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Mast rake 2

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Created by ClaudioD 3 months ago, 19 Jul 2017
ClaudioD
3 posts
19 Jul 2017 2:56PM
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I'm new to this forum and saying hello to everybody.
I started to get some interest about land sailing, but the mast rake is suggesting the simple question : Why ?
The Centering of CE vs CLR-LCB can be obtained with tilted mast as well with up strait mast.
The air wing comparison are difficult to understand since the Speeds are not the same as well the Reynolds numbers.
So far I was reading an old tread so I renewed the same by opening this one.
I do also adds a sketch using a modern shape with elliptical form as used on recent America Cup catamarans.
Thanks a lot

Gizmo
SA, 2815 posts
19 Jul 2017 8:25PM
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Ok this may clarify the situation....but then again it may not.

You are right an elliptical is the most efficient wing shape, the higher the aspect ratio, the greater the lift, Now the angle.
It is known that the 'less' of an angle the more lift is achieved, think 'Sail Planes' (Gliders) soaring birds like eagles, seagulls, albatross etc. BUT with that greater lift it is less efficient at higher speeds.
Swing wing aircraft have been used to gain effect of greater lift on take off i.e.; F1-11 used by the Australian Army / Airforce.

Getting to land yachts.... Back in the 1970's - 80's there was a class that was limited by overall mast height 'above the ground'. This later was changed to hight above the ground at the mast step + mast length.
But in the original rule an advantage was gained by using a longer mast and raking it back to get a longer luff length and thus having a higher aspect ratio = greater lift = better performance. But remember this was done to get around a 'particular' rule that was put in front of builders, an upright mast 'may' have worked better but was never tried as it wouldn't conform with the class rules.

Having a high aspect rig yes gives you greater lift but on the flip side it raises the C/E and makes the yacht less stable and will tip over easier.
The current mast rakes used these days are more about aesthetics / looks rather than overall performance, 10deg is perhaps a very 'nominal' angle.
Having a 'horizontal' boom will probably achieve better results with aerodynamics rather than playing with mast rake.

The design of land yachts is way off the high tech development that has been shown in the America Cup yachts.


GeoffSobering
54 posts
19 Jul 2017 11:34PM
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Select to expand quote
Gizmo said..
Ok this may clarify the situation....but then again it may not.

You are right an elliptical is the most efficient wing shape, the higher the aspect ratio, the greater the lift, Now the angle.
It is known that the 'less' of an angle the more lift is achieved, think 'Sail Planes' (Gliders) soaring birds like eagles, seagulls, albatross etc. BUT with that greater lift it is less efficient at higher speeds.
Swing wing aircraft have been used to gain effect of greater lift on take off i.e.; F1-11 used by the Australian Army / Airforce. ...






Two quick points:
1) The speed range where swept wings have an advantage is around Mach 0.8 (~500kn at sea level). Even at landsailing speed-record speeds (100+kn) an unswept wing is best from a lift/drag perspective.

2) The highest lift/drag is obtained with an elliptical lift distribution. For a wing of constant profile and no twist this corresponds to an elliptical planform. Many wings (and especially triangular sails) approximate the elliptical lift distribution by varying the profile and/or twist. This is one reason why the top of a triangular mainsail often has more draft than the lower sections. The designer is compensating for the loss of width by increasing the coefficient of lift at the top of the sail to get closer to the elliptical distribution.

A really good explanation of the aerodynamics of sails is:
northsails.com/sailing/en/resources/how-sails-work
It discusses the interactions between sweep, planform, and profile very well.

Other good references are:
www.gentrysailing.com/theory.html
If you're interested in wing-sails (i.e. a wing shaped mast with a soft sail attached) Tom Speer has a very good collection of articles:
www.tspeer.com

Cheers,

Geoff S.

ClaudioD
3 posts
20 Jul 2017 1:47AM
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Tanks to both of you. I got the impression that is more a fashion way for the land sailers.
One replied that are nice to see.
Some Military Airplanes modify the wing shape as function of the speed and Reynolds numbers. Take Off and Landing are made at a speed that is at least 5 times less than the cruising speed therefore they need more lift by opening the wings.
Yes I will stick to the elliptical form.
I have to consider a batten soft sail.
For the time being the rigid sail is not part of the project, may one day, who knows .
Cheers

ClaudioD
3 posts
20 Jul 2017 2:27PM
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Probably I have found a possible answer to my question just playing graphically with a side view



GeoffSobering
54 posts
21 Jul 2017 2:43AM
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Select to expand quote
ClaudioD said..
... I got the impression that is more a fashion way for the land sailers.
...
Some Military Airplanes modify the wing shape as function of the speed and Reynolds numbers. Take Off and Landing are made at a speed that is at least 5 times less than the cruising speed therefore they need more lift by opening the wings.
...


One of the advantages of soft sails is that their shape can be varied while sailing by changing the tension applied to them (downhaul, outhaul, mainsheet).
A flexible mast adds the additional control of draft and twist.

In many sail-craft mast rake is used for small adjustments of the CE.

Additionally, with a tapered wing the leading edge (mast) has to be angled back approximately as much as the trailing edge (leech) in order to make the aerodynamic sweep 90 degrees.

It's counterintuitive, but a triangular sail set on a straight mast with no rake is actually swept forward.
The aerodynamic "centerline" of a wing is approximately 25% back from the leading edge, so a line/curve drawn at the "quarter-chord" point on a triangle goes from a point 25% back on the boom to the tip of the sail.




desertyank
1213 posts
23 Jul 2017 12:34PM
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I love this stuff...........

Sylk
WA, 197 posts
2 Aug 2017 2:34PM
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Sailing my modified class 5 my largest sail is 8sm and smallest is 3.
Sailing on a salt lake the surface can be quite variable in terms of grip and roughness. When it is very smooth and wet then fine adjustment to the CE is critical to not spinning out. I find adjustable rake the quickest and easiest way to compensate for changing wind and surface conditions.

US772
302 posts
2 Aug 2017 10:55PM
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Every landsailer design is slightly different. For my designs the main reason for a particular rake is to achieve the proper lateral resistance. If the Center of effort is too far forward you can lose the ability to steer. The front end will get pushed sideways. If the center of effort is to far back the rear wheels crab too much. Here are a few options to balance the landsailer.

Leeward helm
1) move the mast back
2) more mast rake
3) let some air out of the front tire to reduce steering chatter
4) put weigh on the front
5) tighten the back stays if you have stays
6) move rear axle forward

Crabbing
1) move mast forward
2) less mast rake
3) let air out of the rear tires
4) loosen back stays
5) move rear axle back
6) add weight in the back

Its best to make a rig that has adjustment for and aft as well as rake. Most of us have different sail sizes. That makes it tricky to balance a boat if things aren't designed to be adjustable. Out of balance boats manifest them selves most in high winds when it's the most dangerous.

One time I redesigned the section shape on a deep wing masted mast. It was the same side profile. The new section moved the center of effort forward and created bad steering problems in high winds. I have to move the rig back 3" to gain balance again. I would of raked it back but had no room because the boom angle was all wrong. We have found with the solid wing flap combo's that the wings needs have 35 to 40 % of its wing area behind the rear axle.







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"Mast rake 2" started by ClaudioD