Flapping wings could revolutionize aircraft design
A silvery airplane appears on the horizon. At first glance, nothing seems out of the ordinary about the small dot moving across the sky. Only when it's directly overhead do you realize you've never seen a plane quite like this: just like a bird, it arcs its broad wings up and then pushes them down in one continuous, fluid motion. No turbines or propellers, no flaps or rudders interrupt the smooth surface of the plane's flattened body, and it emits barely a whisper as it sweeps past. Even as you struggle to take it all in, the plane furls both wings, plunges forward, and soars out of sight.
This futuristic plane is so far just a concept in the minds of a small research team, of which I am a part. But if we have our way, a flapping-wing plane like this could become a reality within a decade or two. Over the past seven years, our group, scattered in five U.S. locations, has been investigating exactly what it would take to build such an aircraft. With funding from the NASA Institute for Advanced Concepts, in Atlanta, we've completed a feasibility study and worked out an initial design--and even some functional, if crude, proof-of-principle models.
As currently envisioned, the ultraslim vehicle would be unmanned, solar-powered, and made of strong, lightweight materials. Its size could range from a few meters across to perhaps a hundred meters, depending on its mission. Rather than a metal framework covered by riveted plates and hydraulically actuated parts, the plane's body and wings would consist of a plasticlike material called an ionic polymer-metal composite, which deforms when exposed to an electric field. If the voltages are applied just right, the material can be made to flap like a wing. On top of the composite wings would be paper-thin sheets of photovoltaic material and lithium-ion batteries, layered on by thin-film deposition, the same method used today in some semiconductor processes; together, these layers would power the plane. Because it won't have a single moving part, we call it the solid-state aircraft [ ].
But why fly like a bird? First, there's energy efficiency. Large-winged creatures like the albatross can glide great distances and circle over the same area for long periods of time. The solid-state aircraft will do the same, gliding most of the time while collecting power from the sun and flapping only to maintain altitude. Birds are extremely agile fliers, controlling their flight by subtly changing their wings' cross section, length, area, sweep, and inclination. The wings on our plane, too, would be able to adjust some of these characteristics.
The solid-state aircraft would have many potential uses; gathering scientific data, relaying communications, and surveying terrain are but a few. Thanks to its flexible body, it could be stowed, transported, and then deployed in remote places on Earth or even on other planets whose inhospitable atmospheres would doom planes that need oxygen to burn their fuel.
Aircraft designers have long been interested in morphing capabilities. Morphing wings would allow military jets to outmaneuver their adversaries. A passenger airliner with shape-shifting wings, meanwhile, would be able to constantly adapt its aerodynamics to different flying stages--takeoff, landing, acceleration, cruising--thereby reducing noise and saving fuel. Recently, a number of groups in industry and academia have been working on morphing wing concepts [see sidebar, ], a line of research that promises to open a new and exciting chapter in aircraft design.
We began our project by considering two basic aspects of bird flight: the shape of the wings and how they flap. Wing shape is instrumental in getting the bird aloft and keeping it there. In this respect, birds and traditional airplanes are similar. The engines of a fixed-wing plane move the craft forward, forcing air to flow over and under its wings. Because the top and bottom surfaces of the wing are curved differently, air rushes over them at different speeds, creating a pressure difference between the two surfaces. This pressure effect--known as Bernoulli's principle--is what lifts the plane.
Birds rely on the same effect. As they flap, air flows past the wings and a pressure difference forms, pushing the birds aloft. (Insects, by contrast, flap their wings at high speed and rely on different lift mechanisms. See "Fly Like a Fly," IEEE Spectrum, November 2005.)
In both planes and birds, different wing shapes yield different types of flight. The Global Hawk surveillance drone, like the albatross, has wings that are long, thin, and narrow, ideal for long-distance, low-speed flight. Planes that need to maneuver at high speeds, like the F-16 Fighting Falcon, have stubbier, swept-back wings, which produce enough lift but with less drag. Eagles and hawks, likewise, have shorter wings for greater agility.
But birds and planes control their flight very differently. Conventional airplanes maneuver by means of moving surfaces: flaps and ailerons on the wings, horizontal sections called elevators on the tail, and also the rudder. Birds, on the other hand, can bend, twist, and deform their wings and bodies to turn, change their speed, and adapt to unforeseen conditions such as wind gusts. If planes could do the same, they would have more lift and less drag, gaining agility and consuming less fuel.
We studied the wings of various birds in search of an appropriate model for our aircraft. Hawks and eagles looked promising, but what really grabbed our attention was the pteranodon, a carnivorous pterosaur that soared through the skies more than 75 million years ago. With membranous wings each nearly 5 meters long, this animal was a formidable glider.
But was that the ideal wing shape for us? To find out, we turned to a computer program called Wind. Created by researchers at the Air Force's Arnold Engineering Development Center, NASA's Glenn Research Center, and Boeing, Wind is a computational fluid dynamics program that lets you simulate an airplane's wing--or, more accurately, its airfoil, the shape characterized by its teardrop cross section--under varying conditions.
From our studies, we needed a thin and slightly curved airfoil that would approximate that of a pteranodon wing. Consulting databases that catalog thousands of existing airfoils, we found a few with the right profile, bearing the code names Selig 1091, Selig 1223, and Eppler 378. Using Wind, we simulated each airfoil for high-altitude flight at relatively low velocities of up to 64.6 meters per second, or Mach 0.19. The analysis produced two parameters that are essential in any aircraft design: the lift coefficient (which, as the name implies, is a measure of the wing's ability to push the aircraft upward) and the drag coefficient (a measure of unwanted resistance to motion). These two parameters vary according to the wing's angle of attack, its front-to-back inclination relative to airflow. It's kind of like flying a kite: to get it up in the air, you pull it at an angle to the ground, so as to maximize lift.
When designing an aircraft, then, you study how lift and drag vary as you angle the airfoil. Our initial tests showed that for an angle of attack of 10 degrees, the Selig 1091 has a maximum lift coefficient of 1.5 and a drag coefficient of 0.05--good enough, aerodynamically speaking, for our type of aircraft. But our plane, like a bird, will constantly change how it angles its wings forward, so we had to study our wing's aerodynamics over a wide range of angles.
Before we settle on a final airfoil, we'll also test two- and three-dimensional computer models, as well as actual scale models in a wind tunnel, simulating myriad steady-state and turbulent conditions as the plane flaps, glides, and soars.
Flapping-wing airplanes, sometimes called ornithopters, have fascinated humanity for centuries. Leonardo da Vinci proposed a few such designs, to be powered by a human, but whether they were built remains unknown. More recently, inventors have successfully demonstrated both small and large ornithopters. A team at the University of Toronto, for example, developed an ingenious flapping plane, powered by an internal-combustion engine, that even carries a pilot on board.
Despite such successes, we decided to depart altogether from traditional aircraft design paradigms. In most planes, the wing is a cantilevered framework covered by metal plates that give the wing its aerodynamic shape. We rejected the framework structure in favor of layering together different materials to form a compact, nonhollow body.
Enter the aforementioned ionic polymer-metal composite, or IPMC. Outwardly, it looks like ordinary plastic, but its core is made of perfluorinated sulfonic acid, a compound that works as an ion-exchange membrane: when exposed to an electric field of a few tens of volts per millimeter, it allows water molecules and hydrated ions to migrate across it. This flow of water and ions creates internal forces that make one side of the sheet expand and the other contract, resulting in a bending motion. The deformation is proportional to the electric field's strength, and once the field is removed, the sheet returns to its original shape.
The solid-state aircraft will have wing-shaped sheets of the material sandwiched between two metal grids: an anode grid on the bottom and a cathode grid on top, each containing thousands or even tens of thousands of electrodes. A computer control system will supply voltage to the electrodes. By applying different voltage levels to different portions of the sheet, we can make it change its shape to flap. Our hope is that with a very fine grid we'll be able to emulate the wings of flying vertebrates.
We're far from that level of control. In our first test, we started small, with two 5-centimeter-long IPMC strips. We then attached control electrodes to each little wing and applied a variable voltage. It was fascinating to see this primitive contraption beating its wings fairly well, even at high flapping rates.
We then built a larger model with 46-cm-long wings. This required producing newspaper-size sheets of the IPMC material--to our knowledge, the largest ever made. Alas, this proto type responded erratically to the electric field. We weren't surprised by the problems, given that IPMC is still a relatively new material; making it in large sheets requires a good deal of trial and error, with some batches working better than others.
We expect to solve these fabrication problems over the next several years, and then we'll begin the next step: integrating the IPMC material with layers of photovoltaic and lithium-ion or lithium-polymer materials. Our plan is to apply the same thin-film coating technology that is used to make certain semiconductor, optical, and ceramic devices. Thin-film solar cells have been around for years, although they're still expensive, and thin-film batteries are beginning to emerge from the lab. For the solid-state aircraft, we hope to produce sheets of those materials that can bend and twist extensively without breaking.
If we build it, will it fly? To answer that question, we considered two variables: the power the aircraft requires to fly and the power available in the environment where it's deployed. Obviously, if the former is greater than the latter, the plane won't be able to get off the ground.
Because the solid-state aircraft is powered by the sun, the amount of power available will depend on where and when it is flying, as well as on the characteristics of its solar cells. Using high-performance solar cells, with a high specific power rating of 1 kilowatt per kilogram and a conversion efficiency of 10 percent, our calculations showed that the maximum available power on Earth at an altitude from 1 to 35 kilometers would be 90 watts per square meter of solar cell. On Venus, it would be 150 W/m2 at an altitude of 53 to 82 km, and on Mars, 55 W/m2 at 1 to 7 km above the surface.
The aircraft's power needs, on the other hand, depend on its size and weight, the wing's shape and motion, and the power required by onboard systems. Key parameters in this calculation are the wings' lift and drag coefficients (which we determined in our computational-fluid-dynamics analysis) and the plane's weight (2 kilograms per square meter for the IPMC, for example). We used a range of values for the length of the wings and the flapping and gliding durations. Plugging all these variables into a collection of Newtonian physics and Navier-Stokes fluid-dynamics equations tells us the power required to get the aircraft aloft.
So, can the solid-state aircraft fly? Yes, at least in certain scenarios. Consider a version with a 12-meter wingspan flying at an altitude of 10 km above Earth. To cruise at 10 meters per second and glide for 10 seconds between flaps, with each flap lasting around 5 seconds, it would need about 10 W/m2, or about a tenth of what's available.
Redoing the math for different aircraft sizes, we figure that vehicles with wingspans from as short as 3 meters to as long as 100 meters would be able to fly on both Earth and Venus. For Mars, with its thinner atmosphere, the wingspan would have to be significantly larger, on the order of 250 meters or more, so that the aircraft could capture enough sunlight to remain aloft.
With so many variables coming into play, the design process becomes iterative. We set some parameters and then see under what conditions the aircraft would fly. If we'd rather have different conditions, we go back to stage one--the airfoil choice--and start over, repeating it all until we're satisfied with our design.
To reach the skies, the solid-state aircraft ideally would take off from the ground and ascend on its own. But that requires a lot of power and strong wings. So initially we're looking at launching prototypes from a balloon similar to those used in weather research. The aircraft would be compactly stowed in the balloon's gondola, and at the desired altitude it would be unfolded and released.
The aircraft's ability to remain aloft for long periods of time would make it ideal for imaging the Earth's surface from on high, of clear interest for both civilian and military purposes. Although not designed to carry heavy payloads, the plane could easily wield the latest generation of miniaturized cameras developed for unmanned aerial vehicles (UAVs) and micro-UAVs.
Remote sensing is another potential application. Equipped with special sensors and miniaturized mass spectrometers, the solid-state aircraft would ideally fly at an altitude of about 20 km, where Earth's atmospheric conditions are more stable and from where it would be able to monitor the troposphere, just below, where most weather activity occurs.
As a communications platform, the aircraft could carry thin-film antennas or transparent metallic antennas--both now used in automobiles--to relay signals to and from satellites and ground stations, thereby extending transmission ranges.
Perhaps most intriguing is the idea of using the aircraft to explore other planets. Take Venus. Its surface is hot and inhospitable, but at altitudes above 50 km it's actually very Earth-like, with pressure levels similar to ours, temperatures of less than 50° C, and an atmosphere containing carbon, hydrogen, oxygen, nitrogen, and other basic compounds needed for life. Given all that, plus the planet's abundant solar energy, it's one of the more likely places in our solar system to find life.
We still have much work to do to make any of that happen. The next step will be to simulate a full 3-D representation of the aircraft so that we can understand its behavior during all kinds of flight conditions. In particular, we need to understand better how to vary the airfoil's shape throughout the flapping cycle. We also plan to build a 1-kg scale model demonstrator with a 1-meter wingspan. By testing it in a wind tunnel, we'll be able to confirm the lift and drag coefficients. We'll also attach the model to a tether and fly it in a circle-which will help us further assess its aerodynamics.
Overall, the greatest challenge will be to develop a control scheme for the wings. Just getting the IPMC material to replicate the many ways that birds change their wings will be a feat in itself; controlling these intricate variations in real time will be even harder. Flight-control schemes used in conventional planes won't work; those preprogrammed systems use measurements like airspeed, altitude, and rotation to compute the plane's position and orientation, and thus the actions required to maintain stability. The solid-state aircraft will likely need to use neural networks or some other kind of artificial intelligence that will enable the aircraft to "learn" how to control itself.
To overcome these and other challenges, we'll have to recruit a multidisciplinary team of aerospace engineers, materials scientists, biologists, computer scientists, and others. It may take a lot of effort and time, but the end result will be worth it: a machine that can not only fly like a bird, but maybe even do it better.
The author wishes to thank the following for their contributions to the solid-state aircraft project: Mohsen Shahinpoor at the University of New Mexico; Phillip Jenkins, Curtis Smith, and Terri Deacey at the Ohio Aerospace Institute; Kakkattukuzhy Isaac at the University of Missouri, Rolla; Teryn DalBello at the University of Toledo, in Ohio; and David Olinger at the Worcester Polytechnic Institute, Massachusetts.
About the Author
Anthony Colozza is a researcher with Analex Corp., in Fairfax, Va., and NASA's Glenn Research Center, in Cleveland.
To Probe Further
For more technical details, see "Solid State Aircraft," Phase II Final Report, prepared for the NASA Institute for Advanced Concepts, May 2005.
For more on the solid-state aircraft's "artificial muscles" and to see videos of small-scale flapping prototypes, visit http://www.unm.edu/~amri.
For more on solar-powered flying missions on Venus, see Colozza's "Feasibility of a Long Duration Solar Powered Aircraft on Venus," presented at the 2nd AIAA International Energy Conversion Engineering Conference in August 2004 (AIAA-2004-5558).