Four Basics Forces of Flight.

An aircraft in straight and level accelerated  flight has four forces acting on it. These forces are :

1. Lift, an upward acting force

2.Drag,retarding force of the resistance to lift

3.Weight, the downward effect that gravity has on the aircraft

4. Thrust, the forward acting force provided by the propulsion system

LIFT(L): As the wing moves through the air,the airflow will be over and under the wing. As air passes over the upper surface of wing,because of the wing chambered shape,the air has to travel at higher speed. This results in decrease in pressure on the upper surface of the wing. This will create a component of force acting upward perpendicular to the relative wind(Bernoulli's principle)

Down wash Airflow: If the wing is tilted upward,the airflow under wing is deflected downward below and behind the wing of an airplane. This is called down wash. Due to this aerodynamic action,the wing receives an additional upward counter aerodynamics force. The lift of a wing is proportional to the amount of air diverted down time the downward velocity of that air. According to Newtons third law of motion the counter aerodynamics force is produced due to airflow deflected downwards. 

In fixed wing aircraft the speed of the airflow over the wings is determined by the air speed of the airplane. This is not so with (helicopters) rotor wing aircraft. The speed of the airflow over the rotors is determined not only by the airspeed of the helicopters,but also by the speed of the rotation of the rotor blades. To move the helicopters in any horizontal in any horizontal direction,entire rotor disk must be tilted in the desired directions. 

Drag (D):The force is required to move or accelerate any airfoil(airplane) in the air. The application of the force to an airfoil produce equal and opposite force (Newtons Third Law of Motion). 

This opposite aerodynamic force or the resistance an aircraft experiences in moving forward through the air is called Drag. This is the component of force acting in the opposite direction to the line of flight. the force is paralleled to the relative wind an opposes the motion of the airplane through air.

At subsonic speed there are two kinds of drag. 

1. Induced Drag
2. Parasite Drag. 

Induced Drag: The induced drag is unavoidable part of the lift generation. This is created as aresult of the generation of lift. The pressure difference between the upper and lower surfaces of the wing results is downward component"down wash". This results in wing vortex being formed at each wing tip,causing an induced drag. As the angle of attack increases,so does drag,at a critical point the angle of attack can become so great that the airflow is broken over the upper surface of the wing and lift is lost while drag increases. 

Parasite Drag: Parasitic Drag is caused by form resistance(due to shape), skin friction,interference and all other elements that are not contributing to lift (engines,fuselage,landing gear,tail section,antennas and the shape that cause local airflow separations). The higher the airspeed,the greater the parasite drag. The Parasite drag may be further classified into:-

1. Form Drag is created by the form or shape of structure as it resists motion of the aircraft through the air. The drag arises to airflow separations from surface due to shape of the body. The streamline is necessary to decrease parasite drag. 
2. Skin friction is due to viscosity of air. The airflow has tendency to cling its surface of an airfoil. The best way to reduce skin friction is by cleaning and polishing the surface. 

Compressibility Drag: This is the other form of drag due to shock wave build up (on the wing) at higher Mach Number speed. This has considerable effect on jet aircraft performance due increase in drag,loss of lift and change in position of Center of Pressure. 

Lift and Drag Ratio: This aerodynamic parameter is very important in the aircraft performance. 

Speed of Sound: 

Drag Equation is 

 is the drag force, which is by definition the force component in the direction of the flow velocity,

${\displaystyle \rho }$ is the mass density of the fluid,

${\displaystyle u}$ is the flow velocity relative to the object,

${\displaystyle A}$ is the reference area, and

${\displaystyle C_{D}}$ is the drag coefficient – a dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag, in general ${\displaystyle C_{D}}$ depends on the Reynolds number.