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if gravitational force is acting on every object present on earth then why aeroplane is able to fly why dont it get crashed ot land.

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When the air passes over aerodynamically design wings and fuselage, air has low velocity and high pressure on the lower surface than the upper surface of the wing and fuselage. This gives rise a net upward pressure force. This upward vertical force is called Lift. Lift is nothing but a vertical component of resultant force acting upon the airplane. This resultant force is vector summation of all the pressure force and shear force acting on the whole structure of the plane. Magnitude of lift force decides whether aircraft will gain or lose or maintain the altitude. Lift equation is given by following formula

Lift, L = 0.5 × density of air × square of plane velocity × wing surface area × lift coefficient

Where Lift coefficient - is a geometrical property. Its value changes with deflection of Flight control surfaces - as profile of geometry changes with deflection.

Now lets discuss the magnitude of lift in different phases of flight.

In stationery position, velocity of aircraft is zero and hence no lift is produced when plane at stationery.

When airplane starts accelerating on runway, it starts building up lift but lift still remains less than the weight of the airplane till the aircraft reaches Rotation (aeronautics) . At rotation speed , Elevator (aeronautics) - are deflected downward which increases the coefficient of lift. At this point lift becomes greater than aircraft weight and whole plane takeoffs.

In steady level flight, all the forces are in equilibrium. Lift becomes equal to weight, thrust becomes equal to drag and hence in steady level flight, airplane neither gain nor lose altitude. Plane flies at same height with constant speed.

For climbing, lift must be more than the weight force, it can be done by moving the throttle at higher setting of power lever angle which increases the thrust which in turn increases velocity or nose up the airplane by increasing the angle of attack by deflecting the elevator downward or combination of both.

For descending it is necessary to decrease the lift which can be done by reduction of engine power or by deflecting the elevator upward or by combination of both.

But when plane approach for touchdown it fly at minimum possible speed. It is called stall velocity. At stall speed aircraft can fall like stone because lift is not high enough for gradual and slow decrement of altitude. To avoid this situation, trailing edge Flap (aeronautics) - are extended to increase lift coefficient which increases the lift but still remains less than weight of plane. It helps in smooth landing.

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