Square Top vs Pin Head

Technical discussion of ARC products
Bruiser
Professional
Posts: 55
Joined: July 1st, 2013, 6:37 pm
Boat Make/Model: RC 27

Square Top vs Pin Head

Post by Bruiser »

So I have a question on ways to reduce this spanwise flow on the back side of the main. Would twist in the main sail help reduce that?
gahamby
Professional
Posts: 252
Joined: July 24th, 2012, 7:02 am
Boat Make/Model: SuperCat 15#315
Location: Falls Church VA 22042

Re: Square Top vs Pin Head

Post by gahamby »

If I read Bill correctly, the only cure is (more cowbell) an increased chord length at the top of the sail. The airflow needs more surface area to direct it aft and off to leeward instead of over the leech and down the leeward side of the sail.
havliii
Professional
Posts: 272
Joined: March 27th, 2011, 8:12 pm
Boat Make/Model: SuperCat 19, Modded SC20
Location: Fredericksburg Virginia

Re: Square Top vs Pin Head

Post by havliii »

gahamby wrote:If I read Bill correctly, the only cure is (more cowbell)
Okay Geoff, now you'll have to explain to everyone else. Nearly fell outa my chair. LOL
havliii
Professional
Posts: 272
Joined: March 27th, 2011, 8:12 pm
Boat Make/Model: SuperCat 19, Modded SC20
Location: Fredericksburg Virginia

Re: Square Top vs Pin Head

Post by havliii »

there's a terrific picture that illustrates the tip vortex taken by daniel forster I can't find the link
search on these words: sailing cape town fog daniel forster a small thumb nail comes up maybe someone can find a good link.

the picture has been used in several books and articles as well
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Square Top vs Pin Head

Post by Bill Roberts »

Bruiser, our primary objective is to get the most thrust per square foot of sail area out of the sail. This will occur at a sail twist that matches the twist in the relative wind direction from the foot of the sail to the head of the sail. If we increase the twist more than than that, the spanwise flow would become less because the delta P across the sail would be less and the sail thrust would be less. Not so good.

gahamby, if I understand the "cowbell" term correctly, in the Aerodynamics World it is also called 'wing taper ratio'' or 'tip chord to root chord ratio'. The information I am using is right out of an Aeronautical Engineering Text Book. The Span Effectiveness of a pointed wing tip airfoil is 88%. This means this wing is generating 88% as much lift as it could because of spanwise flow. The wing Span Effectiveness can be raised to 98% if the tip chord is 25% of the root chord. The span effectiveness can be raised to 99% if the tip chord is 40% of the root chord. Increasing the tip chord even more is no further benefit. So, the max cowbell effect you want is with the top of the cowbell about 40% as wide as the bottom of the cowbell.
Now gahamby, the next time you fly on a commercial airliner, look out at the wing. Notice that the wing tip is much narrower than the wing where it comes out of the fuselage. The wing tip chord is probably around 20% to 40% as wide as the root of the wing. Now you can say to yourself, "I know why they did that".


havliii what we are talking about here is a loss in sail or wing performance due to leakage flow over the trailing edge of the
wing or sail.
TIP VORTEX has to do with a leakage flow over the tip of the wing and is not affected by the slope of the leech of the sail.
Last edited by Bill Roberts on August 24th, 2016, 6:53 pm, edited 1 time in total.
Kevin Keller
Professional
Posts: 362
Joined: September 24th, 2006, 2:56 am
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Re: Square Top vs Pin Head

Post by Kevin Keller »

It seems like they go back and forth on whether to have a pointy tip or curved tip as apposed to a fat type tip on wings. I thought one reason the WWII Spitfire was such a good airplane was because of the curved tips helped prevent span wise flow.

I also thought that gliders used long aspect wings for efficiency which would seem like a pointy sail and a fat top would be more like a lower aspect ratio wing.
Kevin Keller
Professional
Posts: 362
Joined: September 24th, 2006, 2:56 am
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Re: Square Top vs Pin Head

Post by Kevin Keller »

Bill,
I don't know if you realize it but the post you put up regarding square top sails is locked and cannot be replied to. I was wondering if a small winglet like on a MD-11 at the head of the sail would stop the span wise flow and improve efficiency.
Kevin
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Square Top vs Pin Head

Post by Bill Roberts »

HI Kevin, I was advised that locked was the best way to handle this paper.
Two more are coming thanks to Matt Haberman.

A wingtip plate might help reduce the wing tip vortex, leakage over the tip. A correctly built wing/sail "tip" should have a slight upward slope to it toward the trailing edge. THIS INCLUDES MAST RAKE AND MAST BEND. Both of these two effects tend to droop the aft end/corner of the square top on a sail . This slight uphill slope on the square top edge to the aft end tends to reduce tip leakage and the tip vortex. You can see this on some small private aircraft wing tips

The wing efficiency/drag improvement you are talking about on the MD-11 shows up in the third or fourth number to the right of the decimal point. It reduces drag which reduces thrust which reduces fuel burn. This was worth it when fuel costs were very high. The winglet itself costs money to make. The winglet itself makes drag. The winglet itself adds to the weight of the airplane and the required lift. It is not all sugar and sweetness.

Kevin #2. I don't know who your "they " is but the high aspect ratio, ~ 30% square tip airplane wing became the way to go many years ago. The story about the Spitfire round wing tip being an aerodynamic advantage is an old wives tale. Round tipped airfoils promote a large tip vortex which is drag and wasted energy/thrust/horsepower. This goes for daggerboard and rudder tips also. Ever wonder why a H18 is such a slow 18 footer? The Spitfire was an exceptional fighter airplane because of its very high horsepower to weight ratio in its day.


The change in calculated wing aspect ratio, glider for example, between a pointed tip foil and a 30% tip chord foil shows up in the second or third number to the right of the decimal point in the aspect ratio calculation. The pointed tip airfoil will have like an 88% span effectiveness with much spanwise flow. The square tipped wing will have 99% span efficiency or 11% more effective wing area than the pointed wing tip airfoil for the same wing span.
I'll say it like this. A pointed tip sail on a 30 ft mast will produce the same thrust as a square top sail on a 27ft mast.
This is a 10% loss in span effectiveness for the pointed tip sail.
.
havliii
Professional
Posts: 272
Joined: March 27th, 2011, 8:12 pm
Boat Make/Model: SuperCat 19, Modded SC20
Location: Fredericksburg Virginia

Re: Square Top vs Pin Head

Post by havliii »

"Certainly more technical explanations could be offered for the advantages presented by increased area at the head of a mainsail. Those more familiar with the science underlying these designs would tell you that a broader head tends to stabilize the air flow across that section of the sail by diminishing the tip vortices. Ultimately, the effect is that the foil becomes more efficient."

Tip vortex apparently has something to do with this topic which is "Square top vs pin head"
havliii
Professional
Posts: 272
Joined: March 27th, 2011, 8:12 pm
Boat Make/Model: SuperCat 19, Modded SC20
Location: Fredericksburg Virginia

Re: Square Top vs Pin Head

Post by havliii »

Here's the link to one of Daniel Forster's jpegs, some of you may have already seen this in other publications. Mr Forster took several angles of this same event, this is not the best but the only one I could find a link for.

The aerial view is quite dramatic and the fog illustrates the vortex drag.

http://features.boats.com/boat-content/ ... mg8076.jpg

found the better jpeg!! this link will take you to Mr Forster's website, wait for the site to load and then the vortex pic will pop onto the screen. It's worth the wait.

http://www.yachtphoto.com/#a=0&at=0&mi= ... 00&s=2&p=1
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Square Top vs Pin Head

Post by Bill Roberts »

havliii,
In scientific testing, one parameter at a time is varied. In the span effectiveness tests where the taper ratio was varied over a wide range, SAIL AREA or WING AREA was held constant while tip chord to root chord ratio was varied. If area had been allowed to vary at the same time, no definite conclusion could have been made.
havliii
Professional
Posts: 272
Joined: March 27th, 2011, 8:12 pm
Boat Make/Model: SuperCat 19, Modded SC20
Location: Fredericksburg Virginia

Re: Square Top vs Pin Head

Post by havliii »

Bill Roberts wrote:I'll say it like this. A pointed tip sail on a 30 ft mast will produce the same thrust as a square top sail on a 27ft mast.
This is a 10% loss in span effectiveness for the pointed tip sail.
Is there no benefit to the top 3 feet? Seems like a win/win if we just cut off the top of all our pin head sails. We get rid of three feet of mast (top weight) and gain a more effective thrust producing mechanism. I've got an older SC20 sail that I might just be cutting up, would be a fun project. (Unfortunately I sold my 29 foot mast) I think we talked about this on another thread.
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Square Top vs Pin Head

Post by Bill Roberts »

havliii,
Why would one go to all that trouble and expense to cut down a sail and rig just to break even. To get a new reinforced head put on a mainsail would cost a few hundred dollars. Option 2, make a square top out of your 30ft pin head sail and go faster. This is what the sailing world is doing.
There are two more technical articles coming out on this subject. I think things will be more fully understood when all three have been read.

The Span Effectiveness information I am using came from an Aeronautical Engineering Text Book
by Dommasch, Sherby and Connolly. This is a widely used text book in Aeronautical Engineering schools.
Last edited by Bill Roberts on August 25th, 2016, 1:12 pm, edited 1 time in total.
gahamby
Professional
Posts: 252
Joined: July 24th, 2012, 7:02 am
Boat Make/Model: SuperCat 15#315
Location: Falls Church VA 22042

Re: Square Top vs Pin Head

Post by gahamby »

So Bill, would it be practical to take the Dacron pintop on my '84 SC15 and have it re cut to a square top? Would the luff remain the same dimension while the foot was shortened? Or, would more material be added at the top while maintaining the original length of the foot?
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Square Top vs Pin Head

Post by Bill Roberts »

gahamby, yes your dacron SC !5 mainsail can be made into a square top by adding width at the top. Your sailmaker needs to know what he is doing. Some of the load traveling up the leech needs to continue up to the square top corner and then come across the top to the mast. The upper leech should be two ply with a strip of sailcloth about 4 ins wide and then continue across the top edge of the square top to the mast. The square top material the sailmaker adds to your sail should have a very
slight amount of shaping in it to get the square top to stand well. Back when I was making/experimenting with my first square top sails made from pin head sails, I would split the sailcloth right down the center of the gaff batten for about 18 ins and then sew that cut back together with about 1/8 ins or so of reverse broad seaming in it at the leech. Then glue and sew a doubler strip over that seam for strength. Your gaff batten should be untapered and about 8 to 10 pounds compression.
Good Luck
Post Reply