Understanding Wing Technology

uwt

From Tom Speer, 2/10/10:

Rigid wing rigs have these advantages over soft sails:
- The section shape and twist match the design shape
- Slotted flaps can develop a higher maximum lift coefficient
- Larger cross sectional moment of inertia for greater stiffness and strength
- Less drag at low angles of attack due to avoiding windward side separation

Rigid wing rigs have these disadvantages over soft sails:
- Inability to reduce area as the wind increases
- Heavier (generally)
- Difficult to transport and handle
- Must be "flown" 100% of the time, making mooring problematic
- More expensive (many more parts, labor to construct)

The higher maximum lift coefficient of slotted flaps has been a key factor in their success in C class catamarans, with their sail area limit that precludes the use of spinnakers off the wind.

AFAIK, the most widespread use of rigid wing rigs has been in landsailing. They have dominated the C class cats, but there are comparatively few of those compared to the larger number and greater variety of landyacht wings. The fastest of the rigid wing landyachts run neck-and-neck with the fastest of the wingmast/soft-sail landyachts, so one can't generalize the superiority of one over the other.

In bad weather, rigid wings are taken down and stored indoors. Landyachts at a regatta are typically "moored" by using dollies under the rear wheels that have their axles aimed at the front wheel. This allows the yacht to pivot freely about the front wheel, like a boat on a mooring, but the toed-out dolly wheels resist backward motion. It's not uncommon to see tracks in the morning that indicate the yacht has swung through 360 degrees during the night.

In principle, a rigid wing yacht could live on a conventional mooring. However, when a gust or wind shift hits the wing, it will develop lift and drive, causing it to sail forward and possibly capsize. Aerodynamic control of the wing using a tail to make it aerodynamically balanced allows the wing to react much faster than the hull can move, which will alleviate gust loads. However, the wing must be able to swing through 360 deg (and for an unlimited number of rotations) if the hull is to remained fixed. Aerodynamic control has been successfully used offshore in the form of the Walker Wingsail, although I don't know how extensively they have sailed offshore.

The experience of sailing a rigid wing has some big differences from sailing soft sails. A wing doesn't "talk" to you the way a soft sail does because it doesn't luff. Instead, it backwinds, which can slow you down quickly or even pitchpole backwards bow over stern. When tacking, if the wing is not rotated quickly enough to follow the hull, the angle of attack will become negative and the wing will backwind as the hull turns. If the wing is rotated too quickly during the tack, the wing will backwind. A wing that has positive control (manually rotated instead of being free to turn) takes some practice to tack properly. An aerodynamically stable wing that is free to turn is easier to tack because it can be allowed to feather itself, like a luffing sail. It can also be sheeted conventionally, pulling in on the wing and allowing aerodynamic moments to rotate it outboard.

Especially for high-speed craft like landyachts, flutter is a definite possibility. This is a self-sustained oscillation due to coupling between rotation of the wing about its pivot axis and the side-to-side translation as the wing rotates laterally about its mast step due to flexibility or slack in the rigging. Positive control is sometimes used to minimize flutter, but moving the center of gravity forward, preferably on or ahead of the pivot axis, is a more effective remedy.

The relevant telltales are on the lee side, where they are not visible through an opaque rigid wing. Phil Rothrock solves this problem by having a wind vane forward on the body of his landyacht, with the arms that form the references slaved mechanically to the wing so they rotate with the wing. This allows him to gauge the wing angle of attack as he's looking forward to see where he's going. Another form of telltale that would work with a rigid wing is the close-mounted wind vane at the leading edge, like is used by many wingmast sailors to locate the stagnation point.

Going back to your original question about efficiency, If a softsail rig is given the same height as a comparable rigid wing rig and designed to avoid separated zones, then there's no fundamental reason why it would be less efficient. Those are two big "ifs" though. The flip side is also true. If you design a rigid wing to have a low aspect ratio, it may not give you any performance edge over a soft sail.

The ability to adapt the soft rig to the sailing conditions can give it the edge over the rigid wing rig - adaptability through wing twist was the key to Cogito's success. The ability to add sail area or to reduce it through reefing makes the soft rig the better choice for most application.
__________________
Tom Speer

 

uwt- USA 17=sloop rig

from Tom Speer 2/23/10;

Quote:

"Perhaps it is important to note that the entire wing aft section is actually made up of many unique and independently controlled sections that do not connect to one another at all. ... "


That's not quite right. Each flap is pinned at the trailing edge to its neighbors. The control arm at the base of each flap thus controls that flap and the one below it. The gaps between the flaps were covered with a stretchy material to seal the gaps, forming one continuous surface from an aerodynamic perspective.

In order to reduce weight and maintain stiffness, the spar was straight and located at the maximum thickness of the wing. That meant the hinge line had to be curved. Which meant that there had to be some gap between the flaps because they would come together when deflected.

Really, the wing is not that much different from the soft sail rig. There's a rotating mast and a mainsail attached to that. For the wing, the mast is larger in chord and the mainsail is thicker. But the cross section topology is the same for both rigs - a teardrop shaped mast, a small gap, and a much thinner mainsail that is articulated to be cambered relative to the mast. The hinge points where the flaps attach to the main element/mast are just like batten cars that are fixed in position instead of being on a track. The flap ribs are just thicker battens. The covering is also a woven material, so USA 17 could be said to have a more traditional sail material than A5's molded 3DLs!

Despite its size, the main (forward) element of the wing can be considered to be a leading edge device for the mainsail, just as the mast is for the soft sail rig. Deflecting the flap one degree while holding the main element fixed relative to the apparent wind will produce more than 80% of the lift one would get by rotating the whole wing one degree while holding the flap fixed relative to the main element. Which also means that rotating the main element while holding the same orientation of the flap relative to the apparent wind will only change the lift by less than 20% compared to rotating the whole wing. So qualitatively, it's where the flap points that matters.

That's one reason why twist control on USA 17 is all done with the flap instead of by twisting the main element as was done with Cogito. It was a simpler and lighter way to go for a wing that size. This is another way that the wing is similar to the soft sail, where twist is all in the mainsail, too.

It may look different, but it really is a sloop.
__________________
Tom Speer

 

At least once a day over the past few weeks I have studied the Patient Lady pdf (above) to try to understand the twist system. I've just about got it.
One thing I don't yet understand on Patient lady is: why is the flap angle of rotation not parallel to the wing axis of rotation and what is the significance of that?

 

Why should they be parallel? remember the flap doesnt have a great deal of torsional stiffness

 

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That was exactly Steve Clarks answer to my question. How should I know why they should or should not be parallel. My question was to FIND OUT if there was any design reason for the difference not to advocate for them being parallel!
My experience suggested that since most things in wing design are done FOR A REASON that there might be a reason that escaped me......

As to whether or not that non-parallel flap axis was done for a reason, Mr. Clark said: "no".

 

Doug:
Don't start whining.
Point is that the axis of flap hinges and the axis of the wing rotation are completely different problems. Because the both have the word "axis" doesn't mean they are related.

Wing rotation is about an axis comfortably forward of the quarter chord. The same principles that apply to balanced rudders apply here. Move the axis aft and the sheet loads go down. Go too far and you have a wing that doesn't reliably sheet out... very bad.
It is useful to draw that axis because the hounds design is a bit more complex than on a simple mast. There is the need for some form of rotating gantry or Lazy Susan to get the shrouds outboard away from the wing surface therefore permit the full range of rotation. This should be oriented with regard for this axis of rotation so that the rig tension does not change with rotation.
Rig tension is the primary device for stiffening the platform against wracking. The four stays and the mast form a king post truss. So consistent tension is desirable.

The primary design consideration for the placement of the flap pivots is the movement of the flap relative to the other elements. There are many inputs here, but generally, the further forward the flap pivot, the more the flap will move laterally for a degree of angle. This is because the flap leading edge is traveling along a larger circumference. There is lots of room for experimentation possible here, but much of it can be done by theory or simulation. Flap deflection has two components: lateral movement away from centerline and flap angle. Having the flap on an arm rotating around a single pivot is a practical simplification of these functions. It is in many ways far from ideal, but it is a light and reliable compromise.

The flap hinges on the PL wing are aligned for ease of construction and so that the flap stays straight as the flap is deflected. BMWO wing had a curved array of flap hinges and the flap required a number of expansion joints as a result. Their intent was to get the optimal span wise lift distribution and these expansion joints were a necessary complication. Miss Nylex had a kink in her trailing edge, The flaps required a V shaped gap between them. There is no single correct answer, there are many compromises.
SHC

 

Thanks very much for the info, Steve.

 

Doug, there is a PDF on the net somewhere that seems to have some quite good information on spanwise distribution in relation to RC planes, Ill post a link when I find it again. If you interpolate between the spanwise distribution information and possible engineering solutions the PL wing becomes almost elegantly simple

Maybe if Steve is reading this, or anyone else that knows...
It appears that the latest Cs have twist control on the main element as well, this brings a couple of questions to mind if one were to build a smaller wing (significantly shorter span), would twist control on the main element give a big benefit?
Im guessing the answer to this may be different depending on the prevailing wind conditions and possibly sea state, or more specifically lamina or turbulent boundary layer (wind on sea not wing), with twist control giving a bigger benefit with a turbulent layer due to the larger change in wind speed over the span of the wing.

 

Twist: etc

Cogito pioneered the twisting main element in 1995. Most subsequent wings have had this capacity.
I have not distributed that stuff over the Internet, but I have explained it in public and on tape dozens of time.
It is hard to make specific design decisions based on the exact nature of the boundary layer because the boundary layer is quite variable. Surface temperatures, sea state, and dozens of other conditions have a measurable effect the velocity gradient. Then there is the predicted speed of the boat and the height of the wing, both of which are significant variables.
So the answer is "depends." Not very helpful, perhaps, but then again figuring out the apparent wind vectors for an idealized boundary layer and a target boat speed and heading isn't that tough either. You have to do what we all do. Make a bit of a study, look at the data and then take your best guess.
SHC

 

That is pretty much the answer that I was expecting... The whole thought process started after a few beers at the yacht club last week with one of the R-class guys and after seeing pictures of Adam Mays moth.

It all seems simple until you scratch the surface!

 

This is a paper by Steve Killing with very good wing design information:

 

Understanding Wing Technology-----2010 Wings

Here are some of the pioneering wings of 2010:

Aethon, BMW, Moth(Adam May),
Canaan, BMW again(short wing) and Moth(designed by Magnus Clarke and Steve Killing/pix by Thierry Martinez)-click on image

 

Hi Guys,
I think I understand how the twist is induced and controlled, but I'm struggling to figure how the surfaces are twisted without distorting the covering on the wing?

Cheers
Mojo

 

The flaps are not very rigid in torsion.
Wing surfaces have some strain relief built in. With the film covered wings this is done by having a fairly stretchy film that can absorb the distortion. The molded noses have slotted connections and expansion joints to allow the surfaces to move relative to each other by a bit.
Aethon's 100% molded wing has quite a few of these joints to allow for the distortion of the structure due to twist and sag. Getting the range of these expansion joints right, and making sure they operate properly was one of the things that kept this wing off the water this summer. We are still knobbling away at it, but with much less urgency.
SHC

 

Thanks Steve, Your input is appriciated.
Can you tell me the name of the film generally used?

Cheers
Mojo