Tom Hutchins of Auckland asks :-
Bernoulli's Theorem is the usual explanation for how aircraft wings generate lift, in that an aerofoil causes the air to travel faster over the top of the wing. It seems very odd that a fluid moving at a faster rate is at a lower pressure. One would think that with faster speed there would be more energy involved and there would also be an increase of pressure in the fluid. Why not?
Squadron Leader Jim Rankin, a physicist who led the RNZAF Display Team and hence was the person who flys the upside-down plane, responded.
You can produce lift just by putting a flat surface in the airflow, at an angle to the flow (called the 'angle of attack'). Everyone has put their hand out of a car window, and made their hand 'fly' up or down by twisting their wrist. Part of the lift comes purely from the reactive force of air hitting the bottom surface, increasing the pressure on the bottom. A flat wing is not as efficient at producing lift as a curved one, where some lift comes from the reduced pressure on the upper surface caused by the greater curvature normally found on the top of the wing.
When an aircraft flys upside down, the angle of attack has to be increased enough to make the wing work even though the lower side of the wing is now more curved than the top.
In the photograph of the Air Force Red Checkers 'mirror' formation the top aircraft (NZ1938) is pointing much higher up in the sky than the bottom one. It has a bigger angle of attack, so the wing still works, but not as efficiently as the right way up.
Bernoulli's Theorem is another example of the adage that 'you can't get something for nothing'. By causing air to speed up over the wing the dynamic pressure (i.e. the force you would feel on your face if you were standing on the wing looking into the airflow) does indeed increase. To keep the total energy of the air constant, there is a commensurate reduction in the static pressure of the air (i.e. the pressure you would measure with a barometer) that creates lift. This effect is used in a number of common devices, including the carburettor on most engines.