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Drag - Level 3

 

Drag is the resistance an airplane experiences in moving forward through the air.

For an airplane to maintain steady flight, there must be sufficient lift to balance the weight of the airplane, and there must be sufficient thrust to overcome drag.

Fig. 1 shows an airfoil moving forward through the air and depicts the principle known as the resolution of forces. The vertical component (OL) is the lift and is used to support the weight of the airplane. The horizontal component (OD) is the drag. OR is the resultant reaction of these two components.

Since drag is a force directly opposed to the motion of the airfoil and, as the work of overcoming it is performed by the engine, it is desirable to have it as small as possible, to afford the engine to be more efficient.

Drag is of two principal types.

1. PARASITE DRAG is the term given to the drag of all those parts of the airplane which do not contribute to lift, that is, the fuselage, landing gear, struts, antennas, wing tip fuel tanks, etc. In addition, any loss of momentum of the airstream caused by openings, such as those in the cowling and those between the wing and the ailerons and the flaps, add to parasite drag.

Parasite drag may be divided into two components.

  1. Form drag refers to the drag created by the form or shape of a body as it resists motion through the air.
  2. Skin friction refers to the tendency of air flowing over a body to cling to its surface.
  3. Although parasite drag can never be completely eliminated, it can be substantially reduced. One method is to eliminate altogether those parts of the airplane that cause it. For this reason, retractable landing gear has been developed. Wing struts have been eliminated in favour of fully cantilevered wings. Another method is to streamline those parts that cannot be eliminated. Skin friction can be reduced substantially by the removal of dust, dirt, mud or ice that has collected on the airplane.

    Even the most carefully designed individual parts must, however, be joined together to create the total airplane. Resistance caused by the effect of one part on another (i.e. where the wing is attached to the fuselage, or the struts to the wings) is called interference drag and can be reduced by careful design in the fairing of one shape into another.

2. INDUCED DRAG is caused by those parts of an airplane which are active in producing lift (i.e. the wing). It is the result of the wing's work in sustaining the airplane in flight and is, therefore, a part of the lift and can never be eliminated. It increases as the angle of attack increases and decreases as the- angle of attack decreases.

Induced drag can be reduced only during the initial designing of an airplane. A wing with a high aspect ratio, that is, with a very long span and a narrow chord, produces less induced drag than does a wing with a short span and a wide chord. Gliders and sail planes are therefore commonly designed with high aspect ratio wings.

The phenomenon, known as wing tip vortices, is testimony to the existence of induced drag.

As the decreased pressure over the top of the wing is less than the atmospheric pressure around it, the air flowing over the top surface tends to flow inward. The air flowing over the lower surface, due to the relatively higher pressure around it, tends to flow outward and curl upward over the wing tips.

When the two airflows unite at the trailing edge of the wing, they are flowing contra-wise. Eddies and vortices are formed which tend to unite into one large eddy at each wing tip. These are called wing-tip vortices. This disturbed air exerts a resistant force against the forward motion of the wing. This resistant force is known as induced drag.

In order to support the weight of an airplane, a large amount of air must be displaced downward. This displaced air must have somewhere to go, and tends to flow spanwise outwards, as explained above. It is seeking to escape around the wing tips and flow into the low pressure area created over the upper surface of the wing. It will be become very clear that the heavier the airplane and the higher the span loading on the wing, the more air it will displace downward, therefore the greater will be the circulation of air, and the greater the magnitude of the wing tip vortex created and the greater the induced drag.

Induced drag does not increase as the speed increases. On the contrary, it is greatest when the airplane is flying slowly, a few knots above the stalling speed when maximum lift is being realized at minimum speed.

The induced drag characteristics of a wing are not the same very near the ground as they are at altitude. During landing and take-off, the ground interferes with the formation of a large wing-tip vortex. Induced drag is, therefore, reduced when an airplane is flown very near the ground. This phenomenon is known as ground effect.

Although induced drag cannot be eliminated, it can be reduced by certain design features. As has been stated earlier, less induced drag is generated by a long, narrow wing than by a short, broad one. It has also been found that winglets are effective in reducing induced drag. Attached to the wing tip, the winglet, a small, vertical surface of airfoil section, is effective in producing side forces that diffuse the wind-tip vortex flow.

See Lift and Drag Curves

Aileron Drag

In banking to make an airplane turn, one aileron is depressed and the other is raised. The downgoing aileron, being depressed into the compressed airflow on the underside of the wing, causes drag. The upgoing aileron, moving up into a more streamlined position, causes less drag. The drag on the downgoing aileron is known as aileron drag and if not corrected for in the design of the aileron, tends to cause a yaw in the opposite direction to which the bank is applied.

The Boundary Layer

The boundary layer is a very thin layer of air lying over the surface of the wing (and, for that matter, all other surfaces of the airplane). Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air, the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called the laminar layer.

As the boundary layer approaches the center of the wing, it begins to lose speed due to skin friction and it becomes thicker and turbulent. Here it is called the turbulent layer. The point at which the boundary layer changes from laminar to turbulent is called the transition point (Fig. 3). Where the boundary layer becomes turbulent, drag due to skin friction is relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases, the transition point also tends to move forward.

Fig. 3

Various methods have been developed to control the boundary layer in order to reduce skin friction drag.

Suction Method. One method uses a series of thin slots in the wing running out from the wing root towards the tip. A vacuum sucks the air down through the slots, preventing the airflow from breaking away from the wing and forcing it to follow the curvature of the wing surface. The air, which is sucked in, siphons through the ducts inside the wing and is exhausted backwards to provide a little extra thrust. The laminar flow airfoil is itself a structural design intended to make possible better boundary layer control. The thickest part of a laminar flow wing occurs at 50% chord. The transition point at which the laminar flow of air breaks down into turbulence is at or near the thickest part. The transition point at which the laminar flow of air becomes turbulent on a laminar flow airfoil is rearward of that same point on a conventional designed airfoil (Fig. 4).


Fig. 4

 

Vortex generators are small plates about an inch deep standing on edge in a row spanwise along the wing. They are placed at an angle of attack and (like a wing airfoil section) generate vortices. These tend to prevent or delay the breakaway of the boundary layer by re-energizing it. They are lighter and simpler than the suction boundary layer control system described above.

Streamlining is a design feature by which a body is so shaped that drag is minimized as the body moves forward through the air. A flat plate or a round ball moving through the air disturb the smooth flow of air and set up eddies behind them. The effect of streamlining an object can be seen in Fig. 5.

Fig. 5

COUPLES

The principle of equilibrium has already been discussed in earlier discussions of the forces that act on an airplane in flight. When two forces, such as thrust and drag, are equal and opposite, but displaced parallel to each other rather than passing through the same point, they are said to form a couple.

A couple will cause a turning moment about a given axis.

If the center of gravity of weight is ahead of lift on a wing, the couple created will turn the nose of the airplane down. Conversely, if lift is ahead of weight, the couple created will turn the nose of the airplane up.

If drag is above thrust, the couple formed will turn the nose of the airplane up. Conversely, if thrust is above drag, the couple formed will turn the nose of the airplane down.

 


The material for this section is reproduced from the publication, FROM THE GROUND UP, with the permission of its copyright owner, Aviation Publishers Co. Ltd. No further reproduction is authorized, in any print, electronic or other form of media, without the prior consent of the publisher at http://www.aviationpublishers.com . Any questions regarding this portion of the website should be directed to Dr. Claudius Carnegie. Questions regarding the publication, FROM THE GROUND UP, should be directed to the publisher at info@aviationpublishers.com.

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Updated: May 03, 2008