Ready, Set, Flap!
Birds do it, bees do it. Can two weird aircraft make aviation history doing it?
- By Graham Chandler
- Air & Space magazine, January 2002
(Page 2 of 4)
And while the wing is twisting, it also needs to maintain the right degree of bending rigidity. DeLaurier explains the concept by asking me to imagine holding the ends of a cardboard tube. "Try and twist it," he says, "and it remains rigid. But slit it down the length of one side and overlap the edges and you can twist it while it remains stiff." DeLaurier and Harris pulled off a similar innovation by conceiving a shear-flexing design. Each wing is composed of two flexing sections of polyester; during flapping the sections slide over the wing ribs. The sections are attached to what is essentially a double trailing edge: One section slides over the other, like two edges of the cardboard tube.
Harris and DeLaurier first tested the concept with a 10-foot-span radio-controlled model the named "Mr. Bill" (after the misfortune-prone "Saturday Night Live" creation). It took years of research, they say before Mr. Bill was able to fly with their shear-flexing design and demonstrate their method of three-axis control, which they later used in Big Flapper.
In the full-size ornithopter, the pilot controls pitch by manipulating the horizontal stabilizer. But the third function of a flapping wing, lateral control, hasn't been included on Flapper's wing. The shear-flexing wing design precludes the use of standard ailerons for direct roll control. So Flapper is designed to turn solely by rudder. The pilot will bank by a technique known as yaw-roll coupling. Lateral stick input deflects the rudder, yawing the aircraft. The windward wing experiences an increase in angle of attack and airspeed and therefore enhanced life, which rolls the aircraft in the direction of the turn.
Flapper has yet to demonstrate that it can actually do any of this. During a test in November 1998, as Flapper exceeded 50 mph, it started to lift off the runway, then smacked down on the ground so hard that the nose gear failed. To keep the craft on the runway while the speed increased, the team shortened the nose gear so Flapper had a nose-down angle. That suppressed lift buildup. "What we hadn't accounted for," says DeLaurier, "was that the new down force from the wings was imposing compressive loads in our vertical struts beyond the design limit." During another test, this one in October 1999, Jones-Bowman reached 56 mph—just short of Flapper's predicted 57-mpg takeoff speed—when one of the vertical struts buckled. The team reinforced the struts.
Once we get clearance from the tower, Jones-Bowman comes up on the throttle and Flapper beats its wings down the runway. Taking their cues from the rhythm of the wings, the fuselage pulsates up and down, the tail takes little dips, the main gear does mini-squats, and the wing supports flex. The cadence picks up as Jones-Bowman accelerates to 35 mph, turns around, and does it again. We're in formation at her two o'clock, sitting in the back of a rusty VW pickup, when Flapper's engine sputters and dies.
The crew pushes Flapper off the active runway. After 20 minutes, the problem is declared to be a blockage in the fuel filter. An engine failure in flight has been thought of, and DeLaurier calculates that wing loading—essentially, the difference between the pressure of the air above the wing and that below it—would overcome the engine compression and drive the wings to their full-up position. "Lateral control would be pretty sensitive with all that dihedral," he says. "But with small and tender inputs, it would be flyable."
By now the wind is picking up. Since sideslipping into a crosswind without both aileron and rudder is dicey, testing the aileron-less Flapper is best done in calm, limp-windsock mornings, so testing is called off for the day.
I ask Jones-Bowman how it feels to be inside the Big Flapper. "I'm steering, controlling, bouncing, watching instruments and everything," she says. "Once I rev up, it's up and forward, down and backward, and the stick goes with me. I tell myself not to try and stop it but it's difficult. If I do, PIOs are gonna be a problem." ("PIOs" are pilot-induced oscillations—the pilot's attempts to correct pitching motions actually increase their amplitude, rather than diminish them.)