Factors:
The factors, which would affect the way in which a wind vane works would firstly be which type of materials are used. This is important as different material take in different amounts of wind eg. Plastic vs steel. It would also be the same with card pieces of different lengths. The other factors would be the friction between the knitting needle and glass-rod as this may slow down the speed of the wind vane. If there are more cards added to the cork the air resistance will increase. The force of wind hitting the fan is also important, as you need large amounts to make the fan turn.
Diagram:
Apparatus:
- Pieces of card of different lengths
- Clamp
- Stand
- Boss Head
- String
- Wind Vane (cork, glass-rod)
- Knitting Needle
- Weights (50g)
- Stopwatch
Method:
- Collect all the equipment listed above from the classroom before you begin the experiment.
- Put the stand on a desk and attach the boss head and then the clamp.
- Place a knitting needle into the clamp and tighten it.
- Slide the wind vane consisting of a cork and a glass-rod onto the knitting needle.
- Wrap the string around the wind vane carefully so it can move easily.
- Attach to the end of the string a weight of fifty (50g) grams.
- Cut out cards of different lengths accurately.
- Slot in the shortest piece of card in to the cork.
- Take a stopwatch and time how long it takes for the weight to drop to the desk.
10.Repeat this procedure for the pieces of card with different
lengths.
11.Record the timings into a results table. From this produce a
line graph.
Variables:
Results:
Graph:
Graph showing the time it takes for a weight of 50 grams to travel 50cm by using, one at a time, 7 fans with different lengths in a wind vane
Aerodynamics of Wind Turbines: Stall and Drag
Stall:
Now, what happens if an aircraft tilts backward in an attempt to climb higher into the sky quickly? The lift of the wing will indeed increase, as the wing is tilted backwards, but in the picture you can see that all of a sudden the airflow on the upper surface stops sticking to the surface of the wing. Instead the air whirls around in an irregular vortex (a condition which is also known as turbulence). All of a sudden the lift from the low pressure on the upper surface of the wing disappears. This phenomenon is known as stall.
An aircraft wing will stall, if the shape of the wing tapers off too quickly as the air moves along its general direction of motion. (The wing itself, of course, does not change its shape, but the angle of the wing in relation to the general direction of the airflow (also known as the angle of attack) has been increased in our picture above). Notice that the turbulence is created on the backside of the wing in relation to the air current. Stall can be provoked if the surface of the aircraft wing - or the wind turbine rotor blade - is not completely even and smooth. A dent in the wing or rotor blade, or a piece of self adhesive tape can be enough to start the turbulence on the backside, even if the angle of attack is fairly small. Aircraft designers obviously try to avoid stall at all costs, since an aeroplane without the lift from its wings will fall like a rock.
On the page on power control we shall return to the subject of how wind turbine engineers deliberately make use of the stall phenomenon when designing rotor blades.
Drag:
Aircraft designers and rotor blade designers are not just concerned with lift and stall, however. They are also concerned with air resistance, in technical jargon of aerodynamics known as drag. Drag will normally increase if the area facing the direction of motion increases.
Conclusion:
I have found out through experimenting that our prediction was mostly right as more air resistance is created through increasing the surface area of the card. Also if there is more drag it is harder to rotate the fan, so the fan will turn slower which is better than it rotating fast as it will be more efficient.
As we gradually changed the length of the card to longer size, we noticed that the timings also increased making rotation slower. We also noticed that the weights further helped slower the rotation of the fan.
Evaluation:
I think that we have answered the question that we set out to find out at the start of the investigation, as we investigated factors, which affect the way in which wind vanes can be used to control the rate at which, a weight falls.
Yes, our experiment was fair as we only had two variables the length of the string and the weights. Everything else in the fair test was carried out consistently.
The improvements that I would make if I were to redo the experiment would be to do the experiment for a third time and to use a data logger if possible to get the most accurate reading. I would keep everything else the same.
The results were as accurate as they could get. To make it more clear we decided to get an average. The results came out as expected are evident enough to make a firm evaluation that if one fan creates air resistance, then more fans would create more drag enabling wind vanes to work effectively.
