James Lindsay in Nanaimo, B.C. asks: “How can an airplane fly upside down when the dynamics of the wing profile are inversed?” We put this puzzling question to Jock Williams, a retired RCAF fighter pilot and aviation expert in Toronto. “This explanation is hard to give without the use of gesturing hands, but I’ll try my best,” he says. Williams explains:
There are two ways in which a plane can fly upside down.
Planes that are intended to fly upside down – in airshows for example – can have what is called a symmetrical wing, one with the same curvature top and bottom. When the plane is inverted, the airflow across the “top” (actually the bottom) of the wing is differentiated from the bottom flow only by the angle of the wing relative to the oncoming airflow. As observed from a side view, the plane will look like it has its nose slightly higher than its tail. Lift will be produced by the upper (which is actually the bottom) surface. If the plane were to flip right-side up, the situation would be reversed and the now rightfully upper surface will resume doing the job.
The complicating factor is fuel and oil supply, both of which normally count upon the services of gravity. This limits inverted flight in most planes to about 20 seconds. Planes intended for longer inverted flights have special fuel and oil supply systems to counter this problem.
Planes that only fly upside down for short periods count upon the power of their engines and a major nose up angle to continue in level flight.
While the wing is not generating enough lift to remain level, it is generating a fraction of it, and the engine and propeller supply the extra percentage of total weight requiring support. Some modern sport planes have sufficient engine power to literally hang in the air like like helicopters. Some modern fighters such as the CF18 can balance motionless on the thrust of their engines alone, and then add throttle and climb away vertically. This can be done when thrust equals or exceeds weight. The Wright brothers would have been amazed.