hard sail trim flap

i am a student and am studying boat design i have just started. me and a another friend are planing on designing a yacht with a hard wing sail our lecturer is away on paternity leave.

we were wondering what was the purpose of the trim flaps on the sail and on the arm on the back of the sail.also we are not sure on now to move the sail and is a symmetric or asymmetric more efficient.

Tom

 

The flap is how one gets an asymmetrical section that works on both tacks. Each element is made up of symmetrical sections, but the ensemble is asymmetrical when the flap is deflected. Flap deflection does the same thing as camber in a sail. It increases the maximum lift, and it reduces the profile drag when operating at lift coefficients below maximum lift.

Think of a wingsail as a sloop rig. There's a rotating mast and a mainsail, usually with a small gap between them. The mast is really big compared to normal practice, but the way it works is still basically the same. The angle between the flap and the apparent wind determines how much lift the section makes, just like the angle between the mainsail leech and the apparent wind does.

You will want to be able to control the rotation of the whole wing as it sits on its pivot. This is like the sheet or traveler control on a soft-sail rig. You will also need to be able to control the flap deflection. This is like the outhaul that controls the camber of the sail. And you will want to arrange your control system so that you can vary the flap deflection between the foot and the head, ideally by twisting the flap so you get a smooth variation along the span. This is equivalent to using the vang to control the twist of the mainsail.

Trimming the wing is a lot like trimming a soft-sail rig with a rotating mast. Sheet in and out to get the power you want. When in doubt, use more flap deflection and less angle of attack. Don't try to sail with flat flap deflection unless you are trying to feather it to depower. Flat with lots of angle of attack is draggy compared to cambered at low (even negative) angle of attack to develop the same lift, even if the wing is not stalled.

There is a range of flap deflections and angles of attack that will produce the same lift, but you want the combination of flap and angle of attack that produces the least profile drag for the required lift. Rotate the forward element to windward by increasing the flap deflection (camber) until the stagnation point is near or a little to windward of the leading edge. This will be easier to judge if you mount a dinghy windvane on the leading edge so that the tail just barely clears the leading edge. It will flick from one side to the other, depending on where the stagnation point is. If the tail is mostly to the lee side and flicking occasionally to the windward side, you probably have the flap deflection about right as long as the flap deflection is less than 20 - 25 degrees. This will generally put the forward element at a negative angle of attack relative to the apparent wind. Trimming with the stagnation point near the leading edge may not be the absolute minimum drag configuration, but it will be close.

If you are trimming for max power as opposed to max efficiency, then use a maximum of around 30 deg of flap and sheet in until close to stall. The profile drag will start to rise significantly above about 25 deg flap deflection, but if the wing stalls before you get the power you need, then use more flap.

If overpowered, twisting off the flap at the head will allow you to still develop power down low by using a lot of flap deflection (camber) at the foot.

The flap is typically hinged at 90% - 95% of the forward element chord, with the links just long enough to allow the flap leading edge to clear the trailing edge of the main element as the flap tacks. This will result in a slotted flap when deflected. The flap section should be fairly thin, as the leading edge suction peak that would normally form on a thin section at high lift (causing leading edge stall) can be suppressed by closing the gap (moving the hinge point closer to the forward element trailing edge, while keeping the flap position fixed).

It would be useful to make the hinge line adjustable, as varying the gap is important for tuning the rig for maximum performance. You can provide for multiple holes for the hinge pins. Or you can put the hinges in tubes or rods (think nunchucks) that can be slid into fore-aft holes/tubes in the forward element and flap and secured by pins or set screws to vary the hinge location.

The forward element section needs to be thicker and have the thickness fairly far forward because at high lift the stagnation point will be well back on the windward side from the leading edge, and the flow will accelerate significantly as it goes around the leading edge. A thin section or a sharp leading edge will stall under these conditions.

The sheet can be rigged like a conventional sheet, but you don't need to pull down on it to generate leech tension. So the more horizontal, the better. A continuous sheet that goes to each outer hull/gunwale instead of centerline will allow you to pull the trailing edge to windward.

Control arms on each flap segment are typically connected via cables that run down to the bottom flap segment so the whole flap moves when the bottom flap moves. If the sheet is connected to the trailing edge of the bottom flap, then the pressure on the wing will make it want to camber in the right direction and change automatically when tacking. This also means the control mechanism for camber only has to limit the amount of camber, and the same control (and control setting) will work for both tacks.

Another way to rig the flap is to have a handle at the base of the flap that rotates the whole flap, with the torsional rigidity of the flap controlling the upper sections. A mechanism is needed that holds the handle in position, but allows it to be adjusted - Caterpillar throttle levers are a prized solution for this among landsailors.

Keep it light! And make it tall to minimize the lift-induced drag, which is the main source of drag from the rig.

And don't forget that a wing has to be "flown" 100% of the time when it is up. It cannot be left to fend for itself unless it is capable of swinging 360 degrees for an unlimited number of turns. Someone needs to be with the boat whenever the wing is up.