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A force may be thought of as a push or pull in a specific direction. A force is a vector quantity so a force has both a magnitude and a direction. When describing forces, we have to specify both the magnitude and the direction. This slide shows the forces that act on an airplane in flight.Maintaining a steady flight requires a balance, often described as an equilibrium of all the forces acting upon an airplane. Weight, lift, thrust and drag are the acting forces on an airplane. Assuming a straight and level flight, lift must be equal to weight and drag must be equal to thrust


Weight is a force that is always directed toward the center of the earth. The magnitude of the weight depends on the mass of all the airplane parts, plus the amount of fuel, plus any payload on board (people, baggage, freight, etc.). The weight is distributed throughout the airplane. But we can often think of it as collected and acting through a single point called the center of gravity. In flight, the airplane rotates about the center of gravity.
Flying encompasses two major problems; overcoming the weight of an object by some opposing force, and controlling the object in flight. Both of these problems are related to the object's weight and the location of the center of gravity. During a flight, an airplane's weight constantly changes as the aircraft consumes fuel. The distribution of the weight and the center of gravity also changes. So the pilot must constantly adjust the controls to keep the airplane balanced, or trimmed.weight opposes lift. Weight and lift are equal when a plane flies level at constant velocity. Because excess weight requires more lift, and therefore more thrust, heavy planes are more difficult to get off the ground as compared to lighter planes. Planes with less weight require less thrust. Thus, planes are designed to be as light as possible.


Lift is the force generated in order to overcome the weight, which makes the aircraft fly. This force is obtained by the motion of the aircraft through the air. Factors that affect lift:

Lift force is therefore dependent on the density of the air r, the airspeed V,
the type of airfoil and on the wing’s area according to the formula below:

Lift Force = 0.5 * r * V2 * Wing's Lift Coefficient * Wing Area

Where the Lift Force is in Newton, Wing Area in m2 and the airspeed in m/s.
The standard density of the air is 1.225kg/m3.

The wing's lift coefficient is a dimensionless number that depends on the airfoil
type, the wings aspect ratio (AR),
Reynolds Number and is proportional to the
angle of attack (AoA) before reaching the stall angle.

Imagine sticking your hand out the window of a moving car and flying your hand. As you tilt your hand up slightly, lift is the force that pushes your hand up. (Actually, lift is perpendicular to the direction of movement.) Lift is equal to the weight as your hand flies level at constant velocity. When a plane stalls, lift is lost! Stalling can occur due to insufficient air velocity or an excessive angle of attack.


To overcome drag, airplanes use a propulsion system to generate a force called thrust. The direction of the thrust force depends on how the engines are attached to the aircraft. In the figure shown above, two turbine engines are located under the wings, parallel to the body, with thrust acting along the body center line. On some aircraft, such as the Harrier, the thrust direction can be varied to help the airplane take off in a very short distance. The magnitude of the thrust depends on many factors associated with the propulsion system including the type of engine, the number of engines, and the throttle setting.

For jet engines, it is often confusing to remember that aircraft thrust is a reaction to the hot gas rushing out of the nozzle. The hot gas goes out the back, but the thrust pushes towards the front. Action <--> reaction is explained by Newton's Third Law of Motion.

The motion of the airplane through the air depends on the relative strength and direction of the forces shown above. If the forces are balanced, the aircraft cruises at constant velocity. If the forces are unbalanced, the aircraft accelerates in the direction of the largest force.
Note that the job of the engine is just to overcome the drag of the airplane, not to lift the airplane. A 1 million pound airliner has 4 engines that produce a grand total of 200,000 of thrust. The wings are doing the lifting, not the engines. In fact, there are some aircraft, called gliders that have no engines at all, but fly just fine. Some external source of power has to be applied to initiate the motion necessary for the wings to produce lift. But during flight, the weight is opposed by both lift and drag. Paper airplanes are the most obvious example, but there are many kinds of gliders. Some gliders are piloted and are towed aloft by a powered aircraft, then cut free to glide for long distances before landing. During reentry and landing, the Space Shuttle is a glider; the rocket engines are used only to loft the Shuttle into space.
You can view a short movie of "Orville and Wilbur Wright" explaining how the four forces of weight, lift, drag and thrust affected the flight of their aircraft. The movie file can be saved to your computer and viewed as a Podcast on your podcast player 

The magnitude of the thrust depends on many factors associated with the
propulsion system used:

- type of engine
- number of engines
- throttle setting
- speed

The direction of the force depends on how the engines are attached to
the aircraft.

The glider, however, has no engine to generate thrust. It uses the potential
energy difference from a higher altitude to a lower altitude to produce kinetic
energy, which means velocity.
Gliders are always descending relative to the air in which they are flying.


Drag opposes thrust. Imagine sticking your hand out the window of a moving car and flying your hand. The force that pushes your hand back is called "drag". As your hand pushes on the wind, the wind also pushes against your hand. Isaac Newton would say that force of your hand pushing on the air is always equal to the force of the air pushing on your hand; this is his third law. When the plane flies level at constant velocity, weight = lift! When the engines of a plane quit, drag slows the plane down according to Newton's 2nd Law.

Legs of birds and wheels of planes are tucked in to reduce drag. Drag is unwanted because it makes the plane or bird inefficient. Planes with more drag require more thrust to fly successfully. To reduce drag and increase efficiency, planes are streamlined. Planes also use camber and high aspect ratios to reduce drag.
As the airplane moves through the air, there is another aerodynamic force present. The air resists the motion of the aircraft and the resistance force is called drag. Drag is directed along and opposed to the flight direction. Like lift, there are many factors that affect the magnitude of the drag force including the shape of the aircraft, the "stickiness" of the air, and the velocity of the aircraft. Like lift, we collect all of the individual components' drags and combine them into a single aircraft drag magnitude. And like lift, drag acts through the aircraft center of pressure.

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