High-speed video shows that bats achieve some of their efficiency in flight by pulling their wings inward on the upstroke, as seen above. While this does affect drag forces on the wing slightly, the primary energy savings comes from the inertial ease of lifting the folded wing. Much the way it is easier to lift your arm when it is folded than when you stretch it outright, it takes less energy for the bat to lift a folded wing than one that is fully extended. (via Wired Science)
Search results for: “lift”
Micro Air Vehicle Flow Viz
A smoke wire shows the deformation of streamlines around a swept-winged micro air vehicle (MAV). These crafts typically feature wingspans smaller than one foot and, thus, never develop the type of flow fields associated with larger fixed-wing airplanes. This complicates theoretical predictions of lift and drag for MAVs as well as making them difficult to control. MAVs have numerous commercial and military applications, including search and rescue operations. (Photo credit: Tom Omer)
Vortices on an Airliner
Wingtip vortices form on airplanes due to the finite length of their wings. In general, lift on the wings results from low-pressure, high-velocity air moving over the top of the wing and high-pressure, low-velocity air moving below the wing. Near the wingtips, the high-pressure air is able to slip around the edge to the top of the wing, generating a vortex that then trails behind the airplane. The same thing is occurring in the video above, except the edges of the wing’s control surfaces are serving as the tip of the wing. Similar vortices also exist at the wingtips, but they are not made visible by condensation as the aileron vortices are.
Wind Tunneling Testing for BASE Jumpers
While we usually think of wind tunnel testing airplane models, the truth is that wind tunnels today test a much wider array of subjects. From oil rigs and skyscrapers to athletes and police sirens, if you can imagine it, it’s probably been stuck in a wind tunnel. This video shows some wind tunnel testing of a tracking suit used for BASE jumping. The primary focus seems to be on lift and drag at angle of attack–which can be used to determine glide ratios for the pilot–but there is also some study of localized turbulence generation, as evidenced by the use of smoke generators and the streamers attached to the suit’s arms and legs. (submitted by Jason C)
Circulation Around an Airfoil
As a followup to yesterday’s question about ways to explain lift on an airfoil, here’s a video that explains where the circulation around the airfoil comes from and why the velocity over the top of the wing is greater than the velocity around the bottom. Kelvin’s theorem says that the circulation within a material contour remains constant for all time for an inviscid fluid. Before the airplane moves, the circulation around the wing is zero because nothing is moving. As shown in the video, as soon as the plane moves forward, a starting vortex is shed off the airfoil. As the plane flies, our material contour must still contain the starting position and thus the starting vortex. However, in order to keep the overall circulation in the contour zero, the airfoil carries a vortex that rotates counter to the starting vortex. This is the mechanism that accelerates the air over the top of the wing and slows the air around the bottom. Now we can apply Bernoulli’s principle and say that the faster moving air over the top of the airfoil has a lower pressure than the slower moving air along the bottom, thus generating an upward force on the airfoil. (submitted by jessecaps)
Hawk Moth Hovering
The hawk moth (Manduca sexta) flies quite similarly to a hummingbird, able to hover over the flowers from which it feeds by rotating its wings as it flaps. This constant change in angle of attack allows it to maintain lift while remaining stationary in space. Researchers study the stability of such miniature hovering flight by destabilizing the moths and studying how they react to disturbances like being struck with a miniature clay cannonball. By testing how the moths recover from disturbances, we can learn how to build better robots and micro air vehicles (MAVs). (via supercuddlypuppies)
Dove in Flight
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)
Separation and Stall
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)
High-Altitude Balloon Flight
Tangentially fluids-related, but SpaceWeather has a fun video of a high-altitude helium balloon bursting. Although this balloon carried a space-related payload, it’s the same type of set-up used for weather balloons. With only a few basic assumptions, it’s possible to do some neat calculations on the buoyancy, loading capacity, and behavior of such balloons.
Flow Around a Delta Wing
Smoke visualization in a wind tunnel shows the vortices wrapping around and trailing behind a delta wing. As with more commonly seen rectangular or swept wings, the vortices that form around delta wings affect lift, drag, and control of an aircraft. They can also be hazardous to aircraft nearby. Note that, although delta wings are often seen on supersonic aircraft, this visualization only applies at subsonic speeds. The flow field changes drastically above the speed of sound.
