Angle of Attack:

If IAS remains constant and AoA increases Lift will also increase.

LIFT COEFFICIENT In a wind tunnel it is possible to select a fixed speed for the wind and then rotate the aerofoil through various angles of attack. It is then possible to study the effect of changing the angle of attack with all other factors constant. This cannot be done with a real aeroplane in flight, but it does tell us some important things about the aerofoil. Represents approximately the behaviour of the inboard section of the Cessna 152 wing.

The aerofoil's ability to extract lift from the energy of the airstream is represented as numbers for each angle of attack. These numbers are called lift coefficients. We are not interested in the numbers themselves, it is the general behaviour of the aerofoil that concerns us. Because the aerofoil is cambered, it makes some lift even at no angle of attack. To produce no lift, a small negative angle of attack is required. Lift continues to increase in a straight line until, at about 18° angle of attack, a point is reached where any further increase in angle of attack results in less lift. This angle of attack is known as the stalling [or critical] angle of attack. Note that the lift coefficient is really a measure of how much of the energy available is being converted into lift. It is a measure of the 'degree of success' being achieved in the production of lift. This aerofoil, at about 18° angle of attack, will achieve its best 'degree of success' in extracting lift from the airstream. The aerofoil's ability to extract lift from the energy of the airstream is represented as numbers for each angle of attack. These numbers are called lift coefficients. We are not interested in the numbers themselves, it is the general behaviour of the aerofoil that concerns us. Because the aerofoil is cambered, it makes some lift even at no angle of attack. To produce no lift, a small negative angle of attack is required. Lift continues to increase in a straight line until, at about 18° angle of attack, a point is reached where any further increase in angle of attack results in less lift. This angle of attack is known as the stalling [or critical] angle of attack. Note that the lift coefficient is really a measure of how much of the energy available is being converted into lift. It is a measure of the 'degree of success' being achieved in the production of lift. This aerofoil, at about 18° angle of attack, will achieve its best 'degree of success' in extracting lift from the airstream. The amount of lift actually produced would also depend on how much energy is available i.e. speed and density of the airstream. Lift coefficient is not lift. It is one of the factors that produces lift and it depends upon angle of attack. The change of lift coefficient with angle of attack can be explained in terms of the behaviour of the local airflow as it responds to the pressures produced around the aerofoil. As air flows across the top surface, it accelerates to reach a maximum speed near the point of maximum thickness. According to Bernoulli, this must also be the point of minimum pressure. Since there is a natural tendency for air to flow towards a region of low pressure, as it approaches the point of minimum pressure the pressure gradient acts in the direction of motion and causes it to accelerate After the point of minimum pressure, its momentum carries it on but the pressure gradient is opposing the direction of motion, causing it to decelerate. To a much lesser extent, similar effects occur across the bottom surface. While ever the airflow is slowing down, its value as a lift producing agent is lessening. As angle of attack increases, the minimum pressure region on the top surface moves forward and becomes even lower. This assists the airflow to accelerate over the front section, but also acts to further impede its progress aft of the minimum pressure. For small angle of attack increases, the gain from the extra speed over the front, exceeds the loss from the reduction in speed over the rear, so a net gain in lift coefficient is noticed The centre of pressure continues to move forward with increasing angle of attack until the point is reached where the gain produced by the accelerating airflow near the front is cancelled by the loss due to the decelerating airflow at the rear. Any further increase in angle of attack produces a net loss of lift and the stall has occurred. .

Curiously, the original speed of the relative airflow has almost no effect on the angle of attack at which the stall is encountered. In fact it is meaningless to talk about the stalling speed of an aerofoil, it stalls at a certain angle, not at a certain speed. One way of investigating the behaviour of the airflow as it passes across the wing is by means of a wool tuft experiment. This involves attaching a number of tufts of knitting wool to the surface being considered. As the air passes across the surface, each wool tuft deflects to indicate the behaviour of the local airflow. The result can often be surprising as the following photographs show! These photographs were taken during a wool tuft experiment conducted on a Piper Warrior at Archerfield. The wool tufts show clearly how the airflow behaves in flight.