Can a teardrop shape produce an aerofoil effect?

Question

I have been looking at a lot of diagrams of wind turbine blades. Wind turbine blades use both lift and drag to cause rotation. The diagrams show cross sections of blades and arrows to indicate wind direction. The blades in the diagram have a 'teardrop' cross section.

Whenever I have read about the aerofoil effect, aeroplanes wings are used to instantiate it. Aeroplane wings have a cross sectional shape that makes the air above them travel faster than the air below them. They are like an asymmetrical teardrop shape with one side more rounded than the other. According to bernouli's theorem, this should and does produce a lift effect.

Whenever I look at diagrams of the cross section of wind turbine blades, they don't appear to have this asymmetry. They do, however, appear to be pitched in relation to the airflow.

Can anyone explain to me how the aerofoil effect is produced in this instance?

Answer 1

This whole Bernoulli lift principle is one of the best propagated myths. Most science teachers totally have this wrong as do many books. Lift doesn't result from the pressure differential due to one layer of air moving faster over one surface of the wing. If that were how lift worked a Cessna would have to go a few hundred miles an hour to get off the ground. It gains lift from angle of attack and from Newton's third law. The aerodynamics of the wing throws air downward from the trailing edge of the wing. By newton's law that means there's an upward force on the wing. Here's a very good explanation...

http://home.comcast.net/~clipper-108/lift.htm

Answer 2

Good answer from Brian L and well illustrated to show the principles behind sails on wind turbines.

One of the limiting factors in turbine design is how fast it can turn and air resistance contributes to this, hence the non aerofoil cross section of the blade which is teardrop shaped to offer the least drag as the blade cuts through the air.

Answer 3

The design of a wing is to create a force parallel to the flow of air over it or perpendicular to the wing surface. This is called lift.

A windmill rotor doesn't want to produce such a force in this direction. Instead it is designed to produce a force perpendicular to the wind direction (parallel to the blade) to force it to rotate.

The pitch on these blades enables this (turning) force to be kept more or less constant. At higher wind speeds the pitch is less, at lower wind speeds the pitch is increased. This enables the rotors to rotate at a more or less constant rate.

Windmill blades are similar in design to turbine blades, creating rotational motion from linear motion.

Answer 4

In that instance it's not a matter of lift so much as it is a matter of pushing - think of a sailboat sailing at an angle to the wind.

If wind is going from east to west like this

------------------->>>>>>>>

And the sailboat wants to go north, like this:
^
^
^
|
|
|

The captain will set his sail at an angle, like this:
\
 \
  \
The wind coming from the east will strike the sail and bounce southward because of the angle of the sail.

Angled fan blades work the same way - they impact the air and push it downward by virtue of angle of attack, and it has nothing to do with bernoulli's principle - it's the same as throwing a baseball at a wall and having it bounce off.

Answer 5

It is not possible. THe aerofoil effect is not produced in this occasion!