articles - Venturi and VFR
Venturi
A fluid passing through smoothly varying constrictions experience changes in velocity and pressure, as described by Bernoulli's principle. A Venturi is a system for speeding the flow of the fluid, by constricting it in a cone-shaped tube. They are found in many applications where the speed of the fluid is important, and form the basis of devices like a carburattor.
Venturis are also used to measure the speed of a fluid, by measuring pressure changes from one point to another along the Venturi. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a Venturi is all that is needed. When the fluid flows though the Venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.
VFR
Visual flight rules
The visual flight rules (VFR) are a set of regulations that a pilot may operate under when weather conditions meet certain minimum requirements. The requirements are designed to provide sufficient visibility so that other aircraft can be seen and avoided.
Under VFR, the pilot generally controls the attitude of the aircraft by relying on what can be seen out the window (see visual flight), although this may be supplemented by referring to the instrument panel. A pilot flying under VFR is usually required to stay at least a specified distance away from clouds and must stay in areas where the visibility meets minimum requirements. There may be other requirements which vary from one country to another, such as not flying over a solid layer of clouds, or not flying at night. The pilot is responsible for seeing and avoiding other aircraft, terrain, and obstructions such as buildings and towers. Being in contact with air traffic control is optional in most airspace, and the pilot is usually allowed to select the course and altitude to be flown even when in contact with ATC. The pilot may navigate either visually, or by reference to instruments and electronic aids to navigation.
The minimum meteorological requirements for VFR are called visual meteorological conditions (VMC). If they are not met then the flight must be flown under instrument flight rules (IFR), the pilot must have an instrument rating and meet recency of experience requirements pertaining to instrument flight, and the aircraft must be equipped and type-certified for instrument flight. In some types of airspace, generally at extremely high altitudes, a flight must be flown under IFR regardless of the meteorological conditions.
Visual flight
Visual flight is the control of an aircraft by using the view from the aircraft as the primary reference point. A pilot observes the outside world and uses the flight controls to take the aircraft safely to the place they want to go. The alternative is to fly the aircraft primarily using feedback from onboard instruments such as an artificial horizon.
Except in the simplest aircrafts, such as ultra light aircrafts, hang gliders or balloons, a pilot almost always makes some reference to instruments to determine their exact speed or altitude. A complex aircraft, such as a military jet or an airliner is flown almost exclusively by reference to instruments.
As well as flying the aircraft, a pilot is also expected to avoid other aerial traffic. It is easier to watch for other traffic when using visual flight, since the pilot is looking mainly outside the aircraft. When using instrument flight a pilot is usually in contact with an air traffic control facility, which ensures that other aircraft are avoided. The primary visual reference used is usually the appearance of the natural horizon or the ratio of visible sky to ground, when the pilot is looking straight ahead. Different kinds of aircraft are controlled in very different ways, whether in visual or instrument flight. For an article on visual flight as it relates to airplanes, and to a great extent gliders and hang gliders, see visual flight (airplanes).
Visual meteorological conditions
In aviation visual meteorological conditions are those in which VFR flight is permitted. Generally there will be requirements for visibility, ceilings (for takeoffs and landings), and cloud clearances. Generally, VMC requires greater visibility and cloud clearance in controlled airspace than uncontrolled airspace. In uncontrolled airspace there is no risk of a VFR aircraft colliding with an IFR aircraft emerging from a cloud, so airplanes are permitted to fly closer to clouds.
The exact requirements vary by type of airspace, whether it is day or night (for countries that permit night VFR), and from country to country. Typical visibility requirements vary from one statute mile to five statue miles. Typical cloud clearance requirements vary from merely remaining clear of clouds to remaining at least one mile away from clouds horizontally and one thousand feet away from clouds vertically.