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.
1. Feral cats can minimize rodent problems. While cats cannot hunt rats and mice into extinction, they can keep their populations in check and discourage new rodents from moving into the area. Often feral cats fill in a gap in the current ecosystem. For example bob cats or lynx used to live up and down the East Coast but were hunted ruthlessly and driven away by development. Feral cats are similar in size and behavior to these native feline predators and help to control the same species of small prey animals. 2. Many people enjoy watching feral cats and observing animals has been shown to lower blood pressure in medical studies. 3. People who help to care for feral cats by feeding them and taking them to the vet enjoy many benefits. Often cat caretakers are elderly and live alone, a population at risk for depression, loneliness, and isolation. Cats relieve these conditions and often bring a sense of happiness and purpose to people who help them. Just as companion animals have been shown extend life expectancies, lower blood pressure, and relieve stress, caring for feral cats improves the health of their caretakers. 4. Individuals who cannot take on the full time commitment of adopting a companion animal can participate in programs to help feral cats. This provides a viable alternative to irresponsibly purchasing an animal one is not prepared to care for. 5. An established, stable, vaccinated, and sterilized colony of feral cats will deter other stray and feral cats from moving into the area. This actually decreases the risk that residents will encounter an unvaccinated cat, and will virtually eliminate problem behaviors like fighting and spraying.
