Wind Tunnel          TurboMachine.com home

I constructed this simple wind tunnel from materials from Home Depot and a motor and fan from Grainger.  Material cost are approximately $200.

This wind tunnel is based on the Baals design:  http://www.fi.edu/flights/first/makebigger/index.html

Click on thumbnails pictures below to see larger construction pictures.

For questions email stephen@turbomachine.com

How a Wing Produces Lift

The lift of a wing is equal to the change in momentum of the air it is diverting down. Momentum is the product of mass and velocity (mv). The most common form of Newton’s second law is F= ma, or force equal mass times acceleration. The law in this form gives the force necessary to accelerate an object of a certain mass. An alternate form of Newton’s second law can be written: The lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air.

This picture illustrates the effect of the air being diverted down from a wing. A huge hole is punched through the fog by the downwash from the airplane that has just flown over it. 

So how does a thin wing divert so much air? When the air is bent around the top of the wing, it pulls on the air above it accelerating that air downwards.  This pulling action causes the pressure to be reduced above the wing. Also, depending on the angle of the wing, air from the bottom of wing is deflected downwards.  It is the acceleration of the air in the downward direction that gives lift or in other simple words "lift is produced by forcing air downwards".  The forcing action is caused by net result of both the upper and lower surfaces of the wing.  Newton's third law states that for every action there is an equal and opposite reaction.  Forcing air downwards causes the wing to be forced upwards to support the weight of the aircraft.  A wing is an efficient shape that bends the air and forces it downwards.

The Scoop

I would like to introduce a mental image of a wing. One is used to thinking of a wing as a thin blade that slices though the air and develops lift somewhat by magic. The new image that we would like you to adopt is that of the wing as a scoop diverting a certain amount of air from the horizontal to roughly the angle of attack. For wings of typical airplanes it is a good approximation to say that the area of the scoop is proportional to the area of the wing. The shape of the scoop is approximately elliptical for all wings, as shown in the figure. Since the lift of the wing is proportional to the amount of air diverted, the lift of is also proportional to the wing’s area.

Summary

• The amount of air diverted by the wing is proportional to the speed of the wing and the air density.
• The vertical velocity of the diverted air is proportional to the speed of the wing and the angle of attack (angle of attack is the angle of the wing into the air).
• The lift is proportional to the amount of air diverted times the vertical velocity of the air.
• The power needed for lift is proportional to the lift times the vertical velocity of the air.