In this chapter
- Importance of the Subject
- Flow Physics
- Drag due to Speed
- Drag Reduction Approaches
- Methods of Computation
- Summary of Drag Data (CD-ROM version)
- CD of Various Aerodynamic Systems
- CD Levels of Misc. Aircraft
- CD Levels of Road Vehicles
- Selected References
Drag force is a very large topic in aerodynamics. There are books and conferences entirely devoted to it, along with countless specialized publications.
From a physical point of view, drag is the resultant of forces acting normally and
tangentially to a surface, the former ones being pressure terms, and the latter ones
viscous terms. The mechanism under which these forces are created is ultimately
related to the formation of vortices and shear layers.
Very narrow gains (1 % or less) can translate into a change of technology. It is
widely assumed that the fuel crisis of the 1970s created the need to invest in drag
reduction technology for aircraft transport. But the problem is wider than that,
since all the aerodynamic systems use external power that is partially
dissipated due to drag forces.
For example, a reduction in the drag coefficient of an ordinary passenger car from CD
= 0.4 to CD=0.3 would improve the fuel consumption by 7.5 %. This saving
multiplied by the number of road vehicles in Europe and North America yields a figure
(at least 10 billion gallons/year) that could affect the price of the crude oil in
the world markets.
The reduction of 10 % drag on a large military transport aircraft would save over 10
million gallons of fuel over the life time of the aircraft.
A 15 % drag reduction on the Airbus A340-300B would yield a 12 % fuel saving, other
parameters being constant (Mertens, 1998).
See the Table of Drag Data for more details.
on CD-ROM only
The fundamental mechanisms by which drag is produced in steady state conditions can
be reduced to the following ones
Viscous drag is produced by the effects of viscosity on the aerodynamic
systems, through the thrust that must be applied to overcome the shear layers due to
the non slip condition.
Drag due to lift is the result of the downwash (vertical flow) and to the
strength of the vortices produced at some particular locations (wing tips or other
sharp edges) of many lifting systems.
Vortex drag can be created by both lifting and non lifting bodies (usually of
the bluff variety, ex. road vehicles, airships). Vortices are released during
flow separatio and trail downstream to form structured
or unstructured wake patterns.
Interference is the effect of the presence of one body on the aerodynamics of
a second body. The interference drag is a system drag that is present even in
absence of viscous effects (ideal fluid) and non lifting conditions. Since
interference occurs in many practical situations interference drag is a separate
topic.
Wave drag is created by radiation of disturbances in the fluid by a moving
body. This is the case of transonic and supersonic flows; in hydrodynamics waves are
produced by several means, the most important of which is probably the pattern of
surface waves produced by boats, ships and submerged bodies.
The presence of one or more drag components, along with their respective amounts,
clearly depends on the aerodynamic arrangement and the system operation.
Another classification sometimes used is that according to speed.
The speed (e.g Reynolds and Mach numbers) have, in fact, one of the most
important effects on both the drag build-up and the drag level.
The most effective approach to drag reduction is to concentrate on the components
that make up the largest percentage of the overall drag. Small improvements on large
quantities can become in fact remarkable aerodynamic improvements. This is another
reason why the drag build-up analysis is always made before attempting to study the
drag reduction strategies.
There are several methods used to compute the drag of a lifting body. For example:
The drag of an airfoil at subsonic speeds can be computed by using the Squire-Young
approximation. The method consists in evaluating the drag coefficient by using
boundary layer quantities at the trailing edge.
By using the axial momentumbalance on a large control volume (between two planes
far upstream and downstream the body).
By integration of the surface forces (CFD approach).
There are two contributions: the tangential (due to skin friction) and normal (due to
pressure) contributions. This is the approach followed most by the current research.
Other (simplified) methods include: Integration of circulation in the Treffz plane (induced
drag of large aspect ratio wings); Hayes formula (for linearized supersonic flow); Munk’s
stagger theorems (for linearized multi-body lifting systems), etc. A detailed review
of CFD capabilities has been recently published by van Dam (1999).
Importance of the Subject
Effects of Drag Reduction
Flow Physics
Viscous Drag
Lift-induced Drag
Vortex Drag
Interference
Wave Drag
Speed-induced Drag
Drag Reduction Approaches
Methods of Computation
Selected References
Check the reviews of these references.
