Good point. To boil it down even more, all the forces on the sails, keel, hull, etc., add up to a net forward motion at the base of the mast and the cleat point of the sheets that ”pulls” the boat forward, regardless of point of sail.
r we being PULLED or PUSHED??
- Thread starter gudiss
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The lift on an airfoil is primarily the result of its angle of attack. When oriented at a suitable angle, the airfoil deflects the oncoming air (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection. This force is known as aerodynamic force and can be resolved into two components: lift and drag. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack.
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i design structural systems for buildings, which are essentially sails cantilevered from the ground. in addition to gravity loads on the building i also check and run a wind loading analysis. the wind on the buildings produce two forces that must be addressed. first are positive forces on the structure, wind slamming into the face of the building. then i look at the negative forces resulting from wind passing over or by a building wall and hence pulling on my building. the negative or pulling forces are a mere fraction of the positive forces. this is no different on an "airfoil", 95% of lift is generated from the air passing over the bottom of a foil/ wing or inside face of a sail.
A pilot with a stalled airfoil would probably not agree that 95% of his lift is still being generated (as he falls from the sky). Airfoils, by their design, are unlike blocks. They use the shape of their upper / leeward side to generate significant lift. When the airflow over that side separates the reduction of lift is dramatic.
Here’s an interesting picture from the Wikipedia page on airfoils:
Notice that even when the angle of attack is 0, meaning no downward deflection from the lower side of the wing, lift is being produced.
No. A fluid cannot pull. As the thrust of of the engines accelerates the gasses into the tunnel below the rocket the air pressure surrounding the stream of moving gas drops. This is conservation of energy. When that drop occurs the air surrounding the base of the rocket rushes in to fill the void. That is what you are seeing.
Looks like a good deal of breakthrough fluid dynamicsis being discovered in this thread.....maybe Phil should think about publishing the notes......
Yup.. my old Aero Prof said that air doesn't have tensile strength (can't pull things) but it has compressive strength. An airfoil enhances the low pressure area compared to a different shape.. so the air is trying to fill the low pressure area, but it has to push on the foil to get there.
My dos centavos; now, back to that martini!
My dos centavos; now, back to that martini!
oh yes it can.
Look at this...
https://en.wikipedia.org/wiki/Thixotropy
old ketchup bottles did that.
Jim...
So funny. Post 14 had it. Man has built many theories to help predict behavior, but if we are to be pedantic...
... Air is a collection of particles, and they can only interact with the sails by collision. They either hit the sail harder or they hit the sail less. In a perfect vacuum they don't hit anything (because they are not there), but still do not actually pull. They just don't push. Pressure can be reduced a number of ways. In the case of velocity, it is because temperature ~ average molecule kinetic energy, and if they are moving in one dirrection, the average velocity normal to that dirrection is obviously lower (or the average temperature would increase).
I've always felt the most obvious way to look at it is a problem in conservation of momentum.
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For the followers that talk about the air "going around the leeward side faster because it has farther to go and must reach the leach at the same time" this has been disproven by wind tunnel testing using pulsed smoke. Google it. The air on the leeward side is always late. Clearly, there was never any natural law supporting this assumption in the first place (what would it be?), rather like a lot of things politicians say; it must be true because it gives the result we want.
... Air is a collection of particles, and they can only interact with the sails by collision. They either hit the sail harder or they hit the sail less. In a perfect vacuum they don't hit anything (because they are not there), but still do not actually pull. They just don't push. Pressure can be reduced a number of ways. In the case of velocity, it is because temperature ~ average molecule kinetic energy, and if they are moving in one dirrection, the average velocity normal to that dirrection is obviously lower (or the average temperature would increase).
I've always felt the most obvious way to look at it is a problem in conservation of momentum.
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For the followers that talk about the air "going around the leeward side faster because it has farther to go and must reach the leach at the same time" this has been disproven by wind tunnel testing using pulsed smoke. Google it. The air on the leeward side is always late. Clearly, there was never any natural law supporting this assumption in the first place (what would it be?), rather like a lot of things politicians say; it must be true because it gives the result we want.
The original question is whether the sails are pulling or pushing the boat. The sail is always pulling the boat by its direct connection to the boat through the mast and indirect connection to the boom for the main sail. Likewise via the mast, fore stay and jib sheet with the fore sail.
The wind creates the force that drags or pushes the sails
The wind creates the force that drags or pushes the sails
Which suggests a discussion of fishtail rudders.
Foils deflect fluid. If the flow is attached on both sides, they both work. If the flow detaches, only the windward side works (that blue line will only drop to about 50% of the attached flow values).
If you add flaps to a wing or rudder, a foil will deflect more fluid, particularly when stalling. If the back side is stalled, the fish tail "flap" does not actually harm the lift, because it is in the separation bubble (eddy).
Thus, fish tail rudders are often speced for ships that need very high angle of attack for maneuvering. Lots of research can be found. Most trawlers would benefit. Some sailboats would benefit under power, depending on the keel-rudder configuration.
[The flow acceleration thing was written by the marketing department and is complete nonsense, but several German associations and universities did the basic research, and the high angle lift plots make sence.]
No offense: it's a meaningless distinction. If the force was being applied at the bow, we'd call it pulling. If it were applied at the stern, we'd call it pushing. With a sailboat, it's being applied amidships, so... it's a meaningless distinction.
Just to keep this going....If the sailboat is a catboat, with the mast at the bow, the force is pulling. If the mast is mounted on the stern (I don't know what they are called), then it is pushing. In the words of the great Captain Ron, "Nobody knows!'.