Rotating Wing Mast – theoretical discussion

A dinghy style wing mast, if its just a single fabrication, will be far stiffer fore and aft than a pole. If its un-rotated then the drag is horrible. If it is overrotated then the tip will tend to bend to windward in gusts (because it bends sideways, not fore and aft) So yes, the gust response is both worse and quite different to a conventional pole mast. And this is not good in an overpowered boat.

Bethwaite also contests that the more conventional pear shaped wing sections are very difficult to get in the groove and be faster as opposed to slower. My personal impressions of running with the things also empirically backs this up.

Finally it does seem that the only really successful wing masts in dinghies have been the hyperbolic leading edge square back mast froom Bethwaite, and its successors as used in NS14s etc these days, which could be decribed as hyperbolic front and rear. I am not aware of a "conventional" pear shaped style wing mast that has worked well enough on a monohull dinghy for its builder to persist with it, yet that is what the vast majority of attempts have been with.

Personally I keep thinking that there's unfiinished business with the things, but with all the problems of section, two part construction (part structural, part aerodynamic only) , tuning, weight etc... well its always been s*d that I'll buy a pole and go sailing...

 

My experience with wingmasts is with free-standing designs, which is different from the experiences of CT 249s cat rigs which are undoubtedly stayed. A stayed mast is much slenderer than a free-standing rig, and as a result the stayed rig begs to be "tuned" with the wires. The free-standing rig, of course, has no stays (or only headstay and running backs in the case of a jib-headed sloop) so there is nothing or very little to play with. You have to design the mast to sail on its own and be as forgiving as possible in a wide variety of conditions.

Eric

 

See Aerodynamics Of Teardrop Wingmasts

Bethwaite sent me some tracings of Tasar mast sections, but I've been unable to analyze them because the backward-facing step is too extreme for a panel code like XFOIL. Maybe some day I'll be able to tackle these types of mast sections with a Navier Stokes code like NS2D.

But the principles should be similar. There's going to be a separation bubble behind each step, like the case of the under-rotated teardrop mast. However, the separation point will be further aft. So the design objective would be to maximize the leading edge suction while minimizing the size of the windward and leeward separation bubbles. The pressure recovery after the leading edge suction peak would have its steepest portion in the attached flow on the mast.

The windward side separation point can be moved aft significantly, which may reduce the size of the windward separation bubble despite the aft facing step. But the separation bubble on the lee side will inevitably be larger than for the corresponding teardrop section. This may be why the Bethwaite sections have evolved from a rounded wedge to more of a boat-tail trailing edge that has a reduced base area.

 

True. XFOIL has a inverse boundary layer method that can handle modest amounts of separation. Using it to model the a wingmast is probably at the extreme edge of its validity, if not beyond.

That said, I think it still provides good insight into what's happening around an wingmast, and the trends due to changes in wingmast geometry and trim.

Unfortunately, what published data I've found on wind tunnel tests of wingmasts don't include the coordinates for mast or sail, and they also tend to be 3D models, not 2D. So it's hard to validate XFOIL's prediction of wingmast section characteristics.

It would be interesting to compare the XFOIL results with, say, tose from a Navier Stokes code. Do I smell a MS thesis topic here?

 

One factor to consider with wingmasts is the amount of un-reefable area. Every account I've read of a racing multihull that went to sea with a large wingmast had the skipper coming back home saying, "Never again."

 

This may be an extreme example, but wingmasts of all sizes are de rigueur in landyachts. The sails are cut perfectly flat - no broadseaming at all. The only camber in the sail comes from a bit of luff round and mostly from stretch of the material.

One sailmaker, Charlie O'Leary, uses a wingmast in a pocket, although in his case, the pocket is just bigger than the mast so it's more a case of not having to deal with a luff groove in the mast than any aerodynamic feature. One day, I was in just the right position to sight right down his mast as he came almost right at me on the beat to the finish line. I was amazed to see he had under-rotated the mast to the point that the sail bulged out to leeward of mast. Of course, he was hooked up and moving fast, and he would have gone with more camber for accelerating off the line.

I think a very interesting rig would be an egg-shaped mast, narrow end forward, in an oversized luff pocket. The shape would provide a far better leading edge shape than a round mast, and rounded back to the mast would avoid any crease in the contour where the sail left the mast. The pocket would fill where the windward side separation bubble would be, maintaining attached flow.

The mast may require positive rotation control, depending on where the pivot was located. There would be pro's and con's to terminating the battens at the mast for camber control or at the pocket.

 

I too think an egg shaped mast might be interesting. What you are really looking for at least aerodynamically is the ability to adjust the leading edge diameter. As you rotate a round mast, the leading edge diameter remains the same. An egg shaped mast with nose pointed forward, however, would permit the leading edge radius to increase as the mast is turned. Although on small sailing craft with small mast diameters this may not have a great effect. On larger masts with tracks, I don’t think it wise to have the sail behind a rounded section. A smooth transition between the mast and the sail is essential is it not? A tear drop shaped mast with an egg shaped nose may be beneficial for masts with tracks. This shape is very similar to elliptical mast sections depending on the cord to width ratio.

 

Modern NS14 sticks; must fit inside a 100mm ring.

Although there's no way of isolating mast development from hull, foil and sail development, it must be said that a Tasar just can't compete with a modern NS14 in most conditions. The NS14 is now clearly faster apart from downwind in light to medium winds, so the mast can't be too slow.








http://www.sailinganarchy.com/forums/index.php?act=Attach&type=post&id=31197

 

That carbon mast section is being encouraged by a gentleman that goes by the name Emo on the NS14 forums (http://forum.ns14.org/) He has posted a couple of pictures of test sections built by C-Tech (http://forum.ns14.org/viewtopic.php?p=865&highlight=#865) It looks like a miniature version of the mast designed by Eric for Wobegon Daze; an elliptical section with almost the same chord to width ratio. Wobegon Daze mast 21 x 10.5, at the deck I presume. MK4 90mm x 44mm (I’m not sure I understand the drawing. It has dimensions listed in ml) Of course the actual construction technique is different.

 

I suspect I'm a little-ender while you're a big-ender. I see the egg-shaped mast as a way to angle the leading edge to control the camber. It also provides a much narrower leading edge, which is more typical of high-performance sections, but has to be oriented correctly to avoid developing a leading edge suction peak.

 

I can see where you are coming from, if you are in a racing situation, and you are constantly in tune to your vessel, and able to constantly adjust the rotation to stay in the groove, I think a smaller leading edge has performance benefits, especially if the rotation is adjusting camber at the same time. Then the system can be tuned to give proper leading edge diameter/angle of attack for a range of camber settings. I think you alluded to this in post 36

“The mast may require positive rotation control, depending on where the pivot was located. There would be pro's and con's to terminating the battens at the mast for camber control or at the pocket.”

You are right in saying that I am a big-ender, and I will reiterate for the sake of others who are studying this subject, and also so that my thinking may be brought under scrutiny. Keep in mind as far as my purposes; I am studying to design an efficient Blue water cruiser, so my needs are different from the racer. For instance, I am not always going to be attentive to the mast angle of attack, or even the camber, so I need a system with some forgiveness built into it.

Weick and Scudder have shown that sharp leading edges applied to a Clark Y section has
the following effects; a decrees in Cl max of up to 7.5, A decrease in drag, a shift from trailing edge stall to more of a thin airfoil stall (angles of attack of 15 degrees or less), a delay in separation at the leading edge,(within a narrower range of cambers) with more difficulty reattaching flow once separation has occurred. “The Effect on Lift, Drag, and Spinning Characteristics of Sharp Leading Edges on Airplane Wings”, NACA TN 447, February 1933 radius. This isn’t the best study, there are others, I can’t seem to find at the minuet.

In general larger leading edge radius's have the following qualities:
Most wing sections achieve Cl max with leading edge radius's between 1.5% and 2% of the chord (of the wing section, not the mast alone), fatter nose radius's allow for a wider range of cambers without separation toward the leading edge., wing sections with greater leading edge radius are capable of greater angles of attack, delaying stall. Fatter leading edge helps maintain laminar flow within a wider range of wind speeds. The penalty for all of this is higher drag (but not much higher) These are some of the qualities that a cruising yacht needs.

Ideally a mast needs to constantly rotate to keep an ideal angle of attack. We have auto-helms; in the future we will have auto-mast rotation.

 

I think leading edge radius is a red herring. The NACA needed a way to close out their 4-digit thickness distribution so they spliced in a small circular arc to round off the leading edge.

But this is not the best way to shape a leading edge for may reasons. First of all, there's an abrupt change in curvature between the leading edge radius arc and the remainder of the contour. The biggest reason not to use a circular arc for the leading edge is because now we can shape the entire leading edge based on the aerodynamic requirements.

For example, this Wortmann FX 74-CL5-140 airfoil has a comparatively narrow leading edge, even though it was specifically designed as a high lift section.


What really matters is the position of the stagnation point and the curvature in that immediate region. If the stagnation point is located where the curvature is high, the flow can be turned around the leading edge without generating a high suction peak that can promote separation. This is just like taking a turn at low speed with your car - you can turn sharply without excessive lateral acceleration. But if the stagnation point moves away from the high curvature area, the flow accelerates away from the stagnation point and then has to negotiate the leading edge - this leads to trouble.

Modern inverse design methods, like the ones in XFOIL, can properly shape the leading edge for the design operating range of angles of attack. This may result in a narrow leading edge shape like the Wortmann section above, or it may produce a surprisingly blunt section that can actually have a flat spot at the leading edge and rounded corners with higher curvature before settling into the gentler curve of the main part of the secition - you'd never get that kind of shape by specifying a leading edge radius. These methods also take into account the effect of the trailing edg on the leading edge, which is a massive influence.

If you've no idea of what the flow is actually doing at the leading edge, then sure, a large radius will help to avoid producing a pressure peak that can lead to leading edge stall. But it's so much better to specify the angle of attack range and let the leading edge shape fall out from that. With the free availability of tools like XFOIL and the graphical user interfaces that have been developed for it, there's really no need to stick with an approach that was convenient in the 1930's when they were in the early stages of understanding what made different sections tick.

 

"With the free availability of tools like XFOIL and the graphical user interfaces that have been developed for it, there's really no need to stick with an approach that was convenient in the 1930's when they were in the early stages of understanding what made different sections tick."

This is true, when you read through some of the old reports, often times test data did not always produce results that where consistent with theory. It is evident that there were other design issues that, at the time could not be taken into account, at least not without further testing. Modern design and analysis tools are very helpful in that several design criteria can be considered simultaneously. For instance, as you have pointed out, trailing edge effect and a whole host of other criteria. I don’t mean to discount these other issues; the focus has been on the leading edge because this thread is about rotating wing masts, of which the mast makes up the leading edge (at least for the main sail).

One note of caution; as we deliberate back and forth, you will notice for simplicity sake, we will use a single wing section as an example for discussion. Sails are capable of producing hundreds of different wing sections. Within a single hour a sail may change its span, chord, camber, thickness, thickness distribution, leading edge angle of attack, trailing edge angle, etc. It becomes evident that what ever design criteria we pick for an optimal mast section, of utmost importance is its ability to perform well across a broad range of possible wing sections. It seams a constantly varying radius from the nose aft is at least one desirable design criteria. Egg shaped, teardrop shaped and elliptical all have this characteristic at least to some degree. It would be nice to see some test data. Until then I think I will focus on other aspects of the mast, like sail attachment, construction details, sail plan.

 

I agree with the thrust here MO,

A cruiser is not going to adjust everything contuously. And there are real limits for racers too.

Generally the fastest boats are set up to automatically adjust - to reduce the time the crew spend with "their heads in the boat".

I can't imagine any useful cue that would allow changing of mast trim in this way - how would it be possible to SEE that the flow wasn't quite right.

With catamaran rigs you can get the rotation right as some of the leeward telltales flatten out when the angle hits a certain point - but I suspect this "looks nice" rather than is an optimum solution.

Upwind anyhow most of us try to minimise the conscious changes apart from tiller angle and some mainsheet trim - at least when settled down after tacking. I might extend to adjusting the vang/cunningham through the gust cycle with some boats that have stiffer masts where things happen less automatically.

But to monitor an accurate mast rotation would have my head facing the wrong way altogether for efficient sailing - if steering I should be splitting between the tufts and oncoming wind and waves and tactics and the rest.

A control that takes a lot of concentration is likely to be slow compared to self adjusting systems.

MIK

 

There are a number of ways to see the correct mast trim. One way is to put short telltales in a horizontal row on the luff of the sail and on the mast. These alow you to see how far back the windward separation bubble extends and if you're developing a bubble on the lee side. The closer you sail to the wind, the less mast rotation you need, just like flattening the sail using outhaul tension.

Another very useful device popular with landyacht sailors is a short windvane positioned right at the leading edge of the mast such that the tail of the vane just barely clears the mast. This vane is dominated by the flow at the mast, not the freestream wind direction. It will tend to stay fixed on one side, then suddenly pop to the other side and maintain its angle to the mast as the mast rotation is changed.

What's happening is the vane's tail will be on the opposite side from the stagnation point. When the vane is intermittently popping from one side to the other, the mast is rotated such that the stagnation point is right on the nose. A little more rotation, and the vane will move to the lee side and stay there. A little less rotation and the vane will move to the windward side and stay there. If the vane is mostly on the lee side and occaisonally popping to the windward side, the mast rotation is about right.

And then there's Skeeter class iceboat champion Don Clapp's method of trimming his sail from within a closed cockpit ahead of the mast, where he can't see the sail or feel the wind: "I change something, and if my head snaps forward, that's bad. If my head snaps back, that's good."