Square top mains?

Well, that is true to a point in that for fast aircraft, their operational Cl is very low at speed and only high while landing. As induced drag is proportional to Cl^2, the effect at low Cl in normal filght is small. However aircraft that normally operate closer to Cl max like a glider in min sink trim, induced drag is a much greater issue. In sailing, it is the the major componant of drag so proportionally is much more important. In aircraft, the complexity and cost of constructing an accurate eliptical wing is very high compared with a tapered one that is a good approximation. This is not such an issue with sails.

Ken

 

Hi

This is not an issue if the wing is designed correctly. If the tip camber is increased so it will stall at a higher Cl, then the wing twisted to keep all the profiles zero lift angles aligned along the span, the root will stall 1st safely despite the lower Reynolds number at the tip. This keeps the entire span operating most efficiently close to the same Cl and is common practice on full size sailplanes which typically have very small tip chords even down to 40% of the root but is a detail seldom observed in RC models which tend to tip stall when the tip chord is less than 60% or the root.

Regards,
Ken

 

square top mains

Ken , I've been trying to round up some of the info that helped me become convinced since the early 70's of the validity of the square top. I found some info on rectangular planform sails tested by Marchaj in Sail Performance on page 191 and I think there is more in the Aerohydrodynamics of Sailing. There is more on http://www.sailboat-technology.com but I can't open pdf's-check it out.
I just read a story about Bill(?) Hansen a Windsurfer sail designer credited with comming up with the "truncated tip". And I've had the privilege to discuss this with Barry Spanier who designed the rig for Long Shot and is a Caleema sail designer. He is a believer in the square top especally the peaked up square top I described earlier(at least as I showed him on a 14' foiler sailplan Eric Sponberg and I are working on). Speaking of aspect ratio: the windsurfer sails that benefit from this technology aren't that high in aspect ratio.
I haven't been able to find an article I ran into about 4 years ago by some French Aerdynamicists that claimed that their research showed the tip has a lot more influence on induced drag than thought up to that time-basically that the "peaked up" squaretop if done correctly would be superior to just a square top. I'll keep looking and post what I find.
Remember that the way this kind of sail reacts "mechanically" is one of it's most important assets: I described the gust response earlier-it's real important and can be done with a simpler rig(lighter, less windage) than a Bethwaite flex top....
I mean no disrespect at all but if the square top isn't working in your class I have to believe that it isn't being done right-or that other factors are blurring the comparison.

 

Adding washout to the wing would help prevent a tip stall, but it in turn introduces ineficiency in that it is virtually impossible to properly load a wing with washout with any planform. All light gliders and all full scale gliders I have seen or flown have had square wingtips, and only the fastest have elliptical wings.

 

I mean no disrespect at all but if the square top isn't working in your class I have to believe that it isn't being done right-or that other factors are blurring the comparison.


Hi Doug
That is one of my concerns in all this as well.
I appreciate any referances you can give me.
Incidently the NS/MGs use a wind mast so is not your typical mast flex situation.

Ken

 

Ah but we are being much more cleaver than just adding washout.
The trick is keeping the zero lift angles aligned along the span. Therefore while the wing may have geometric washout it is aerodynamically completely untwisted. and that is very important because as we then increase angle of attack, each section then maintains the same Cl along the span and that keeps the downwash constant along the span so keeps induced drag at the same min value. The tip though has a higher Cl max because of it's greater camber. Most CL glider designs neglect the camber issue.

There is no impact on induced drag with this only a very insignificant impact on form drag.

Ken

 

My point is that with the changing airfoil and changing angle of attack, the Cl will vary. I agree that wimple washout is a disaster and that the careful alignment that you propose would not lead to any inherent inefficiencies, it is just that keeping Cl constant with a blended airfoil and changing angle of attack would at least be extremely difficult, especially on a surface as difficult to form presisely as a sail. For the same reason, I believe that preventing a tip stall is more important on a sail than attempting to keep the Cl constant.

 

OK but you were talking RC glider wings. In the case of a wing, its simple because Cl is in a direct linear relationship with angle of attack from zero lift (not the profile datum) and zero lift angle can for practicality also be considered in a linear relation to camber within the small range of camber change. Also a wing ought to be a relatively tortionally rigid structure.


In a sail, it's really a vastly different situation because the luff angle is so critical, camber directly affects AoA and chord affects the luff angle by induced incidence. We can't isolate variables with much latitude at all.
So now that you bring this up, we really can't isolate plan form from camber and angle of attack in the way of considering wings. So that's a good point I think.

Regards,
Ken

 

Ken, if you're near a good library (ie the Maritime Museum) you may want to look up Aust Sailing May 1988. It has a relevant article by Neil Pollock, an aerodynamacist who sailed an NS14 and therefore it's pretty much on target I'd say.

I'm certainly no expert but I think (from memory) Pollock and Speer make the same point about full ellipses not being ideal when righting moment is limited. There are articles on the net about the fact that planes have the same problem when the loading on the wing root is a limiting factor.

Interesting discussion on rigs on the 12 footer's forum, too.

 

Look at the top part of a typical triangular main. The chord length is so short in relation to mast thickness that it can't be doing much except creating drag. More advanced rigs have tapered masts, but not tapered enough. Unfortunately the only way to increase the effective lifting area of a triangular main is to increase the extraneous draggy bit as well.

The only way to change this is to add enough sail behind the mast in relation to mast thickness to start acting like an airfoil again. With a triangular main this takes battens or gaffs, and/or to bend the mast. Thus the square-topped main. Even if the uppermost part is still just drag-producing, the effective lifting area is maximized.

 

You can experiment yourself using this spreadsheet. Go to the Analysis page and you can put in an aribtrary planform of your choice. I've currently got it set up to use the same planform from the Design sheet so you can look at off-design conditions. But just replace the chord formulae with your own - there are even some blank input variable fields you can use for convenience.

The section camber shape is captured by the zero lift angle of attack. If you don't know what it is for your sail shape, just draw a line tangent to the camber at the 3/4 chord point and figure that's the zero lift line. The zero lift angle will be the (negative) angle between the 3/4 chord tangent and the chord. You get some effective aerodynamic twist by using a different camber shape at the head and foot.

You can add whatever twist you want, too. The program assumes a uniform wind, so if you want to account for the rotation of the apparent wind with height it has to be factored into the net twist. Likewise, you can crudely model a variation in the wind speed with height by changing the lift curve slope according to the square of the velocity ratio.

The Chord_a sheet shows the chord distribution (planform shape). GammaPlot_a shows the spanwise lift distribution, and ClPlot_a shows the local lift coefficients along the span. Wplot_a shows the induced velocity distirbution - the self-created header that causes induced drag.

Of course, since it's a spreadsheet, you can add additional sheets to do whatever calculations you wish.

You can vary the twist to simulate the square-headed rig in light winds vs heavy to see how the spanwise lift distribution changes, and you can use the Design sheet to calculate the optimum for comparison.

 

Has anybody taken photos of their square-top sails in different conditions so the camber and twist can be determined? This would be especially useful for a cat rig like a A-class catamaran. It would be interesting to plug the measured shape into the spreadsheet to see what it says is going on with the rig.

 

Hello,

I am new to this forum, so please forgive the very late entry to this topic.

There are a number of advantages to a square top mainsail for certain types of craft.

1. It is definitely better than the traditional triangular bermudan sloop, especially as the aspect ratio increases (when so much of a triangular sail is of too short a chord length to be effective). Hence they have become popular on catamarans and RC yachts.

2. At certain geometries, a square top sail is a reasonable approximation of an ellipse. A tip:root (or head:foot in sailing parlance) ratio of 0.4 is not that inferior to an ellipse. Again this ratio is easiest to create on high aspect ratio rigs.

3. Even sails with elliptically shaped roaches are unlikely to have an optimally shaped ellipse. Certainly the leading edge i.e. the mast, is too straight and upright. Bethwiate boats come closest to dealing with this but even they do not have truly elliptically curved masts. In my opinion, this is one area that could lead to significant improvements (albeit setting a decent size jib on a well raked and pre-bent mast might prove tricky).

4. Perhaps most importantly square top sails offer good gust response - the usefulness of which will vary depending on the type of boat you sail. There is very little tension in the leech of a square top sail due to the discontinuity at the head.
In comparison an elliptical sail transmits mainsheet loads through the leech all the way to the mast head.
Low leech tension allows the sail to fall open as a gust hits. The amount of tension in the leech is easily altered by adjusting the downhaul (more downhaul tension = less leech tension). Obviously Bethwaite has shown that you can get good gust response from an elliptical sail, but the square top is probably simpler and mor effective.

In a relatively modestly powered boat like the MG14 it may not be so important to have this gust response, but I would be surprised if the square top was actually noticably slower in any circumstance.

Apologies if I have repeated what others have said, I have just skimmed through the whole topic so have probably missed some interesting points in my haste.

 

Upper outhaul

On my models and on my 16 footer's square topped jib the outhaul was adjustable at the top of the sail. I'm considering adding an adjustable(while sailing) upper outhaul to the square topped main on my new boat. It would require a "gaff" instead
of a diagonal upper batten(to support the "peaked up" square top) but that could be extremely light and obviate the need for full battens(at least at the top of the sail). Setting up the mechanics of the adjustment from the cockpit shouldn't be too hard.
Does anyone have an opinion on whether or not using an adjustable outhaul at the top of a square topped main would be worth the trouble?
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Model version square top w/"gaff" and adjustable upper outhaul:
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Address:http://www.microsail.com/images/a1e.jpg Changed:11:14 PM on Friday, July 28, 2006

 

James Wharram's "Tiki" design has his well known wingsail. This is high aspect ratio gaff sail where outhaul is definitely adjustable at the gaff. The gaff and straight leach obviates the need for full length battens. Some of his sails are boomless as well, but I think that the wingsail would work better on all points of sail with a boom.
The gaff if stiff will not allow change of sail draft near the head if used as a "sleeve luff" irrespective of sail tension at the gaff. The other possibility is not to use a stiff gaff but manufacture one that sleeves into a sail pocket and flexes correctly under load.
I have built such a sail with a flexy gaff but havn't tested it out yet.