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- Principles of Flight.
However, the balance of the lift needed to support the aircraft comes from the flow of air above the wing. Herein lies the key to flight. It is neither accurate nor useful to assign specific values to the percentage of lift generated by the upper surface of an airfoil versus that generated by the lower surface.
These are not constant values. They vary, not only with flight conditions, but also with different wing designs Different airfoils have different flight characteristics. Many thousands of airfoils have been tested in wind tunnels and in actual flight, but no one airfoil has been found that satisfies every flight requirement.
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The weight, speed, and purpose of each aircraft dictate the shape of its airfoil. The most efficient airfoil for producing the greatest lift is one that has a concave or "scooped out" lower surface. As a fixed design, this type of airfoil sacrifices too much speed while producing lift and is not suitable for high-speed flight. Leading edge Kreuger flaps and trailing edge Fowler flaps, when extended from the basic wing structure, literally change the airfoil shape into the classic concave form, thereby generating much greater lift during slow flight conditions On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground.
Thus, modern airplanes have airfoils that strike a medium between extremes in design. The shape varies according to the needs of the airplane for which it is designed. This lowered pressure is a component of total lift. Aerodynamic force acts through this CP. At high angles of attack, the CP moves forward, while at low angles of attack the CP moves aft. In the design of wing structures, this CP travel is very important, since it affects the position of the air loads imposed on the wing structure in both low and high AOA conditions.
In fact, many lifting airfoils do not have an upper surface longer than the bottom, as in the case of symmetrical airfoils. These are seen in high-speed aircraft having symmetrical wings, or on symmetrical rotor blades for many helicopters whose upper and lower surfaces are identical.
In both examples, the only difference is the relationship of the airfoil with the oncoming airstream angle. A paper airplane, which is simply a flat plate, has a bottom and top exactly the same shape and length. Think of a hand being placed outside the car window at a high speed. If the hand is inclined in one direction or another, the hand will move upward or downward.
This is caused by deflection, which in turn causes the air to turn about the object within the air stream. The amount of induced drag varies inversely with the square of the airspeed Wingtip Vortices: An airfoil wing or rotor blade produces the lift force by making use of the energy of the free airstream.
As a result, the air tends to flow from the high pressure area below the tip upward to the low pressure area on the upper surface. Another important idea is that when an object moves through a stationary fluid, the science is pretty much the same as if the fluid moved and the object were still. That's why it's possible to study the aerodynamic performance of a car or an airplane in a wind tunnel : blasting high-speed air around a still model of a plane or car is the same as flying or driving through the air at the same speed.
When you empty water from a plastic bottle, you've probably noticed you can do it in two very different ways. If you tip the bottle at a shallow angle, the water comes out very smoothly; air moves past it, in the opposite direction, filling the bottle with "emptiness. Sometimes the water wins and rushes out, sometimes the air wins and rushes in, briefly stopping the water flow.
The fight between water exiting and air entering gives you the characteristic "glug-glug" sound as you pour. Photo: Pour water slowly from a bottle and you get smooth, laminar flow. Tip the bottle up more and the flow will become turbulent. Also, can you see how the spout of water dripping down from this bottle gets narrower toward the bottom where the water moves faster after being accelerated by gravity?
Forward flight of birds revisited. Part 1: aerodynamics and performance
That's an example of fluid continuity, which is explained below. What we see here are the two extreme types of fluid flow. In the first case, we have the water and the air sliding very smoothly past one another in layers, which is called laminar flow or streamline flow because the fluid flows in parallel lines called streamlines. In the second case, the air and water move in a more erratic way, which we called turbulent flow.
If we're trying to design something like a sports car, ideally we want to shape the body so the flow of air around it is as smooth as possible—so it's laminar rather than turbulent. The more turbulence there is, the more air resistance the car will experience, the more energy it will waste, and the slower it will go.
The speed at which a fluid flows past an object varies according to how far from the object you are. But there's not a sudden, drastic discontinuity between the stationary car and the fast-moving air. Right next to the car, the air speed is actually zero: the air sticks to the car because there are attractive forces between the molecules of the car's paintwork and the air molecules that touch them. The further away from the car you get, the higher the wind speed.
The region surrounding the car where the air speed increases from zero to its maximum is known as the boundary layer. We get laminar flow when the fluid can flow efficiently, gently and smoothly increasing in speed across the boundary layer; we get turbulent flown when this doesn't happen—when the fluid jumbles and mixes up chaotically instead of sliding past itself in smooth layers.
Photo: Wind speed increases with distance from the ground. In theory, a wind turbine tower has to be high enough to ensure the rotors are operating outside the boundary layer. In practice, turbine designers have to compromise: very high turbines may be unacceptable for all kinds of environmental and safety reasons. The idea of the boundary layer leads to all kinds of interesting things.
Introduction to Aeronautical Engineering
It explains why, for example, your car can be dusty and dirty even though it's racing through the air at high speed. Although it's traveling fast, the air right next to the paintwork isn't moving at all, so particles of dirt aren't blown away as you might expect them to be.
The same applies when you try to blow the dust off a bookshelf. You can blow really hard, but you'll never blow all the dust away: at best, you just blow the dust the upper layers of dust particles off the dust the lower layers that stay stuck to the shelf! The boundary layer concept also explains why wind turbines have to be so high.
The closer to the ground you are, the lower the wind speed: at ground level, on something like concrete , the wind speed is actually zero. Build a wind turbine that's way up in the sky and you're hopefully reaching beyond the boundary layer to the place where the air speed is a maximum and the wind has higher kinetic energy to drive the turbine's rotors.
Photo: The faster you go, the harder you have to work against the air. Air resistance doesn't really matter when you walk, because you're not traveling fast enough. But if you cycle, you need to think about aerodynamics much more and adopt a streamlined posture that disturbs the airflow as little as possible. Like this rider, you might want to get a tear-shaped cycling helmet and wear tight-fitting clothes?
Air resistance—drag, as it's usually known—follows on from the distinction between laminar and turbulent flow. When a sports car speeds through the air, the flow remains relatively laminar; when a truck plows through it, there's much more turbulence.
Drag is the force that a moving body feels when the flow of air around it starts to become turbulent. If you ride a bike or you've ever run a sprint race, it'll be very obvious to you that drag increases with speed. But a very important point is that it doesn't increase linearly as your speed increases but according to the square of your speed.
In other words, if you double your speed, roughly speaking you quadruple the drag. This doesn't just apply to land-speed record cars but to ordinary drivers as well: for stop-start city driving, you waste most of your energy in braking ; when you speed along on the highway, most of your energy is being lost pushing aside the air. Photo: Top: Friction drag: The aerodynamic shape of this car allows the airflow around it to remain reasonably laminar.
There is drag, but mainly caused by friction between the layers of air moving past one another at different speeds. Notice how the air becomes more turbulent behind the car and vortices start to occur in the wake. Bottom: Form drag: A box-shaped vehicle such as a large truck makes no attempt to divert air around itself. As soon as the air smacks into it, it starts to become turbulent. Why does drag happen?
The science of aerodynamics
There are two types called friction drag and form drag and they have different causes. Imagine a car sitting still as the wind speeds past it. If the car is smoothly shaped, the air next to its paintwork isn't moving at all. The layer just beyond that is moving a little bit, and the layer beyond that is moving a little bit more. All these layers of air are sliding past one another in exactly the same way that your foot might slide across the floor: they have to overcome the mutual attraction between one another's molecules, which causes friction. Friction drag happens because it takes energy to make layers of air slide past one another.
The rougher or more obstructive the object, the more turbulent the air flow becomes, the greater the friction between the layers, and the greater the drag. At low speeds, the air flows splits when it meets an object and, providing the object is reasonably aerodynamic, flows right around it, closely following its outline. But the faster the air flow and the less aerodynamic the object, the more the air flow breaks away and becomes turbulent.
Wing - Wikipedia
That's what we mean by form drag. Photo: Supersonic planes create a sonic boom when they overtake their own sound waves. The faster you go, the harder it is go faster still—that's the tricky science that makes it so difficult to break speed records in cars, boats, and planes. In theory, the laws of fluid dynamics of which aerodynamics is a part apply in much the same way, whether you're speeding over salt flats in a rocket-propelled car, skimming over the waves in a hydrofoil boat, or screaming through the air in a military jet.
Planes, however, are in a different category to cars and boats because they can go 5—10 times faster. Once they hit a certain speed, the speed of sound , different rules of aerodynamics come into play. Push your jet plane through the sound barrier and huge, cone-shaped shock waves form at the nose and the tail, where they can greatly increase drag.
That's why supersonic faster-than-sound jet planes have sharp noses and sharp, swept-back wings.
Go faster still and the aerodynamic rules change once again. At hypersonic speeds around five times faster than sound , shorter wings work best and they need to be positioned further back from the nose than on a supersonic plane.
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