Sails

The key to understanding sails is aerodynamics, understanding is better than any amount of information on the subject since it enables you to make informed decisions based on facts. I will attempt to explain the aerodynamics in the best way I know how, coming from the world of aviation I might have a different view on the subject than some but ultimately the forces involved are the same.

A sail is a horizontal wing, it functions like any other wing wether it be a Boeing 747, Bell helicopter rotor blades or a propeller on a hover craft.

The picture on the right shows three different kinds of wing, the one on the top is a slow flying aircraft wing, in the middle is a symmetrical wing typical for aerobatic aircraft and the one on the bottom is a typical mainsail. The lower surface of the wing is not strictly required although it is beneficial.* What we need to understand are the main three forces acting on every one of these wings, lift, drag and the driving force. All wings are designed for a given balance of these forces, any change in one of these forces has an equal and opposite effect on the other two

The Lift is the most important force, it is what enables aircraft to fly, makes propellers efficient and it pushes our sailboats forward**. The lift is produced when a wing moves through a fluid or air, to best explain consider the following. When a wing moves forward it separates the air into two streams, one flows above the wing and the other below. Lift is created by deflecting air downwards, the amount of lift is determined by the volume of air, it's velocity and the angle relative to the angle of the wing. The most obvious form of lift is generated when the wing is at an angle to the air stream and air hitting the underside of the wing is deflected at the same angle downwards, this however only contributes a minority of the lift. Most of the lift is generated by the air travelling over the wing which will follow the contours of the wing and be deflected down at a steeper angle than the air deflected by the underside of the wing.

Drag is used to describe any unwanted side effects of moving matter, we usually hear it used to describe friction but it applies to other forces. With wings we are concerned with two kinds of drag***, Parasitic drag and Lift induced drag. Parasitic drag can be further split into three subcategories; skin friction, form drag and interference. Form drag makes up the largest part of parasitic drag, it's generated by any solid object traveling through a fluid/air, it can be reduced by using aerodynamic forms and reducing the forward profile of the object.**** Skin friction is fairly well known, it is the resistance of any two materials to move across the surface of each other, on supersonic jets this friction causes enough heat to melt steel!. Interference is specific to fluids/air, it is caused by the generation of vortices whenever fluid/air is forced to pass over a sharp corner/edge. Wing-tip vortices are caused by this effect, when high-pressure air under the wing tries to flow to the low pressure area on top of the wing. When the air crosses the wing-tip a vortex is generated. Lift induced drag is caused by the deflection part of lift, as the angle of attack***** is reduced so is lift induced drag.

Drive is the source of power, in regards to aircraft this is usually called thrust but in the case of sailboats the drive is generated by the wind which the sails convert into thrust.

To sum up, Lift = Good, the more the better, Drag = Bad but will always match the opposing forces, Force = The wind and hence ...... Force-Drag=Lift

There is still one concept which we have not discussed, the stall, as we know the amount of lift generated can be increased by increasing the angle of attack. This also increases drag but at low airspeeds this is often favorable. A stall happens when the angle of attack is raised beyond what the wing is designed for. The airflow over the wing dislodges itself and forms vortices, this causes an instant loss of the lift (only the deflection lift is left). For the wing to recover the angle must be reduced and stable airflow reestablished. This is incidentally the same phenomenon as caitation where you have a stalled propeller blade.

Commercial aircraft designers are left with a problem, lean narrow wings cause less drag and are favorable at high airspeed while big thick wings create more lift which is required at lower airspeeds for landing. The engineers had their cake and ate it by creating the flaps and slats, it made possible high speed wings which can be transformed into a high chamber wing in-flight! The two are shown on the right, the first is an extension on the back, extending from under or within the wing backwards. This effectively creates a semi-single-surface wing with a higher chamfer, this is clearly seen as the wing has been tilted to demonstrate the new wing form. The second type (called slats) moves the leading edge****** downwards creating the semi-single-surface wing seen before, it also has the advantage of causing a slot-effect which is the source of much confusion within the sailing community. Many commercial aircraft have flaps and slaps, in effect this enables them to use higher angles of attack and resulting in a lower stall speed.

To finish our aerodynamics session I want to apply these concepts to sailing and sails. I have found the world of sailing to be full of misconceptions and theories based on anecdotal evidence. Here are some theories I want to debunk, keep the pictures on the right as a reference.

	Two wings on top of each other are always less efficient than one wing (low pressure for one wing is high pressure for other wing)
	Slot Effect is only beneficial in allowing a higher angle of attack without stall occurring (better pointing ability)
	Double surface wings are more efficient than single surface wings, the only reason for single surface is ability to work reversed (both ways)
	Wings that are "pointy" towards the end are only beneficial for supersonic flight because of wave drag.
	The ideal sail/wing irregardless of stability/strength/ and ability to sail on both port/starboard is one shaped like the wing of a glider, very long and sleek.

All this being said we are going with Mylar sails and a cutter rig, practical and easy are the key elements.

Wikipedia:

en.wikipedia.org/wiki/Lift_%28force%29

en.wikipedia.org/wiki/Induced_drag

en.wikipedia.org/wiki/Parasitic_drag

en.wikipedia.org/wiki/Wingtip_vortices

Flaps:

142.26.194.131/aerodynamics1/controls/Page5.html

Aerodynamics:

www.grc.nasa.gov/WWW/K-12/airplane/presar.html

* Hang gliders originally had single surface wings, they later developed double surface wings which led to better glide angles, less drag and more speed.

** Both the propeller and the sails on the boat use the same effect to drive the boat through the water

*** There is a third kind of drag which is only applies when approaching supersonic speeds, this is called "Wave Drag"

**** Think about sticking your hand out the window of a moving car, try flipping your hand from a vertical to a horizontal position, the difference in force felt is the difference in form drag.

***** Angle of attack is the difference of angle from the apparent wind/travel direction and where the wing points.