At small angles of attack, air flows smoothly around an airfoil, providing lifting force through the difference in pressure across the top and bottom of the airfoil. As the angle of attack increases, the lift produced by the airfoil increases as well but only to a point. Increasing the angle of attack also increases the adverse pressure gradient on the latter half of the top surface, visible here as an increasingly thick bright area. Over this part of the surface, the pressure is increasing from low to high—the opposite of the direction a fluid prefers to flow. Eventually, this pressure gradient grows strong enough that the flow separates from the airfoil, creating a recirculating bubble of air along much of the top surface. When this happens, the lift produced by the airfoil drops dramatically; this is known as stall.
Icing on airplane wings remains little understood and a major hazard. These photos show examples of ice formation along the leading edge of a swept wing. If an aircraft flies through a cloud of supercooled water droplets, the droplets will freeze shortly after impact with the aircraft’s wings. As ice continues to build up in strange shapes, the aerodynamic profile of the wing changes, which can lead to disastrous effects as the stall and control characteristics of the wing shift. (Photo credit: NASA Glenn Research Center)
(Source: facilities.grc.nasa.gov)
This spectacular high-speed video shows a dove in flight. Note how its wings flex through its stroke and the way the wings rotate over the course of the downstroke and reversal. There is incredible beauty and complexity in this motion. The change in wing shape and angle of attack is what allows the bird to maximize the lift it generates. Note also how the outer feathers flare during the downstroke. This promotes turbulence in the air moving near the wing, which prevents separated flow that would cause the dove to stall. (See also: how owls stay silent. Video credit: W. Hoebink and X. van der Sar, Vliegkunstenaars project)
(Source: vliegkunstenaars.nl)
This flow visualization of a pitching wind turbine blade demonstrates why lift and drag can change so drastically with angle of attack. When the angle the blade makes with the freestream is small, flow stays attached around the top and bottom surfaces of the blade. At large (positive or negative) angles of attack, the flow separates from the turbine blade, beginning at the trailing edge and moving forward as the angle of attack increases. The separated flow appears as a region of recirculation and turbulence. This is the same mechanism responsible for stall in aircraft. (Submitted by Bobby E)
Bats use tiny hairs on their wings to sense the direction and speed of air flow. Researchers found that removing these hairs caused bats to fly faster and make wider turns, likely because the bat believed it was on the verge of stalling and losing lift. Engineers are considering whether artificial versions made of flexible polymers that respond to strain could provide improved stall sensing on fixed-wing aircraft. # (Photo credit: justynk)
The flippers of humpback whales include bumps—called tubercles—on their leading edges. The tubercles create vortices that prevent the boundary layer from separating, which causes stall and a loss of lift. New research shows that adding similar bumps to the leading edge of tidal turbine blades results in greater energy production at low flow speeds compared to conventional designs. See Scientific American for more. #
Researchers at MIT are studying stall to understand how birds land and come up with new ways for gliders to perch instead of requiring a runway. This photo shows a smoke visualization of the glider stalling. #
Celebrating the physics of all that flows. 