Flying Upside Down
Devices an aerobatic airplane uses to defy gravity--and convention.
- By Patricia Trenner
- Air & Space magazine, May 2002
(Page 2 of 3)
Most airfoils are cambered, or curved, on top but flat on the bottom. As a result, they fly better upright than inverted. Symmetrical airfoils, which have the same curvature on both surfaces (3 in diagram below), perform exactly the same upright or inverted, and so are favored by aerobatic pilots. In order to fly at all, however, a symmetrical airfoil must be positioned at a slight positive angle-leading edge high-with respect to the flight path; otherwise the airflow around the upper and lower surfaces would be the same, and no lift would be created.
These shovel-shaped surfaces, rigidly mounted on arms forward of the ailerons, provide "aerodynamic balance," reducing the effort needed to roll the airplane. Aerobatic airplanes need aerodynamic balances because their control surfaces are large and their speeds are sometimes high. When the ailerons are neutral, the spades are aligned with the airstream and do nothing. But when an aileron is deflected upward, for example, its spade tips downward. Air presses against it, (4 in diagram above) helping the aileron along, just as the weight of a small person on one end of a teeter-totter helps a larger person at the other end push off the ground. The farther the aileron is deflected, the larger the force supplied by the spade. Aerobatic pilots describe spades as akin to power steering.
"Spade design is a black art," says airshow pilot Patty Wagstaff. "You see all kinds of shapes and all sizes, depending on the airplane. Akro pilots are always tweaking them to get the control feel just right-not too light and not too heavy. I've flown without spades, and it was like driving a Mack truck."
An aerobatic airplane has either a fixed-pitch or constant-speed propeller. The pitch of the blades is the angle at which they "bite" into the air. On airplanes with a fixed-pitch propeller, engine rpm (revolutions per minute) is the primary power gauge. Advancing the throttle increases combustion, which spins the driveshaft faster and increases rpm. When airspeed increases, the relative airflow from the airplane's forward motion reduces the angle of attack for a given pitch of the propeller blade, which reduces drag and lets the propeller spin faster. Too much airspeed can result in engine overspeed, so the pilot must keep an eye on the tachometer to make sure engine rpm does not exceed redline.
On a constant-speed propeller, which has been likened to a car's automatic transmission, blade pitch is adjusted by a governor, an engine-driven pump that monitors engine rpm and uses oil pressure to vary the pitch of the blades to maintain that rpm, regardless of changes in airspeed or power settings.