The reference area depends on what type of drag coefficient is being measured. For automobiles and many other objects, the reference area is
the projected frontal area of the vehicle
. This may not necessarily be the cross sectional area of the vehicle, depending on where the cross section is taken.
What reference area is considered in calculating the drag coefficient of a sphere?
The reference area A is typically
the cross-sectional or frontal area of the object (pR
2
for a sphere)
, but may also be the surface area (wetted area) or other representative area describing the object. Notice that according to this formula, a higher cruise speed V requires a larger thrust force to counteract F
D
.
What is reference area for drag?
For airfoils, the reference area is
the nominal wing area
. Since this tends to be large compared to the frontal area, the resulting drag coefficients tend to be low, much lower than for a car with the same drag, frontal area, and speed.
How does area affect drag coefficient?
The total aerodynamic force is equal to the pressure times the surface area around the body. Drag is the component of this force along the flight direction. Like the other aerodynamic force, lift,
the drag is directly proportional to the area of the object
. Doubling the area doubles the drag.
What is the unit for coefficient of drag?
| Variable Identity Metric Units | D Drag Newtons | Cd Drag Coefficient No units | r Density of air kg/m 3 | V Velocity m/s |
|---|
How is drag calculated?
The drag equation states that drag D is
equal to the drag coefficient Cd times the density r times half of the velocity V squared times the reference area A
. For given air conditions, shape, and inclination of the object, we must determine a value for Cd to determine drag.
What shape has the lowest coefficient of drag?
It is generally accepted that
some variation of the teardrop/airfoil shape
has the lowest drag coefficient.
What shape has the highest drag coefficient?
A quick comparison shows that
a flat plate
gives the highest drag, and a streamlined symmetric airfoil gives the lowest drag–by a factor of almost 30!
Is drag coefficient constant?
The drag coefficient can depend upon velocity, but
we will assume that it is a constant here
. Table 1 lists some typical drag coefficients for a variety of objects. Notice that the drag coefficient is a dimensionless quantity. At highway speeds, over 50% of the power of a car is used to overcome air drag.
What is the drag coefficient of a cube?
| Type of Object Drag Coefficient – c d – Frontal Area | Cube 0.8 s2 | Bike – Racing 0.88 3.9 ft 2 (0.36 m 2 ) | Bicycle 0.9 | Tractor Trailed Truck 0.96 frontal area |
|---|
Can coefficient of drag be negative?
Drag coefficient cannot be negative in steady flow
and a negative value of drag coefficient in steady flow indicates a computational error. It can however be zero in unsteady flow instantaneously. Negative drag is thrust.
How do you reduce drag coefficient?
Methods of decreasing the drag coefficient of a vehicle include
re-shaping the rear end
, covering the underside of the vehicles, and reducing the amount of protrusions on the surface of the car.
What is a high drag coefficient?
A low drag coefficient implies that the streamline shape of the vehicle’s body is such as to enable it to move easily through the surrounding viscous air with the minimum of resistance; conversely a high drag coefficient
is caused by poor streamlining of the body profile so that there is a high air resistance when the
…
What units are for velocity?
| Derived quantity Name Symbol | speed, velocity meter per second m/s | acceleration meter per second squared m/s 2 | wave number reciprocal meter m – 1 | mass density kilogram per cubic meter kg/m 3 |
|---|
What is coefficient of lift and drag?
The lift to drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag. The lift/drag ratio is used to express the relation between lift and drag and is determined by dividing the lift coefficient by the drag coefficient,
CL/CD
. A ratio of L/D indicates airfoil efficiency.
Does drag depend on mass?
Drag
depends directly on the mass of the flow going past the aircraft
. The drag also depends in a complex way on two other properties of the air: its viscosity and its compressibility. These factors affect the wave drag and skin friction which are described above.
