does the speed of the fan differ with respect to the length of its wings.?

Question

the wings in the wind mills are large. but their speed is very slow. There is any connection between the electric fan and the fan in the wind mills.

Answer 1

Actually it all depends on the force or the torque. You might get confused because due to size of blades the area may change and the air resistance may change which might change the speed if everything else including the material and the force remains the same.

But it all comes down to the force or torque.
In the case of windmill the force is of the wind over which we dont have control ie. it can vary greatly.
Whereas an electric fan is run by an electric motor which convert electricity to motion. So here the speed can be controlled.


Regards,
if you still have a doubt please notify through your question's additional details.

Answer 2

You seem to be a young poster, please excuse me if I am wrong.

Absolutely the fan blade length makes a difference. It has to do with the mass of air, and the fact that it is constant (for the sake of simplicity for you PhD posters) - pretty much the same density.

The bigger a fan blade (lets call it a "wing", as this is what it really is), the thinner the air is compared to a smaller one, all other things being equal.

The equivalent wind speed for a windmill and a small desk fan are hard to understand. If the small desk fan was rotating at 100 revolutions per minute (RPM), this would be very slow. If the large windmill was doing the same rpm, it would probably explode in a matter of seconds - although mostly to do with other things than the wind itself. If the windmill were doing 100 rpm, by wind alone, the wind would probably be more than the worst hurricane/cyclone ever experienced on earth.

Essentially, the windmill is geared down to do a job - mostly to perform tasks that have a rotating movement, like grinding (milling) grain or other things like crushing ore, running factory equipment etc. This needs to happen slowly due to the type of mechanical structures employed - most gears were made from wood in the "old days".

Long and thin wings (like windmill blades / sails) are more efficient at extracting power from the wind than short broad wings - at low speeds. Look at the difference between the wing on a modern airplane glider and a modern fighter jet. Google "wing induced drag". The glider uses the earths gravity only as its power source, where the modern fighter might have 100,000 HP (equivalent).

There is another good example of this idea: helicopter blades and an airplane propeller. They do exactly the same thing - produce thrust to do work. The helo blade is thrust upwards to counter gravity, the airplane to produce forward movement (and some gravity depending on the angle it is flying at - never mind).

The number one thing in designing each of the blades is the speed of sound. The tip of the blade must never (ideally) travel at more than the speed of sound - 340m/s or 1200 fps. The long blade has a longer path to follow, so it goes faster at the tip, than does the shorter airplane prop. Typical speeds for helo blades might be 250 rpm, and airplane props (cessna etc) about 2500rpm.

Now think about how long the blades are in the huge wind power generators we see on TV - or you may have seen them around the place. Some blades can be 50 meters - 160 feet long or more. What rpm could they do before having issues with the speed of sound do you think? They are very efficient.

This is a fascinating subject. Start with Boyles Law and Reynolds Numbers.

Hope this helps.

Answer 3

Hi Jerry -

Yes there is. It is really a question of stress in the rotating fan. The tensile stress at the fan blade root is generally proportional to AN^2, where A is the annular area swept out by the blade in a single rotation, and N is the rotational speed in units like rpm or frequency. The faster you turn, the higher the blade root stress. The longer the blade, the greater the swept area and the larger the blade root stress. Of course, the aerodynamic shape of the airfoil also has an effect, but this is assuming similar blade shapes to avoid confusion. So big blades have to turn slowly to avoid failure.

Another issue that is critical to turbines (not so much windmills) is the stress in the bore or hub that holds the blades together. These stresses are also proportional to the square of the speed and the square of the radius (both inner and outer radius). Since the hub of a fan is a lot different - and actually a lot more structural - than that of a windmill, I suspect that this slows the windmill down even further.

ADDED: two clarifications -

1. The loads and stresses due to aerodynamic and torque forces are insignificant compared to those due to rotation.

2. Fan blade tips exceed the speed of sound on a regular basis in aircraft engines. That's why you hear the fans begin to "moan" when they approach max throttle.

Answer 4

The speed of fans does not necessarily depends on the length of its wings because some have more power or force behind them while others have less. For instance, wind mills have large fans, but also don't have that much force behind them because it depends solely on the winds. Those fans are also big because they are meant to create power and the bigger wings cause more power to be generated because it takes more work for the wind to make them spin. But, yes, it is true that the area of the wings cause it to take more force to make it move at the speed of a smaller wing. The electric fans and windmills aren't really connected because one is used to make energy and the other uses energy to make the room cooler.

Answer 5

Wow, what a lot of complicated answers.
When you drive a fan, it is normally turned very fast with small blades to move air as far as possible while running the motor at an efficient speed. But it is noisy and focused. A ceiling fan uses a special motor and much slower speed with much larger blades (up to 20 feet long www.bigassfans.com) to move a lot of air quietly at lower velocity.
Old style windmills have big blades that move slowly because the air moves slowly and they want to capture all the force and apply it as torque for moving grindstones etc. It it moved too fast, it would out pace the wind. Farm windmills have as many narrow blades, much shorter, set at a different angle to move as fast as possible and are actually geared down to do work of pumping water.
Power generating windmills have complicated blades that are wide at the hub to get the most out of the slow moving root, while they get thin and twisted more at the end because the long blade moves many more feet per second thru the air at the same rpm.
I build fun windmills - whirlygigs as they are called - and using a fan blade doesn't work well as the angle and size is wrong for wind speeds.
http://users.ticnet.com/mikefirth/whirljig.htm

Answer 6

u r very well right

do u know the law of conservation angular momentum.
it says that angular momentum always remains constant.

angular momentum - L
moment of inertia - I (nothing but mass in rotational motion)
angular velocity - w


L= I x w

When I i.e. moment of inertia increases i.e. size of the fan blade increases w tha angular velocity decreases. since the angular momentum is always constant or conserved

it also happens vice-versa

this is the reason why big fan rotate slow.

Answer 7

hi!
yes, centrifuge force depends on area..
shorter the area more the force........
and vice versa
short finned fan will spin speed......
keep smiling:-)