Heavier-than-air aircraft must find some way to push air or gas downwards, so that a reaction occurs (by Newton's laws of motion) to push the aircraft upwards. This dynamic movement through the air is the origin of the term aerodyne. There are two ways to produce dynamic upthrust: aerodynamic lift, and powered lift in the form of engine thrust.
Aerodynamic lift is the most common, with aeroplanes being kept in the air by the forward movement of wings, and rotorcraft by spinning wing-shaped rotors sometimes called rotary wings. A wing is a flat, horizontal surface, usually shaped in cross-section as an aerofoil. To fly, the wing must move forwards through the air; this movement of air over the aerofoil shape deflects air downward to create an equal and opposite upward force, called lift, according to Newton's third law of motion. A flexible wing is a wing made of fabric or thin sheet material, often stretched over a rigid frame. A kite is tethered to the ground and relies on the speed of the wind over its wings, which may be flexible or rigid, fixed or rotary.
With powered lift, the aircraft directs its engine thrust vertically downwards.
The initialism VTOL (vertical take off and landing) is applied to aircraft that can take off and land vertically. Most are rotorcraft. Others, such as the Hawker Siddeley Harrier, take off and land vertically using powered lift and transfer to aerodynamic lift in steady flight. Similarly, STOL stands for short take off and landing. Some VTOL aircraft often operate in a short take off/vertical landing regime known as STOVL.
A pure rocket is not usually regarded as an aerodyne, because it does not depend on the air for its lift (and can even fly into space), however many aerodynamic lift vehicles have been powered or assisted by rocket motors. Rocket-powered missiles which obtain aerodynamic lift at very high speed due to airflow over their bodies, are a marginal case.
Ann Thomas on 08 Dec 08Good for body Some of the studies show that owning a pet can have very real physical benefits for their owners. These include: * Pet owners have lower blood pressure. It has long been known that the act of stroking a pet can reduce blood pressure. But a study at the State University of New York at Buffalo found that the beneficial effects continue even when the pet is not present. The study, which looked at a group of stockbrokers with hypertension, concluded that just owning a pet can help lower blood pressure -and keep it lower, even in stressful settings. * Pet owners have lower blood cholesterol levels. A study of more than 5,400 people conducted by Australia's Baker Medical Research Institute found that pet owners had not only lower blood pressure, but also lower levels of blood cholesterol and triglycerides in comparison to the non-pet owners, reducing the risk of heart disease. * Pet owners have a higher survival rate after serious illness. Two studies have found that heart attack patients who owned the pets were significantly more likely to be alive a year after they were discharged from the hospital than those who didn't. What's more, a study conducted at City Hospital in New York found that the presence of a pet affected survival rate even more than having a spouse or friends. Patients in hospitals or nursing homes who have regular visits from their pets have shown to be more receptive to treatment. The need to care for their pet gives them reason to recover and the will to live. * Pet owners have fewer doctors' visits. Studies conducted by Cambridge University in England and at the University of California at Los Angeles have found that pet ownership corresponds to overall improved health and fewer medical care visits. A study of Medicare patients also found that seniors who own dogs go to the doctor less than those who do not. Even the most highly-stressed older dog owners in the study had 21 % fewer physician's visits than non-dog ow
