Re: How can I calculate the force of wind on a surface area?

Greetings:

Calculating the force of moving air on standard shapes such as flat
-plates , spheres and cylinders is straight forward. However, your question is
related to the science of aerodynamics of complex shapes which is one of the
most difficult sciences to mathematically model. For this reason scientists and
engineers still use wind tunnels and pressure measurements in the flow
systems to gather design information for aircraft, jet engines, automobiles,
and motor cycles. Wind tunnel testing is an expensive procedure and pressure
measurements on actual vehicles operating at high speeds are often used in
place of wind tunnels.

In turbo-supercharged engines, the air intake turbines must turn at very high
RPMs and they consume a large amount of the available engine power to
deliver compressed air to the engine air intake. Thus a ram air system would
be desirable for small engines such as those used on motor cycles.

I have found the following web site that has information on the testing of
various ram-air- equipped airboxes on motorcycles that were conducted by
Sport Rider Magazine and reported in the October 1999 and December 1999 issues.

http://www.factorypro.com/magazine_articles/mag_sportrider,Ra mAir,part2.html

For some reason I could not access the web site directly from the above URL
and I had to search for “ Sport Rider” and “ram air” on Altavista.com to
connect to it.

Sport Rider made pressure measurements, versus engine RPM, at speed and
on a dynamometer test stand for a number of popular racing bikes. The
difficulties that they encountered in the tests are described in detail in the
articles.

Air is a compressible fluid and increased compression and air flow into the
motorcycle engine is what a ram-air induction system is attempting to do.
However, it turns out that air behaves more like an incompressible fluid at
speeds less that Mach 0.3. Mach 1 is the speed of sound at the local
temperature and air pressure. At standard temperatures and pressures (20 C,
760 mm Hg) on the earth’s surface, the speed of sound is 1236 km/hr (768
miles per hour). Thus an air velocity greater than 370 km/hr (230 mph) would
be required to begin to observe air compression in an air intake duct. To
accomplish this in a ram air system requires that the air intake duct be tapered
down to a smaller diameter to increase the velocity of the air flow to be
greater than Mach 0.3 into the engine. The design of these type ducts is
highly dependent on the inlet pressure of the engine (- 11 milibars in the
article) and the ram air compression (+11 millibars in the article) at 12 to 14
thousand RPM for a YAMAHA YZF-R6 bike where they measured a 5 horsepower
increase using the ram-air system. Also, compressing air
increases its temperature, and this changes the Mach number as the air passes
through the various stages of the intake system making the design problem
even more complex.

Best regards, Your Mad Scientist
Adrian Popa