Theory of flight debunked

[boomerangben]

What's this got to do with sailing?

Uffa Fox said, in 1960, that rather than work as an aerofoil:

"... sails are deflectors: [they] turn the wind backward towards the stern and [the] reaction pushes the vessel forward."

He also says, in the same book, that in heavy weather it's best to take seas beam-on.

Do people still believe in Uffa?

I think this "new" explanation is much the same as what Uffa said. The shape of the upper surface of a wing causes airflow to be deflected down and when the AoA is greater than zero, so does the lower surface. The interesting aspect is the explanation as to why laminar flows adhere to a convex surface. I suspect that is down to pressure gradients, surface tension and internal friction.

 

[Little Rascal]

The example shown in the video is wrong. A wing shape has a flat bottom surface, and a curved upper surface. It also faces straight into the airflow, not at an angle shown in the video.

No it ain't.

There are plenty of aircraft with symetrical airfoils. A symetrical airfoil (like on your rudder) needs an angle of attack to make lift.

A wing with 'camber' (assymetrical) will make lift at zero geometric angle of attack - but only a small amount, roughly a third of it's potential. It also needs some angle of attack to make more lift.

The 'proof' in the video isn't new... it's just a good explanation of what we've known for years.

Much as I love Uffa Fox's designs I have to take issue with that explanation. The sail (with attached flow) provides a lift force roughly perpendicular to the boom. Which is exactly where you'd go if you didn't have another 'wing' making lift to windward under the boat. The resultant vector of these two forces is what propels the boat in a forward direction.

 

[jimbaerselman]

Sails, propellers, wings, keels are all devices for changing the momentum of a fluid. They do so with varying efficiencies depending on to their shapes. The change in momentum creates a force. That's all one needs to know, really. Simples, eh?

Until you see the windmill in "Warhorse". This extraordinary machine turns steadily when there's no wind (cigarette smoke rises vertically, character comments on lack of wind). It's sails are in rags, so there's nothing to catch what wind exists. Yet the sails revolve.

Now that really is an aerodynamic masterpiece. Certainly de-bunks the theory of flight. Beats the bumblebee. Proves the shape of the aerofoil is irrelevant. Or perhaps proves that Spielberg's art trumps science - and common sense. Fancy using all that spontaneous energy to grind nothing.

 

[Danny Jo]

Much as I love Uffa Fox's designs I have to take issue with that explanation. The sail (with attached flow) provides a lift force roughly perpendicular to the boom. Which is exactly where you'd go if you didn't have another 'wing' making lift to windward under the boat. The resultant vector of these two forces is what propels the boat in a forward direction.

Concise, clear and accurate - qualities that I wish I could master.

Thread drift, of course*, but a lot of people forget that the lift to windward created by the keel depends on having enough water flowing past it. You can stall a keel, just as you can stall an aircraft, by going too slowly. So don't pinch up tight immediately after a tack - concentrate on getting the speed up.

*I gather that the Scuttlebutt convention is that acknowledgement of thread drift is the closest one is allowed to come to an apology.

 

[oldbilbo]

http://www.telegraph.co.uk/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html

So how might this affect our understanding of sail trim?

I've told you before, 'Pugwash'. Flying IS an act of faith. Don't do it if you don't believe....

Same with sailboats.

 

[Little Rascal]

Cheers Danny Jo! See what happens when you stand around on atmospheric mountains for too long...

 

[grumpy_o_g]

I was taught that the effect was due to Bernoulli's Theorem back in the mid-70's - I don't where this "the air has to meet up again after it's been round aerofoil" idea comes from but it's patently rubbish. If you want proof that the lower surface has nothing to do with it then take a tablespoon and hold it by the tip of the handle as loosely as you can (without dropping it) between your thumb and forefinger. Then dangle it over the sink so that the convex surface of the bottom of the spoon has a stream of water from the tap running over it. You've now effectively got an aerofoil (OK hydrofoil) with a flow only over the upper surface. The spoon will be deflected away from you and into the stream of water. If you do it carefully you can do this without getting any water on the inside of the spoon.

 

[marklucas]

I was taught that the effect was due to Bernoulli's Theorem back in the mid-70's - I don't where this "the air has to meet up again after it's been round aerofoil" idea comes from but it's patently rubbish. If you want proof that the lower surface has nothing to do with it then take a tablespoon and hold it by the tip of the handle as loosely as you can (without dropping it) between your thumb and forefinger. Then dangle it over the sink so that the convex surface of the bottom of the spoon has a stream of water from the tap running over it. You've now effectively got an aerofoil (OK hydrofoil) with a flow only over the upper surface. The spoon will be deflected away from you and into the stream of water. If you do it carefully you can do this without getting any water on the inside of the spoon.

When, many years ago I was instructing and asked about aerofoils, I simply picked up any old piece of paper (nice virgin piece of A4 is best, but you don't find those lying around on a yacht) and simply hold with the shorter edge in front of your mouth - blow - the natural sag will lift into a reasonable aerofoil shape. A drier version of the demonstration suggested above.

WRT keels, yes they are often forgotten, but they are indeed the reason that yachts have to have some leeway in order to create opposing lift to that produced by the sails - keels don't have to be as large as water is 1,000 times denser than air - and are of course symmetric.

 

[Little Rascal]

You have to be a bit careful with some of these demonstrations.

The one where you blow over the top surface of the paper demonstrates the Coanda effect not Bernoulli's theorem. The Coanda effect is when a jet of air is bent round a convex surface, and the lift force is being provided by a kind of vectored thrust. When you blow you are providing a jet of air not an established flow (like the wind). The jet has it's own special properties.

The teaspoon trick is more about surface tension, not Coanda or Bernoulli.

If you want a demonstration of lift vs angle of attack then stick your hand out of the car window...

WRT keels, yes they are often forgotten, but they are indeed the reason that yachts have to have some leeway in order to create opposing lift to that produced by the sails - keels don't have to be as large as water is 1,000 times denser than air - and are of course symmetric.

Exactly. The degree of leeway roughly equates to the angle of attack the keel is seeing. If the keel foil section was cambered the leeway angle would be reduced (but not on the other tack!). Some racing yachts have twin assymetric dagger boards for this reason.

 

[oldbilbo]

Exactly. The degree of leeway roughly equates to the angle of attack the keel is seeing. If the keel foil section was cambered the leeway angle would be reduced (but not on the other tack!). Some racing yachts have twin assymetric dagger boards for this reason.

I looked carefully at a couple of AC45 high-tech catamarans at Plymouth. Their cambered foils are canted athwartships, such that the immersed, leeward foil provides a modest component of vertical lift at speed, in addition to horizontal 'lift to windward'. The other foil, on the airborne hull, is winched up mostly clear of the water, reducing drag, when the boat is sailed with one hull just flying at around 8-10°.

I could see no obvious 'toe-in', or preset AoA, and the NACA-section camber is moderate.

I'm wondering about the purported benefits of curved foils, which are now appearing on some big catamaran racers. There is one currently in build at Millbrook, by Plymouth. First thoughts are that these might give some of the benefits of lifting foils - reducing the immersed volume, thus reducing form drag, thus increasing speed - without the complexity of fully foil-borne operation....

 

[maxi77]

Who cares what the theory is, sails work, that is all we need to know.

 

[grumpy_o_g]

You have to be a bit careful with some of these demonstrations.

The one where you blow over the top surface of the paper demonstrates the Coanda effect not Bernoulli's theorem. The Coanda effect is when a jet of air is bent round a convex surface, and the lift force is being provided by a kind of vectored thrust. When you blow you are providing a jet of air not an established flow (like the wind). The jet has it's own special properties.

The teaspoon trick is more about surface tension, not Coanda or Bernoulli.

If you want a demonstration of lift vs angle of attack then stick your hand out of the car window...



Exactly. The degree of leeway roughly equates to the angle of attack the keel is seeing. If the keel foil section was cambered the leeway angle would be reduced (but not on the other tack!). Some racing yachts have twin assymetric dagger boards for this reason.

The hand out of the car window really proves nothing as a significant proportion of the force is actually due to the airflow striking the under-side of the angled hand. There is a component of surface tension in the spoon experiment but it's not large enough to explain the deflection. It's done more properly by flowing a fluid around a curve and measuring the load on the inside wall of curve. It's still not perfect but fits predictions pretty well. The Coanda effect and Venturi effect both rely on the Bernoulli Effect which, in a nutshell, says that, whenever the flow of fluid speeds up to pass through a restriction but the mass being moved is the same, then the pressure has to drop or you'd be creating the energy to accelerate the mass from nowhere.

In the case of an aerofoil section where the flow is accelerated around the longer surface what is actually happening is that slower clear air above the wing provides the "opposite" side of the venturi. It can only do this if the fluid possesses mass and frictional resistance to itself (effectively viscosity) which is what was correctly posited many posts ago.

 

[Little Rascal]

The hand out of the car window really proves nothing as a significant proportion of the force is actually due to the airflow striking the under-side of the angled hand.

We could argue 'Newton vs Bernoulli' (deflection vs pressure difference) all day but the real explanation is rather more complex and involves both anyway.

In the case of an aerofoil section where the flow is accelerated around the longer surface what is actually happening is that slower clear air above the wing provides the "opposite" side of the venturi. It can only do this if the fluid possesses mass and frictional resistance to itself (effectively viscosity) which is what was correctly posited many posts ago.

I'm afraid NASA doesn't agree.

You're absolutely right about viscosity though - without it flight/lift would be impossible.


I'm starting to agree with Maxi77!

 

[grumpy_o_g]

We could argue 'Newton vs Bernoulli' (deflection vs pressure difference) all day but the real explanation is rather more complex and involves both anyway.



I'm afraid NASA doesn't agree.

You're absolutely right about viscosity though - without it flight/lift would be impossible.


I'm starting to agree with Maxi77!

NASA is welcome to dis-agree with me. Actually NASA both agree and dis-agree as they can't even agree amongst themselves (or at least they couldn't 10 years ago when I was last involved).

 

[Bosun Higgs]

I think you've missed the point - its rather the kg is described as the SI unit of WEIGHT when of course it is the SI unit of MASS

No. The KG is a unit of weight. Ask your local weights and measures or look in the supermarket.

 

[SamSalter]

Kg is the unit of mass / Newton is the unit of weight.
This thread is starting to disappear up it's own posterior!!!

 

[grumpy_o_g]

Kg is the unit of mass / Newton is the unit of weight.
This thread is starting to disappear up it's own posterior!!!

Just to shove it a bit further - Kg is the unit of mass and Newton is the unit of force. Weight is the result of gravity acting on a mass. 1 Kg of mass only weighs 1 Kg in 1g (earth gravity) - on the moon 1Kg of mass "weighs" about 170g..

 

[SHUG]

Does this mean that a slight amount of back-winding in the main will deflect the airflow more and give greater drive. From my own observation,the top racers seem to think so!