ADTO # 90 - December 10, 1999
QuestChats require pre-registration. Unless otherwise noted, registration is at: http://quest.arc.nasa.gov/aero/chats/#chatting Friday, December 17, 1999 10 AM - 11 AM Pacific Aerospace Team Online QuestChat with the Wright Brothers Step into the QuestChat "time machine" to chat with Orville & Wilbur Wright in celebration of their 96th anniversary of the first successful, powered, piloted flight in history.
Happy Holidays! Due to school holidays, celebrations, family gatherings, and other fun stuff at this time of year that we find our chats poorly attended and our experts out on holiday or scrambling to finish research papers to share at January conferences. After the December 17th chat we will be taking a break from chats and updates until the New Year, 2000. We return full force with the futuristic vision of flight on Mars! What could be more exciting for the new millenium? Best wishes, Susan Lee
[Editors Note: Sometimes our experts answer Email questions so well that I hate to leave them stored away in archives. This answer was provided by Steve Smith, Aerospace Research Engineer. Read his bio at: http://quest.arc.nasa.gov/aero/team/smith.html To see more questions or to ask questions go to: http://quest.arc.nasa.gov/aero/chats/ ]
HOW CAN YOU REDUCE DRAG ON A MODERN PLANE?
ANSWER from Steve Smith
December 6, 1999: This is a BIG question. There are lots of ways to try to reduce drag, and lots of people exploring the possibilities. Hopefully this will give you a first idea about where drag comes from, and what design changes can be made to reduce the drag. But also, it should give you some idea of what other things you would need to give up, called trade-offs. First, you have to understand what makes the drag, and then, see what can be done to change it. There are 4 main causes of drag: 1) skin friction on surfaces 2) "induced drag" from wings making lift 3) pressure drag from flow on shapes where the flow doesn't stay attached 4) compressibility drag. The first, is just air friction on the surfaces. If you want to make that drag smaller, you can make less surfaces, like try to make the wing smaller, or get rid of the fuselage and have a "flying wing". Of course, the design trade-off for the wing size must include the ability to fly slow to land on runways, so you can't just make the wing smaller, unless you have better high-lift systems (flaps) to allow landing. Or, you can try to reduce the skin friction itself. You can try to use laminar flow, which has less drag than turbulent flow. There are ways to help the laminar flow stay longer, but there are trade-offs with more weight and complexity. There are also things like "riblets" that reduce the turbulent friction. The second is a basic relationship that in order to generate the lift, there is some wasted energy, called induced drag. It decreases proportional to the square of the wingspan and it increases in proportion to the square of the weight. This is why sailplanes have such long wingspans. But if the wings are too long, they get heavier, so there is a balance where the added benefit is lost. Also, the longer wingspan means you can't park them at the airport gates unless you re-do the airport to make more room. There are a few things that can help induced drag without changing the wingspan, like winglets. You can see an example of a winglet on the wing tip of a 747-400. Winglets are one of my specialties. The third is reduced by good design - try to make streamline shapes. pressure drag is high on a bus or truck, medium on a car, and very small on an airplane. Again, if you have a flying wing, and get rid of the fuselage, you get rid of some pressure drag. The trade-off is where to put the people. The airplane needs to be very very big in order to let people stand up inside the wing. The fourth is the extra drag that comes when you try to fly fast, close to the speed of sound. Most jets now fly about Mach 0.8, or 80% of the speed of sound. They fly at the point where the small amount of compressibility drag starts to cost more fuel than the benefit from going faster. Better airfoils, called "supercritical airfoils" have helped to let airplanes fly a little faster for the same compressibility drag. Adding more wing sweep also helps, but there are trade-offs for the flying qualities if you have too much sweep. So, the current generation of jet transports have been developed to do everything they can to reduce drag, but to keep the right balance on other things too.