Variable-Geometry Wings

November 14, 2011 at 5:13 pm

If you have ever sat in a window seat on a plane, you might have noticed the flaps, spoilers, aileron, and slats (as seen here: Plane Parts) moving throughout the flight, during takeoff, and especially landing. These features are the results of engineers attempting to mimic the wings of birds, like the wings of the owl seen in this video.

Two key abilities belonging to the owl in this video and many other birds are the ability to take off without a running start and the ability to have completely decelerated by the time its feet touch down. Birds do this by changing the geometry of their wings to deal with lift and drag. Many characteristics of the wings are mimicked by engineers, but so far no wing has been produced that as able to vary in sweep, camber, and incidence, as demonstrated by the owl in this video. Sweep is the degree at which the wings protrude from the fuselage when looked at from above. Camber is the shape of the wing when taking a cross section of it when looking at it from the side. The angle of incidence is the angle of the wing relative to the fuselage (ex. if the wing is horizontal and the fuselage is horizontal, the angle of incidence is 0).

There are many example of variable-sweep planes. “With different wing positions allowing for greater efficiency and performance in various flight modes, these aircraft are more versatile than aircraft with fixed wings” (Switchblade). The F-14 Tomcat is a plane that employs variable-sweep technology. For slower speeds and landing, especially on small runways like aircraft carriers, the F-14 Tomcat’s wings extend to be nearly perpendicular to the fuselage. For faster, supersonic speeds, the wings contract to minimize surface area.

variable-wings
(This is not an F-14, this is a conceptual design for a variable-sweep plane. This is a picture from the website http://history.nasa.gov/SP-468/ch10-4.htm)

The best attempt at replicating variable-camber wings have been the addition of plane flaps and aileron. These parts do change the camber of the wing, but only slightly and do not compare to the changes in camber that birds’ wings are capable of. NASA experimented with variable-camber wings from 1984 to 1992 on models of the X-29 plane. The picture below shows how the adjustment of the aileron can change the camber.

aileron

Variable-incidence wings are the most unique. Almost all planes have their wings attached at a fixed angle to the fuselage along the longitudinal axis. No modern planes have experimented with variable-incidence wings since the F-8 which flew in active duty until 1987. The main reason variable-incidence wings were tested was to alter landing speeds.

Takeoff and landing are two of the most difficult parts when it comes to mimicking nature. As seen in the blog post about Festo, flapping flight has been mimicked by engineers, but to takeoff and landing involved human intervention to prevent the robot from damaging itself. Combining variable-sweep, variable-camber, and variable-incidence has not yet been achieved. Instead of attempting this feat, alternate forms of takeoff and landing have been devised. V/STOL (Vertical and/or Short TakeOff and Landing) appears to be what engineers are currently pursuing. Aircraft such as the Harrier and the V-22 Osprey (a tiltrotor) employ this method of takeoff and landing.

Harriers have vector thrust engines which means the engine redirects fast moving air through nozzles that can be adjusted to point in many directions. When the nozzles are pointed down, the force of the air leaving the nozzles is greater than the weight of the harrier, allowing vertical acceleration.

Skip to 0:23 to see the takeoff.

The V-22 Osprey has two rotor systems at the end of each wing. This allows the V-22 Osprey to take off vertically like a helicopter. Once it is in the air, the rotor systems can rotate along the pitch axis so that the V-22 Osprey can travel in any direction the rotor systems are pointing.

Skip to 0:55 to watch the V-22 Osprey takeoff vertically and eventually the rotors point forward, allowing it to fly off quickly.

What does it mean that engineers are currently pursuing V/STOL technology? It could mean that combining variable-sweep, camber, and incidence is too difficult with current technology. It could also mean that V/STOL is far more efficient than landing like a bird. The energy needed for V/STOL could be far less than the energy needed to mimic bird landing.

It is interesting though that tiltrotors and rotor craft are popping up in sci-fi movies and games.

Avatar
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Halo
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Is this suggesting that V/STOL is the way of the future?

Sources:

  • http://www.grc.nasa.gov/WWW/k-12/airplane/airplane.html
  • Switchblade http://science.howstuffworks.com/transport/flight/future/switchblade-plane.htm
  • http://www.aerospaceweb.org/question/dynamics/q0045.shtml
  • http://aboutfacts.net/Things3.htm
  • http://en.wikipedia.org/wiki/Variable-incidence_wing
  • http://web.mit.edu/2.972/www/reports/harrier_jet/vectored_thrust_engine.html
  • http://www.navy.mil/navydata/fact_display.asp?cid=1200&tid=800&ct=1
  • http://james-camerons-avatar.wikia.com/wiki/Scorpion_Gunship
  • http://halo.wikia.com/wiki/VTOL

By Aidan | Posted in student post | Tagged , , , , , , , , , , , , , , | Comments (4)

Da Vinci’s Flying Machines

November 14, 2011 at 2:36 pm

Leonardo da Vinci was a great artist who lived during the Italian Renaissance. Today he is remembered mainly for his artistic contributions, such as the Mona Lisa. Da Vinci however, was much more than just an artist, he was also an incredible inventor and engineer. He had written up designs for everything from bridges, to tank, to helicopters. Originally Da Vinci wished to emulate birds and bats, so he designed a contraption that would allow its wearer to flap their wings in order to create thrust.

Da Vinci's design of a man-powered ornithopter.

Da Vinci's design of a man-powered ornithopter.

It is not clear whether he built this device or if it was left as design. It is clear however that da Vinci realized it was too heavy to take off on its own. Modern day engineers have learned through trial and error that fully copying nature is not a good idea.

Da Vinci's glider. Image from the British Library.

Robert Full mentioned when he spoke to our class how evolution has created animals that have many traits, and not all of them are specified towards a task. In fact sometimes animals can perform a task because a random set of evolutions occurred, and not because they needed some specific job to be completed. Rather than copying nature we should take aspects of nature, and try to make them work for us.

Da Vinci saw that a creature the size of a man could not take off the same way that a bird does, so he focused his efforts on the gliding aspect of birds. His next design was much lighter and much more practical. Da Vinci likely did not build this either, but a few modern day people tried to replicate his design. This is what happened:

As you can see Da Vinci’s design was successful! He didn’t have calculus or wind tunnels, but by observing the way birds glide he was able to replicate their coasting. There is some evidence that da Vinci flew, and if he did this was likely how he did it.

After realizing the success he could/did have with gliding da Vinci came up with one more “flying” invention. The parachute.

Da Vinci's parachute.

Da Vinci's parachute.

This would potentially allow him jump from a great height without being injured. The parachute builds up air resistance, and allows for a slowed descent. Like with most of his idea’s da Vinci likely observed something control its descent, and did his best to emulate it. The materials, including the fabric used, during da Vinci’s time lead scientist to believe that a parachute made during the Renaissance would be too heavy. Da Vinci’s parachute also did not have a hole in the top to stabilize it. Even so, a team of English inventors wanted to try it out.

Amazingly, Leonardo’s design was a success.

Da Vinci lived in a time when basic aeronautics were understood by few if any. Simply by using his observations and the blueprints that nature gave him, Da Vinci was able to design inventions that fly. Many of them were not built until hundreds of years later, and after hundreds of years we have come up with only slight variations to his designs. Powered flight was Da Vinci’s goal, and thanks to the internal combustion engine we have achieved it. Da Vinci saw that we are not birds, but he believed that we would one day fly.

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By Ted Sirota | Posted in student post | Tagged , | Comments (1)

Flight Dynamics of Dragonflies

November 14, 2011 at 3:20 am

The dragonfly is one of natures most impressive flyers. Some can fly at speeds up to 35 mph, can decelerate from 35-0 mph in less than a second, and fly backwards. They are extremely agile and can make dramatic lateral movements in the blink of an eye. These amazing abilities come from dragonflies unique flying style. First dragonfly use aerodynamic drag instead of  aerodynamic lift to propel themselves through the air. They flap their wings up and down perpendicularly to the direction of flight.

CFD simulation by Jane Wang

Computational simulation of dragonfly wings during hovering, by Z. Jane Wang. The left vortex rotates clockwise and the right vortex rotates counter-clockwise.

The angle to which the wings are twisted (angle of attack) affects many aspects of the flight, most notably the flight speed. As the angle of attack of the wing increases, the flight speed of the fly increases. Another unique attribute of dragonfly flight is that they have two sets of wings each separately controlled. This attribute is what makes them exceptionally agile.  They vary the phase difference  between the forewing and the hindwing to perform different aspects of flight. A phase difference between 55-100 degrees (angle one would see between the forewing and hindwing if the dragonfly was flying straight at you) is commonly used for straight forward flight. A 180 degree phase difference is used for hovering  and 0 degree phase difference (both wings flapping together) is used for accelerating or maneuvering.

These two very unique features make the dragonfly one of natures most proficient flyers. If engineers could apply this idea to a flapping aircraft, it would be an enormous breakthrough and a huge step towards more agile aircraft.

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By Brad Burke | Posted in student post | Tagged , , | Comments (1)

Triassic Delta Wing Gliders

November 14, 2011 at 1:46 am

Delta-winged aircraft have been around for several decades, and their use has revolutionized the aerospace industry.  Yet apparently nature had already designed a delta-winged glider long before humans had lifted themselves off of the ground.  Delta wing designs are named for their triangular shape, and roughly 225 million years ago nature had produced a small, bipedal, lizard-like glider with a similarly triangular shape.

Sharovipteryx was first discovered in 1965 by Alexander Sharov while he was collecting fossilized insects in Kyrgyzstan.  Dating back to the Triassic period, it predated the adaptations of pterosaurs and other flyers by having the first known example of membranes attached to its hind limbs.  The fossil of Sharovipteryx, shows that the hind legs possessed membranes that connected to the base of the tail, essentially giving it a pair of wings in the center of its body as opposed to its center of gravity.

Sharovipteryx Fossil

Many researchers have questioned the effectiveness of an arrangement where the wings are between the hind legs and the base of the tail, and for obvious reasons.  If the wings are not positioned close to the center of gravity, the glider/flyer in question is likely to tip one way or the other.  In the case of Sharovipteryx, the greater mass in front would have caused a noticeable downward pitch, making the glide far less efficient.

Skeletal Interpretation. Image: Wiki Commons.

Researchers from the University of Dublin and the University of Leeds have examined the gliding mechanism of Sharovipteryx in detail.  They have come to believe that the fossil also shows a membrane in front of its hind legs, possibly even connecting to the front legs, which would resemble the membrane of a flying squirrel, in addition to the rear membranes.

Researchers' Model

Such a shape would create a delta wing design where the profile of the animal mid-glide would show a distinctive triangular shape.  In order for the animal to have gained maximum efficiency with this design, a third membrane has been proposed to have stretched from the front arms to the base of the head, given credence by the fact that the head region is poorly preserved.  This means that in order to control flight speed for landing, it would only have had to alter the shape, and thereby the area, of its wings by positioning its legs accordingly.  Such a shape would also have greatly improved balance during flight, and the model created by the team yielded greater flight performance than today's gliding lizards of the Draco genus.

Sources:

By Andrew Chalifoux | Posted in student post | Tagged , | Comments (2)

Pros and Cons of the Avian Skeletal System

November 14, 2011 at 12:17 am

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Avian Skeletal System. Image from Feisty Feathers website.

Over the course of millions of years, birds have evolved a particularly unique skeletal system among the animal world.   Naturally, much of the difference between mammalian and avian skeletal systems arises due to most birds’ capability of flight.  Granted, flight is a highly advanced evolutionary adaptation; however, birds sacrifice a good deal to achieve it.  The major adaptations regarding flight in the avian skeletal system are pneumatic bone structure, elongated necks, small skulls, modified ribs, and a serious degree of rigidity.

Many bones in the avian skeletal system are fused into single ossifications, such as the “Pope’s nose” or pygostyle at the birds’ posterior. The fusion of bones in the skeletal system provides a heightened degree of rigidity to the birds’ skeleton, providing for some uniquely awkward biomechanics, particularly whilst on the ground.  Rigidity is just as important in engineered flight as it is in avian flight. Most of the earliest attempts at mechanical flight tried to mimic the complex motions of avian flight; however, fixed wing structure and a different means of propulsion has been proven as a much simpler way to achieve lift.  There are very few moveable parts on the exterior of an airplane.   The fusion of bones and a modified rib structure seem to be natures answer to the need for rigidity in powered flight.  The major drawback of all this rigidity is that most birds are horrible at maneuvering on land.  They can certainly get from points ‘a’ to ‘b’, but at a heavy loss in dexterity.

Bird_bone_trabeculae.jpg

Bird bone. Image from Ornithology course webpage by Gary Ritchison

The pneumatic bone structure of birds refers to their porous nature.  Mammalian bones are mostly solid, with a hollow section in the middle for bone marrow.  Certain avian bones are instead nearly hollow, with the inside filled with an arrangement of trusses and struts of thin bone to provide some structural support.  These pneumatic bones have evolved for two reasons: facilitating their unique respiratory system and reducing weight.  Flight requires a higher rate of metabolism, which in turn requires a more efficient gas exchange in breathing.  The avian respiratory system has a number of unique adaptations, including a constant supply of “fresh” oxygen rich carbon dioxide pure air.  None of which would be possible if it were not for their porous bones.  Cleary, porous bones a requisite for this adaptation.  As far as reducing weight is concerned, it seems that nature has run into the same issue as engineers on a daily basis.  For example, when designing a Formula One car, every gram counts, which is why  you will never see a hood ornament on a racecar.  In the case of a bird, less weight translates into less energy spent flying.  Interestingly enough, the pneumatic bone adaptation carries over to flightless birds.

Finally the elongated necks and small heads have important physical and biological implications with regards to flight.  The elongated necks in most birds allow for adjustments in the center of gravity mid-flight.  Since the moment of inertia is a function of center of mass, the ability to adjust that center helps in maneuvering, as seen in the gecko videos shown in class.  As far as the size of their heads, birds have proportionally smaller heads regarding their bodies than do mammals.  This adaptation makes flight a bit less awkward, and reduces weight further.  In addition, birds have beaks rather than a traditional mandible with teeth.  This adaptation reduces weight, yet again, but makes eating a bit more crude than it is for most animals.  Everything is swallowed whole, no chewing.   As far as I am concerned, flight and all these other drawbacks associated with it seem to be a fair trade.

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By Craig Kelley | Posted in student post | Tagged , , | Comments (2)