New Posts New Posts RSS Feed: Mast rake: explain please!
  FAQ FAQ  Forum Search   Register Register  Login Login

Mast rake: explain please!

 Post Reply Post Reply Page  <1 45678>
Author
I luv Wight View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 28 Jan 05
Location: United Kingdom
Online Status: Offline
Posts: 628
Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Topic: Mast rake: explain please!
    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.

http://www.bloodaxeboats.co.uk
Andy P
foiling Int Moth GBR3467
Freedom 21 Codling
Back to Top
I luv Wight View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 28 Jan 05
Location: United Kingdom
Online Status: Offline
Posts: 628
Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 14 at 10:10pm
29ers have a fixed length forestay, but you adjust the rig tension/rake by the shroud adjustment as it gets windier.

http://www.bloodaxeboats.co.uk
Andy P
foiling Int Moth GBR3467
Freedom 21 Codling
Back to Top
craiggo View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 01 Apr 04
Location: United Kingdom
Online Status: Offline
Posts: 1810
Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 14 at 10:20pm
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.
Back to Top
NHRC View Drop Down
Posting king
Posting king


Joined: 13 Dec 13
Location: Lymington
Online Status: Offline
Posts: 102
Post Options Post Options   Quote NHRC Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 14 at 10:25pm
Originally posted by Daniel Holman

In aerodynamics, a swept aerofoil cannot derive as large a lift coefficient as an upright one, so when not overpowered you get more "bite" from an upright rig.



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...?

Back to Top
Daniel Holman View Drop Down
Really should get out more
Really should get out more


Joined: 17 Nov 08
Online Status: Offline
Posts: 997
Post Options Post Options   Quote Daniel Holman Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 14 at 10:38pm
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.
Back to Top
NHRC View Drop Down
Posting king
Posting king


Joined: 13 Dec 13
Location: Lymington
Online Status: Offline
Posts: 102
Post Options Post Options   Quote NHRC Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 14 at 10:56pm
Originally posted by Daniel Holman

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.



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
Back to Top
I luv Wight View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 28 Jan 05
Location: United Kingdom
Online Status: Offline
Posts: 628
Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 14 at 10:57pm
OK i know it's different - but that's why it's interesting - and why the usual 'obvious' explanations aren't always right :-)

http://www.bloodaxeboats.co.uk
Andy P
foiling Int Moth GBR3467
Freedom 21 Codling
Back to Top
sargesail View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 14 Jan 06
Location: United Kingdom
Online Status: Offline
Posts: 1459
Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 14 at 12:08am
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.....? 
Back to Top
redman View Drop Down
Newbie
Newbie


Joined: 29 Nov 11
Online Status: Offline
Posts: 26
Post Options Post Options   Quote redman Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 14 at 5:31am
Originally posted by Daniel Holman

Yep its the same part as the D One!
Somebody asked why people don't rake all the time if its so great.
In aerodynamics, a swept aerofoil cannot derive as large a lift coefficient as an upright one, so when not overpowered you get more "bite" from an upright rig.
As for raking, big long old subject that could fill best part of a book.
It is fast in the wind in most stuff.
49ers don't do it as fixed length forestay I believe. Not many i14s do it as if you pull a string you lose 10 boat lengths minimum, and you can have a set that works over a very broad range.


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.
REDMAN
Back to Top
Guests View Drop Down
Guest Group
Guest Group
Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 14 at 7:46am
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.
Back to Top
 Post Reply Post Reply Page  <1 45678>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy