. How does a gas turbine engine (i.e., an aircraft engine) work?
A gas turbine uses energy stored in pressurized or rapidly moving gas to do work on a rotating mechanism. This rotary work can then be used to propel a vehicle or to generate electricity. Whether the gas is pressurized or rapidly moving doesn't really matter much. What is important is that the gas tends to flow from one region to another through a series of turbine blades. If the gas is pressurized, it is propelled through the blades by the unbalanced pressures (gases always accelerate toward lower pressure). If the gas is rapidly moving, it flows through the blades because of inertia.
As the gas flows through the turbine blades, it flows over and under each blade. The blades are shaped so that the gas goes faster over each blade than under each blade, and an imbalance of pressures results as a consequence of Bernoulli's effect. Each turbine blade acts like the wing of an airplane and experiences a lift force. This lift force pushes on each blade and twists the turbine around and around. The turbine blades effectively fly through the flowing gas stream and extract energy from it. The blades and turbine gain energy while the gas stream loses energy. The gas leaves the turbine at a lower pressure and/or speed than it had when it arrived.
. If air in a turbofan engine bypasses the jet engine after going through the turbofan, why does the jet engine even exist in the system?
The turbofan engine has a giant fan at its inlet, with a much smaller turbojet engine behind it. That turbojet engine is what provides the mechanical work needed to turn the giant fan. About 5 to 10% of the air passing through the fan then passes into and through the turbojet behind it. The turbojet uses this air for its operations: compressing it, burning fuel in it, and then extracting most of that hot air's energy as rotational work. This rotational work is used to power the giant fan.
. In a high bypass ratio turbofan engine, does the fan turn at the same RPM as the power turbine section — or is it geared down to run slower?
A turbofan engine uses a small turbojet engine to turn a giant fan and it is this fan that provides most of the engine's propulsion. The question asks whether the fan is turned directly by the turbojet engine or whether gears are use to allow the larger fan to spin more slowly than the smaller turbojet. This is an interesting question, particularly since many of the parts inside a jet engine are spinning almost as fast as they can tolerate without ripping themselves apart.
A turbofan engine contains two separate rotating assemblies or "spools," each of which is powered by hot exhaust gases flowing out of the combustion chamber through some turbine discs and each of which spins some compressor disc that push air toward the combustion chamber. The shorter of the two spools is hollow and the lower spool passes through its center.
The shorter spool, which spins at about 12,000 rpm, derives its power from high speed gas flowing through its turbine blades just after the combustion chamber and it powers a high pressure compressor just in front of the combustion chamber. The longer spool, which spins at about 4,000 rpm, derives its power from low-pressure gas flowing out of the high-pressure turbine and it powers both a low-pressure compressor in front of the high-pressure compressor and the actual turbofan blades. Overall, there is a rapidly turning hollow spool right around the combustion chamber and a more slowly spinning solid spool that extends both in front of and behind the high-speed spool. It's the low speed spool that spins the turbofan itself.
. How do helicopters fly with such small wings without them breaking off?
As you suggest, the blades of a helicopter are really rotating wings. But unlike the wings of a normal airplane, the helicopter blades are always moving through the air, even when the helicopter's body is not. That's why a helicopter can obtain an upward "lift" force from the air while it's hovering motionless—the wings keep moving and obtaining that lift force. A second difference between a helicopter's rotating blades and the wings of a normal aircraft is that a helicopter's blades are under enormous tension. Were it not for this tension, the end of each blade would naturally travel in a straight line at constant speed, a behavior that we associate with inertia—objects that are free of outside forces travel at constant velocity (they follow straightline paths at constant speeds). To make the end of its blade travel in a circle (which is certainly not a straight line), the helicopter must pull the end of the blade toward the pivot about which the blade is turning. Thus as the blades turn, each blade experiences an enormous tension pulls the parts of the blade toward the pivot. This tension is what stiffens the blade, just as tension stiffens the strings of a guitar or a violin. Just as it's hard to break a guitar string by bending it, it's hard to break a helicopter blade by bending it. However, both guitar strings and helicopter blades will snap if they're exposed to more tension than they can tolerate. The manufacturers of the blades work hard to make each blade strong enough to withstand the enormous tension it experiences in use. As long as the blades can tolerate this tension, they won't break and will have no trouble supporting the body of the helicopter.
. I've heard two explanations for how flight is achieved: (A) through lift generated by differential pressure (Bernoulli's effect) and (B) through elastic collisions between air molecules and the underside of a wing. Which is correct? How does the fact that planes can fly upside down enter into this picture?
Explanation A is entirely correct and explanation B is partly correct. If you extended explanation B to include all collisions between air molecules and the entire wing, then it would also be correct. Explanation A is the continuous fluid picture of flight and the revised explanation B is the granular fluid picture of flight. To the extent that gases are incompressible fluids (as required for Bernoulli's equation to be completely valid), these two explanations are essentially equivalent.
The lift experienced by a plane's wing depends on its shape and on its tilt or "angle of attack" into the wind. In general, wings are airfoils—curved shapes that are designed to obtain significant lift forces while experiencing minimal drag forces. Most airplane wings are more highly curved on their tops than on their bottoms and obtain upward lift forces as a result. These lift forces occur because the stable airflow that forms around such a wing involves faster-moving and thus lower-pressure air above the wing than beneath it. However, some airplane wings are symmetric—they have equal curvatures on top and on bottom. These symmetric wings compensate for their symmetry by attacking the air at an angle. When they are tipped so that their leading edges are higher than their trailing edges, these wings also experience upward lift forces. The air again flows more rapidly over than under the wings and the pressure is lower above the wings than beneath them. Even an inverted non-symmetric wing can adjust its angle of attack to obtain an upward lift force, which is how a plane can fly upside down.
In all of these cases, the forces are really exerted on the plane's wings by the impacts of countless air molecules. These air molecules hit harder and more often beneath the wings than above them and thus exert a net upward force on the plane. The fact that some wings have more surface area on their highly curved tops doesn't lead to larger downward forces because many of the collision forces exerted by molecules on the top surface of the wing cancel one another, in the same way that forces exerted on opposite sides of a sheet of paper cancel one another.
. If airplane cabins are pressurized to provide adequate oxygen for the passengers to breathe, what provides this compressed air? - EL
The air that you breathe inside an airplane is actually pumped into the cabin through the jet engines. The first component of a jet engine is a compressor that takes the low-density air outside and boosts its pressure and density. While most of this air then continues through the engine to the combustion chamber, part of it is diverted to the cabin. But before it can be released into the cabin, the air must be chilled by an air conditioner. That's because compressing air adds energy to it and raises its temperature. The compressed air leaving the jet engine's compressor is hot, even though no combustion has taken place yet. So the air is first cooled and then sent into the cabin.
. What is a shockwave and a sonic boom? - EL
A plane that is flying faster than the speed of sound is outrunning its own sound. As a result, its sound spreads out behind it as a conical structure, with the plane located at the apex of that cone. This cone moves along with the plane. Since the planes sound is all contained inside the cone, you can't hear the plane until the cone passes by you. When the edge of the cone does pass you, you hear a great deal of sound all at once. In fact, there is a pressure jump right at the surface of the cone (sound and pressure are closely related) and this cone itself is a shockwave. As the shockwave (or cone surface) passes you, you hear a loud booming sound, a "sonic boom". Note that the sonic boom occurs when the shockwave passes your ears, not when the plane "breaks the sound barrier". When you hear the sonic boom depends on where you are relative to the moving plane, so different people hear it at different times.
. What is the "sound barrier"? - EL
The "sound barrier" is more a psychological barrier than a real impediment. In the early days of high-speed flight, there was concern that a plane flying at or beyond the speed of sound in air would encounter unanticipated phenomena that would rip it apart. However, when Chuck Yeager finally did exceed the speed of sound for the first time in 1947, he found the transition from subsonic to supersonic uneventful. The only way that he could tell he was traveling faster than the speed of sound was with the help of his instruments.
. How do the different jet engines work on aircraft—the turbojet, the turbofan, and the turboprop? — KB, Charlottesville, VA
All three engines start with a turbojet engine. In a turbojet engine, a stream of air is first compressed by a rotary compressor. The air is then mixed with fuel and the mixture is burned. Finally, the hot burned gases are allowed to expand through a rotating turbine and they flow out of the back of the engine at very high speed.
To understand how all of this works, let's follow the flow of energy through the turbojet engine. Assuming the plane is moving forward, the air is moving fast when it encounters the engine's inlet duct. This inlet duct slows the air down substantially and the change in its speed causes the air's pressure to rise—an effect observed by Bernoulli. The air's energy doesn't change, but its kinetic energy (energy of motion) is partially converted to pressure potential energy. The now pressurized air is further pressurized by its passage through the rotary compressor at the front of the turbojet. The compression process adds energy to the air by doing mechanical work on that air. Now fuel is added to the high-pressure air and the mixture is burned. This combustion adds an enormous amount of energy to the air. The exhaust gases immediately expand and their speeds increase substantially as they pour out of the combustion chamber. These gases flow through a rotating turbine on their way out of the back of the engine. Even though the gases do work on the turbine, they still have lots of energy and flow out of the jet engine at a much greater speed than the air had when it arrived. Much of the fuel's chemical potential energy has become kinetic energy in these exhaust gases. The turbine provides the mechanical work that operates the rotary compressor, or the fan of a turbofan or the propeller of a turboprop. Overall, the exhaust gases leave the turbojet engine traveling faster than the air did when it arrived. Since the gases carry backward momentum with them as they leave the engine, they have evidently pushed the engine forward to give the engine and the plane forward momentum.
That's all there is to a turbojet engine. A turbofan engine uses the mechanical work from an enlarged turbine to operate a large fan that's in front of the turbojet engine itself. This fan takes air that has slowed down on entry into the jet's inlet duct and adds energy to this air. The air then speeds up as it flows out the jet's outlet duct and the air leaves the engine traveling faster than when it arrived. Once again, the engine experiences a forward thrust force as it pushes this air backward.
A turboprop engine uses mechanical work from an enlarged turbine to operate a propeller. The propeller pushes air flowing past the engine backward and the air pushes the engine and airplane forward. Because there is no duct around the propeller blades, the air passes the blades at full speed (a turbofan engine uses its duct to slow the air down before pushing on the air with its fan blades).
. If you were in freefall from a jet airplane, would the airplane overtake you in the fall (assuming that the plane started freefalling as you jumped out the door)?
It would overtake you immediately. Airplanes are designed to experience extremely small drag forces and are remarkably aerodynamic as a result. In contrast, you would experience severe air drag (air resistance) once you left the plane. The plane would coast past you at high speed while you would slow enormously in the first second or two of exposure to the air.