Skin-Friction Drag An important aerodynamic force during low ...

Skin_Frictionhttp://www.centennialofflight.gov/essay/Theories_of_Flight/Skin_Frict...2 of 3 1/1/2011 8:35 AMA the velocitybecomes a constantvalue V. The layer offluid where thevelocity is changingfrom zero to aconstant value isknown as theboundary layer.Figure (c) compareslaminar andturbulent flows. Asis usual for turbulentflow, there is arandom motion inthe boundary layer.Real fluid flow aboutan airfoil. Thethickness of theboundary layers andwake are greatlyexaggerated. Thebottom flow alonglower surface is thesame as on theupper surface.Figure (a) comparesthe ideal fluid casestatic-pressuredistribution at theairfoil surface andcenterlinestreamline with thereal fluid case.Figure (b) illustratesthat, in the idealfluid case, the netstatic-pressureforce acting on thefront surface of theairfoil (up to theshoulder) parallel tothe free streamexactly opposesand cancels thatacting on the rearsurfaces of theairfoil. In the realfluid case, shown infigure (c), the netresult is a drag forcedue to theasymmetricpressure distributionaircraft wing is generally less than a centimeter (2.5 inches). Yet, the velocitymust vary from zero at the surface of the wing to hundreds of meters per secondat the outer edge of the boundary layer. It is evident that tremendous shearingforces (internal friction) must be acting in this region. This gives rise to theskin-friction drag.Applied to an airfoil in a real airflow, the same free-stream velocity V¥ andfree-stream static pressure p¥ apply. The field of air ahead of the airfoil is onlyslightly modified and for all practical purposes, the velocities and staticpressures are the same as for the ideal fluid case. Again a stagnation point (apoint with no motion) occurs at the leading edge of the airfoil and the pressurereaches its maximum value of p t at this point (total or stagnation pressure).From this point on along the airfoil, the picture changes.As noted earlier in the example of the flat plate, a boundary layer begins toform because of viscosity. This boundary layer is very thin and outside of it, theflow acts very much like that of an ideal fluid. Also, the static pressure actingon the surface of the airfoil is determined by the static pressure outside theboundary layer. This pressure is transmitted through the boundary layer to thesurface and thus acts as if the boundary layer were not present at all. But theboundary layer feels this static pressure and will respond to it.Over the front surface of the airfoil up to the shoulder, an assisting favorablepressure gradient exists (pressure decreasing with distance downstream). Theairflow speeds up along the airfoil. The flow is laminar and a laminar boundarylayer is present. This laminar boundary layer grows in thickness along theairfoil. When the shoulder is reached, however, the air molecules are movingslower than in the ideal fluid case. This is an unfavorable condition because theprevious ideal flow just came to rest at the trailing edge of the airfoil. It wouldappear now, with viscosity present, that the flow will come to rest at somedistance before the trailing edge is reached.As the airflow moves from the shoulder to the rear surface of the airfoil, thestatic-pressure gradient is unfavorable (increasing pressure with downstreamdistance). The air molecules must push against both this unfavorable pressuregradient and the viscous forces. At the transition point, the character of theairflow changes and the laminar boundary layer quickly becomes a turbulentboundary layer. This turbulent boundary layer continues to thicken downstream.Pushing against an unfavorable pressure gradient and viscosity is too much forthe airflow, and at some point, the airflow stops completely. The boundarylayer has stalled short of reaching the trailing edge. (Remember that the airflowreached the trailing edge before stopping in the ideal fluid case.)This stall point is known as the separation point. All along a line starting fromthis point outward into the airflow, the airflow is stalling. Beyond this line, theairflow is actually moving backward, upstream toward the nose before turningaround. This is a region of eddies and whirlpools and represents “dead” air thatis disrupting the flow field away from the airfoil. Thus, the airflow outside thedead air region is forced to flow away and around it. The region of eddies iscalled the wake behind the airfoil.Up to the separation point, the difference between the static-pressuredistribution for ideal fluid flow and real airflow is not very large but onceseparation occurs, the pressure field in greatly modified. In the ideal fluid case,the net static-pressure force acting on the front surface of the airfoil (up to theshoulder) parallel to the free stream exactly opposed and canceled that actingon the rear surfaces of the airfoil. Under real airflow conditions, however, thissymmetry and cancellation of forces is destroyed. The net static-pressure force