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| 22 Apr 2008 08:43:18 |
| Steven M-M |
| Lift question |
I have a question about lift. In a moving shell at the moment of blade entry do the forces of lift develop immediately merely by placing the blade in at a good angle or do you need to apply pressure on the handles to generate lift? I=92m focusing here on the first few moments of the entry: the time it takes to bury the blade. (I=92m sorry if this is not clear as I find it difficult to describe my question.) Steven M-M |
| 22 Apr 2008 09:58:06 |
| Re: Lift question |
On Apr 22, 8:43=A0am, Steven M-M <s...@ku.edu > wrote: > I have a question about lift. In a moving shell at the moment of blade > entry do the forces of lift develop immediately merely by placing the > blade in at a good angle or do you need to apply pressure on the > handles to generate lift? I=92m focusing here on the first few moments > of the entry: the time it takes to bury the blade. =A0(I=92m sorry if this= > is not clear as I find it difficult to describe my question.) > > Steven M-M Until there is some force at the handle, there is no "lift". i.e. The blades do not move in the direction of the hull (unless placed beyond the point at which they would be parallel with the hull, or likely a bit beyond due to the curved face of the blade.) I'm sure that an engineering type will be along shortly with a proper definition of lift and how it conforms to the "equal and opposite" principles. This is the exact reason that the angle of attack (AOA) should be looked at at going from high to low during the first part of the Drive, rather than the more conventional low to high. In fact, the higher and quicker the onset of force the longer the high AOA will be maintained, increasing the temporal "length" of the stroke (the important measure of "length"). Arc length is often cited as being meaningful, but I'd contend that that alone is nearly useless as an indicator of anything, since even if no pressure is brought to bear on the handles, they will still go through the full "normal" range if the boat is moving, yet have no positive effect on accelerating the system. - Paul Smith |
| 23 Apr 2008 20:42:55 |
| Carl Douglas |
| Re: Lift question |
paul_v_smith@hotmail.com wrote: > On Apr 22, 8:43 am, Steven M-M <s...@ku.edu> wrote: > >>I have a question about lift. In a moving shell at the moment of blade >>entry do the forces of lift develop immediately merely by placing the >>blade in at a good angle or do you need to apply pressure on the >>handles to generate lift? I’m focusing here on the first few moments >>of the entry: the time it takes to bury the blade. (I’m sorry if this >>is not clear as I find it difficult to describe my question.) >> >>Steven M-M > > > Until there is some force at the handle, there is no "lift". i.e. The > blades do not move in the direction of the hull (unless placed beyond > the point at which they would be parallel with the hull, or likely a > bit beyond due to the curved face of the blade.) > > I'm sure that an engineering type will be along shortly with a proper > definition of lift and how it conforms to the "equal and opposite" > principles. > > This is the exact reason that the angle of attack (AOA) should be > looked at at going from high to low during the first part of the > Drive, rather than the more conventional low to high. In fact, the > higher and quicker the onset of force the longer the high AOA will be > maintained, increasing the temporal "length" of the stroke (the > important measure of "length"). Arc length is often cited as being > meaningful, but I'd contend that that alone is nearly useless as an > indicator of anything, since even if no pressure is brought to bear on > the handles, they will still go through the full "normal" range if the > boat is moving, yet have no positive effect on accelerating the > system. > > - Paul Smith I may be being dense today, Paul, but I'm not quite sure I follow you in all of the above No matter. Must be 'cos it's St. George's day (he's our patron saint, borrowed from the Balkans or some such typically English region). Where we unambiguously agree is that "lift" is not something got for free; it is the fluid-dynamic mechanism, & its resulting force reaction, by which a foil passing at some angle of incidence through a fluid & under an applied load is prevented from falling face first through that fluid. Lift cannot exist without an applied load & it generates itself in reaction to that load as a result of the consequent change of the foil's angle of attack under load. Lift is equal and opposite to the applied load. An increase of load does increase angle of attack, with lift normally proportional to AoA. Cheers - Carl -- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK Find: http://tinyurl.com/2tqujf Email: carl@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers) |
| 23 Apr 2008 22:23:18 |
| Ted van de Weteringe |
| Re: Lift question |
Carl Douglas wrote: > Where we unambiguously agree is that "lift" is not something got for > free; it is the fluid-dynamic mechanism, & its resulting force reaction, > by which a foil passing at some angle of incidence through a fluid & > under an applied load is prevented from falling face first through that > fluid. An asymmetric foil may have no angle of attack (zero) and still generate lift. You think the blade is an asymmetric foil or in practice undistinguishable from a flat plate? |
| 23 Apr 2008 22:08:42 |
| Carl Douglas |
| Re: Lift question |
Ted van de Weteringe wrote: > Carl Douglas wrote: > >> Where we unambiguously agree is that "lift" is not something got for >> free; it is the fluid-dynamic mechanism, & its resulting force >> reaction, by which a foil passing at some angle of incidence through a >> fluid & under an applied load is prevented from falling face first >> through that fluid. > > > An asymmetric foil may have no angle of attack (zero) and still generate > lift. You think the blade is an asymmetric foil or in practice > undistinguishable from a flat plate? I was attempting, foolishly, to keep it simple. Too much information may create confusion, so I didn't want to get into a detailed definition of AoA :( You're absolutely correct. An assymmetrically cambered foil will generate positive lift into negative AoA's - how far negative the AoA at which lift ceases will depend on camber, etc. And, yes, an oarblade is significantly cambered (curved one way). Unlike normal cambered foils, however, the blade has minimal thickness. A typical cambered foil would be the wing of a passenger aircraft in normal-lift/cruising speed configuration, i.e. pretty flat underneath & convex on top. You could make an oarblade that shape, but it's resulting buoyancy might make it hard to keep it immersed, & it might not perform too well at catch & extraction. Please Sir, may I go now? Cheers - Carl -- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK Find: http://tinyurl.com/2tqujf Email: carl@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers) |
| 23 Apr 2008 23:37:00 |
| Ted van de Weteringe |
| Re: Lift question |
Carl Douglas wrote: > Unlike normal cambered foils, > however, the blade has minimal thickness. But just like a sail. Are you saying the minimal thickness of the blade is a (or the main) reason for saying that it is, in practice, a flat plate? |
| 23 Apr 2008 23:01:30 |
| Carl Douglas |
| Re: Lift question |
Ted van de Weteringe wrote: > Carl Douglas wrote: > >> Unlike normal cambered foils, however, the blade has minimal thickness. > > > But just like a sail. Most sails have thickened & rounded leading edges (= the mast), or sometimes a wing-section mast of substantial chord width. My firm built an entirely solid, laminar-flow wing-sail, complete with control surfaces too, for a high-performance catamaran. Square sails do not have thickened leading edges - their leading edges being cloth alone - & thus are more like oars. Those who sailed such vessels sometimes did pretty clever things (e.g. the Vikings) with their sail's leading edge, doubtless introducing a lot of leading edge curvature when needed. But teh Vikings knew an awful lot about sailing & boat design in general - for instance, the steering oar is a pretty advanced concept, mistakenly abandoned by other designers & replaced by the barn-door rudder for many centuries. Where a people's existence really depends on their boats' performance, boats evolve in some intelligent ways. Are you saying the minimal thickness of the blade > is a (or the main) reason for saying that it is, in practice, a flat plate? No, it is not - in theory or in practice - a flat plate. Nor did I suggest it might be. It ain't. Cheers - Carl -- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK Find: http://tinyurl.com/2tqujf Email: carl@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers) |
| 23 Apr 2008 18:58:17 |
| Steven M-M |
| Re: Lift question |
Before we get too far off on another tangent (one I will likely find interesting), thank you, Paul and Carl, for answering my question. smm |