Compared to birds, manmade aircraft tend to be quite limited and inelegant. Fixed-wing aircraft, for example, require long, flat areas for take-off and landing, whereas birds of all sizes are adept at maneuvers like perching. This video examines the perching behaviors of large birds and extends the physics to a small unmanned aerial vehicle (UAV). As a bird approaches a perching location, it pitches its body and wings upward. This places the bird in what’s known as deep stall, where air flowing over the upper surface of the wing separates just after the leading edge. This move dramatically increases drag on the bird, slowing it for landing. At the same time, the speed of the pitch maneuver generates a vortex on the wing that helps the bird maintain lift despite the drop in speed. With the help of both forces, the bird can make a graceful, controlled landing in only a short distance. (Video credit: J. Mitchell et. al.)
Tag: lift generation
Frisbee Physics
Frisbees are a popular summertime toy, but they involve some pretty neat physics, too. Two key ingredients to their long flight times are their lift generation and spin. A frisbee in flight behaves very much like a wing, generating lift by flying at an angle of attack. This angle of attack and the curvature of the disk rim cause air to accelerate over the top of the leading edge. Airflow over the top of the disk is faster than that across the bottom; thus, pressure is lower over the top of the frisbee and lift is generated. Aerodynamic lift and drag aren’t enough to keep the frisbee aloft long, though. Spin matters, too. If the frisbee is launched without spin, gravity acts on it through its center of mass, but lift and drag act through a point off-center because lift tends to be higher on the front of the disk than the back. This offset between gravitational forces and aerodynamic forces creates a torque that tends to flip the frisbee. By spinning the frisbee, the thrower gives it a high angular momentum acting about its spin axis. Now instead of flipping the disk, the torque caused by the offset forces just tips the angular momentum vector slightly. Physically, this is known as spin stabilization or gyroscopic stability. Tomorrow we’ll take a closer look at airflow over the frisbee. (Image credit: A. Leibel and C. Pugh, source video; recommended papers by: V. Morrison and R. Lorentz)
