Using Research Tools to Design an Airplane
When we look at or fly on an airplane, we sometimes take for granted the hard work of the aerospace engineers, scientists, and technicians that made that flight possible! It takes a lot of time and effort to design a new airplane or to make an existing airplane more safe or more efficient. Here is your chance to look at the design process every airplane goes through and to find out how engineers use wind tunnels, computation, flight simulation, and research flights as tools to do the job! Before going on, you might want to first read about what aeronautics is and how an airplane flies.
Every airplane goes through many changes in design before it is finally built in a factory. These steps between the first ideas for an airplane and the time when it is actually flown make up the design process. Along the way, engineers think about four main areas of aeronautics: Aerodynamics, Propulsion, Structures and Materials, and Stability and Control.
Aerodynamics is the study of how air flows around an airplane. In order for an airplane to fly at all, air must flow over and under its wings. The more aerodynamic, or streamlined, the airplane is, the less resistance it has against the air. If air can move around the airplane easier, the airplane's engines have less work to do. This means the engines do not have to be as big or eat up as much fuel which makes the airplane more lightweight and easier to fly. Engineers have to think about what type of airplane they are designing because certain airplanes need to be aerodynamic in certain ways. For example, fighter jets maneuver and turn quickly and fly faster than sound (supersonic flight) over short distances. Most passenger airplanes, on the other hand, fly below the speed of sound (subsonic flight) for long periods of time.
Propulsion is the study of what kind of engine and power an airplane needs. An airplane needs to have the right kind of engine for the kind of job that it has. A passenger jet carries many passengers and a lot of heavy cargo over long distances so its engines need to use fuel very efficently. Military airplanes flying secret missions do not want to give away their location in the sky so their engines minimize their exhaust to prevent detection. Engineers are also trying to make airplane engines quieter so they do not bother the passengers onboard or the neighborhoods they are flying over. Another important concern is making the exhaust cleaner and more environmently friendly. Just like automobiles, airplanes' exhaust contains chemicals that can damage the Earth's environment.
Structures and Materials is the study of how strong the airplane is and what materials will be used to build it. It is really important for an airplane to be as lightweight as possible. The less weight an airplane has, the less work the engines have to do and the farther it can fly. It is tough designing an airplane that is lightweight and strong at the same time. In the past, airplanes were usually made out of lightweight metals like aluminum, but today a lot of engineers are thinking about using composites in their designs. Composites look and feel like plastic, but are stronger than most metals! Engineers also need to make sure that airplanes not only fly well, but are also easy to build and maintain. The less time it takes to build an airplane, the less it costs to make. What does this mean to you? If airlines need to spend less money to buy or maintain an airplane, they can offer customers cheaper tickets!
Stability and Control is the study of how an airplane handles and interacts with the human pilot. Pilots in the cockpit have a lot of data to read from the airplane's computers or displays. Some of this information could include the airplane's speed, altitude, direction, and fuel levels as well as upcoming weather conditions and other instructions from ground control. The pilot needs to be able to process the correct data quickly, to think about what kind of action needs to be taken, and to react in an appropriate way. Meanwhile, the airplane should display information to the pilot in an easy-to-read and easy-to-understand way. The controls in the cockpit should be within easy reach and just where the pilot expects them to be. It is also important that the airplane responds quickly and accurately to the pilot's instructions and maneuvers.
Are these four areas of aeronautics related to each other?
Definitely! That's what makes designing an airplane such a challenge. Each of these areas of aeronautics: Aerodynamics, Propulsion, Structures and Materials, and Stability and Control affect each other. An improvement in one of these areas sometimes comes with a decrease in performance in another area. For example, if engineers make an airplane a lot stronger by using heavier materials to build it, the airplane might not be able to fly as far and the pilot might have a harder time controlling it. Engineers look at these trade-offs and then decide what design has the best performance overall. The tools that engineers use to do this job are wind tunnels, computation, flight simulation, and research flights.
What are wind tunnels?
Just as its name suggests, a wind tunnel is a tube or tunnel that has man-made wind blown through it at a certain speed. Scientists and engineers put a model of an airplane in the tunnel and then study the way air moves around the model. By looking at the way this smaller model acts in the wind tunnel, they get a pretty good idea of how a real life-sized airplane of the same design will probably fly. It is a lot easier, cheaper, and safer to build and test a model than to build and fly a real airplane.
How do wind tunnels work?
Wind tunnels work on the idea that a stationary model with air moving around it behaves the same way a real, full-scale airplane moving through stationary air does. Sometimes only a part of an airplane, like a wing or an engine, is tested in a wind tunnel. The models, usually made out of steel or aluminum, that are tested are loaded with many instruments and sensors that report back to the computers in the control room. It's there that scientists, engineers, and technicians can begin to understand how the airplane is performing.
How are wind tunnels used in aerospace research?
Scientists and engineers use wind tunnels to study the pressures, forces, and air flow direction affecting an airplane. Pressure is measured by small devices called pressure taps that are placed at various locations on the surface of the model. Forces are recorded by sensors in the structures that support the model in the test section. The direction that air flows around the model can be seen by the way tufts, small yarn-like strands attached to the model, flap around. Sometimes smoke is blown into the test section to make it easier to see how the air is flowing. From these different kinds of measurements, a great deal can be learned about the model being tested.
Wind tunnels vary in size according to their function. Some of the smallest wind tunnels have test sections that are only a few inches large and therefore can only be used with tiny models. Other wind tunnels have test sections that are several feet big. The largest wind tunnel in the world is at the National Full-Scale Aerodynamics Complex at NASA Ames Research Center. Its 80 foot by 120 foot test section can fit a life-sized Boeing 737 inside!
What else can wind tunnels be used for?
Wind tunnels aren't just used to test airplanes. Anything that has air blowing around or past it can be tested in a wind tunnel. Some engineers have put models of spacecraft, cars, trucks, trains, even road signs, buildings, or entire cities in wind tunnels to see how to improve their designs. Here are the different aircraft and spacecraft that have been tested in NASA's Unitary Plan Wind Tunnels.
What is computation?
Computation is when scientists use computers to solve math and science problems. As supercomputers become faster and more powerful, they are able to save researchers a lot of time by doing complicated airplane design calculations quickly and accurately. Computers can predict things like how air will flow around an airplane or how the structures inside the airplane will support its weight - before the airplane is actually built.
How does computation work?
The computer first needs to create a mathematical model of the airplane being tested. Next, a computer program is run that calculates specific numerical values at different points on the model. For example, if the program is written to figure out the pressure around an airplane model, it finds the individual pressures at several points on the model to make a good prediction at what the overall pressure looks like.
Computers can also be used to estimate the temperature, speed, direction, and density of the air around an airplane. Usually, the different numerical values that the computer finds are assigned to different colors so scientists only need to look at the variety of colors in an image to get an idea of the different numerical values represented. Going back to the pressure example, let's say "high" pressure is assigned colors like red or orange and "low" pressure is assigned colors like blue and purple. Just by looking at a computer-generated image, scientists can easily see where on the airplane there are higher or lower pressures - by searching for the appropriate colors.
As you can imagine, computation is a very powerful tool in the design process. Calculating all those numerical values by hand would take a very long time and would include several human errors. A computer can work on millions of equations at the same time making very few mistakes.
How accurate is computation?
The results from computation are so accurate that some scientists and engineers doing airplane design believe that computation should completely replace wind tunnel research. Afterall, it's a lot easier to test something on the computer than to build a model and then do several wind tunnel tests. Other scientists think that experimental research, such as wind tunnel testing, and computational research are both needed. Many times the results from a wind tunnel test are compared to the results predicted by the computer. If one set of values is very different from the other set, then we know that something is wrong. By using both of these tools correctly engineers and scientists can insure that a design is ready to move on to the next steps: flight simulation and research flights.
What else can computation be used for?
Scientists use computation tools to solve many different types of problems. Computers can predict how automobile engines mix air and fuel together, how rivers and oceans move water around the world, how blood is pumped through the human heart, or even how winds on one part of Earth can affect the weather somewhere else in the world.
What is flight simulation?
Flight simulation, or "flight sim" for short, is a way for pilots to fly an airplane without having to leave the ground. Pilots sit in a simulator that looks just like the cockpit of a real airplane. As the pilot moves the control stick, the entire simulator moves up, down, and around and the computer-generated scenery outside the windows changes, giving the pilot the feeling that he or she is actually flying. Flight sim is not just used during airplane design. New airline, combat, and even space shuttle pilots get trained in flight simluators before they fly the real thing. The world's largest motion-based simulator is the Vertical Motion Simulator at NASA Ames Research Center.
How does flight simulation work?
Because it stays on the ground the whole time, the simulator has to provide different cues or signals to the pilot to make the experience as authentic as possible. To make the computer-generated scenes look even more real, several screens are placed end-to-end all around the simulator so that everywhere the pilot looks, he or she sees the sky above or the terrain below. To accompany these sights, the pilot hears simulated sounds like the wind blowing by, or the flaps, engines, or landing gear at the appropriate times. Sometimes the pilots wear special suits so that they can feel different pressures on their bodies as they "fly" at different altitudes.
What can engineers learn from flight simulation?
Engineers can learn about how easy and safe it is for a pilot to control a certain airplane. Simulators can also reproduce bad weather, emergency situations, or other procedures to see how pilots react. Using the information that comes from flight simulation, engineers can make additional adjustments before going on to building an actual airplane for research flights.
What are research flights?
A research flight is usually one of the last steps in the airplane design process. After a new type of airplane has been planned out on paper and on the computer screen, it's time to build a full-size working version, or a prototype. Because only one or two prototypes are made at a time, they are usually quite expensive to build. Before engineers can be sure a design is really the one they were looking for, and before being made in large quantities, these prototypes need to be tested out several times.
How do research flights work?
A specially trained research pilot is chosen to fly the prototype airplane. Research pilots are selected based on their previous flying experience. For example, if a certain research pilot has flown many fighter jets before, he or she would be a good person to test-fly a fighter jet prototype because they know what to expect and can compare the prototype to the other airplanes they have flown.
Once the prototype is in the air, the pilot goes through several different, planned maneuvers to try out different characteristics of the airplane. Computers onboard the airplane and back on the ground record how the airplane and the pilot perform throughout the research flight. Later on, the engineers and the research pilot get together to figure what features of the design still need to be improved.
Is it safe to fly a prototype airplane?
Because a prototype airplane is still in its experimental stages, engineers pay a lot of attention to safety. A prototype is flown only after its design has been thoroughly tested in the wind tunnel, on the computer, and in flight simulators. Only research pilots with a lot of flying experience and special training are allowed to fly these one-of-a-kind airplanes! There are a lot of safety concerns, but being a research pilot can be very exciting! Can you imagine being the first person ever to fly a new type of airplane?
Since a prototype's exact flight characteristics are not always known, research flights are usually done in remote areas, away from crowded cities. At NASA Dryden Flight Research Center in the desert of southern California, research pilots have hundreds of square miles of restricted air space to fly around in. Here's a list of current, recent, and past flight research projects at Dryden.
What can engineers learn from research flights?
Research pilots who fly prototypes have a lot of important information to share with the engineers back on the ground. It gives the engineers the chance to hear firsthand descriptions about how the airplane performed and what to improve before building a final version.