how does an aircraft fly upside down . explain with physics of fluids theory, please?

Answer 1

Lift is determined not just by the shape of an airplane's airfoil but also by its angle of attack.

When an airplane is upside down, it can still fly tilted toward the sky and produce lift the same way it would if it would fly rightside up. It just happens that a plane must have a higher angle of attack when upside down than it would rightside up (because the shape of the airfoil is optimized for rightside up flying).

Let's assume we're talking about subsonic flying. The physics is identical regardless of which way the plane is flying. Laminar flow will separate over the wing and join together at the end of the wing. When the wing is tilted upward with respect to the direction of thrust, the flow separation over the wing forces the air over the higher part of the wing to travel a longer path than the air under the lower part of the wing. This makes the effective volume of a small unit of air over the wing smaller, and this decrease in volume comes with a decrease in pressure. That decrease in pressure on the higher side of the wing allows the pressure on the lower side of the wing to provide upward lift. (note: this principle of decreased volume over the top of a wing is a result of the conservation of energy; the airflow over the top of the wing is traveling faster and thus has more kinetic energy. To conserve energy, it's potential energy must decrease, and thus its pressure decreases)

Also note that this principle isn't the only thing causing lift. There is also a lift component from the normal force of air impacting the bottom of the airfoil. This combined with the effect described above produce the lift required.

Just stick your hand out the window. If you tilt it away from the road, it doesn't matter whether or not your palm is facing up or down. In either way, your hand will feel a lift force. In fact, even if you change the camber of your hand to make it shaped like an airfoil, you'll still be able to flip it over and get upward lift as long as you tilt your hand upward enough.

Of course, if the airfoil (or your hand) needs to be tilted too far upward to get enough of an angle of attack to provide lift, it will stall when laminar flow ceases. In this case, the turbulent flow results in "stall" and you stop getting any lift.

Answer 2

Hi. It has to be designed to take the load and the wing needs to create lift at a high angle on attack, inverted. The flow of air over the bottom of the wing surface must create enough pressure difference (from the inverted top surface) to overcome drag.