Welcome to Aerospace Engineering

You are now at the first screen of the Design-Centered Introduction (DCI) to the Aerospace Digital Library.  For fast access to subject areas beyond the Introduction course, please use Table 1, below. For guided access, skip Table 1 and proceed with the introduction course.

Table 1: Direct Access to the Sub-Disciplines of Aerospace Engineering

Aerodynamics; Fluids
 Structures; 
Solids
Materials
 Propulsion
 Astronautics
  Flight Mechanics 
Controls; Avionics
 Design;
Manufacturing

 DESIGN-CENTERED INTRODUCTION TO

AEROSPACE ENGINEERING

  • Text books and data sources on aerospace engineering
  • Advanced Courses on Design Decision Making, Processes and Robustness
  • Topics

    1. Course Organization
    2. Today's Dreams in Various Speed Ranges
    3. Designing a Flight Vehicle: Route Map of Disciplines
    4. Mission Specification & TakeOff Weight
    5. Force Balance during flight
    6. Earth's Atmosphere
    7. Aerodynamics
    8. Wing Loading
    9. Propulsion
    10.Performance
    11. Stability
    12. Flight Control
    13. Structures and Materials
    14. High Speed Flight
    15. Space Flight

    1. Course Organization

    For over a century, aerospace engineers have led the progress of human technology, and brought the world closer together. Most simply, aerospace engineering is the realization of grand dreams through careful scientific thinking and planning, bold but informed innovation, and  dedicated pursuit of  perfection. It is the broadest of engineering disciplines, because it takes the best of all human knowledge to design, build, sell and operate a new (and always better!) aircraft or spacecraft, and to use it to the best advantage. Many aerospace projects appear so "far-out" that most people dismiss them as impossible, until they actually see them working: it is up to the AE to figure out these dreams, and reduce them to simple, step-by-step designs which are clean, simple, safe, cheap and reliable, so commonplace that anyone can use them and feel at home.

    Click here to scare yourself thinking about the simple process of flying home for Christmas

    So don't be surprised  when you read  that you can learn to design an airliner, starting out with a high-school background. The approach we take in this course is called the "Runway across Canyons".

    The various disciplines of aerospace engineering, such as aerodynamics, propulsion, etc. are like mountain ranges. Sometimes we feel like we have to climb down into a canyon and then up a steep wall to get to another discipline, i.e., to really understand all the things that people have figured out over the years. In this course, we lay out a "runway", bridging these canyons, so that we can go at high speed from aerodynamics to propulsion to flight mechanics, etc., on our way to developing our own conceptual design for an aircraft. We do have a few resources, shown on the control panel of our craft, as we start the takeoff roll...

    In this course we will use the motivation of designing a specific vehicle to learn about the various areas of aerospace engineering. So we will go off into one area after another, but always come back at the end of that detour, and do some more calculations or refinement of our design. All that you need is a notebook and pencil, a calculator for elementary calculations, and a spreadsheet.

    2. Some Dreams of Today

    The above picture is from the Clip Art  provided with Deneba Canvas 5 software.
    (Question: Which is the front end of this contraption?)
    The Wright Flyer must have looked incredibly sophisticated to the people of 1903. And it was. It was a canard design, with wings which had variable camber and twist, and the pilots had to perform extremely well. It could take off from an unprepared runway, under gusting wind conditions, and fly nap-of-the earth (very low altitude), an operation calling for precise flight control. Fortunately the beach at Kitty Hawk was quite flat.

    Likewise, today's designs look extremely sophisticated to us. They can fly over 100 times as fast as the Wright Flyer, and go right out into Space, circle the earth every hour or so, and return to precise touchdowns on earth. Have we reached the limits of aerospace engineering? Many people, even in the 1920s, thought that airplanes had reached the limits of speed and altitude, and had detailed theories proving that not much more could be gained by investing in thought or development of these wierd machines.  And today, still, we are just beginning. We have only about 100 years of powered flight experience, whereas the birds and insects that we see have evolved through maybe a million years of experience. We can't yet match them for control precision, landing versatility, payload fraction, engine weight fraction, fuel costs, maneuverability, reconfigurable geometry, or structure weight fraction.  Our machines are fragile and clumsy: if their engines quit or a piece breaks off,  they fall down quickly or even catch fire. They have stiff, rigid wings that can't flap, twist, fold or thrust to any significant degree. They need long runways and complex traffic control systems. You have to drive through 2 hours of downtown traffic and spend an hour and a half at the airport and another 30 minutes on the taxiway to make a flight of 200 miles. When we launch spacecraft, only about 30% of the structure and 10% of the total launch mass ever reaches orbit: the rest is wasted.

    The picture is of a rumored  "Aurora" aircraft which is rumored to be in flight testing from super-secret Air Force Bases. Some years ago, when the F-117 was still super-secret, there were plastic hobby kits of the F-117 available, and they had elegant shapes like that above. When unveiled, the F-117 did not look much like those shapes.

    Here are some dreams to consider: some are a lot closer than the others. In each case, try writing out a mission specification, and a typical mission profile, and then maybe you'll keep going, and figure out the detailed design. Someone will, sooner or later, and most of these things will get much closer to reality within the careers of today's students. Consider that when today's professors were born, no human had ever reached orbit (well, excluding anyone kidnapped by green-costumed visitors from the Andromeda Galaxy..)

    There are many kinds of flying vehicles today: helicopters, balloons, fixed-wing aircraft (the X-29 is shown), and the Space Shuttle are examples of designs which look drastically different from each other, and are designed for very different missions.

    Table 2: Today's Dreams
    Dream  Technical Requirements
    Fly like a bird 0 - 100mph; land anywhere, hover, cross mountains & rivers
    Commute by air Garage to parking lot to garage. 1 million cars per day above I-85, 300mph, all-weather, safety & traffic management
    City-city, doorstep service 400mph; VTOL with mild downblast and noise
    Cross the world in a day Mach 3, approximately 1800 mph + range of 10,000 miles.
    Visit low earth orbit  18000 mph; re-usuable spaceliner; comfortable takeoff, acceleration, re-entry and landing. Cheaper than $50 /lb.
    Visit nearby planets 36,000 to 500,000 mph; months of endurance.
    Visit nearby star systems Proxima Centauri, 6 light-years: 5.7E14 km
    Deep space travel Millions of light-years. 
    Nano-probes 10E-9 meters size. Numerous applications.

    National Aerospace Plane (NASP) hypersonic airbreathing vehicle concept from NASA, presented along with President Reagan's call for "the Orient Express", a vehicle which can  fly across the Pacific Ocean in less time than it takes to get from Atlanta suburbs to the Atlanta airport. The Space Shuttle is certainly a hypersonic vehicle, except that it takes a whole army to get it ready for each flight, and several weeks to turn it around for the next flight, and it uses rocket propulsion, where all the fuel and the "working fluid" has to be carried on-board from ground level. Picking up the oxygen-laden air en-route should make hypersonic flight much cheaper, if this can be figured out completely: this is called "airbreathing propulsion".  One difficulty with a hypersonic passenger craft is that the acceleration and deceleration phases would be quite "interesting" for most passengers if one flies a direct route.  This can be made less stressful by going around the earth once, which would add another 2 hours or so to the flight time, but would require going to an even higher speed and altitude. It might also raise the expectations of the passengers with respect to the food service (the direct route will have a very short cruise segment,  which only merits peanuts/pretzels by today's standards). Another interesting statistic (Aerospace America, Oct. 1998) is that roughly 75% of astronauts, who are all superbly fit and trained professionals, get various symptoms of motion sickness during space missions, despite medical precautions. So it is likely that the initial hypersonic "airbreathing" vehicles to be revealed will in fact be (or already are) missiles, uninhabited bombers, and perhaps later, some missions flown by military pilots. The "Orient Express" that President Reagan described is still a few years away, and will probably be replaced by a High Speed Civil transport flying at lower supersonic Mach numbers (1.7 to 3.5). Yet another interesting issue (one of very many) is that the surface temperatures generated during high-speed flight might make it difficult to open the doors for some extended duration after landing, so people might get very tired standing up in the aisles with their hand-baggage after the "fasten seat-belt" light goes out at the airport gate.

    Of course these are not unprecedented problems: flight on the venerable  DC-3 Dakota airliner , which was the best option available to many of us when we were younger, also used to make many people sick from the continuous buffeting, and caused piercing ear-aches, partly from the pressure changes, and partly from the pleasure of sitting for hours  close to something that sounded like five diesel locomotives  at full  power.


    3. Designing a Flight Vehicle: A Route Map of  Disciplines

    Use this link to see how the various subject areas come into the design of an aircraft.

    4. (a) Mission Specification

    Where does one start, to go about designing one of these grand contraptions? The answer is quite easy when one stops to think about it. First, we have to decide what we want  the contraption to do. We will write out a wish-list, then think  about it and perhaps constrain those wishes just a little. Then we will think of what a "typical mission profile" might be. For example, we will consider the design of a large airliner, one which is slightly bigger and faster and can go further in greater comfort, and cheaper, than the best of today's airliners. Our aircraft is to carry 400 passengers, non-stop, 10,000 miles, and do this with the comfort-level of today's Business Class for everyone. As we write out the mission profile, various other requirements occur to us. The aircraft must be able to take off from any large city airport, in hot weather. Such as Denver or Mexico City (5000+ feet above sea-level), where the temperature may be 100 deg. F and very humid (well, at least in Mexico), and still fly the full range and payload. And be able to take off, no problem, even if one engine quits just as the aircraft is lifting off the runway. And land safely even if one engine quits when the aircraft is as far as it can be from any airport. And have enough fuel left at the destination to be able to fly another 500 miles, or loiter for 1 hour, because the weather may be bad at the destination... And....And...
    The list gets much, much longer as we think about the detailed design, later. Aerospace engineers think about everything that can possibly go wrong, and many things beyond that. And then they worry, and plan, and check their calculations, and talk to other people about  how to improve their estimates and calculation procedures. They develop simulators to test out every eventuality. Nothing is left to chance. And yet, we know that things still go terribly wrong sometimes, so there's always more to think about...

    Table 1: Simplified Design sequence
     
    Step Issues
    Define the mission What must the vehicle do?
    Survey past designs What has been shown to be possible? (don't worry about WHY yet)
    Weight estimation How much will it weigh, approximately? 
    Aerodynamics Wing size, speed, altitude, drag
    Propulsion and engine selection How much thrust or power is needed? How many engines? How heavy? How much fuel will they consume?
    Performance Fuel weight, take off distance, speed/altitude boundaries
    Configuration How should it look? Designerís decisions needed!
    Stability & Control Locate & size the tail, flaps, elevators, ailerons etc. Fuel distribution.
    Structure Strength of each part, material, weight reduction, life prediction. 
    Manufacturing: concurrent engineering Design each part, see how everything fits, and plan how to build and maintain the vehicle. Break this down into steps involved in manufacturing. 
    Life-cycle cost Minimize cost of owning the vehicle over its entire lifetime. 
    Iteration Are all the assumptions satisfied? Refine the weight and the design.
    Flight Simulation Describe the vehicle using mathematics. Check the "flight envelope".
    Testing Build models and measure their characteristics, verifying the predictions. Explore uncertain regions. Build & test first prototype.
    Iteration and refinement Keep improving, reducing cost and complexity, and extending performance, safety and reliability.

    4 (b) Weight Estimation

    One simple way to start the conceptual design is to realize that we are designing something that must lift some weight and carry it a certain distance. The mass to be carried is the "payload": the load which we (hopefully) get paid to carry. Once the payload is determined (as simple as figuring out how much the passengers, their bags, food, etc. will weigh), we ask: "Haven't others tried to do something similar or close to this? How much did their aircraft weigh? We know we are smarter than anyone else, but maybe they too thought carefully, and maybe we can learn something from the results that they got". This is called "benchmarking". From this, we can get a rough idea of the weight fractions of the various systems involved. For example, it is a rough "rule of thumb" that the fuel weight may be as high as 50% of the take-off weight of a large airliner which is to fly a very long distance. This  applies also to birds flying across oceans (Ref: Tennekes): they eat until they can barely get off the ground even with a long takeoff run on the beach, running into the wind to increase airspeed.

    Table 3: How the Take-off Gross Weight (TOW) of an Aircraft is broken out among the systems
      Component Fraction of TOW
    Payload Fraction: passengers+ crew, baggage, food&water (including peanuts& pretzels), cargo Wpl/Wto
    Propulsion Fraction: Engines, engine control systems, nacelles, fuel lines, fuel pumps, fuel tanks We/Wto
    Structure and Controls: Everything else fixed to the aircraft: wings, fuselage, control surfaces, instruments, landing gear, hydraulic systems, servo motors, airconditioning system, ducting, lights, interior furnishings, movie screens... Ws/Wto
    Fuel:  Wf/Wto
    Total:  1.0
    Now, if we can figure out the payload weight (which only we know, based on the intended mission specification), and we can get the payload fraction from somewhere, (maybe by taking an average of existing designs and being slightly more ambitious) the Takeoff Gross Weight is simply the Payload divided by the Payload fraction.

    For example, if the Payload is 30,000lbs, and the Payload fraction is 0.15, then the TOW is 30,000 / 0.15  = 200,000 lbs.

    This is of course an estimate. The rest of the design is to make sure we come in under this estimate, when we calculate everything else. When we have a rough calculation of all the other things, we'll go back and "iterate", many times: refine our estimates, so that the whole vehicle gets better and better.

    4 (c) Benchmarking

    Hunting through the available data on various aircraft, we find that there is a wide range of answers to our question on the payload fraction. Some craft weigh only 5 times their payload; others weigh 90 times the payload. As we look closer, we see that there is some similarity between these "payload fractions"  for aircraft which have similar "missions" and payloads. So in our case, we ignore the fighter designs and the space launcher designs and the helicopters, and the birds, and focus on large airliners, like the Boeing 747, 767, 777, Airbus A340, A320, McDonnell-Douglas MD11, MD90, and the Lockheed L-1011. These are all aircraft meant to carry large numbers of passengers (100 to 500) over long distances (upto 8000 miles). We find, however, that no one has quite designed an aircraft which can do all the things that we want our aircraft to do. So we are on our own in that respect, going out into the unknown.

    Click below for aircraft and engine specifications:
     
    Aircraft Specs by Manufacturer
    Engine Specs from Pratt& Whitney (not related to left-hand column)
    * Airbus Industrie * 1 -- Engines
    * The Boeing Company * 2 -- Engines
    * Douglas Aircraft Company * 3 -- Engines
    Details and Abbreviations * 4 -- Engines
    Extra Abreviations * 5 -- Engines

    *  The aircraft and engine characteristics provided above were provided by:
    Pratt & Whitney
    Marketing Operations and Support
    January 1997
     

    Go to the other topics in the course.

    Acknowledgements:

    The pictures are from the  Pratt & Whitney  Web Page, the Boeing Web Page, NASA, and the U.S. Air Force Times Library.
    http://www.pratt-whitney.com/lce/lce.html
    http://www.boeing.com/defense-space/military/fa18/images/fa1835.htm
    http://www.pratt_whitney.com
    http://www.boeing.com/companyoffices/gallery/index2.html