# A sail is not a wing

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Feb 7, 2021.

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### Will GilmoreSenior Member

What does that tell us about how to state the relationships of motion, force, bodies, and fluid dynamics with regard to sailboats and aeroplanes that we haven't already been saying?

'Lift' is recognized as the force generated perpendicular to free air motion, as correctly stated by DCockey. Sailor Al objects to the analogy of an airplane wing to a sailboat's sail. In agreement to Sailor Al, I stated that I can see a fundamental difference between how an airplane works and how a sailboat works based upon the direct use of that 'lift' to drive the vessel forward as opposed to using some other force to drive the vessel forward.

I can't say that anything you've stated is wrong. Indeed, it is, I think, quite accurate, but how does that relate? We are ultimately talking about a change in motion, not motion itself.

A plane, without something to drive it against the drag of air, will ultimately be no different than a balloon and there will be no 'lift' force to hold the plane against the constant force of gravity. Air flow is necessary to hold a plane aloft and some outside force is necessary to create air flow, but, in the case of an airplane, airflow does not contribute to more airflow because it does not contribute to forward motion.

That is quite different for a sailboat. Airflow is necessary for forward motion and that airflow is obtained by the use of the 'lift' force induced upon the sail from the change in motion between a liquid and gas interface. If you use the liquid as the frame of reference, you get motion from the air movement across the sail which then changes both the magnitude and direction of the vector force of that air to the vessel. This is what I see as a basic difference between how these two ideas (wing/sail) work.

If you seek to correct this view, I need more than that.

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### gonzoSenior Member

I think that the misunderstanding comes from using different frames of reference. For the airplane, you are using the wing or foil as the x,y.z=0,0,0. However, on the boat you are using the air as x,y.z=0,0,0. If you were to use the air as reference on both, the lateral resistance of the keel would be the equivalent to the force of gravity.

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### GlueandcoffeeJunior Member

But to remove the confusion that the engine in a powered plane take the case of a glider. let it be a sail plane, hang glider or paraglider. Where does the thrust come from. It comes from the a forward component of lift. The fact that it can use both forward and upward components of lift just makes it more efficient than a powered plane or sailboat.

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### gonzoSenior Member

There is no thrust on a glider plane. The potential energy gets converted into kinetic energy as the planes descends.

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### GlueandcoffeeJunior Member

And in the case when it's in an up draft it will maintain altitude while the energy from the external source brings the glider analogy closer to a sailboat.

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### TANSLSenior Member

If the plane were to be dropped freely, all right. But that's not the case (thankfully)

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### Sailor AlSenior Member

Here's a picture of my proposition:

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### gonzoSenior Member

1. Sails do not have leeway or heel.
2. Both sails and wings have an apparent wind angle or angle of attack (synonyms)
3. Wings have lift and drag, but so do sails
Everyone here understands your proposition. It is wrong.

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### gonzoSenior Member

Sure, the energy of the updraft gets converted into potential energy. In a boat it is the forward momentum.

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### GlueandcoffeeJunior Member

That diagram assumes the angle of attack on the running sail is 180°. Its not . It's at 90 degrees.. 180degrees to the boat centerline not the sail. An aerobic plane can fall level and have an angle of attack 90degrees to the wing. The wing is stalled but so is a running sail. A boat can reach with sails having 45degree angle of attack. But a plane can also have an angle of attack greater than 45degrees in high G maneuvers.. cut out the left hand quadrant of the diagram and extend the yellow portion to 90 degrees. Let's assume that planes don't fly backwards and boats dont sail dead downwind fully sheeted in.

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### GlueandcoffeeJunior Member

The big difference is boats dont gather potential energy. They gather kinetic energy in the leeway direction. Just because the boat isn't going directly forward does not mean it is not going somewhere.

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### GlueandcoffeeJunior Member

A glider is given energy from an external source being air moving against the opposing force gravity. A sailboat is given energy from an external source being air moving from high atmospheric pressure to low atmospheric pressure. The plane now has the potential to fly back down to the ground. The boat now has the potential to sail back to the low pressure. A glider spiraling upwards is just a 3d version of a boat beating in 2d to windward.

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### gonzoSenior Member

No, a glider is losing potential energy as it descends. The air does not move, but the glider is moving through a fluid which creates lift. Lift is a force vertical and upwards. However, if the glider has no forward velocity, there will be no lift; it will fall straight down.
No, the boat is transforming the energy in the moving air into momentum. However, boats operate in the interface of two fluids of very different density and viscosity. The comparison breaks down as you describe it. Airplanes have gravity acting on them and the only force keeping them from falling straight down is lift. Boats have the same force of gravity. However, their displacement keeps them from falling down (called sinking).
Potential energy is different from momentum. It can be stored in a stationary body. Momentum is a function of velocity.

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### KeithOSenior Member

The OP's diagram is a joke. Comparing a sail to a wing is like comparing a hang glider single skin wing to an "ordinary" wing which has an upper and lower surface fully developed and constrained. You can do the same things with both devices but the performance of a sail, like the single skin wing, is compromised. People choose sails because they offer operational benefits like being easy to raise, lower, reef, furl which are not easily available on the wing shaped sails. The reference I posted seems to offer reefing, but requires a self supporting and potentially rotating mast. No videos are shown regarding raising and lowering of the wing sail. So I have to believe that they are probably not ready for ocean crossings, but fine for a race around the buoys. Regardless of the type of sailplan, aerodynamic principles will determine how much thrust (lift) is developed and that is not going to change since it used the laws of nature that annoyed Australians cant repeal...

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### KeithOSenior Member

Look at the apparent wind angles at different points of sail. Only in running downwind is the sail primarily a drag device. Of course one cannot point directly into the wind either, all you will do is go backwards...

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