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Mast rake: explain please! |
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I luv Wight ![]() Really should get out more ![]() ![]() Joined: 28 Jan 05 Location: United Kingdom Online Status: Offline Posts: 628 |
![]() ![]() ![]() ![]() Posted: 03 Dec 14 at 10:08pm |
Yeah but gliders have zero rake - with apparent winds of at least 30 kts - to 100 kts? and some have negative rake so they can see down out of the cockpit.
Swept wings only make sense at transonic/supersonic speeds. |
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I luv Wight ![]() Really should get out more ![]() ![]() Joined: 28 Jan 05 Location: United Kingdom Online Status: Offline Posts: 628 |
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29ers have a fixed length forestay, but you adjust the rig tension/rake by the shroud adjustment as it gets windier.
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craiggo ![]() Really should get out more ![]() ![]() Joined: 01 Apr 04 Location: United Kingdom Online Status: Offline Posts: 1810 |
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Andy, gliders dont tend to be righting moment/power limited hence ultra high aspect ratio and zero sweep. Forward swept wings on gliders are swept fwds only enough to ensure that the 1/4 chord line is unswept, such that the effective sweep is zero. This ensures they produce the maximum L/D.
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NHRC ![]() Posting king ![]() Joined: 13 Dec 13 Location: Lymington Online Status: Offline Posts: 102 |
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Hi Daniel Out of interest is the lift coefficient really that applicable compared to the change in helm balence by moving the centre of effort...? |
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Daniel Holman ![]() Really should get out more ![]() Joined: 17 Nov 08 Online Status: Offline Posts: 997 |
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Hi - TBH the CoE move is opposite to what you'd want - as you feather a sail, the CofPressure moves back giving weather helm which is slow - unless this is all matched by moving CLR back by raising dagger or centreboard.
Can't give numbers off the top of my head but have a gander in any of the basic aerodynamics texts on stall angles for finite aspect ratio wings at different sweep angles. |
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NHRC ![]() Posting king ![]() Joined: 13 Dec 13 Location: Lymington Online Status: Offline Posts: 102 |
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Not sure I have ever read many that relate to low enough wind speeds that would be apparent. That said Bethwaite Snr showed some interesting effects of wind sheering between 1m and 6m above water at low speeds enough to disprove that coefficient effects only apply with greater force. The theory you mentioned earlier completely stands up, be nice to see someone take this up as an experiment. By the way I would say that feathering and moving coefficient giving you weather helm is due to the drag of feathering not applicable by rake. Raking gives an efficient aerofoil reducing drag, increasing lift but also displacing unwanted high up the mast force. Centre of Effort moved aft in this case reduces helm. Think windsurfer more than laser dinghy. Edited by NHRC - 03 Dec 14 at 10:57pm |
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I luv Wight ![]() Really should get out more ![]() ![]() Joined: 28 Jan 05 Location: United Kingdom Online Status: Offline Posts: 628 |
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OK i know it's different - but that's why it's interesting - and why the usual 'obvious' explanations aren't always right :-)
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sargesail ![]() Really should get out more ![]() ![]() Joined: 14 Jan 06 Location: United Kingdom Online Status: Offline Posts: 1459 |
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Not sure if it's relevant here but it always strikes me that sideways bend is too often ignored in the models and descriptions of how rigs work.
Certainly on soft tension rigs (Firefly, 2000) where there is a need to maintain sheet tension to hold the rig back (and the boom on the centreline to open the slot), it is my opinion that the speed that comes from extra rake is as much about the interaction of clunky sails on coarse rigs than all the detail of aerodynamics. But why.....?
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redman ![]() Newbie ![]() Joined: 29 Nov 11 Online Status: Offline Posts: 26 |
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Thanks Daniel, that's as good a response to the why you don't rake at all times question as I'll ever need. Understand that raking is a big enough subject to cover a book on it's on. For those of us who don't have what it takes to chew through aerodynamics textbooks, is there any chance you could give us a summary of your top 5 effects of raking, ranked in order of importance? I'm sure I'd be one of many who would enjoy reviewing such a list. Thanks Again - redman. |
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REDMAN
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I'm not totally convinced by that though. Yes, I know that lift is reduced with sweep (subject to taper ratios etc) but... Many boats actually have fwd sweep of the 1/4 chord point with a near upright mast so a bit of rake would help improve the sweep angle and anyway even when raked the sweep angle of most rigs is not more than a degree or two. So it's hard to imagine the effect is significant enough to be all of the answer all of the time.
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