Asked by: BBC Focus Magazine
To fly upside down, you need a wing design that can still provide lift even when inverted. On a conventional aircraft, the aerofoil is curved on the upper side and flat on the underside. That causes air over the top to flow more quickly than underneath. The difference in pressure gives the wing lift, forcing it upwards. Upside down, that effect would reverse, but the pilot could still achieve lift by altering the angle of attack, in other words by pointing the plane’s nose skyward. Depending on the aerofoil this could restore level flight and even allow the plane to climb.
But wings on aerobatic planes are curved on both the upper and lower sides. With this symmetric design, the plane can fly either normally or inverted. The pilot can flip from one to the other by altering the angle of attack. While great for stunts, the downside of this design is that it reduces the plane’s fuel efficiency.