil to course and constant real wind because of the progressive changes speed and direction of the current.

Apparent wind and aviation

A Plane to hold in the air must reach a minimum speed which is determined by its aerodynamic characteristics.

To reach on takeoff this speed, the pilots and the air-traffic controllers of the airports make take off, as far as possible, the planes vis-a-vis the wind. The apparent wind felt by the apparatus, and thus its speed in the airs, will be then the sum of the wind obtained by the speed of the plane compared to the ground and the real wind. The landing vis-a-vis the wind makes it possible to increase the bearing pressure when speed compared to the ground decreases.

A Aircraft carrier, to take off its planes, is put vis-a-vis the wind and fact road at maximum speed (the aircraft carriers are designed to reach speeds going up to 35 node S for this purpose). The apparent wind felt by the plane on takeoff is then the sum of the wind created by its own speed on the bridge of the ship, the speed of the ship and the speed of the real wind.

This apparent wind is described as relative Vent in Aerial navigation.

a plane to take off must reach 200 km/h
the aircraft carrier at maximum speed reaches 50 km/h
With a blowing wind of face to 20 km/h, the plane takes off as soon as it reached 130 km/h

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