33.Ground (screen) effect. Influence of screen effect to the aerodynamic characteristics of a wing.

In fixed-wing aircraft, ground effect is the increased lift and decreased drag that an aircraft's wings generate when they are close to a fixed surface.[1] When landing, ground effect can give the pilot the feeling that the aircraft is "floating". When taking off, ground effect may temporarily reduce the stall speed. The pilot can then fly level just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached.

When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircraft's wingspan or helicopter's rotor diameter, there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft.[3][4]

A wing generates lift, in part, due to the difference in air pressure gradients between the upper and lower wing surfaces. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static air pressure. These air pressure differences also accelerate the mass of air downwards. Flying close to a surface increases air pressure on the lower wing surface, known as the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. As the wing gets lower, the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result,[5] which accounts for the "floating" effect. Ground effect will also alter thrust versus velocity, in that reducing induced drag will require less thrust to maintain the same velocity.[5]

34.The geometrical characteristics of body of revolutions. Features of the flow of a rounded body.

Sometimes we are not interested in generating lift, just reducing drag, and when we have to reduce the drag of a given volume, the best shape is often a slender body -- and often nearly a body of revolution. The aerodynamics of such shapes is quite different from airfoils and wings, but follows some of the the same basic principles.

Bodies including fuselages are important because they produce drag, lift and moment. They also produce important interference effects with wings and substantially change the stability of an airplane.

The flow over more general fuselages and bodies can be predicted in much the same way as the flow over airfoils and wings. Superposition of sources and doublets in the form of panel methods or simpler forms (ala thin airfoil theory) are often used. Navier Stokes equations are used when flow separation is suspected.

The maximum velocity is given by:

The figure below (from Schlichting) illustrates the pressure distribution on bodies of revolution. D/L = 0.1

As indicated below, fuselages in inviscid flow produce a nose-up pitching moment when the angle of attack is increased. This effect is destabilizing and is an important consideration in the sizing of the horizontal tail.