Wingsail Flap & Direction Control

I am trying to determine the simplest, most reliable method to control the movable wing surfaces (forward slot and aft flap) and direction (tail feathers) for a rigid wingsail I would like to build for a Motorsailer. The concepts for the Motorsailer and the wing may be found on my web site: www.wingsailor.com by clicking on the "Wingsails" and "The Ideal Motorsailer" tabs. I have a very good idea regarding the wing cross section and moveable surfaces but control methods for these surfaces offer lots of opportunities. The actual movement could be accomplished using mechanical (wires, cables), Hydraulic or electrical. Modern aircraft use hydraulic and electrical and, I would hope, are very reliable. The controller would be human, not a special computer program as used in the Walker Wing Sail.
Has anyone messed about with wings and the systems that can be used to control the surfaces?

 

I've sailed a landyacht with a free-rotating rigid wing rig. The yacht was wind tunnel tested at the University of California San Diego.

The wing rotation was controlled aerodynamically. Unlike the Walker Wingsail, which can be classified in the "big tail, short arm" approach to achieving a given tail volume (tail area times moment arm), we used the "small tail, long arm" approach. Since aerodynamic damping goes up as the square of the tail arm length, the latter provides much higher damping than the former.

We used Morse cables to connect a bar to the flap and tail. The bar could be split in two to control each surface separately or linked together so one control was used for both (with the flap and tail rotating in opposite directions). Worm screws with small cranks allowed the pilot to vary the attachment points of the cables so as to adjust the sensitivity of the controls and the relative movement of flap compared to the tail.

In practice, we found the linked flap and tail controls worked. We found separate flap and tail controls worked. We found that fixed surfaces worked. Most pilots preferred to set the tail at neutral, and then physically position the wing by grabbing the tail boom (which passed just over the pilot's head) and holding it where they wanted it to be. Because the wing was aerodynamically balanced by the tail, it only required finger-tip pressure to trim it.

We initially started with a canard configuration, but we found the aft tail worked much better. The problem is the canard has to be more heavily loaded than the wing in order to have a stable system. When the canard stalls, the drag contributed to the rotational moment, and the rig slewed out of control. With an aft tail, drag on the tail is stabilizing (like the cloth tail on a kite).

Most rigid wing landyacht sailors use positive control of the wing, meaning the wing is not free to rotate, but is held at a fixed angle by the control system. The most popular control consists of a large sprocket or sheave attached to the wing, and a wheel with a small sprocket that drives a chain connected to the wing. The flap is adjusted by means of a handle and locking mechanism at the base of the flap. Caterpillar tractor throttle levers are highly prized for this purpose. They have a friction mechanism that allows the lever to move easily in either direction, but lock in position when released.

For a cruising boat, I would go with aerodynamic control, especially of the small surface/long tail variety. We found the gust response to be very good - too good for a racing boat, because when the gust hit, the wing would sheet out and miss the "punch" of acceleration as a gust hits a fixed rig. Aerodynamic control handles the active management of the wing automatically, and the manual control is used to adjust the trim. A wing must be "flown" 100% of the time. You can't just furl the sail, so the wing has to be active even when moored, even if you are swinging to a mooring on the water. That's why, for a cruiser, I would rule out positive control that results in a wing locked in position.

It is very important that the wing be mass-balanced to put the center of gravity on or slightly ahead of the axis of rotation. The purpose is to avoid flutter and minimize disturbances due to motion in a seaway. An arm sticking out ahead of the wing can hold the ballast mass. It must be very strongly built. A large mass on a short arm will have less moment of inertia than a small mass on a long arm, but will be heavier. If you look at a Walker Wingsail, you'll see a big arm sticking out ahead of the wing, with a solar panel on it, IIRC. The solar panel mounting is opportunistic - the real reason for the arm is to mass balance the wing. Personally, I would mount the balance arm much higher, to take advantage of lateral motion of the wing.

The effective windage of a wing includes not only the parasite and induced drag of the wing, but also the negative lift that can occur as the wing lags behind the changing wind direction or if the wing is swung by wave motion. This can be very high - I've pitchpoled a landyacht backwards when a wingmast fluttered on a tack. Repeatedly. The windage will be far higher than you're postulating with the airfoil vs rod comparison on your website.

For example, say you have the tail and flap neutralized to feather the wing into the wind. When the wind shifts, there will be an initial difference between the wind angle and the orientation of the wing, creating an angle of attack, delta-alpha. The normal force on the wing will be approximately qbar*area*CN_alpha*delta-alpha. The normal force on the wing will be tilted downwind by delta-alpha, so the additional drag qbar*area*CN_alpha*(delta-alpha squared). The effective drag coefficient for this added drag is approximately CN_alpha*(delta-alpha squared), which can easily be far greater than the minimum profile drag coefficient you may be assuming will characterize the windage.

If the wind were to hold the new direction, the wing will rotate to match it, and delta-alpha will go to zero. But the wind doesn't hold its direction, it is constantly changing. The wing will be constantly lagging behind what the wind is doing. And if the wing gets near 180 degrees out of phase with the wind shifts, the oscillations can get quite wild. For this reason, the wing must track the wind closely. The natural frequency of the wing needs to be significantly faster than the frequency content of the gusts you want it to track. The natural frequency is largely determined by the tail volume and the moment of inertia of the wing. The dynamics of a freely rotating wing in a seaway will be a lot of fun to work out analytically, and will have a big influence on the effective windage.

It is important that the wing on a watercraft be able to rotate 360 degrees, especially when tied to a dock. You can achieve this with mechanical controls if they come up through the pivot axis. For example, a vertical pushrod can come up through a tube at the pivot, and then activate the surfaces via bellcrank and additional rods or cables. The rod can be allowed to rotate at either end (or both) to allow the wing to rotate freely without restriction. Two controls can do the same thing by having the two pushrods be tubes that are concentric with each other.

If you have a slat, it's not necessary to have any controls at all. You can arrange the hinge moments on the slat so that it will tack itself from one side to the other, and come up against hard stops at the desired position. You may want to consider a four-bar linkage to allow the slat to clear the wing's leading edge and move back to achieve overlap with the leading edge, while rotating the the slat so that the trailing edge moves more than the leading edge.

However, it should not be necessary to use a slat or forward slot at all. Good airfoil design will allow you to have a leading edge that will operate well at high lift without a slot. Aircraft use slats because their sections are primarily design for efficient cruise at high speed and low lift, and have to reconfigure for takeoff and landing. Drag is actually an asset when landing because it steepens the approach angle. Wing rigs don't really operate in the high-speed/low-drag regime except when feathered. Their sections can be optimized for high lift/drag ratios at high lift. A slat adds complexity, parasite drag from the mechanism, and moving parts that degrade reliability.

 

Thanks, Tom

Tom,
I have seen your posts on other sites. You are the best. It is immeasurably helpful to get suggestions from one who has been there and actually done it.
Many of the solutions you have found have been running through my mind (and on Yacht Club Bar napkins) for 15 to 20 years. I am finally ready to get started finalizing the wing cross section and start building.
The reason for the forward slot was to create "twist" as much as to increase lift. The twist would be created by using a symetrical center structural surface with a flap aft and slot forward. The aft flap would be wider at the bottom and the slat leading edge wider at the top. The whole wing would look like a rhomboid with the top forward. It seems from your calculations that the slot would add more drag than it is worth. In addition it would add a little more weight increasing momentum.
Thanks for the info. It will shorten the build.
The experimental trimaran for the first iteration may be viewed on my website: www.wingsailor.com
The boat is called "FloWinGo". It is completed with wing bearings in place. I have also build the strut (2" thick wall aluminum tube with unidirectional carbon over a wooden clothes pole stuffed inside. I have the large pulltruded styrofoam blocks, the hot wire cutters and transformer on the shelf at my warehouse. I hope to get started this summer.
Regards and thanks again,
Captain Roger