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Drag

in Aerodynamics

There are two basic types of drag, parasite drag and induced drag. Parasite drag is named so, because it in no way aids in flight. Induced drag is created (or induced) by the wings developing lift.

Parasite drag is composed of disruption of the streamline flow (things like antennas poking up from the airplane) and the resistance of skin friction. The profile drag of a streamlined object held in a fixed position relative to the airflow increases approximately as the square of it’s velocity. Or, in English, doubling the airspeed increases drag four times, tripling the airspeed increases drag six times, etc.

Induced drag is drag created by the lifting forces of the wings.

Whenever the wing is producing lift, the pressure on the lower surface is greater than the pressure on the upper surface (Bernoulli Effect).  As a result, the air tends to flow from the high pressure area below the wing to the low pressure area above the wing.  At the wing tips the air pressure equalizes resulting in the air flowing from the bottom to the top of the wing creating wingtip vortices at each wingtip.  From the tail of the airplane, the vortices will circulate counterclockwise from the right wingtip and clockwise from the left wingtip.  The greater the size of the vortices, the greater the induced drag.

As airspeed decreases, the total drag becomes greater, due mainly to the increase in induced drag created by having to have a higher angle of attack to maintain straight and level flight.

Understanding the relationship between speed and drag is important in calculating maximum endurance and the range of the airplane.  When drag is at a minimum, power required to overcome drag is also at a minimum.  Take a look at the chart below to see the relationship in drag versus speed.

To understand the effect of lift and drag on an airplane in flight, both must be combined and the lift/drag ratio considered.  With the lift and drag data available for various airspeeds of the airplane in steady, unaccelerated flight, the proportions of CL (Coefficient of Lift) and CD (Coefficient of Drag) can be calculated for each specific angle of attack. The resulting plot for lift/drag ratio with angle of attack shows that L/D increases to some maximum, then decreases at the higher lift coefficients and angles of attack, as shown in figure 3-6. Note that the maximum lift/drag ratio, (L/D max) occurs at one specific angle of attack and lift coefficient. If the airplane is operated in steady flight at L/D max, the total drag is at a minimum. Any angle of attack lower or higher than that for L/D max reduces the lift/drag ratio and consequently increases the total drag for a given airplane’s lift.

The location of the center of gravity (CG) is determined by the general design of each particular airplane. The designers determine how far the center of pressure (CP) will travel. They then fix the center of gravity forward of the center of pressure for the corresponding flight speed in order to provide an adequate restoring moment to retain flight equilibrium.

51UFncHi9pL._SX390_BO1,204,203,200_Learn more about airplane aerodynamics with the Illustrated Guide to Aerodynamics. This unique introductory guide, which sold more than 20,000 copies in its first edition, proves that the principles of flight can be easy to understand, even fascinating, to pilots and technicians who want to know how and why an aircraft behaves as it does.

 

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