# Session 4

#### Why It Is Possible to Fly

Use this Session to get into direct contact with a fundamental problem of science, which has only recently been resolved, using basic tools of DigiMat. To start consider the following propositions:

• Somehow the downward gravitational force on a cruising airplane or bird in gliding flight,  must be balanced by an upward force on the wings from the air flowing around the wings.
• The upward force on the wings must be balanced by a downward (reaction) force from the wings on the flow of air.
• The flow of air must be diverted down by the wing, referred to as down-wash.
• Newton computed the lift force from particles hitting the lower side of a tilted wing and thereby being redirected, but found a lift 10 times too small.
• Major contribution must come from the upper side of the wing pulling air down by suction (low pressure).
• Decreased pressure on the upper side and increased pressure on the lower side contribute to lift, most evidently from suction on upper side. From where does the suction come?

Watch Potential Flow and Secret of Flight video and website.

One of the great mysteries of science is how the lift keeping birds and airplanes in the air, is generated in the flow of air around a wing. Newton tried an argument but the lift he found was way too small to balance the gravitational pull. Another argument named d’Alembert’s paradox indicated that in fact the lift was zero. Only recently (2008) has the true mechanism for generation of sufficient lift been exhibited, which you can now discover yourself with the help of the following two code examples: (i) potential flow without lift and (ii) Euler flow with lift.

See that in Euler flow there is net downwash as downward flow redirection by the action of the wing with corresponding upward reaction force creating lift. See that in potential flow there is no redirection and so no downwash and so no lift (compare with potential flow around skew square). See that the redirection  directly connects to different separation patterns at the trailing edge.