15.4: Display Distribution of Cl
Cl (lift coefficient), this abbreviation probably heard everyone, who had
contact with aviation. With Cl one can describe lift characteristics of
airfoils. The lift coefficient of an airfoil varies with the angle of attack.
Having a table with angles of attack and Cl values, one can draw with them a
curve (Cl-alpha-polars). From this polars one can also see, with which angle of
attack the airfoil will stall.
Click the marked button to make the Cl distribution over the span width visible
in the 2D-graphic.
If a vortex computation is on hand, the 2D-graphic looks like shown below. The
drawn through green line shows the momentary Cl load of the wing. Dotted green
lines above and below indicate the Cl-max and the Cl-min. In the picture below
the load is OK, the wing still has enough reserves upward and downward.
If Cl-max is exceeded, a stall will occur. In the 2D-graphic these areas are
marked in red. In case of such a figure becomes visible, a too big angle of
attack is chosen. One has to decrease the angle of attack step by step, until
the stall warning will vanish.
In a modern airliner the on-board computer would emit a stall warning, before
it comes to such a situation. In the most favourable case the airplane simply
stalls and takes the nose down. In addition, it can happen that the airplane
suddenly begins to roll, e.g. if the wing tips are involved in the stall. Or it
rises itself up (flying wings with strong sweep and stall at the wing tips). If
this happens with insufficient altitude (landing approach), it comes to a
crash.