A paper airplane flies for roughly the same reason that a normal airplane flies: the air pressure below its wings is somewhat higher than the air pressure above its wings. As a result of this pressure difference, the paper airplane experiences an overall upward force due to air pressure and this upward force is strong enough to balance the airplane's downward weight.
In a paper airplane, the most important effect is a rise in pressure below the wings. To understand why this pressure rise occurs, think about the movement of air from the perspective of a bug that's riding on the airplane. To the bug, the air is flowing toward the front of the airplane. As this stream of air encounters the undersurface of the wing, the air slows down. You can think of this air as hitting a slanted wall. Whenever a moving stream of air slows down, its pressure rises. You experience this pressure rise when you hold your hand out of the window of a moving car and feel the slowing air push your hand toward the back of the car.
The dynamics of the air above the wing is more complicated and depends on the design of the wing. But in any case, the air above the wing doesn't slow down and its pressure never rises above atmospheric pressure. In a well-designed wing, it actually drops below atmospheric pressure! Since the air pressure rises under the paper airplane's wing and doesn't rise above the airplane's wing, the wing experiences an upward force due to pressure. It's this upward force that supports the airplane.
A pitot tube determines airspeed by measuring the pressure rise that occurs when the airstream is slowed to a stop. Any time moving air encounters a closed chamber head-on, the air stops and it exchanges its kinetic energy—its energy of motion—for pressure potential energy—energy stored in the form of an elevated pressure. By measuring this elevated pressure, you can determine what the air's kinetic energy was while it was moving and thus how fast it was moving.
Pitot tubes are used to measure airspeed in airplanes. They're the cigar-shaped objects that project forward from the undersurfaces of airplanes near their noses. I suppose that you could use a pitot tube to measure the speed of air flowing through an air duct, but to determine the volume of air flowing through that duct, you'd need to know the dimensions of the duct. The relationship between pressure in the pitot tube and the airspeed is complicated and so is the relationship between airspeed in a real duct and the volume of air it's carrying. Overall, this doesn't look like an easy job.
The wings of a normal airplane obtain upward lift forces from the air as the airplane moves forward through the air. That's because the shape and angle of the wings is such that air flows faster over the top surface of each wing than under the bottom surface of that wing and the air pressure above the wing drops below the air pressure below the wing. Each wing experiences a net upward pressure force and these upward forces are enough to support the weight of the plane.
A helicopter spins its wings around in a circle so that they move through the air even when the helicopter itself is stationary. Normally, these rotating wings are called blades. Again, the air flows faster over each blade than beneath it and there is a net upward pressure force on each blade. These upward forces support the helicopter and they also allow it to tilt itself—by adjusting the angle of each blade as the blades turn, the helicopter can obtain twists from the air so that it tilts one way or the other. Once the helicopter has tilted, it can use some of the lift force from its blades to push it horizontally so that it accelerates forward, backward, or toward the side.
Their altimeters don't read zero once they have landed—they read the altitude of the airport! Each airport's altitude is reported on the navigational maps that pilots use. As the pilot approaches the runway, the pilot watches the altimeter and expects it to reach the airport's altitude about the time that the plane touches the runway. Before the next take off, the pilot adjusts the altimeter using the airport's official altitude as a calibration point for the altimeter. Some modern planes also used radar equipment to determine the distance to the ground beneath the plane. These devices do read zero at landing. The satellite-based Global Positioning System (GPS) also provides altitude information to pilots. Since this system reports altitude above sea level, it gives the altitude of the airport at landing, not zero.
When a bullet emerges from the barrel of a gun, the high-pressure gas that is propelling it from behind abruptly enters the atmosphere. This sudden burst of pressurized gas is like that released by an exploding firecracker and produces a loud "pop" sound. A silencer slows down this gas's entry into the atmosphere. Before leaving the gun, the bullet passes through a series of air-filled chambers. The gas behind the bullet must enter each chamber, one at a time, and with each passage, its pressure and energy decrease. By the time the gas emerges from the last chamber, its pressure is low enough that it makes only a weak "whoosh" sound as it enters the atmosphere. This same technique is used by an automobile muffler. However, a silencer is only effective with low-velocity bullets. If the bullet itself travels faster than the speed of sound (331 m/s), it will create shock waves as soon as it enters the atmosphere and will generate its own explosion noise—miniature "sonic booms."
Even though a paper airplane's wings are flat, they experience all of the aerodynamic forces found in more sophisticated wings. For example, when the air flowing past the paper airplane encounters the lower surfaces of its wings, this air slows down and its pressure rises above atmospheric pressure. However, while the air flowing over a sophisticated airplane wing experiences a substantial increase in speed and consequently a drop in pressure, this effect is very small in a paper airplane's wing. Depending on how the air flows over or around the wing's leading edge and whether or not it breaks away from the wing's upper surface, the air pressure above the wing will be at or slightly below atmospheric pressure. Nonetheless, the air pressure below the wing is always slightly higher than that above the wing and the wing experiences a net upward aerodynamic force—a lift force. If you examine the airflow around a well-designed paper airplane wing, all of the flow features that occur around a sophisticated wing will be present but weak. Bowing the wing outward, as is done in a sophisticated wing, simply enhances those features so that the wing can lift a larger load.
An airplane wing's main job is to generate a large upward lift force while experiencing as little backward drag force as possible. To obtain the lift force, a wing must make the air flowing over its top to speed up while the air flowing under its bottom slows down. The wing must also avoid introducing turbulence into the main airstream because that will result in severe pressure drag. There are many cross sectional shapes for wings that achieve both large lift forces and small drag forces, but some are better suited to each style of airplane than others. For example, private propeller-driven planes travel relatively slowly and need broad, highly curved wings to obtain enough lift to support them. In contrast, commercial jets have much narrower, less curved wings because they travel faster and produce lift more easily. But during takeoff and landing, even jets need to increase the curvatures of their wings. That's why many jets have slats and flaps that extend from the leading and trailing edges of their wings to increase the wings' breadths and curvatures for low-speed flight.
The sound barrier is something of a myth that dates to the early days of transonic flight. As early airplanes approached the speed of sound, they suffered various flight instabilities—a significant rise in air drag and a tendency for supersonic shock waves to interfere with the operations of control surfaces. Exceeding the speed of sound appeared problematic at the time and the expression "the sound barrier" came into common use. However, there is no real sound barrier. Once Yeager had exceed the speed of sound in an experimental plane, it became clear that the speed of sound was not a firm barrier.
However, there is one peculiar thing that does happen once a plane has exceeded the speed of sound. You can no longer hear the plane coming because it is outrunning its own sound waves. Instead of having its sound spread out in front of it, the plane has its sound swept back in a cone behind it. The edges of this cone are a shock wave and you experience a sudden pressure rise as this cone passes across you—you hear a sonic boom. A supersonic plane carries this conical shock wave with it at all times and everyone hears a sonic boom as this shock wave sweeps across them. What you should remember is that the sonic boom doesn't occur when the plane "breaks the sound barrier"; the sonic boom is a continuous feature of a supersonic plane that you hear as its shockwave passes you by.
Airplanes travel faster from west to east in the United States. That's because the prevailing winds at out latitudes are eastward and they blow the airplane toward the east. When the airplane flies toward the east, it has a tail wind and travels faster with respect to the ground. When the airplane flies toward the west, it has a headwind and travels slower with respect to the ground.
During flight, an airplane wing obtains an upward lift force by making the air flowing over its top surface travel faster than air flowing under its bottom surface. When the air over its top speeds up, that air's pressure drops. Since the pressure of the slower moving air under the wing is larger than the pressure of the faster moving air over the wing, there is a net upward force on the wing due to this pressure imbalance and the wing is lifted upward. A wing also experiences drag forces—or air resistance—that tend to slow the plane down. But as long as an airplane wing doesn't cause the airstreams flowing around it to separate from its surface, it will experience relatively little pressure drag force; the most important drag force for a large, fast-moving object.
The details of the airplane wing's surfaces have relatively subtle affects on the wing's performance. While most wings are asymmetric, with broadly curved top surfaces and relatively flat bottom surfaces, that isn't essential. It's quite possible to use wings that are symmetric, with the same curvature on their tops as on their bottoms. But a symmetric wing won't obtain an upward lift force unless it's tilted upward, while an asymmetric wing can obtain lift even when it's horizontal. A broader, more highly curved wing can also obtain more lift at a lower speed, as required for slow moving propeller planes. So wing shapes are often dictated by the desired flight angle and speed of a particular airplane and its wings.
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