The Aerodynamics Of Sail Interaction - Gentry Sailing

.The Aerodynamics of Sail InteractionBy Arvel GentryProceedings of the Third AIAA Symposium on the Aero/Hydronautics of SailingNovember 20, 1971Redondo Beach, CaliforniaAbstractThis paper deals with the basic problem of the interaction between a mainsail and the jib. Since this paperDecemberis written for the sailor rather than the aerodynamicist, all aerodynamic terms and concepts are developedand explained as they are needed. The characteristics of the flow about the jib and mainsail airfoils whenthey are each used alone and when they are used together are discussed and illustrated. Results from theseflow field studies give a very complete and accurate description of the jib-mainsail interaction problem.1. IntroductionThe general subject of the interaction of sails, both onthe same boat and on boats sailing near each other, hasbeen of interest to competitive sailors for sometime. Thesailing literature abounds with attempts to explain justwhat is involved in the interaction between two sails. Suchterms as slot-effect, Bernoulli's equation, the venturieffect, mast effect, and even safe leeward position appearfrequently in what are apparently very rationalexplanations of this very basic sailing problem. However,the explanations are usually different in many respectsand as a result, this general problem has been the subject ofmuch argument and discussion over the years.As an aerodynamicist by profession and a weekendsailor (by preference), I have been interested, amazed andamused by this situation. After talking to a number ofsailors and reviewing both the sailing and aerodynamicliterature, I realized that there were several reasons whyarguments on this subject had persisted for so long.First, some of the difficulties of the sailor inunderstanding this problem stem from some completelyfalse and misleading ideas published in standard sailingreferences. Second, the explanations given in the literaturewere based on educated guesswork and practicalexperience and only a very limited amount of actualtheoretical calculations or measurements, not fromdetailed test measurements or valid and accuratetheoretical calculations. And third, attempts byaerodynamicists to properly explain the problem have notreached the sailor because of the technical language usedand also because the aerodynamicist's descriptions werenot always completely accurate themselves.Although it is recognized that a single paper such asthis cannot completely erase the years of argument toeveryone's satisfaction, the most important effects that arebrought about when two airfoils are used close togetherare illustrated and discussed. Important aerodynamicterms and concepts will be introduced and explained asthey are needed. The results shown in this paper wereobtained with the aid of some of the same techniques andC1999tools used by the aerodynamicist in the design of modernjet aircraft. Any educated guesswork will be avoided or, atthe least, clearly identified as such. Since this paper iswritten for the sailor, not the engineer, it will not dwell onthe mathematical details of the aerodynamic theories used(this type of information is well-covered in the referencedaerodynamic literature).2. Important Definitions and ConceptsA number of important terms and concepts should beclearly understood by the non-aerodynamicist before themain body of this paper is approached. A step-by-stepdevelopment of this fundamental aerodynamicknowledge is necessary before the reader can appreciatethe significance of the basic arguments and explanations tobe presented later.2.1 Division of Flow RegionsWhen the aerodynamicist studies the airflow about ashape, he recognizes that the flow can usually be dividedinto two basic types of flow areas, the external flow region,and the boundary layer region. These two flow regions areillustrated in Figure 1.lowal fnreExtTransitionalTurbulentLaminarFigure 1. Definition of flow regions about a thin airfoil.The boundary layer flow region is that layer of air thatlies very close to the airfoil. The thickness of this layer of airis greatly exaggerated in the Figure 1 for clarity. Air hasviscosity (even though it is very small when comparedwith other substances), and it is in the boundary layer thatthe viscous characteristics of air come into play. Because ofthis viscosity, the air that touches the airfoil is actuallycarried along by the airfoil (the air has zero speed withrespect to the surface of the airfoil). The air just a small1971 Arvel Gentry

.distance fromthe airfoil moves with some finite velocityRanger23 Newsletterwith respect to the airfoil. The air at the edge of theboundary layer moves with the speed of the external air atthat point on the airfoil. The remainder of the airflow willbe identified as the external flow. The viscosity of the airdoes not affect the aerodynamic calculations for this part ofthe flow. The techniques used by the engineer incalculating what happens to the air in these two types offlow are different.The boundary layer itself is usually divided into threeseparate types of flow. Near the leading edge of an airfoilthere is a very smooth change of airspeed within theboundary layer from the airfoil surface to the edge of theboundary layer. This is the laminar boundary layer.Eventually, because of the development ofunsteadiness within the boundary layer and because ofdisturbances introduced into the flow by roughnesses (jibhanks, cloth seams, etc.), the smooth changes in speedwithin the laminar boundary layer start to give way to amuch more erratic type of flow. This is called thetransitional region of the boundary layer. After this shorttransitional region, the boundary layer becomes fullyturbulent.The external flow is not appreciably affected by this sothe lift on the airfoil does not change much. The mostsignificant effect is that the skin friction drag is greater inthe turbulent area of the flow than it is in the laminarportion. For the portions of the boat that are underwater,great care is usually taken to keep the bottom clean andsmooth to delay the transition from laminar flow toturbulent flow as far back on the hull or keel as is possible.Laminar separationTrubulentSeparationFigure 2. Separation flow areas on a thin airfoil.Figure 2 shows another part of the airflow about a sail ,separated flow. Separated flow regions occur when theboundary layer is no longer able to follow the surface of theairfoil. When this point is reached, the boundary layerseparates from the surface and a very confused and mixedup type of flow is formed. (Note that from theaerodynamic standpoint, we refrain from calling this typeof flow turbulent, since the word turbulent is reserved foruse in describing the turbulent boundary layer. Instead,we will use to the term, “separated flow”).There are two types of separation: laminar separationand turbulent separation. A boundary layer will separatewhen the external pressure along the surface starts toincrease too rapidly. The more rapid the increase inpressure, the more likely it is that the boundary layer willseparate. The rate of change of pressure along the surfaceis called the pressure gradient. When the pressure is-2-increasing, the pressure gradient is called an adversepressure gradient.Whether or not the boundary layer separates whensubjected to a given adverse pressure gradient dependsupon: the character of the boundary layer, laminar orturbulent, what has happened to the boundary layerbefore reaching the adverse pressure gradient, and whatthe speed of the airflow is a the edge of the boundary layer.The speed-distance factor is expressed by theaerodynamicist in a term called the Reynolds number.In general, the laminar boundary layer is more proneto separation than the turbulent boundary layer. In somecases the separated flow will reattach itself to the surface ofthe airfoil. This is frequently the case for a laminarseparation where a small separated bubble may form andthe flow reattach as a turbulent boundary layer.If the flow separates from the airfoil (either a laminaror turbulent separation) and does not reattach, weexperience what is defined as a stalled condition. The lifton the airfoil no longer increases with angle of attack butactually reduces. We spot this situation by watching theyarn telltails on the lee side of our jib start to twirl wildly aswe bear off from a close-hauled course.The most important fact to remember from the abovediscussion is that we need an increase in pressure (anadverse pressure gradient) to cause separation. If thesteepness of this gradient is decreased, then theprobability of getting separation will be reduced. We willsee the importance of this later on when we examine theinfluence of the jib on the mainsail.2.2 Potential FlowThe term potential flow is used by the aerodynamicistto describe airflow that is not affected by viscous effects(the boundary layer or by separation). Potential flowtheory is a way of solving the external flow when we areable to neglect the boundary layer or separation. Since wealways try to shape our airfoils (sails) so as to avoidseparation and since the boundary layer is relatively thin,we will find that potential flow solutions have many veryuseful applications.By the use of potential flow theory, we will be able todetermine completely how the air flows past a singleairfoil or around a combination of airfoils such as amainsail and a jib. Potential flow solutions my be obtainedwith elaborate and advanced digital computer programssuch as in Reference 1, or by the use of the rheoelectricanalogy as applied with the analog field plotter. The use ofthe field plotter will be discussed in more detail later.2.3 StreamlinesThe concept of a streamline is very simple and weneed only to look briefly at the example in Figure 3 on thenext page to get the basic idea. The streamlines tell thedirection of the airflow at different points in the flow fieldabout an airfoil. The airflow between two particularstreamlines will always stay between the two streamlines.The stagnation streamline is the streamline that separatesthe airflow that goes on one side of the airfoil (the top orleeward side) from the airflow that goes on the other side