More info:

Aerodynamics of Boomerang. Chapter 1

APR 21, 2006

by Saulius Pakalnis

Click here if you don't see all 7 chapters

The purpose of the article is to give you fundamental knowledge of aerodynamics of boomerangs and explain principles which let  you to build your own boomerang constructions. This will encourage you to make high quality returning boomerangs for sport. 


Ideal and Real gases. Ref (pdf).

Ideal gases

Basic assumptions of Ideal gases model:
1. Gas consists of molecules in ceaseless motion.
2. The size of the molecules is negligible in the sense that their diameters are much smaller than the average distance travelled between collisions.
3. The molecules do no interact, except during collisions.

Real gases


From the ideal gas equation:
• The molecules of gas have finite volume.
• Molecules of gas do attract each other.
• The smaller the distance between the gas molecules, the more likely that attractive forces will
develop between the molecules.



Viscosity of air

It seems natural to see the origin of viscosity in terms of the attractive and repulsive forces between molecules. However, gases have substantial viscosity even though their inter-molecular forces are weak, suggesting some other mechanism. Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow. A lot of fluid dynamics is concerned with inviscid flow, but the role of viscosity is crucial to understanding some of the most important fluid phenomena, such as lift produced by a wing. (Author: Fred Senese Ref.

Touch a lift force in water 

Inner surface
The water "reflects" from spoon surface. According to the third  Newton's law ("For every action there is an equal and opposite reaction") spoon moves to the opposite direction. The motion is shown by arrows.

Outer surface
The water follows the surface of the spoon. According to the third  Newton's law the spoon moves to the opposite direction.

Why water sticks to the surface of spoon?
You can replace the surface tension's attractive force by a repulsive one. Just put oil or fat film on spoon surface. See that static forces (as capillarity) make no changes to water stream. It still  attracts the spoon. The effect has a dynamic origin. It is explained by Coanda effect.

Does Coanda effect also "work" in free streams?

 See interaction of oil and water streams. The streams are centered. They pull each other below a point of convergence. The pull turns into push when speed of both streams becomes equal. Then oil begins to collapse into drops, and the two phase system becomes unstable. Second picture shows oil drops, which split from water. Note, that static surface tension force is repulsive for water - oil interface.

The streams are directed  in the way, that oil stream just touches surface of water stream. Above the convergence point water stream is more slow than oil one.  An attractive force of water and oil streams is explained by Coanda effect.

Touch a lift force in air

Take a shield of paper (A4 format). Cut it into 2 pieces.

Take hair dryer and one of the paper strips. Make an experiment.

Inner surface
Direct air blow to inner surface. Lift force pushes the strip up.

Outer surface
Air is blowed at small angle to a curved surface. Lift force pushes the strip up.

Outer surface
Almost no lift? You can push the strip down, but how to pull it up?
Note1: Lift force appears as the result of surface and air flow interaction. It seems that some special conditions should be met to get a lift.


Peculiarities of dynamic ant static pressure

Dynamic pressure is the component of fluid pressure that represents fluid kinetic energy (i.e., motion), while static pressure represents hydrostatic effects.

Static pressure is isotropic - the same in all x,y,x directions. In air it equals the atmospheric pressure and does not depend on the wing speed.


Note1. Pillow phenomenon may explain why air flow speed near the curved upper surface is accelerated and is higher if compared to the air flow  near the lower surface.

Note2: Air stream "sticks" to surface and bends down the near upper surface of the wing not due to Pillow phenomenon but due to Coanda effect.



Weltner, Klaus and Ingelman-Sundberg, Martin. Ref.. Misinterpretations of Bernoulli's Law:

Static pressure in a free air stream.


Measurement of static pressure within a free stream 
