Hello,

it is well know fact that the aerofoil achieves its best L/D ratio at 4 degrees angle of attack (for most normal aerofoils), that is also where the magnitude of drag is also minimum (correct me if I'm wrong) . So if angle of attack was increased from zero to beyond the stalling angle the magnitude of drag would decrease towards 4 degrees then increase till beyond the stalling angle.

This is with relation to question number 12 on exercise A2 on your Aerodynamics text book. Your answer states that the magnitude of drag increases continuously from zero to beyond the stalling angle. Please correct me if my statement is wrong

Thank you

There is a big difference between the magnitude of drag when considering an aerofoil in a wind tunnel and the behaviour of the drag acting on an aircraft in flight.

In a wind tunnel, the speed is kept constant and the aerofoil is rotated through a range of angles of attack. In that case, drag would simply increase throughout from zero angle of attack to beyond the stalling angle.

For an aircraft in level flight, lift is not negotiable - it must equal weight. So various speeds require particular angles of attack to produce the necessary lift. In that case, when the speed is such that the best lift/drag ratio angle of attack is required, the total drag acting will be at a minimum. By the way, even though 4° is often used as an example, there is nothing magic about the number 4. What matters is that there will always be a particular angle of attack (about 4° for the average GA wing) that will produce the best lift/drag ratio.

Above the best lift/drag ratio speed, drag increases faster that lift so the best lift drag ratio is begins to decrease.

Thank you for clarifying sir, it makes sense now

Have a blessed day