Fluids – Lecture 8 Notes

Fluids – Lecture 8 Notes 1. Wing Geometry 2. Wing Design Problem Reading: Anderson 5.3.2, 5.3.3 Wing Geometry Chord and twist The chord distribution is given by the c(y) function. Each spanwise station also has a local geometric twist angle αgeom (y), measured from some reference line which is common to the whole wing. The freestream angle α is also defined from this same common reference line. The choice of the reference line for all these angles is arbitrary, although a common choice is the wing-center chord line. −αL=0 zero lift li ne local relative velocity αi α aero chord line α geom α common reference line for whole wing V How the geometric twist varies across the span is loosely described by the terms washout and washin: Washout: αgeom (y) decreases towards the tip. Washin : αgeom (y) increases towards the tip. washout washin If the wing has a spanwise-varying camber, the local zero-lift angle αL=0 (y) will also vary. It is useful to define an overall aerodynamic twist angle as αaero (y) ≡ αgeom (y) − αL=0 (y) The local αL=0 (y) and hence αaero (y) can be changed by a flap deflection δ. zero l zero lift li ne ift lin −αL=0 α aero α aero αgeom e −αL=0 αgeom δ Local loading/angle relations The local lift/span can be given either in terms of the local circulation Γ(y), or the local chord-cℓ product. L′ (y) = ρ V∞ Γ(y) L′ (y) = 1 ρ V 2 c(y) cℓ (y) 2 ∞ 1 Equating these gives the circulation in terms of the chord and cℓ . Γ(y) = 1 V∞ c(y) cℓ (y) 2 (1) Assuming the airfoils are not stalled, the local cℓ is proportional to the angle between the local relative velocity and the zero lift line. cℓ (y) = ao [α + αaero (y) − αi (y)] (2) The constant of proportionality ao = dcℓ /dα is nearly 2π for thin airfoils, but is somewhat larger for thick airfoils. It can be obtained from either experimental data or calculation. Finally, the relation between αi at any one location yo , and the entire spanwise circulation distribution, was obtained previously using the Biot-Savart law. 1 αi (yo ) = 4πV∞ � b/2 −b/2 dΓ dy dy yo − y (3) Equations (1), (2), and (3) form the basis of wing analysis and design. The general analysis problem is rather complicated and is somewhat beyond scope here. However, the design problem is simpler, and an example design problem is considered next. Wing Design Problem Elliptic loading A typical basic wing design problem is to determine the geometry of a wing so that it will have some specified load (or circulation) distribution Γ(y). As an example, consider the simple case where the span b and flight speed V∞ are given, and the circulation is to be elliptic. � � �2 2y Γ(y) = Γ0 1 − b Although we don’t yet know what the wing looks like, we do already know its lift and induced drag from Γ(y) alone. From previous results, π L = ρ V ∞ Γ0 b 4 (L/b)2 Di = 1 ρ V∞2 π 2 , Planform definition The wing chord distribution, or planform, is partially given by equation (1). This now becomes � � �2 2y 2Γ0 1− c(y) cℓ(y) = V∞ b which states only that the c × cℓ product must be elliptic. How c or cℓ vary individually is not determined, but rather must be chosen by the designer. The possibilities are unlimited, but it’s useful to consider two particularly simple choices. Choice 1: Pick a spanwise constant cℓ . In this case the chord distribution must be elliptic, c(y) = c0 c0 = � 2y 1− b 2Γ0 V ∞ cℓ 2 � �2 where c0 is the center chord. Such a wing is aerodynamically attractive, but the curved outlines may be impractical for construction. Choice 2: Pick a simple constant wing chord, c(y) = c. Using equation (1) again we have cℓ (y) = cℓ0 cℓ 0 = � 1− 2Γ0 V∞ c � 2y b �2 where cℓ0 is the wing-center lift coefficient. Aerodynamic twist definition Whatever choice is made for either c(y) or cℓ (y), the required corresponding wing twist distribution can now be obtained. We have determined earlier that for the elliptic loading case, equation (3) produces a spanwise-constant induced angle, given by αi (y) = Γ0 2bV∞ (spanwise constant) although a non-constant αi does not present any complications. The aerodynamic twist distribution is then given by rearranging equation (2). cℓ (y) + αi (y) ao α + αaero (y) = Only the sum α + αaero (y) is defined at this point, since it is the relevant angle which directly affects the lift. How this sum is split up between αaero (y) and α is arbitrary, and will be defined as a final step. The figure shows the two design choices described here, and the resulting wing shapes and aerodynamic twist distributions. Note that the elliptic-planform wing is aerodynamically flat, meaning that it has a constant aerodynamic twist (i.e. the zero-lift lines at all spanwise locations are parallel). In contrast, the “simple” constant chord wing has not turned out so simple after all, with an elliptic aerodynamic twist distribution. α + α aero Γ cl /ao αi 1: y α + α aero y cl /ao αi 2: y Geometric twist definition Definition of the geometric wing twist requires that the airfoil be selected, possibly varying across the span. This selection fixes the αL=0 (y) distribution, given airfoil −→ 3 αL=0 (y) which in turn determines the required geometric twist, apart from the constant α offset. α + αgeom (y) = cℓ (y) + αi (y) + αL=0 (y) ao (4) Reference line selection The overall angle of attack α is finally determined by selection of a common angle reference line for the whole wing. A convenient choice is to set the geometric twist angle at the wing center y = 0 to take on some specific value, say αgeom (0) = α0 . Applying equation (4) at y = 0 then defines α. α = cℓ (0) + αi (0) + αL=0 (0) − α0 ao When this α is substituted into equation (4), the geometric twist distribution is finally obtained. αgeom (y) = α0 + cℓ (y) − cℓ (0) + αi (y) − αi (0) + αL=0 (y) − αL=0 (0) ao It must be stressed that the choice of α0 does not alter the wing twist, or the wing’s orientation relative to the freestream velocity (i.e. the physical flow situation is unaffected). As shown in the figure, changing α0 merely sets the arbitrary reference line at a different orientation on the wing/freestream combination. The flowfields are identical. αgeom αgeom α V α reference line α0 = 9 V 4 reference line α0 = 5