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#132
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push vs pull vis a vis rudders
On Thu, 1 Apr 2004 11:45:50 +0100, "JimB"
wrote: Steven Shelikoff wrote in message ... On Wed, 31 Mar 2004 10:20:59 +0100, "JimB" So I hung the spatula just behind the fan. Lo and behold, the same thing happens but just a little less. When I rotate the spatula to the left, there is a noticable *left* motion to the blade... i.e., it's not only drawn forward into the blade but it also moved to the left from where it was when the spatula blade was perpendicular to the fan. When I turn it to the right, the spatula swings to the right. Steve, that was the experiment I first did. Then I realised that, to yaw the boat, I had to look solely at lateral force. To do this I had to constrain the card so that it could only hinge laterally (no fore and aft motion permitted). This is where the bits of wire came in. The card had a bit of wire attached rigidy to the top, sticking at 45 deg horizontal angle to the card. The card end of the wire bent down to stop the card swinging around the wrong end of the wire. I hung the card (your spatula I guess!) through two loops (hinges) first mounted parallel to the centre line of the fan, then at right angles. This gave a different result, very little lateral swing, lots of fore and aft swing. Of course (a weakness in the experiment) it Your experiment seems to be flawed if you're trying to look solely at lateral force with no fore and aft motion permitted and yet you get a lot of for and aft swing. To prove to myself again that there is a lateral force even with no fore and aft movement, I put a string around the bottom end of the spatula which would allow it to swing laterally but hold it from being moved toward the fan. So, we have a plastic spatula hung by the little hanging hole at the top from a hook which allows it to swing in all directions like a pendulum but I can firmly control the angle of the blade by turning the hook. And there is a string looped around the handle just above the blade which I can hold to prevent the blade from moving towards the fan so there's no fore and aft motion. Result: same thing. When it's behind the fan and you turn the blade so that it's not perpendicular to the fan, the spatula swings *only* laterally since there's a string keeping it from moving toward the fan. My initial conclusion has only been reinforced. Steve |
#133
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push vs pull vis a vis rudders
On Thu, 1 Apr 2004 11:45:50 +0100, "JimB"
wrote: Steven Shelikoff wrote in message ... On Wed, 31 Mar 2004 10:20:59 +0100, "JimB" So I hung the spatula just behind the fan. Lo and behold, the same thing happens but just a little less. When I rotate the spatula to the left, there is a noticable *left* motion to the blade... i.e., it's not only drawn forward into the blade but it also moved to the left from where it was when the spatula blade was perpendicular to the fan. When I turn it to the right, the spatula swings to the right. Steve, that was the experiment I first did. Then I realised that, to yaw the boat, I had to look solely at lateral force. To do this I had to constrain the card so that it could only hinge laterally (no fore and aft motion permitted). This is where the bits of wire came in. The card had a bit of wire attached rigidy to the top, sticking at 45 deg horizontal angle to the card. The card end of the wire bent down to stop the card swinging around the wrong end of the wire. I hung the card (your spatula I guess!) through two loops (hinges) first mounted parallel to the centre line of the fan, then at right angles. This gave a different result, very little lateral swing, lots of fore and aft swing. Of course (a weakness in the experiment) it Your experiment seems to be flawed if you're trying to look solely at lateral force with no fore and aft motion permitted and yet you get a lot of for and aft swing. To prove to myself again that there is a lateral force even with no fore and aft movement, I put a string around the bottom end of the spatula which would allow it to swing laterally but hold it from being moved toward the fan. So, we have a plastic spatula hung by the little hanging hole at the top from a hook which allows it to swing in all directions like a pendulum but I can firmly control the angle of the blade by turning the hook. And there is a string looped around the handle just above the blade which I can hold to prevent the blade from moving towards the fan so there's no fore and aft motion. Result: same thing. When it's behind the fan and you turn the blade so that it's not perpendicular to the fan, the spatula swings *only* laterally since there's a string keeping it from moving toward the fan. My initial conclusion has only been reinforced. Steve |
#134
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push vs pull vis a vis rudders
schlackoff wrote:
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#136
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push vs pull vis a vis rudders
On Thu, 1 Apr 2004 12:24:46 +0100, "JimB"
wrote: Limitations of the experiment: It didn't check for associated force changes at the fan The scale of 'rudder' against fan size is way out The wire had a little flexibility Fag ends produced smoke which rose too fast Reynolds numbers were wrong. And, just in case you mis-understood, my hinges were pendulum hinges which did not allow the 'rudder' to rotate around its vertical axis (except in the 'rudder kick' experiment). They only allowed pendulum movement laterally, or when re-oriented, fore and aft (subject to wire flexibility). ..... JimB An experimental rig for visualizing fluid flow over rudders etc., is easy to make and provably representative of 2-D flow. It consists of an inclined board with side rails to stop the water film dripping off. A reservoir at the top, into which water from a hose pipe flows, and a sump at the other end to lead the waste water to a drain. At the top of the incline, permanganate crystals trail stream lines down the incline. The model (a rudder cross section, for instance) is placed in the stream. The stream lines tilt sidewards ahead of the rudder, when it is inclined at a modest angle to the flow, and tilt sidewards the other way after the model trailing edge. This is an easy way to show the "molecules give lift by hitting the proximal surface" enthusiasts how fluid dynamics really works. (about two thirds of the side force from the distal surface, and one third from the proximal surface.) You can work it out from the streamline spacing over both surfaces. Brian Whatcott Altus OK |
#137
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push vs pull vis a vis rudders
On Thu, 1 Apr 2004 12:24:46 +0100, "JimB"
wrote: Limitations of the experiment: It didn't check for associated force changes at the fan The scale of 'rudder' against fan size is way out The wire had a little flexibility Fag ends produced smoke which rose too fast Reynolds numbers were wrong. And, just in case you mis-understood, my hinges were pendulum hinges which did not allow the 'rudder' to rotate around its vertical axis (except in the 'rudder kick' experiment). They only allowed pendulum movement laterally, or when re-oriented, fore and aft (subject to wire flexibility). ..... JimB An experimental rig for visualizing fluid flow over rudders etc., is easy to make and provably representative of 2-D flow. It consists of an inclined board with side rails to stop the water film dripping off. A reservoir at the top, into which water from a hose pipe flows, and a sump at the other end to lead the waste water to a drain. At the top of the incline, permanganate crystals trail stream lines down the incline. The model (a rudder cross section, for instance) is placed in the stream. The stream lines tilt sidewards ahead of the rudder, when it is inclined at a modest angle to the flow, and tilt sidewards the other way after the model trailing edge. This is an easy way to show the "molecules give lift by hitting the proximal surface" enthusiasts how fluid dynamics really works. (about two thirds of the side force from the distal surface, and one third from the proximal surface.) You can work it out from the streamline spacing over both surfaces. Brian Whatcott Altus OK |
#138
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push vs pull vis a vis rudders
Brian, you just described a prop pushing a water stream over a rudder. pull is
different. An experimental rig for visualizing fluid flow over rudders etc., is easy to make and provably representative of 2-D flow. It consists of an inclined board with side rails to stop the water film dripping off. A reservoir at the top, into which water from a hose pipe flows, and a sump at the other end to lead the waste water to a drain. At the top of the incline, permanganate crystals trail stream lines down the incline. The model (a rudder cross section, for instance) is placed in the stream. The stream lines tilt sidewards ahead of the rudder, when it is inclined at a modest angle to the flow, and tilt sidewards the other way after the model trailing edge. This is an easy way to show the "molecules give lift by hitting the proximal surface" enthusiasts how fluid dynamics really works. (about two thirds of the side force from the distal surface, and one third from the proximal surface.) You can work it out from the streamline spacing over both surfaces. Brian Whatcott Altus OK |
#139
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push vs pull vis a vis rudders
Brian, you just described a prop pushing a water stream over a rudder. pull is
different. An experimental rig for visualizing fluid flow over rudders etc., is easy to make and provably representative of 2-D flow. It consists of an inclined board with side rails to stop the water film dripping off. A reservoir at the top, into which water from a hose pipe flows, and a sump at the other end to lead the waste water to a drain. At the top of the incline, permanganate crystals trail stream lines down the incline. The model (a rudder cross section, for instance) is placed in the stream. The stream lines tilt sidewards ahead of the rudder, when it is inclined at a modest angle to the flow, and tilt sidewards the other way after the model trailing edge. This is an easy way to show the "molecules give lift by hitting the proximal surface" enthusiasts how fluid dynamics really works. (about two thirds of the side force from the distal surface, and one third from the proximal surface.) You can work it out from the streamline spacing over both surfaces. Brian Whatcott Altus OK |
#140
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push vs pull vis a vis rudders
On Fri, 02 Apr 2004 01:59:44 GMT, Brian Whatcott
wrote: On Thu, 1 Apr 2004 12:24:46 +0100, "JimB" wrote: Limitations of the experiment: It didn't check for associated force changes at the fan The scale of 'rudder' against fan size is way out The wire had a little flexibility Fag ends produced smoke which rose too fast Reynolds numbers were wrong. And, just in case you mis-understood, my hinges were pendulum hinges which did not allow the 'rudder' to rotate around its vertical axis (except in the 'rudder kick' experiment). They only allowed pendulum movement laterally, or when re-oriented, fore and aft (subject to wire flexibility). .... JimB An experimental rig for visualizing fluid flow over rudders etc., is easy to make and provably representative of 2-D flow. It consists of an inclined board with side rails to stop the water film dripping off. A reservoir at the top, into which water from a hose pipe flows, and a sump at the other end to lead the waste water to a drain. At the top of the incline, permanganate crystals trail stream lines down the incline. The model (a rudder cross section, for instance) is placed in the stream. The stream lines tilt sidewards ahead of the rudder, when it is inclined at a modest angle to the flow, and tilt sidewards the other way after the model trailing edge. This is an easy way to show the "molecules give lift by hitting the proximal surface" enthusiasts how fluid dynamics really works. (about two thirds of the side force from the distal surface, and one third from the proximal surface.) You can work it out from the streamline spacing over both surfaces. A refinement of this setup is the Heale-Shaw device, in which the flow is enclosed between two parallel transparent plates. The models are the same thickness as the spacers that close the sides. This keeps the flow truly 2D without any surface waves to distub it. Rodney Myrvaagnes NYC J36 Gjo/a "Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab |
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