But if Dr Jayant S. Kudva of California-based NextGen Aeronautics has his way, radical designs with flexible wings for advanced aircraft may change that. That’s because Kudva is investigating a morphing wing that would convert between the extremes of a swept-back "bat wing" for combat and a narrow, wing for cruising.

Kudva, an Indian-American aerospace engineer who led the Smart Wing program when he was at Northrop Grumman in El Segundo, Calif., founded NextGen in January 2003. NextGen researchers are experimenting with so-called morphing wings, or sophisticated structures that automatically reconfigure their shapes and surface textures to adapt to changes in flying conditions.

Such capabilities will in some ways mimic the subtle, nearly instantaneous adjustments that birds make instinctively to their wings, tails and feathers when aloft.

This is where future aircraft is heading.

Consider this: Last week, two Indian students at Penn State -- Deepak Ramrakhyani, doctoral candidate in aerospace engineering, and Smita Bharti, doctoral candidate in mechanical engineering – report having developed morphing aircraft wings that change shape like a bird’s to maximize a plane´s efficiency over a broad range of flight speeds. They have also come up with the idea of incorporating an outer skin like the scales of a fish.

The students, who detailed their concept in a paper, "Tendon Actuated Compliant Cellular Truss For Morphing Aircraft Structures," on Tuesday, April 20, at the 45th Structures, Structural Dynamics and Materials Conference in Palm Springs, Calif., suggest that future aircraft may fly more like birds, adapting the geometries of their wings to best suit changing flight conditions.

They received guidance for their work from Dr. George Lesieutre, professor of aerospace engineering, and Dr. Mary Frecker, associate professor of mechanical engineering. The project is supported by grants from NASA and the Defense Advanced Research Projects Agency (DARPA).

The idea is to develop and demonstrate models of morphing flight vehicles, says Ramrakhyani who will earn his Ph.D. this year. He figures they must be capable of carrying realistic loads and not be substantially heavier than structures that perform similar functions today.

According to Bharti, ‘The project involves optimal design of tendon-actuated compliant cellular truss structures for morphing aircraft. New design methodologies are being developed to optimize truss members, tendons and actuators within the wing structure. Shape changes in span and cross-section of wings are being considered.’

NASA is known to have its own Morphing Project, which is an attempt to couple research across a wide range of disciplines to integrate smart technologies to actual aircraft and spacecraft. The program bridges research in several technical disciplines and combines the effort into applications that include active aerodynamic control, active aeroelastic control, and vehicle performance improvement.

In the context of the project, the word ´morphing´ is defined as ´efficient, multi-point adaptability´ and may include macro, micro, structural and/or fluidic approaches. The project includes research on smart materials, adaptive structures, micro flow control, biomimetic concepts, optimization and controls.

In the last decade, smart technologies have become important enabling technologies that cut across traditional boundaries in science and engineering. Here smart is defined as the ability to respond to a stimulus in a predictable and reproducible manner.

While successes have been achieved in the laboratory, we have yet to see the general applicability of smart technologies to actual aircraft and spacecraft.

Lesieutre, who leads the Penn State project, says, "Airplanes today are a design compromise. They have a fixed-wing structure that is not ideal for every part of a typical flight. Being able to change the shape of the wings to reduce drag and power, which vary with flight speed, could optimize fuel consumption so that commercial planes could fly more efficiently."

Morphing wings can also be useful for military defense and homeland security when applied to unmanned surveillance planes that need to fly quickly to a distant point, loiter at slow speed for a period of time and then return, Lesieutre explains. Flying efficiently at high speed requires small, perhaps, swept wings. Flying at slow speed for long periods requires long narrow wings.

The morphing wings designed by the Penn State team can change both wing area and cross section shape to accommodate both slow and fast flight requirements.

Lesieutre and the wing design team detailed their concept in a paper, "Tendon Actuated Compliant Cellular Truss For Morphing Aircraft Structures," on Tuesday, April 20, at the 45th Structures, Structural Dynamics and Materials Conference in Palm Springs, Calif.

The essential features of the Penn State concept are a small-scale, efficient compliant cellular truss structure, highly distributed tendon actuation and a segmented skin. The cellular truss structure is the skeleton of the wing. The skeleton is formed of repeating diamond-shaped units made from straight metal members connected at the angles with bendable or "compliant" shape memory alloys. Tendons in each unit, like the ropes that shape a tent, can pull the units into new configurations that will spring back, thanks to the shape memory alloys, when the tendon tension is released. Since the underlying structure can undergo radical shape change, the overlaying skin of the wing must be able to change with it.

Lesieutre says a concept that he thinks holds great promise is a segmented skin composed of overlapping plates, like the scales of a fish. He notes that conveyers on the baggage carousel in airports are composed of a similar pattern of plates.

So far, the design team at Penn State has built a tabletop model of the compliant cellular truss structure and a computer graphic model of the wing structure.

Several companies are exploring major wing transformations, and, as a result, much more radical morphing is just beginning to come off some aeronautical engineers´ drawing boards.

At Skunk Works in Palmdale, Calif., the aeronautics research and development arm of Lockheed Martin, engineers have recently proposed an aircraft that would lift and fold its wings while simultaneously drawing them together.

Raytheon Missile Systems in Tucson, is exploring a telescoping wing for a cruise missile.

Participants in a year-old DARPA program called Morphing Aircraft Structures (MAS) have pledged to create, by 2005, functional, scale-model morphed wings that can vary in area or length by 50 percent. That´s a huge change, considering that the control surfaces of a conventional, fixed-wing aircraft modify wing areas by no more than about 5 percent, says DARPA’s Terry A. Weisshaar, who heads the program.

Going beyond wings that merely flex, scientists and engineers have also been developing aircraft surfaces capable of molding themselves from one shape into another, much as arm muscles bulge and flatten. These possibilities arise largely from the use of so-called smart materials, a broad range of substances that can shorten, elongate, flex, and otherwise respond mechanically to electricity, heat, light, or magnetic fields. Even on a modest scale, such reshaping of aircraft contours could greatly enhance vehicle control and performance.

Looking yet further down the air lanes, far more drastic and complicated transformations—for instance, wings that can telescope, curl, or fold—may be on the way, yielding extraordinarily versatile airplanes and missiles that change their shapes according to the missions they are expected to perform.

If research programs that are just starting eventually reveal that such large-scale morphing is feasible, the first of those aircraft may streak across the skies 20 to 30 years from now.