Wednesday, August 1, 2012

Angle of Attack in Turbulent Air

Updrafts

What happens to an airplane when the weight (gravity) and lift force (force of air pushing the air molecules up) in equilibrium are disturbed by neither change of airspeed nor a change of the airplanes weight?

Updrafts cause a change in the angle of attack. Remember, the angle of attack is the angle at which the wing meets the relative wind. When the plane is in level flight where weight and lift forces are in equilibrium the plane is flying toward the relative wind where the angle of attack is just enough to create the equilibrium. 

The upward flowing air in an updraft (think relative wind) temporarily increases the angle of attack. The angle of attack increase creates a large surge of lift force. You feel the airplane balloon upward against your body. 

The upward motion of the airplane causes the wind of flight blow at the wing from slightly above. This produces a lower angle of attack. The lower angle of attack restores lift equilibrium. The plane, with everything else unchanged except for the updraft, resumes its steady upward flight in the updraft.

Downdraft

In turbulent air the plane may exit an updraft and enter a downdraft of air. The downdraft, as the air meets the wing,  temporarily decreases the angle of attack. The lift is suddenly not able to create the lift force to hold the plane up. You feel the sinking airplane  because you exert pressure on your seat belt.

The sinking of the plane causes the plane to temporarily increase its angle of attack to restore lift. It now returns to equilibrium but continues downward as long as it remains in the down draft.

Keep in mind  the attitude of the plane was always level in both the updraft and the downdraft. The airplane always seeks the flight path that that results in equilibrium between its weight and lift. The attitude of the plane remains the same but the flight path will change.