Laminar micro wind tunnel

To study the aerodynamic aspect of different shapes or scale models of 5 to 6 centimeters long

Choose a closed room

The object to study

A cigarette

A cigarette lighter

A ash-tray

A piece of copper wire

A pin

A mass

An halogen light source

A black paper

A camera

Light the cigarette and live it on the ash-tray. Place the model in the smock

Laminar micro wind tunnel

Use incense for non smoker

Aerodynamic drag force formula

Fa = 1/2  r  S  Cd  v2

Fa is aerodynamic drag in newton N

r is air density in kg/m3

S is frontal section of the volume in m2

Cd is the shape coefficient without dimension (Cd cyclist = 0.9 - Cd airplane = 0.027)

v is speed in m/s

The ideal shape

It is seen immediately that the Cd is important when designing vehicle

The shape to the Cd of 0.04 is represented on figure below

Profile of the best aerodynamic revolution volume

Where "a" represents the width and "b" the length

For a Cd of 0.04 it is necessary to comply with this two rules :

- 1° the maximum width is were the third of the length b

- 2° For a maximum volume, the optimum b/a ratio is 5

The form is traversed by three types of air flow :

- 1° At the front one, a laminar flow where the airflows are parallel to surface and resistance is low and proportional to the speed

- 2° In the center, a transitory flow, the dashes tend to deviate from surface

- 3° A the back, a turbulent flow, the nets tend to follow loops trajectories and resistance is proportional to the square of the speed

They exist shapes known as laminar which have laminar fields longer and which are better to receive a human body lengthened with the shoulders on the level of the fields transient

These shapes are used for the fuselages of competition sailplane or the body of recumbent bicycle

An example is the Acabion

These shapes are optimal to receive a human body and to offer the minimum of resistance

Half profiles turbulent and laminar

Moreover aerodynamic shapes require a regularity and an irreproachable surface quality

Ideas and suggestions

• Car with a self-stabilizing profile

• Car with biconvex profile

Oldsmobile Aerotech 1987, 413 km/h on circuit, turbo engine 2 liters

Back center of masses

Position of the center of masses of a vehicle

The position of the centre of masses is imposed

It is located between the front quarter and third of the body

The important masses are thus forwards: thermal engine or electric batteries

If the centre of gravity is too back

In case of takeoff the rear passes in front

The vehicle finishes on the roof as shown in the figure below

This rule is important for all vehicle especially for race boats

MG EX 181 "Teardrop" record breaker on the Bonneville Salt Flats 1959