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Some simple Experiments
Blowing over a sheet of paper.
Some people blow over a sheet of paper to demonstrate that the accelerated air over the sheet results in a lower pressure.
They are wrong with their explanation.
The sheet of paper goes up because it deflects the air, by the Coanda effect, and that deflection is the cause of the force lifting the sheet.
To prove they are wrong I use the following experiment:
If the sheet of paper is pre bend the other way by first rolling it, and if you blow over it than, it goes down.
This is because the air is deflected the other way.
Airspeed is still higher above the sheet, so that is not causing the lower pressure.
The next (theoretical) experiment can be used to prove the "path over the wing is longer, so the air has to move faster" theory wrong.
This wing then should have an excellent lift, but it does not.
A wing delivers a liftforce, even when it is not at an angle.
I have heard the argument that the theory I use does not explain why a wing does give lift when it is levelled.
I feel that that "levelled" is not really levelled.
This wing below is an example of what that people mean.
Why I feel this wing is not levelled. Let me use a profile like below:
And rotate that a little:
And then I change the leading edge just a littlebit:
This is the same picture as the first picture!
Blowing out a candle behind a bottle.
This is an experiment to show the Coanda effect.
Take a normal bottle and place a lighted candle just behind it (please fill with water to prevent fire in case it is a plastic bottle)
If you blow on the other side of the bottle the candle will go out because the flow will follow the curved surface of the bottle.
When you roughen the bottle the air close to the bottle will be slowed down and a thick boundary layer will be present.
This thick boundary layer will make the flow separate, and the candle will be much harder to blow out.
To roughen the surface I used a heavy wrinkled rough paper (toilet paper or some other wrinkled paper tissue will do the job.
Please have some water near in case the paper catches fire.
This is one of the reasons why pilots are afraid of ice on their wings.
The rough ice makes the wing stall much easier, and the weight of the ice makes it necessary to have some more lift.
Why a balloon stays in a flow of air.
Or why a table tennis ball stays above an airflow of a hairdryer.
Ball in air copied from http://www.nal.go.jp/eng/newsletter/98autumn/m106.htm)
The often heard explanation is that the air moves faster and this creates a lower pressure sucking the balloon to the middle of the flow.
I disagree with this explanation.
I have the following explanation:
When the balloon is half in the main flow, the fast moving air close to the main flow tends to follow the surface further, and is thereby deflected, creating a force.
If one does this experiment with a piece of paper wrinkled to a ball it will not stay in the main flow because the surface is very rough, and will create a thick boundary layer, what results in the flow not following the surface.
Blowing a light ball out of a funnel
Take a table tennis ball and try to blow it out of a funnel.
It is very hard.
(You can also use a piece of paper folded into a funnel shape.)
How to explain this.
The air tends to follow the surface of the funnel, and is thereby deflected to the side.
The ball will try to deflect this air back again, causing the ball back into the funnel.
A ball of wrinkled paper is easier to blow out of the funnel.
A drop of water is not "drop shaped"
A drop of water is not in the shape of a drop as most people think.
It will flatten out.
A similar effect can be seen at bubbles in air.
There it is much easier to see because the speed is much lower.
And I would like to mention that a boundary layer is very thin.
For large object like ships this is not true.
At the stern of a 50m ship the boundary layer is approx. 200mm thick.
For a small yacht the boundary layer can be seen in "dirty water",
so water with a lot of small object in it making the flow visible.
The boundary layer of your sail is much thinner, because it is not that long and air is not so "sticky" as water is.
With some cigarette smoke to make the airflow visible you will see that at the leech of your sail the boundary layer is already a few mm.
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