Editing Flight (section)

=== Forces ===
{{Main|Aerodynamics}}
[[File:Forces2.gif|thumb|Main forces acting on a heavier-than-air aircraft]]

Forces relevant to flight are<ref>[http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html "Four forces on an aeroplane."] ''NASA.'' Retrieved: January 3, 2012.</ref>
* [[Air propulsion|Propulsive thrust]] (except in gliders)
* [[Lift (force)|Lift]], created by the reaction to an airflow
* [[Drag (physics)|Drag]], created by aerodynamic [[friction]]
* [[Weight]], created by gravity
* [[Buoyancy]], for lighter than air flight

These forces must be balanced for stable flight to occur.

==== Thrust ====
{{Main|Thrust}}
[[File:aeroforces.svg|thumb|Forces on an [[aerofoil]] cross section]]

A [[fixed-wing aircraft]] generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a [[Propeller (aircraft)|propeller]], or a rotating [[Mechanical fan|fan]] pushing air out from the back of a [[jet engine]], or by ejecting hot gases from a [[rocket engine]].<ref>{{cite web| url = http://www.grc.nasa.gov/WWW/k-12/airplane/newton3.html| url-status = dead| archive-url = https://web.archive.org/web/19991128055408/http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html| archive-date = 1999-11-28| title = Newtons Third Law}}</ref> The forward thrust is proportional to the [[mass]] of the airstream multiplied by the difference in [[velocity]] of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a [[Thrust reversal|thrust reverser]] on a jet engine. [[Rotary wing aircraft]] and [[thrust vectoring]] [[V/STOL]] aircraft use engine thrust to support the weight of the aircraft, and vector sum of this thrust fore and aft to control forward speed.

==== Lift ====
{{Main|Lift (force)}}
[[File:Airfoil lift and drag.svg|thumb|right|300px|Lift is defined as the component of the [[aerodynamic force]] that is perpendicular to the flow direction, and drag is the component that is parallel to the flow direction]]

In the context of an [[fluid flow|air flow]] relative to a flying body, the '''lift''' force is the [[Vector (geometric)#Vector components|component]] of the [[aerodynamic force]] that is [[perpendicular]] to the flow direction.<ref>[http://www.lerc.nasa.gov/WWW/K-12/aerosim/Manual/fsim0020.htm "Definition of lift."] {{webarchive|url=https://web.archive.org/web/20090203074439/http://www.lerc.nasa.gov/WWW/K-12/aerosim/Manual/fsim0020.htm |date=2009-02-03 }} ''NASA.'' Retrieved: May 6, 2012.</ref> Aerodynamic lift results when the wing causes the surrounding air to be deflected - the air then causes a force on the wing in the opposite direction, in accordance with [[Newton's third law of motion]].

Lift is commonly associated with the [[wing]] of an [[Fixed-wing aircraft|aircraft]], although lift is also generated by [[Helicopter rotor|rotors]] on [[rotorcraft]] (which are effectively rotating wings, performing the same function without requiring that the aircraft move forward through the air). While common meanings of the word "[[wikt:lift#English|lift]]" suggest that lift opposes gravity, aerodynamic lift can be in any direction. When an aircraft is [[cruise (flight)|cruising]] for example, lift does oppose gravity, but lift occurs at an angle when climbing, descending or banking. On high-speed cars, the lift force is directed downwards (called "down-force") to keep the car stable on the road.

==== Drag ====
{{Main|Drag (physics)}}
For a solid object moving through a fluid, the drag is the component of the [[Net force|net]] [[Aerodynamic force|aerodynamic]] or [[hydrodynamics|hydrodynamic]] [[force]] acting opposite to the direction of the movement.<ref>French 1970, p. 210.</ref><ref>[http://www.ucmp.berkeley.edu/vertebrates/flight/physics.html "Basic flight physics."] ''Berkeley University.'' Retrieved: May 6, 2012.</ref><ref name=NASAdrag>[http://www.grc.nasa.gov/WWW/k-12/airplane/drag1.html "What is Drag?"] {{Webarchive|url=https://web.archive.org/web/20100524003905/http://www.grc.nasa.gov/WWW/K-12/airplane/drag1.html |date=2010-05-24 }} ''NASA.'' Retrieved: May 6, 2012.</ref><ref>[http://lorien.ncl.ac.uk/ming/particle/cpe124p2.html "Motions of particles through fluids."] {{webarchive|url=https://web.archive.org/web/20120425070933/http://lorien.ncl.ac.uk/ming/particle/cpe124p2.html |date=2012-04-25 }} ''lorien.ncl.ac.'' Retrieved: May 6, 2012.</ref> Therefore, drag opposes the motion of the object, and in a powered vehicle it must be overcome by [[thrust]]. The process which creates lift also causes some drag.

==== Lift-to-drag ratio ====
{{Main|Lift-to-drag ratio}}
[[File:Drag curves for aircraft in flight.svg|thumb|Speed and drag relationships for a typical aircraft]]
Aerodynamic lift is created by the motion of an aerodynamic object (wing) through the air, which due to its shape and angle deflects the air. For sustained straight and level flight, lift must be equal and opposite to weight. In general, long narrow wings are able deflect a large amount of air at a slow speed, whereas smaller wings need a higher forward speed to deflect an equivalent amount of air and thus generate an equivalent amount of lift. Large cargo aircraft tend to use longer wings with higher angles of attack, whereas supersonic aircraft tend to have short wings and rely heavily on high forward speed to generate lift.

However, this lift (deflection) process inevitably causes a retarding force called drag. Because lift and drag are both aerodynamic forces, the ratio of lift to drag is an indication of the aerodynamic efficiency of the airplane. The lift to drag ratio is the L/D ratio, pronounced "L over D ratio." An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. The lift/drag ratio is determined by dividing the lift coefficient by the drag coefficient, CL/CD.<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html</ref>

The lift coefficient Cl is equal to the lift L divided by the (density r times half the velocity V squared times the wing area A). [Cl = L / (A * .5 * r * V^2)] The lift coefficient is also affected by the compressibility of the air, which is much greater at higher speeds, so velocity V is not a linear function. Compressibility is also affected by the shape of the aircraft surfaces.
<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/liftco.html</ref>

The drag coefficient Cd is equal to the drag D divided by the (density r times half the velocity V squared times the reference area A). [Cd = D / (A * .5 * r * V^2)] 
<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/dragco.html</ref>

Lift-to-drag ratios for practical aircraft vary from about 4:1 for vehicles and birds with relatively short wings, up to 60:1 or more for vehicles with very long wings, such as gliders. A greater angle of attack relative to the forward movement also increases the extent of deflection, and thus generates extra lift. However a greater angle of attack also generates extra drag.

Lift/drag ratio also determines the glide ratio and gliding range. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it. The only effect weight has is to vary the time that the aircraft will glide for – a heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time.<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center
https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html</ref>

==== Buoyancy ====
{{Main|Buoyancy}}
Air pressure acting up against an object in air is greater than the pressure above pushing down. The buoyancy, in both cases, is equal to the weight of fluid displaced - [[Archimedes' principle]] holds for air just as it does for water.

A cubic meter of air at ordinary [[atmospheric pressure]] and room temperature has a mass of about 1.2 kilograms, so its weight is about 12 [[Newton (unit)|newtons]]. Therefore, any 1-cubic-meter object in air is buoyed up with a force of 12 newtons. If the mass of the 1-cubic-meter object is greater than 1.2 kilograms (so that its weight is greater than 12 newtons), it falls to the ground when released. If an object of this size has a mass less than 1.2 kilograms, it rises in the air. Any object that has a mass that is less than the mass of an equal volume of air will rise in air - in other words, any object less dense than air will rise.

==== Thrust to weight ratio ====
{{Main|Thrust-to-weight ratio}}
'''Thrust-to-weight ratio''' is, as its name suggests, the ratio of instantaneous [[thrust]] to [[weight]] (where weight means weight at the [[Earth]]'s standard acceleration <math>g_0</math>).<ref name="sutton">Sutton and Biblarz 2000, p. 442. Quote: "thrust-to-weight ratio F/W<sub>0</sub> is a dimensionless parameter that is identical to the acceleration of the rocket propulsion system (expressed in multiples of g0) if it could fly by itself in a gravity free vacuum."</ref> It is a dimensionless parameter characteristic of [[rocket]]s and other jet engines and of vehicles propelled by such engines (typically space [[launch vehicle]]s and jet [[aircraft]]).

If the [[thrust-to-weight ratio]] is greater than the local gravity strength (expressed in ''g''s), then flight can occur without any forward motion or any aerodynamic lift being required.

If the thrust-to-weight ratio times the lift-to-drag ratio is greater than local gravity then [[takeoff]] using aerodynamic lift is possible.