XFOIL 6.94 User Guide - club
Contents
1. 140 1 0000 0 1500 0 2000 Npan PPanel TErat REFrat 1 0000 1 0000 1 0000 1 0000 XrefSi XrefS2 XrefP1 XrefP2 10 0000 0 5500 0 0150 0 8500 Size plotAR CHsize ScrnFr 11 0000 8 5000 0 0000 0 0000 Xpage Ypage Xmargn Ymargn F T Lcolor Lcursor 1 0000 2 0000 0 5000 CPmax CPmin CPdel 0 0900 0 7000 XoffAir ScalAir BLUwt 0 0000 1 5000 0 5000 CLmin CLmax CLdel 0 0000 0 0200 0 0100 CDmin CDmax CDdel 4 0000 10 0000 2 0000 ALmin ALmax ALdel 0 0000 0 3000 0 1000 CMmin CMmax CMdel 1 0 0000 0 0100 MAtype Mach Vaccel 1 0 0000 9 0000 REtype Re 10 6 Ncrit 1 0000 1 0000 XtripT XtripB Line 1 Paneling parameters from the PPAR menu Line 2 Paneling refinement locations Line 3 Specifies the absolute plot size the plot aspect ratio and scales the character number and symbol heights Line 4 Defines page size and page margins in inches Line 5 Flags for color PostScript output cursor screen input Line 6 Defines the Cp axis annotations Line 7 x offset and scale for airfoil on Cp vs x plot BL u profile scale Line 8 CL axis annotations on polar plot Line 9 CD axis annotations on polar plot Line 10 alpha axis annotations on polar plot Line 11 CM axis annotations on polar plot Line 12 Mach CL dependence type Mach number solution acceleration parameter Line 13 Re CL dependence type Reynolds number transition parameter Line 14 Forced transition x c locations on top bottom sides This file will be read at any time with the2. 27 is the independent parameter going around the airfoil The z and w functions are rather complicated but this is not important here The key to the full inverse method is that the mapping coefficients Cn can be computed from a known contour angle 0 w arctan dy dz OR from a surface speed Q w u iv w The other quantity then follows In summary the operations and their commands are a Direct problem 6 Cy gt u iv Q QSET b Inverse problem Qspee Cn gt iy 0 EXEC 7 1 Creation of seed surface speed distribution MDES performs QSET and sets Qspec Q automatically upon entry if Qspec does not exist This default initialization in effect makes MDES a redesign method in which the surface speed distribution of an existing airfoil is used as a starting point to generate a new speed distribution A pure design code which requests the entire surface speed distribution every time is often less natural to use since airfoil design is invariably an iterative process involving repeated analyze fix cycles The MDES menu is shown below lt cr gt Return to Top Level Redo previous command INIT Re initialize mapping QSET Reset Q_spec lt AQ r Show select alpha s for Qspec CQ r Show select CL s for Qspec Symm Toggle symmetry flag TGAP r Set new TE gap TANG r Set new TE angle Modi Modify Qspec MARK Mark off target segment for smoothing SMOO Smooth Qspec inside target segment FILT Apply Ha
3. 29 31 31 32 1 General Description XFOIL is an interactive program for the design and analysis of subsonic isolated airfoils It consists of a collection of menu driven routines which perform various useful functions such as Viscous or inviscid analysis of an existing airfoil allowing forced or free transition transitional separation bubble s limited trailing edge separation lift and drag predictions just beyond Cymax Karman Tsien compressibility correction Airfoil design and redesign by interactive specification of a surface speed distribution via screen cursor or mouse Two such facilities are implemented Full Inverse based on a complex mapping formulation Mixed Inverse an extension of XFOIL s basic panel method Full inverse allows multi point design while Mixed inverse allows relatively strict geometry control over parts of the airfoil Airfoil redesign by interactive specification of new geometric parameters such as new max thickness and or camber new LE radius new TE thickness new camber line via geometry specification new camber line via loading change specification flap deflection explicit contour geometry via screen cursor Blending of airfoils Drag polar calculation with fixed or varying Reynolds and or Mach numbers Writing and reading of airfoil geometry and polar save files Plotting of geometry pressure distributions and polars Versaplot
4. Normally the modified piece of Qspec s is blended into the current Qspec s with matching slopes at the piece endpoints This slope matching can be turned on off with the SLOP toggle command If slope matching is turned off the modified piece will match only the existing value but a slope discontinuity will be allowed 7 2 3 Smoothing Qspec can be smoothed with the SMO0 command which normally operates on the entire distribution but can be confined to a target segment whose endpoints are selected with the MARK command The smoothing acts to alleviate second derivatives in Qspec S so that with many consecutive SMOO commands Qspec s will approach a straight line over the target segment If the slope matching flag is set the endpoint slopes are preserved The FILT command is an alternative smoothing procedure which acts on the Fourier coefficients of Qspec directly and is global in its effect It is useful for cleaning up the entire Qspec s distribution if noise is present from some geometric glitch on the airfoil surface Also unintended noise might be introduced into Qspec from a poor modification via the cursor FILT acts by multiplying the Fourier coefficients by a Hanning window filter function raised to the power of a filter parameter F This tapers off the high frequencies of Qspec to varying degrees A value of F 0 0 gives no filtering F 1 0 gives the standard Hanning filter F 2 0 applies the Hanning filter twice etc T
5. Cp distribution solid line and the inviscid Cp distribution dashed line is due to the modification of the effective airfoil shape by the boundary layers This effective airfoil shape is shown superimposed on the actual current airfoil shape under the Cp vs x plot The gap between these effective and actual shapes is equal to the local displacement thickness 14 9 which can also be plotted in the VPAR menu This is only about 1 3 to 1 2 as large as the overall boundary layer thickness which can be visualized via the BL or BLC commands which diplay velocity profiles through the boundary layer BL displays a number of profiles equally spaced around the airfoil s perimeter while BLC displays profiles at cursor selected locations The zooming commands Z U may be necessary to better see these small profiles in most cases If the Cp reference data overlay option is enabled with CREF initiating a Cp vsz plot will first result in the user being prompted for a formatted data file with the following format x 1 Cp 1 x 2 Cp 2 The Cp vs x plot is then displayed as usual but with the data overlaid If FREF has been issued previously then numerical reference values for Cr Cp etc will be requested and added to the plot next to the computed values Boundary layer quantities are plotted from the VPLO menu H Plot kinematic shape parameter DT Plot top side Dstar and Theta DB Plot bottom side Dstar and Theta UE Plot edge velocity CF Plot
6. RDEF command thus avoiding the manual entry of all the information 11 Caveats The XFOIL code is not foolproof and requires some level of aerodynamic expertise and common sense on the part of the user Although the inviscid analysis OPERi geometry design GDES and Full Inverse MDES routines are nearly invulnerable to failure the Mixed Inverse QDES design routines and especially the viscous analysis OPERv routines will fail if a reasonable problem is not specified Typical failure scenarios are e Viscous Analysis OPERv Massive separation from excessive airfoil thickness flap deflection or angle of attack 32 Inherently unsteady flow von Karman vortex street etc Poor resolution of leading edge pressure spike Poor resolution of small viscous features e g separation bubbles Reynolds number too low e Mixed Inverse Surface Speed Design QDES Re entrant airfoil shape negative thickness A possible consequence of these occurences is an arithmetic fault causing program failure This is unlikely but it does happen occasionally It is therefore a good idea to save any previous work before an ambitious calculation is attempted The following situations may give problems strictly due to numerical roundoff e Excessively small panel s somewhere on the airfoil e Airfoil located too far from origin e Airfoil too thin These situations will rarely result in an arithmatic failure but
7. application This assumption is strongly violated in the near wake behind an airfoil with trailing edge separation but is always reasonable some distance behind the airfoil Hence the usual application of Squire Young at the trailing edge is questionable with separation present but its application at the last wake point typically 1 chord downstream is always reasonable Also application at the last wake 16 point also results in the formula having a smaller effect in any case since there u V and hence 0 29 In most 2 D airfoil experiments drag is measured indirectly by measuring 20 c in the wake often within one chord of the airfoil s trailing edge For consistency this should be compared to the Theta value predicted by XFOIL at the same wake location rather than the true Ca 26 c value which is effectively at downstream infinity In general 0 will be smaller than 0 In most airfoil drag measurement experiments this difference may amount to the drag measurement being several percent too large unless some correction is performed In addition to calculating the total viscous Cp from the wake momentum thickness XFOIL also determines the friction and pressure drag components C p Cp of this total Cp These are calculated by Cp Cs da Cp Cp Cp Here Cf is the skin friction coefficient defined with the freestream dynamic pressure not the BL edge dynamic pressure commonly used in BL theory Note that Cp is
8. derivative plot package used XFOIL is best suited for use on a good workstation A high end PC is also effective but must run Unix to support the X Windows graphics The source code of XFOIL is Fortran 77 The plot library also uses a few C routines for the X Windows interface 1 1 History XFOIL 1 0 was written by Mark Drela in 1986 The main goal was to combine the speed and accu racy of high order panel methods with the new fully coupled viscous inviscid interaction method used in the ISES code developed by Drela and Giles A fully interactive interface was employed from the beginning to make it much easier to use than the traditional batch type CFD codes Several inverse modes and a geometry manipulator were also incorporated early in XFOIL s development making it a fairly general airfoil development system Since version 1 0 XFOIL has undergone numerous revisions upgrades hacks and enhancements These changes mainly originated from perceived shortcomings during actual design use so XFOIL is now strongly geared to practical airfoil development Harold Youngren provided the Xplot11 plot package which is a vast improvement over the grim Versaplot type package used initially Enhancements and suggestions from Youngren and other people were also incorporated into XFOIL itself along the way Over the past few years bug reports and enhancement suggestions have slowed to practically nil and so after a final few enhancements from version 6
9. elements each one separated by the line 999 0 999 0 The user is asked which of these elements is to be read in 3 2 Buffer airfoil normalization XFOIL will normally perform all operations on an airfoil with the same shape and location in cartesian space as the input airfoil However if the normalization flag is set toggled with the NORM command the buffer airfoil coordinates will be immediately normalized to unit chord and the leading edge will be placed at the origin A message is printed to remind the user 3 3 Buffer airfoil generation via interpolation The INTE command is new in XFOIL 6 9 and allows interpolating or blending of airfoils in various proportions The polar shape of an interpolated airfoil will often be quite close to the interpolated polars of its two parent airfoils Extrapolation can also be done by specifying a blending fraction outside the 0 1 range although the resulting airfoil may be quite weird if the extrapolation is excessive A good way to use INTE is to augment or tone down the modifications to an airfoil performed in MDES or GDES For example say airfoil B is obtained by modifying airfoil A A MDES gt B Suppose the modification changed A s polar in the right direction but not quite far enough The additional needed change can be done by extrapolating past airfoil B in INTE Airfoil 0 A Airfoil 1 B Interpolating fraction 0 1 1 4 Output airfoil C 10 Plot
10. every Newton itera tion The coefficient matrix of this system is 1 3 full although most of its entries are very small Substantial savings in CPU time factor of 4 or more result when these small entries are neglected SUBROUTINE BLSOLV which solves the large Newton system ignores any off diagonal element whose magnitude is smaller than the variable VACCEL which is initialized in SUBROUTINE INIT and which can be changed at runtime from the VPAR menu with the VACC command A nonzero VACCEL parameter should in principle degrade the convergence rate of the viscous solution and thus result in more Newton iterations although the effect is usually too small to notice For very low Reynolds number cases less than 100000 it may adversely affect the convergence rate or stability and one should try reducing VACCEL or even setting it to zero if all other efforts at convergence are unsuccessful The value of VACCEL has absolutely no effect on the final converged viscous solution if attained 5 5 Polar calculations and plotting The polar calculation facility driven from the OPER menu deserves a detailed description It has been considerably upgraded from previous XFOIL versions The simplest way to create a polar is to issue the PACC command which sets the auto polar ac cumulation toggle and asks for the optional save and dump filenames If either filename is given each computed operating point will be stored internally and also written to the spec
11. imperceptible 2 Data Structure XFOIL stores all its data in RAM during execution Saving of the data to files is not normally performed automatically so the user must be careful to save work results before exiting XFOIL The exception to this is optional automatic saving to disk of polar data as it s being computed in OPER described later 2 1 Stored airfoils and polars XFOIL 6 9 stores multiple polars and associated airfoils and parameters during one interactive session Each such data set is designated by its stored polar index polar 1 x y CL a CD a Re Ma Ncrit polar 2 x y CL a CD a Re Ma Ncrit Not all of the data need to be present for each stored polar For example x y would be absent if the CL CD polar was read in from an external file rather than computed online Earlier XFOIL versions in effect only allowed one stored airfoil and stored polar at a time The new multiple storage feature makes iterative redesign considerably more convenient since the cases can contain multiple design versions which can be easily overlaid on plots 2 2 Current and buffer airfoils XFOIL 6 9 retains the concept of a current airfoil and buffer airfoil used in previous versions These are the airfoils on which the various calculations are performed and they are distinct from the polar x y coordinates described above The polar x y are simply archived data and do not directly participate in com
12. present in a viscous solution then it is very important to have good panel resolution in the region of the bubble s The large gradients at a bubble tend to cause significant numerical errors even if a large number of panels is used If a separation bubble appears to be poorly resolved it is a good idea to re panel the airfoil with more points and or with points bunched around the bubble region The paneling is controlled from the PPAR menu A good rule of thumb is that the shape parameter Hj just after transition in the bubble should not decrease by more than 1 0 per point Likewise the surface velocity Ue V should not change by more than 0 05 per point past transition otherwise there may be significant numerical errors in the drag The point values can be observed by issuing SYMB from the VPLO menu Moderate chord Reynolds numbers 1 3 million say usually require the finest paneling since the bubbles are still important but very small On many airfoils especially those with small leading edge radii the development of the small bubble which forms just behind the leading edge can have a significant effect on Crmax For such cases the default paneling density at the bubble may not be adequate In all cases inadequate bubble resolution results in a ragged or scalloped Cr vs Cp drag polar curve so fortunately this is easy to spot 5 3 2 Differencing order of accuracy The BL equations are normally discretized with two point central d
13. smoothness of the resulting splined airfoil 9 2 Modifying buffer airfoil Once the buffer airfoil is suitably initialized most of the GDES commands can then be used to modify it The resulting new shape will usually be replotted immediately in a highlighted color The plot can be refreshed anytime with the PLOT command Sometimes a sequence of commands is necessary to achieve the desired effect For instance suppose an airfoil with the current thickness envelope is to be given an entirely new camber line Issuing TSET and inputting a new thickness of 999 keep same thickness and a new camber of 0 will result in the current thickness envelope unchanged and the current camber eliminated so that a symmetrical airfoil remains The new camber line can then be added in the CAMB sub menu 29 lt cr gt Return to GDES TFAC rr Scale existing thickness and camber TSET rr Set new thickness and camber HIGH rr Move camber and thickness highpoints WRTC Write airfoil camber x c y c to file RDAC Read added camber x c y c from file SETC Set added camber x c y c from camberline INPC Input added camber x c y c from keyboard MODC Modify added camber x c y c with cursor INPP Input added loading x c DCp from keyboard MODP Modify added loading x c DCp with cursor SLOP Toggle modified camber dCp slope matching flag SCAL r Scale the added camber CLR Clear the added camber ADD Add added camber to the existing camberline DCPL Toggle DCp plot CPLI rr Change DCp
14. 8 XFOIL 6 9 is officially frozen and has been made public Although any bugs will likely be fixed no further development is planned at this point Method extensions are being planned but these will be incorporated in a completely new next generation code Note to code developers and code enhancers XFOIL does not exactly have the cleanest implementation but it isn t too bad considering its vast modification history Feel free to muck with the code as you like provided everything is done under the GPL agreement Drela and Youngren will not be inclined to assist with any code modifications at this point however since we each have a dozen other projects waiting So proceed at your own risk 1 2 Theory References The general XFOIL methodology is described in Drela M XFOIL An Analysis and Design System for Low Reynolds Number Airfoils Conference on Low Reynolds Number Airfoil Aerodynamics University of Notre Dame June 1989 which also appears as a chapter in Low Reynolds Number Aerodynamics T J Mueller Editor Lecture Notes in Engineering 54 Springer Verlag 1989 ISBN 3 540 51884 3 ISBN 0 387 51884 3 The boundary layer formulation used by XFOIL is described in Drela M and Giles M B Viscous Inviscid Analysis of Transonic and Low Reynolds Number Airfoils ATAA Journal 25 10 pp 1347 1355 October 1987 The blunt trailing edge treatment is described in Drela M Integral Boundary Layer Formulation for Bl
15. L versions this had to be done with the equivalent command sequence LOAD GDES EXEC With XFOIL 6 9 the GDES EXEC commands after LOAD are now superfluous The NACA command automatically invokes the paneling routine to create a current airfoil with a suitable paneling 3 6 Saving airfoil coordinates A coordinate file in any one of these four formats can be written with the PSAV SAVE ISAV or MSAV command respectively When issuing the MSAV command the user is also asked which element in the file is to be overwritten XFOIL can thus be used to easily edit individual elements in MSES multielement configurations Of course normalization should not be performed on an element if it is to be written back to the same multielement file Only the current airfoil coordinates can be saved to a file If the buffer or polar x y coordinates need to be saved they must first be copied into the current airfoil 11 4 Units Most XFOIL operations are performed on the airfoil s cartesian coordinates x y which do not necessarily have a unit chord c Since the chord is ambiguous for odd shapes the XFOIL force co efficients Cz Cp Cm are obtained by normalizing the forces and moment with only the freestream dynamic pressure the reference chord is assumed to be unity Likewise the XFOIL Reynolds number RE is defined with the freestream velocity and viscosity and an implied unit chord Cr big V freestream speed Cp D q v freestream
16. The default setup assumes color X Windows graphics if available and B amp W PostScript These defaults are controlled by the IDEV and IDEVRP flags in SUBROU TINE INIT in xfoil f 21 The Xplot11 library should work on all Unix systems The Makefile in the plotlib directory requires some modifications for some machines The default X graphics window is in Landscape mode with a black reverse video background A white normal video background can be selected by setting the Unix shell variable setenv XPLOT11_BACKGROUND white before Xfoil is started The nicer reverse video is restored with unsetenv XPLOT11_BACKGROUND See the plotlib Doc file for more info on the plot library Xplot11 provides a built in Zoom Unzoom capability which can be applied to whatever is on the screen Zooming Unzooming can be perfomed with the Z and U commands from nearly all the menus these commands are not listed to reduce clutter Some of the menus also have their own Blowup Reset commands The distinction is that XFOIL s plots don t try to adjust themselves to Zoom parameters so a highly Zoomed plot may show nothing at all In contrast Blowup Reset instructs XFOIL to change its own plot scales so a highly Blown up plot will at least show the axes 6 1 Plot Hardcopy For hardcopy the current screen plot can be echoed to a PostScript file plot ps with the HARD command The size of the plot objects on the screen and on hardc
17. XFOIL 6 94 User Guide Mark Drela MIT Aero amp Astro Harold Youngren Aerocraft Inc 10 Dec 2001 Contents General Description Ts E a Pik E RO 1 2 Theory References a t3 Anvis id Formulation ori ai Bel ae A ai ne ee a 1 4 Inverse Formulation 2 ee 15 Viscous Formulation ir A A RR Rae ES OO A a ES Data Structure 2 1 Stored airfoils and polars 00 ee ee 2 2 Current and buffer airfoils 2 2 o e ee ee Program Execution solv Airtel flesformatsy custodia ek wee HE abe aed Shae opie des OR BRE Se eB a 3 1 1 Plain coordinate file 2 2 0 e e o 3 1 2 Labeled coordinate le or sa a A RA 321 3 ISHSicoordinate ile te a A kina ta 3 1 4 MSES coordinate fille 2 2 0 0 02 e o 3 2 Buffer airfoil normalization ee 3 3 Buffer airfoil generation via interpolation 0 e 3 4 Further buffer airfoil manipulation o 3 5 Generation of current airfoll 2 ee 3 6 Saving airfoil coordinates ee Units Analysis Routine OPER 5 1 Horcercal ulatiOn ct ee en i RE ee A Pe a ee E 5 2 Transition criterion a A a a a a aa e aa a A a a A 5 3 Numerical accuracy 5 3 1 Panel density requirements e 5 3 2 Differencing order of accuracy 2 ee 5 4 Viscous solution acceleration ooa eoa a a a a 5 5 Polar calculations and plotting a 56 Ofline polar plotti
18. axis plot limits BLOW Blowup plot region RESE Reset plot scale and origin SIZE r Change absolute plot object size ANNO Annotate plot HARD Hardcopy current plot Added camber line incidence at LE 0 00 deg Added camber line incidence at TE 0 00 deg Max added camber y c 0 0000 at x c 0 000 CAMB gt INPC takes the new camber line as a sequence of x c y c coordinate pairs which are splined INPP takes a sequence of 2 c delta C pairs instead This delta C i e loading distribution defined as is then used in Glauert s thin airfoil relations to define the z c y c camber line With INPC and INPP a slope discontinuity in y x or Cp x can be specified with two identical consecutive z c values which prevents splining across this point INPP can thus easily generate a camber line with a piecewise linear delta C loading distribution as for example an a 0 8 NACA delta C Colomer Co upper 6 digit airfoil x c delta Cp 0 0 0 5 0 8 0 5 0 8 0 5 1 0 0 0 30 This results in a constant delta C 0 5 for 0 0 lt x c lt 0 8 then decreasing linearly to delta C 0 0 for 0 8 lt a c lt 1 0 Once a suitable added camber is input it is added to the existing buffer airfoil camber via the ADD command The various GDES commands for modifying camber line thickness leading edge radius etc should suffice for most geometry modification tasks If truly frustrated the user can draw the new con
19. bly means that the gap was not exactly zero The fix is to first set the gap to zero and then set it again to the desired value After a new gap size is input a blending distance c will also be requested This controls how rapidly the new TE blends into the original airfoil and is essentially the length scale for the blending function which is exponential in z c The limiting values are distance c 0 Only the upper and lower surface TE points are changed 1 A linear wedge is added or subtracted from the airfoil 9 3 Saving buffer airfoil into current airfoil Once the desired buffer airfoil is created a new current airfoil is set directly from the buffer airfoil with the EXEC command equivalent to PCOP at top level Alternatively the new current airfoil can be re paneled from the buffer airfoil with the PANE command at top level The new current airfoil can then be analyzed in OPER If the buffer airfoil has any doubled corner points the doubled points will be eliminated but a current airfoil node will fall exactly on each buffer airfoil corner 10 Start up Defaults XFOIL has hardwired parameters in SUBROUTINE INIT controlling the paneling plotting and viscous execution Most of these can be changed at runtime in the various menus To avoid the 31 need to change the parameters everytime XFOIL is executed they can be saved to the default file xfoil def with the WDEF command at TOP LEVEL This file has the format
20. ch fairly accurately accounts for base drag The total velocity at each point on the airfoil surface and wake with contributions from the freestream the airfoil surface vorticity and the equivalent viscous source distribution is obtained from the panel solution with the Karman Tsien correction added This is incorporated into the viscous equations yielding a nonlinear elliptic system which is readily solved by a full Newton method as in the ISES code Execution times are quite rapid requiring a few seconds on a fast workstation for a high resolution calculation with 160 panels For a sequence of closely spaced angles of attack as in a polar the calculation time per point can be substantially smaller If lift is specified then the wake trajectory for a viscous calculation is taken from an inviscid solution at the specified lift If alpha is specified then the wake trajectory is taken from an inviscid solution at that alpha This is not strictly correct since viscous effects will in general decrease lift and change the trajectory This secondary correction is not performed since a new source influence matrix would have to be calculated each time the wake trajectory is changed This would result in unreasonably long calculation times The effect of this approximation on the overall accuracy is small and will be felt mainly near or past stall where accuracy tends to degrade anyway In attached cases the effect of the incorrect wake trajectory is
21. d Inverse modification is performed on the current airfoil directly in contrast to Full Inverse which generates the buffer airfoil as its output In fact it is important not to issue the PANE or PCOP commands at top level after doing work in the QDES menu as the new current airfoil will be overwritten with the old buffer airfoil 9 Geometry Design Routine Executing the GDES command from the top level menu will put the user into the GDES routine It has a rather extensive menu lt cr gt Return to Top Level Redo previous command GSET Set buffer airfoil lt current airfoil eXec Set current airfoil lt buffer airfoil SYMM Toggle y symmetry flag ADEG r Rotate about origin degrees ARAD r Rotate about origin radians Tran rr Translate 27 Scal LINS DERO TGAP LERA TCPL TFAC TSET HIGH CAMB Flap Modi SLOP CORN ADDP DELP MOVP UNIT Dist CLIS CPLO CANG CADD Plot INPL Blow Rese TSIZ TICK GRID GPAR Over SIZE ANNO HARD NAME NINC GDES rr rr rr rr rr rr rrr ri c gt Scale about origin Linearly varying y scale Derotate set chord line level Change trailing edge gap Change leading edge radius Toggle thickness and camber plotting Scale existing thickness and camber Set new thickness and camber Move camber and thickness highpoints Modify camber shape directly or via loading Deflect trailing edge flap Modify contour via cursor Toggle modifie
22. d contour slope matching flag Double point with cursor set sharp corner Add point with cursor Delete point with cursor Move point with cursor Normalize buffer airfoil to unit chord Determine distance between 2 cursor points List curvatures Plot curvatures List panel corner angles Add points at corners exceeding angle threshold Replot buffer airfoil Replot buffer airfoil without scaling in inches Blowup plot region Reset plot scale and origin Change tick mark size Toggle node tick mark plotting Toggle grid plotting Toggle geometric parameter plotting Overlay disk file airfoil Change absolute plot object size Annotate plot Hardcopy current plot Specify new airfoil name Increment name version number 28 9 1 Creating seed buffer airfoil The first command typically executed is GSET which sets the temporary buffer airfoil from the current airfoil Sometimes it might be desired to operate directly on the coordinates of an already existing buffer airfoil It typically contains coordinates read in from a disk file by LOAD at Top Level or coordinates generated by EXEC from the MDES menu depending on what was done last In either of these cases GSET is skipped 9 1 1 Point addition typ to Eppler and Selig airfoils If the buffer airfoil has an excessively coarse point spacing additional points can be added with the CADD command Using the PANE command at top level also does this but CADD allows the point additi
23. deduced from Cp and Cp instead of being calculated via surface pressure integration This conventional definition Co Codi is not used since it is typically swamped by numerical noise 5 2 Transition criterion Transition in an XFOIL solution is triggered by one of two ways free transition e criterion is met forced transition a trip or the trailing edge is encountered The e method is always active and free transition can occur upstream of the trip The e method has the user specified parameter Merit which is the log of the amplification factor of the most amplified frequency which triggers transition A suitable value of this parameter depends on the ambient disturbance level in which the airfoil operates and mimics the effect of such disturbances on transition Below are typical values of nerit for various situations Situation Nerit sailplane 12 14 motorglider 11 13 clean wind tunnel 10 12 average wind tunnel 9 dirty wind tunnel 4 8 The choice nerit 9 corresponds to the standard e method and is the most common choice It must be pointed out that the e method in XFOIL is actually the simplified envelope version which is the same as the full e method only for flows with constant H x If H is not constant the two methods differ somewhat but this difference is typically within the uncertainty in choosing Merit The e method is only appropriate for predicting transition in situations where
24. defaults are used Like the RGET FREF commands in OPER PPLOT permits reference data to be overlaid A reference polar data file has the following form CD 1 CL 1 CD 2 CL 2 999 0 999 0 alpha 1 CL 1 alpha 2 CL 2 999 0 999 0 alpha 1 Cm 1 alpha 2 Cm 2 999 0 999 0 Xtr c 1 CL 1 Xtr c 2 CL 2 999 0 999 0 The number of points in each set CD CL alpha CL etc is arbitrary and can be zero The contents of a polar dump file can be selectively plotted with the separate menu driven program PXPLOT It is executed with pxplot lt dump filename gt This allows surface plots of Cp vs x H vs x etc for any or all of the saved operating points Of course these plots can be generated in XFOIL for any individual operating point so PXPLOT and the dump file itself are somewhat redundant in this respect 5 7 Re Mach dependence on Cz A few comments are in order on the TYPE command which allows the user to set the dependence of the Mach and Reynolds numbers on Cz Any Cr Cp polar can be of the following three types 20 Type parameters held constant varying fixed 1 M RE lift chord velocity 2 MVC REVCL velocity chord lift 3 M REC chord lift velocity 5 7 1 Type l This corresponds to a given wing at a fixed velocity going over an angle of attack range as in a wind tunnel test alpha sweep or a sudden aircraft pullup This is also the common form for an airfoil polar 5 7 2 Type 2 This correspon
25. ds to an aircraft in level flight at a given altitude undergoing trim speed changes This is the most useful airfoil polar form for determining a drag polar for an aircraft at 1 g For this case The Mach number input with the MACH command is actually interpreted as the product M Cy_ and the Reynolds number input with the VISC or RE commands is actually interpreted as REYC For a wing in level flight these products can be computed from the following exact relations with RE based on the mean chord 1 2 1 2 MVG ei mayo te YP v p AR W weight p ambient pressure S wing area v kinematic viscosity AR aspect ratio p ambient air density b span y 1 4 5 7 3 Type 3 This corresponds to a wing of rubber chord with a given lift at a given speed This is best used for selecting an optimum Cy for an airfoil while taking Reynolds number changes into account The product RE Cr can be computed from the following 2W REC vpVb Caution must be used with Types 2 and 3 so as to not allow the Cr to go negative In addition with non zero Mach and Type 2 the Cz must not fall below that value which makes Mach exceed unity Warning messages are printed when these problems occur 6 Output All output goes directly to the terminal screen H Youngren s plot package Xplot11 1ibP1t a used by XFOIL drives monochrome and color X Windows graphics and generates B amp W or color PostScript files for hardcopy
26. e This is done only for the user s convenience In the input and output labeling x y always refer to the cartesian coordinates while x c y c refer to the chord based coordinates which are shifted rotated and scaled so that the airfoil s leading edge is at z c y c 0 0 and the airfoil s trailing edge is at x c y c 1 0 The two systems cooincide only if the airfoil is normalized 5 Analysis Routine OPER Issuing the OPER command will produce the prompt 12 OPERi c gt Typing a will result in the full OPER analysis menu being displayed lt cr gt l Visc VPAR Re Mach Type ITER INIT Alfa CLI Cl ASeq CSeq SEQP CINC HINC Pacc K rrr rrr PGET f PWRT i PSUM PLIS PDEL PSOR PPlo APlo ASET PREM PPAX RGET RDEL GRID CREF FREF CPx CPV VPlo ANNO HARD ii ii i ir Return to Top Level Redo last ALFA CLI CL ASEQ CSEQ VELS Toggle Inviscid Viscous mode Change BL parameter s Change Reynolds number Change Mach number Change type of Mach Re variation with CL Change viscous solution iteration limit Toggle BL initialization flag Prescribe alpha Prescribe inviscid CL Prescribe CL Prescribe a sequence of alphas Prescribe a sequence of CLs Toggle polar Cp x sequence plot display Toggle minimum Cp inclusion in polar Toggle hinge moment inclusion in polar Toggle auto point accumulation to active polar Read new polar from save fi
27. ers and also the wake It will be seen that if the flow is separated at the trailing edge much of the drag contribution energy dissipation of Ch occurs in the wake As mentioned earlier all forces are normalized with freestream dynamic pressure only Cr Cp Cm are the usual chord based definitions only if the airfoil has a unit chord in general they will scale with the airfoil s chord Also Cm is defined about the cartesian point ref Yrer 0 25 0 0 which is not necessarily the airfoil s 1 4 chord point 5 1 Force calculation The lift and moment coefficients Cz Cm are calculated by direct surface pressure integration Cr L q Cpa Cu M q f o x rep de y Yrer dy where z xcos a ysin a y ycos a xsin a The integrals performed in the counterclockwise direction around the airfoil contour The pressure coefficient C is calculated using the Karman Tsien compressibility correction The drag coefficient Cp is obtained by applying the Squire Young formula at the last point in the wake not at the trailing edge Cp D q 20 20 u V 4 5 2 where 60 momentum thickness u edge velocity at end of computed wake H shape parameter and momentum thickness f f V freestream velocity Te pdo ean The Squire Young formula in effect extrapolates the momentum thickness to downstream infinity It assumes that the wake behaves in a asymptotic manner downstream of the point of
28. escribe inviscid CL Cl r Prescribe CL ASeq rrr Prescribe a sequence of alphas CSeq rrr Prescribe a sequence of CLs The A command is the short alternative form of ALFA and C is the short alternative of CL Likewise AS and CS are the short forms of ASEQ and CSEQ The CLI command has no short form as indicated by all capitals in the menu and must be fully typed Hopefully most of the commands are self explanatory For inviscid cases the CLI and CL commands are identical For viscous cases CLI is equivalent to specifying alpha this being determined a priori from the specified lift coefficient via an inviscid solution CL will return a viscous solution with the specified true viscous lift coefficient at an alpha which is determined as part of the solution prescribing a CL above Cymax will cause serious problems however The user is always prompted for any required input When in doubt typing a will always produce a menu After an ALFA CL or CLI command is executed the Cp vs x distribution is displayed and can be displayed again at any time with CPX If the viscous mode is active the true viscous Cp is shown as a solid line and the inviscid Cp at that same alpha is shown as a dashed line Each dash covers one panel so the local dashed line density is also a useful visual indicator of panel resolution quality If the inviscid mode is active only the inviscid Cp is shown as a solid line The difference between the true viscous
29. he standard Hanning filter appears to be a bit too drastic so a filter parameter of F 0 2 is currently used Hence issuing FILT five times corresponds to the standard Hanning filter The SPEC command displays the mapping coefficient spectrum at any time 24 7 2 4 Symmetry forcing The symmetry forcing option SYMM toggle is useful when a symmetric airfoil is being designed If active this option zeroes out all antisymmetric camber Qspec changes and doubles all symmetric thickness changes This unfortunately has the annoying side effect of also doubling the numerical roundoff noise in Qspec every time a MODI operation is performed This noise sooner or later becomes visible as high frequency wiggles which double with each MODI command Occasionally issuing FILT keeps this parasitic noise growth under control 7 2 5 New geometry computation The MODI BLOW MARK SMOO SLOP FILT commands can be issued repeatedly in any order until Qspec is modified to have the desired distribution In general the speed distributions actually plotted will not exactly match what was input with the cursor since corrections are automatically added to maintain the specified trailing edge gap and to enforce consistency with the freestream speed These are known as the Lighthill constraints on the surface speed The trailing edge gap is initialized from the initial airfoil and can be changed with TGAP To reduce the corrupting effect of the constraint dr
30. ifferencing i e the Trapezoidal Scheme which is second order accurate but only marginally stable In particular it has problems with the relatively stiff shape parameter and lag equations at transition where at high Reynolds number the shape parameter must change very rapidly Oscillations and overshoots in the shape parameter will occur with the Trapezoidal Scheme if the grid cannot resolve this rapid change To avoid this nasty behavior upwinding must be introduced resulting in the Backward Euler Scheme which is very stable but has only first order accuracy Previous versions of XFOIL allowed a specific constant amount of upwinding to be user specified Currently XFOIL automatically introduces 18 upwinding into the equations only in regions of rapid change typically transition This ensures that the overall scheme is stable and as accurate as possible Since only a minimal amount of upwinding is introduced in the interest of numerical accuracy small oscillations in the shape parameter H will sometimes appear near the stagnation point if relatively coarse paneling is used there These oscillations are primarily a cosmetic defect and do not significantly affect the downstream development of the boundary layer Eliminating them by increasing upwiding would in fact produce much greater errors in the overall viscous solution 5 4 Viscous solution acceleration The execution of a viscous case requires the solution of a large linear system
31. ified file If no filename is given the automatic writing is not performed The polar s operating points can be computed individually with ALFA or more conveniently en masse with ASEQ One can also use CL or CSEQ although these will not work close to Cymax The polar can be plotted anytime with PPLO If previous polars have been computed or read in with PGET they can be plotted as well If a polar is deemed incomplete additional points can be computed as needed If automatic writing of a polar was not chosen no filename was given for PACC the polar can be written later all at once with the PWRT command The only drawback to this approach is that if the program crashes during a polar calculation sweep for whatever reason the computed polar and all other stored information will be lost If existing filenames are given to PACC the subsequent computed points will be appended to these files but only if the airfoil name and flow parameters in the file match the current parameters This is to prevent clobbering of the polar file with wrong additional points Messages are always produced informing the user of what s going on 19 5 6 Off line polar plotting Polar save file s can also be plotted off line with the separate program PPLOT This is entirely menu driven and is simply executed pplot The file pplot def contains plotting parameters and is read automatically if available If it s not available then internal
32. iven corrections a good rule of thumb is that the Qspec distribution should be modified so as to preserve the total Cz The Cz is simply twice the area under the Qspec s curve 2 x circulation so that this area should be preserved 7 2 6 Multipoint surface speed display A very useful feature of the MDES facility is the ability to display and modify a number of Qspec distributions corresponding to different alpha or inviscid CL values These values are displayed and or selected via the AQ or CQ commands When any one Qspec distribution is modified the result of modification is also displayed on all the other distributions This allows rapid design at multiple operating points When the Qspec curves correspond to specified Cz values the alpha for each curve will be adjusted after each Qspec modification so as to preserve that curve s Cy The resulting Qspec will therefore not match the input cursor points exactly because of this alpha correction 7 3 Generation of new geometry The EXEC command generates a new buffer airfoil corresponding to the current Qspec distribution If subsequent operations on this airfoil are to be performed SAVE OPER etc it is necessary to first generate a current airfoil from this buffer airfoil using PANE at the top level menu This seemingly complicated sequence is necessary because the airfoil points generated by EXEC are uniformly spaced in the circle plane which gives a rather poor point panel node spaci
33. kinematic viscosity Cu M q p freestream density RE V v q 0 5pV The conventional definitions are C L qc Ca D qc Cm M ac Re Ve v so that the conventional and XFOIL definitions differ only by the chord factor c or c For example a NACA 4412 airfoil is operated in the OPER menu at RE 500000 a 3 first with c 1 0 and then with c 0 5 changed with SCAL command in the GDES menu say The results produced by XFOIL are c 1 0 Cr 0 80 Cp 0 0082 RE 500000 Re 500000 c 0 5 Cr 040 Cp 0 0053 RE 500000 Re 250000 Since Cr is not normalized with the chord it is nearly proportional to the airfoil size It is not exactly proportional since the true chord Reynolds number Re is different and there is always a weak Reynolds number effect on lift In contrast the Cp for the smaller airfoil is significantly greater than 1 2 times the larger airfoil Cp since chord Reynolds number has a significant impact on profile drag Repeating the c 0 5 case at RE 1000000 produces the expected result that Cr and Cp are exactly 1 2 times their c 1 0 values c 0 5 Cr 040 Cp 0 0041 RE 1000000 Re 500000 Although XFOIL performs its operations with no regard to the size of the airfoil some quantities are nevertheless defined in terms of the chord length Examples are the camber line shape and BL trip locations which are specified in terms of the relative x c y c along and normal to the airfoil chord lin
34. lation REST Restore geometry from buffer airfoil CPXX CPxx endpoint constraint toggle VISC Qvis overlay toggle REFL Reflected Qspec overlay toggle Plot Plot Qspec line and Q symbols Blow Blowup plot region Rese Reset plot scale and origin SIZE r Change absolute plot object size ANNO Annotate plot HARD Hardcopy current plot QDES c gt 8 1 Creation of seed surface speed distribution The QDES menu above is intentionally geared for the redesign of a segment of an existing airfoil with its surface speed distribution calculated previously in OPER rather than the generation of a totally new airfoil When QDES is entered the specified speed distribution Qspec is initialized to the current speed distribution Q last set in OPER If a direct solution for the current airfoil hasn t been calculated yet QDES goes ahead and calculates it using the last set angle of attack If this isn t the desired angle it can be set in OPER using ALFA QSET can then be used to set Qspec from the current Q distribution 26 8 2 Modification of surface speed distribution Qspec can be repeatedly modified with the screen cursor and the MODI command exactly as in MDES It is also necessary to mark off the target segment where the geometry is to be modified with the MARK command 8 3 Generation of new airfoil geometry EXEC modifies the airfoil over the target segment to match Qspec there as closely as possible The remainder of the airfoil geome
35. le Write polar to save file Show summary of stored polars List stored polar s Delete stored polar Sort stored polar Plot stored polar s Plot stored airfoil s for each polar Copy stored airfoil into current airfoil Remove point s from stored polar Change polar plot axis limits Read new reference polar from file Delete stored reference polar Toggle Cp vs x grid overlay Toggle reference Cp data overlay Toggle reference CL CD data display Plot Cp vs x Plot airfoil with pressure vectors gee wiz BL variable plots Annotate current plot Hardcopy current plot 13 SIZE r Change plot object size CPMI r Change minimum Cp axis annotation BL i Plot boundary layer velocity profiles BLC Plot boundary layer velocity profiles at cursor BLWT r Change velocity profile scale weight FMOM Calculate flap hinge moment and forces FNEW rr Set new flap hinge point VELS rr Calculate velocity components at a point DUMP f Output Ue Dstar Theta Cf vs s x y to file CPWR f Output x vs Cp to file CPMN Report minimum surface Cp NAME s Specify new airfoil name NINC Increment name version number OPERi c gt The commands are not case sensitive Some commands expect multiple arguments but if the arguments are not typed prompts will be issued The most commonly used commands have alternative short forms indicated by the uppercase part of the command in the menu list For example the menu shows Alfa r Prescribe alpha CLI r Pr
36. lt value of 10 with the ITER command in OPER One should always be wary of trusting solutions which show regions of supersonic flow Such flows can be reliably predicted only with a truly nonlinear field method such as the MSES code Asa rule of thumb if the maximum Mach number doesn t exceed 1 05 anywhere shock losses will be very small the Cp distributions will be reasonably accurate and the drag predicted by XFOIL is likely to be accurate 33
37. mats Plain Labeled ISES MSES All data lines are significant with the exception of lines beginning with which are ignored 3 1 1 Plain coordinate file This contains only the x y coordinates which run from the trailing edge round the leading edge back to the trailing edge in either direction X 1 Y 1 xQ Y 2 X N YN 3 1 2 Labeled coordinate file This is the same as the plain file except that it also has an airfoil name string on the first line NACA 0012 X 1 Y xX 2 YO This is deemed the most convenient format to use The presence of the name string is automatically recognized if it does not begin with a Fortran readable pair of numbers Hence 00 12 NACA Airfoil cannot be used as a name since the 00 12 will be interpreted as the first pair of coordinates 0012 NACA is OK however Some Fortran implementations will also choke on airfoil names that begin with T or F These will be interpreted as logical variables defeating the name detection logic Beginning the name with _T or F is a workable solution to this feature 3 1 3 ISES coordinate file This has four or five ISES grid domain parameters in addition to the name NACA 0012 2 0 3 0 2 5 3 0 X 1 Y xX 2 YO If the second line has four or more numbers then these are interpreted as the grid domain param eters 3 1 4 MSES coordinate file This is the same as the ISES coordinate file except that 1t can contain multiple
38. ng ctas aoisi asi a eae BO Aa BES eae ee 5 7 Re Mach dependence on Cr aooaa Srl Type Tzar Stag gee Bk goa RN E EA De 2 VEY PO 2 s eyen A al ane S A E ae Ped ead eee ea ee el ea eS Output 6 1 Plot Hardcopy a tani aos dad a ang Ge kg eS a ke ee ak dom wats dat eS Full Inverse Surface Speed Design Routine MDES 7 1 Creation of seed surface speed distribution 2 2 ee ee es 7 2 Modification of surface speed distributions 0 e 7 2 1 Cursor input of modifications ee es 7 2 2 Modification endpoint blending o o e e TRS lt SMOOthINg E a a aS Pe ee ed 7 2 4 Symmetry force 7 2 5 New geometry computation 0 0 0 2 ee eee ee ee 7 2 6 Multipoint surface speed display 2 a 7 3 Generation of new geometry ee Mixed Inverse Surface Speed Design Routine QDES 8 1 Creation of seed surface speed distribution 0 2 002 002 ee eee 8 2 Modification of surface speed distribution 2 000000 ee eee 8 3 Generation of new airfoil geometry ee ee Geometry Design Routine 9 1 Creating seed buffer airfoil 2 2 o ee ee 9 1 1 Point addition typ to Eppler and Selig airfoils 9 2 Modifying buffer airfoil 2 ee ee ee 9 3 Saving buffer airfoil into current airfoil o o 10 Start up Defaults 11 Caveats 21 22 22 23 24 24 24 24 25 25 25 25 26 26 27 27 27 29 29
39. ng distribution on the physical airfoil This sequence also prevents the current airfoil from being overwritten immediately when EXEC is issued Once the new current airfoil is generated with PANE it can then be analyzed in OPER modified in GDES or whatever The PERT command allows manual input of the complex mapping coefficients Can which determine the geometry These coefficients are normally determined from Qspec s this is the essence of the inverse method The PERT command is provided simply as a means of allowing generation of geometric perturbation modes possibly for external optimization or whatever 25 The manually changed Cn values result in changes in geometry as well as the current Qspec s distributions The QSET command will restore everything to its unperturbed state The Full Inverse facility is very fast after an initialization calculation of several seconds on a RISC workstation it requires only a fraction of a second to generate the new buffer airfoil 8 Mixed Inverse Surface Speed Design Routine QDES XFOIL s Mixed Inverse facility QDES is useful in certain redesign problems where parts of the airfoil cannot be altered under any circumstances The Mixed Inverse menu is shown below lt cr gt Return to Top Level QSET Reset Qspec lt Modi Modify Qspec MARK Mark off target segment SMOO Smooth Qspec inside target segment SLOP Toggle modified Qspec slope matching flag eXec i Execute mixed inverse calcu
40. nning filter to entire Qspec SLOP Toggle modified Qspec slope matching flag eXec Execute full inverse calculation Plot Replot Qspec line and Q symbols VISC Qvis overlay toggle REFL Reflected Qspec overlay toggle SPEC Plot mapping coefficient spectrum Blow Blowup plot region Rese Reset plot scale and origin SIZE r Change absolute plot object size ANNO Annotate plot HARD Hardcopy current plot 23 PERT Perturb one Cn and generate perturbed geometry MDES c gt As described above the initial Qspec distribution is taken from Q the speed distribution corre sponding to the current geometry at the last angle of attack employed in OPER Qspec can be set back to this Q with QSET anytime 7 2 Modification of surface speed distributions 7 2 1 Cursor input of modifications Qspec can be modified to whatever is desired with the MODI command by specifying points with the screen cursor which are then splined The points can be entered in any order The previously input points can be erased one by one by clicking on the Erase button or simply typing e in the graphics window The input sequence is terminated by clicking on the Done button or by typing d in the graphics window The Abort button or typing a aborts the MODI command and returns to the MDES menu The BLOW command can be used to enlarge regions of interest at any time by specifying opposite corners of the blowup region 7 2 2 Modification endpoint blending
41. on to be restricted to locations with excessive corner angles displayed with CANG and also to locations which fall within a specified x range Different spline parameters can also be used to determine the inserted spline points For example the command GDES c gt CADD 10 0 2 0 1 0 2 will add spline points adjacent to each existing point whose panel angle exceeds 10 degrees and only if the added point will fall within the interval 0 1 lt x lt 0 2 The 2 indicates that an arclength spline parameter is to be used The PANE command will always use the arclength spline Some archived airfoils notably the Eppler airfoils and some of the Selig airfoils have an excessively coarse point spacing around the leading edge The spacing has apparently been tailored for a uniform parameter spline and often produces a badly shaped leading edge with the arclength parameter spline used in Xfoil The following command will insert additional points giving a much smoother shape for subsequent analysis GDES c gt CADD 10 0 1 0 1 1 1 The 10 0 degree angle tolerance can be varied as needed 1 2 of the max angle is the default The 1 argument also a default specifies a uniform parameter spline for the interpolation since this works best for Eppler airfoils and the default z range indicates that the entire airfoil is to be treated The CADD command can be repeated to keep reducing the max panel angle but this may or may not improve the
42. opy can be changed with the SIZE command from most menus The number requested is the width of the plot in inches If the plot ps file is to be previewed with some X Windows PostScript viewer or imported into word processing systems XFOIL must be exited with QUIT in order for the plot ps file to be properly terminated For just printing this may or may not be necessary For the geometry plot in GDES and the Qspec s plots in QDES and MDES described below the hardcopy plot size will also be affected if the graphics window is resized with the cursor at the window manager level This is because the plot is always scaled so that it fills up as much of the window as possible If the window size is left at its start up size the hardcopy plot widths will come out with the specified size in inches If any window dimension is increased from its default value then a subsequent hardcopy plot will probably not fit on a standard 8 5 x 11 0 sheet 7 Full Inverse Surface Speed Design Routine MDES XFOIL s Full Inverse complex mapping facility MDES takes as input a speed distribution Qspec specified over the entire airfoil surface modifies it somewhat to satisfy the Lighthill constraints and generates a new overall geometry First a bit of the underlying theory The geometry and the surface velocities can both be computed from a set of complex mapping coefficients Cn in the form iy 2 w Cy u iv w w Cy a 22 where w 0
43. putations The polar z y must first be transferred into the current airfoil if they are to be used for computation The figure below indicates the data flows resulting from user commands described in the remainder of this manual XFOIL 6 9 Data Flow Airfoil x y coordinate file CSAVE CINTE Buffer o Current Airfoil Airfoil mon CAMB ie Soe A meee GDES __ ee co Polar save file MDES Lo 124 E Li gt 3 Program Execution XFOIL is simply executed with xfoil When the program starts the following top level menu and prompt appear QUIT Exit program OPER Direct operating point s MDES Complex mapping design routine QDES Surface speed design routine GDES Geometry design routine SAVE f Write airfoil to labeled coordinate file PSAV f Write airfoil to plain coordinate file ISAV f Write airfoil to ISES coordinate file MSAV f Write airfoil to MSES coordinate file REVE Reverse written airfoil node ordering LOAD f Read buffer airfoil from coordinate file NACA i Set NACA 4 5 digit airfoil and buffer airfoil INTE Set buffer airfoil by interpolating two airfoils NORM Buffer airfoil normalization toggle BEND Display structural properties of current airfoil XYCM rr Change CM reference location currently 0 25000 0 00000 PCOP Set current airfoil panel nodes directly from buffer airfoil points PANE Set current airfoil panel nodes 140 based on cur
44. s of inverse methods incorporated in XFOIL Full Inverse and Mixed Inverse The Full Inverse formulation is essentially Lighthill s and van Ingen s complex mapping method which is also used in the Eppler code and Selig s PROFOIL code It calculates the entire airfoil geometry from the entire surface speed distribution The Mixed Inverse formulation is simply the inviscid panel formulation the discrete governing equations are identical except that instead of the panel vortex strengths being the unknowns the panel node coordinates are treated as unknowns wherever the surface speed is prescribed Only a part of the airfoil is altered at any one time as will be described later Allowing the panel geometry to be a variable results in a non linear problem but this is solved in a straightforward manner with a full Newton method 1 5 Viscous Formulation The boundary layers and wake are described with a two equation lagged dissipation integral BL formulation and an envelope e transition criterion both taken from the transonic analysis design ISES code The entire viscous solution boundary layers and wake is strongly interacted with the incompressible potential flow via the surface transpiration model the alternative displacement body model is used in ISES This permits proper calculation of limited separation regions The drag is determined from the wake momentum thickness far downstream A special treatment is used for a blunt trailing edge whi
45. skin friction coefficient CD Plot dissipation coefficient N Plot amplification ratio CT Plot max shear coefficient RT Plot Re_theta RTL Plot log Re_theta X rrr Change x axis limits Y rrr Change y axis limits on current plot Blow Cursor blowup of current plot Rese Reset to default x y axis limits SIZE r Change absolute plot object size ANNO Annotate plot HARD Hardcopy current plot GRID Toggle grid plotting SYMB Toggle node symbol plotting LABE Toggle label plotting CLIP Toggle line plot clipping VPLO gt This menu is largely self explanatory The skin friction coefficient plotted with the CF command is defined as Ch 7 0 5pV2 15 This differs from the standard boundary layer theory definition which uses the local Ue rather than V for the normalization Using the constant freestream reference makes C x have the same shape as the physical shear stress T z The dissipation coefficient C this is NOT the drag coefficient is plotted with the CD command Ch x is proportional to the local energy dissipation rate due to viscous shear and turbulent mixing Hence it indicates where on the airfoil drag is being created It is in fact a much better indicator of drag production than C x since Cf does not account for pressure drag Cp on the other hand accounts for everything Its relationship to the total profile drag coefficient is simply as f 2Chas 0 with the integration performed over both boundary lay
46. ted along the modification axis the airfoils are A B C 0 0 1 0 1 4 So airfoil C has 40 more of the change received by B in the redesign Airfoil C s polar will also be changed about 40 more as intended 3 4 Further buffer airfoil manipulation The GDES facility allows very extensive manipulation of the buffer airfoil This will be described in much more detail in a later section If only analysis is performed the GDES facility would not normally be used 3 5 Generation of current airfoil When the buffer airfoil coordinates are read from a file during startup or read in via the LOAD command they are by default also copied directly into the current or working airfoil Hence no special action is needed to start analysis operations However if the starting buffer airfoil has a poor point distribution too many too few poorly spaced etc one can use PANE to create a better panel node distribution for the current airfoil on the splined buffer airfoil shape The paneling routine increases the point density in areas of high curvature i e the leading edge and at the trailing edge to a degree specified by the user The user can also increase panel density over one additional interval on each airfoil side perhaps near transition The current airfoil paneling can be displayed and or modified with PPAR In some cases it is desirable to explicitly re copy the buffer airfoil into the current airfoil via PCOP In previous XFOI
47. the growth of 2 D Tollmien Schlichting waves via linear instability is the dominant transition initiating mechanism Fortunately this happens to be the case in a vast majority of airfoil applications Other possible mechanisms are 17 e Crossflow instabilities These occur on swept wings with significant favorable chordwise pres sure gradients e Attachment line transition This requires large sweep large LE radius and a large Reynolds number Occurs primarily on big jets e Bypass transition This occurs in cases with sufficient wall roughness and or large freestream turbulence or vibration levels The linear instability phase predicted by the e method is bypassed giving relatively early transition Usually occurs in favorable pressure gradients while the linear instability mechanism usually dominates in adverse pressure gradients If any of these alternative transition mechanisms are present the trips must be set to mimick their effect The bypass transition mechanism can be mimicked to some extent by the e method by setting Neri to a small value herit 1 or less This will cause transition just after linear instability begins For very large freestream turbulence or roughness in favorable pressure gradients bypass transition can occur before the linear instability threshold and in this case trips will have to be set as well 5 3 Numerical accuracy 5 3 1 Panel density requirements If strong separation bubbles are
48. tour with the MODI command which accepts cursor inputs in the same manner as the MDES and QDES procedures Slope matching at the modified piece endpoints can likewise be enabled disabled with the SLOP toggle command The only important difference is that here the points must be entered in consecutive order along the new contour One can erase a just entered point by typing e in the graphics window A point can be doubled with the CORN command A doubled point is useful wherever a sharp corner is required such as at a flap break Normally the spline routine enforces slope continuity at all points effectively preventing sharp corners A doubled point marked by a small diamond symbol on the plot causes separate splines to be generated on each side of the corner thus allowing the slope break The doubled point is eliminated by clicking on it after issuing the DELP command Using DELP on a normal single point will delete that point entirely The TGAP command sets the thickness or gap of the blunt trailing edge The gap As is defined as the distance between the upper and lower coordinate endpoints As Ax Ay If the gap is already nonzero then the new TE base vector Ax Ay will have the same orientation as the old one i e Ay Ag _ Ay new Ax old If the gap is zero to begin with then the new base vector will be perpendicular to the trailing edge bisector If the base orientation comes out in an unexpected way it proba
49. try is left unaltered EXEC requests the number of Newton iterations to be performed in the inverse calculation Although as many as six iterations may be required for convergence to machine zero it is not necessary to fully converge a Mixed Inverse case Two iterations are usually sufficient to get very close to the new geometry In any case the new surface speed distribution Q which actually results from the inverse calculation will typically differ somewhat from the specified distribution Qspec by function modes which are added to Qspec At least two modes are added with their magnitudes determined by geometric closure requirements at the inverse segment endpoints As with the MDES complex mapping routine the necessary modifications to Qspec will be smallest if Qspec is modified so that Cz the area under the Qspec s curve is roughly preserved Issuing PLOT after the EXEC command finishes will compare the specified Qspec and resulting Q speed distributions If extra smoothness in the surface speed is required the CPXX command just before EXEC will enable the addition of two additional modes which allow the second derivative in the pressure at the endpoints to be unchanged from the starting airfoil The disadvantage of this option is that the resulting surface speed Q will now deviate more from the specified speed Qspec It is allowable to repeatedly modify Qspec set or reset the CPXX option and issue the EXEC command in any order The Mixe
50. unt Trailing Edges Paper AIAA 89 2200 August 1989 Other related literature Drela M Elements of Airfoil Design Methodology Applied Computational Aerodynamics P Henne editor AIAA Progress in Aeronautics and Astronautics Volume 125 1990 Drela M Low Reynolds Number Airfoil Design for the MIT Daedalus Prototype A Case Study Journal of Aircraft 25 8 pp 724 732 August 1988 Drela M Pros and Cons of Airfoil Optimization Chapter in Frontiers of Computational Fluid Dynamics 1998 D A Caughey M M Hafez Eds World Scientific ISBN 981 02 3707 3 1 3 Inviscid Formulation The inviscid formulation of XFOIL is a simple linear vorticity stream function panel method A finite trailing edge base thickness is modeled with a source panel The equations are closed with an explicit Kutta condition A high resolution inviscid calculation with the default 160 panels executes in less than one second on a workstation Subsequent operating points for the same airfoil but different angles of attack are obtained nearly instantly A Karman Tsien compressibility correction is incorporated allowing good compressible predictions all the way to sonic conditions The theoretical foundation of the Karman Tsien correction breaks down in supersonic flow and as a result accuracy rapidly degrades as the transonic regime is entered Of course shocked flows cannot be predicted with any certainty 1 4 Inverse Formulation There are two type
51. vature PPAR Show change paneling PLOP Plotting options WDEF f Write current settings file RDEF f Reread current settings file NAME s Specify new airfoil name NINC Increment name version number Z Zoom available in all menus U Unzoom XFOIL c gt The commands preceded by a period place the user in another lower level menu The other com mands are executed immediately and the user is prompted for another top level command The lowercase letters i r f s following some commands indicate the type of argument s expected by the command i integer r real f filename s character string Commands will be shown here in uppercase although they are not case sensitive Typically either the LOAD or the NACA command is issued first to create an airfoil for analysis or redesign The NACA command expects an integer argument designating the airfoil XFOIL c gt NACA 4415 As with all commands omitting the argument will produce a prompt XFOIL c gt NACA Enter NACA 4 or 5 digit airfoil designation i gt 4415 The LOAD command reads and processes a formatted airfoil coordinate file defining an arbitrary airfoil It expects a filename argument XFOIL c gt LOAD e387 dat The NACA or LOAD commands can be skipped if XFOIL is executed with a filename as an argument as for example xfoil e387 dat which then executes the LOAD procedure before the first menu prompt is given 3 1 Airfoil file formats LOAD recognizes four file for
52. will typically result in a ragged Cp distribution Examine the paneling in the GDES menu making the GSET command if neceesary to set the current paneling Eliminate excessively small panels my deleting one or more panel nodes with the DELP command When performing viscous analysis calculations it is always a good idea to sequence runs so that alpha does not change too drastically from one case to another The Newton solution method always uses the last available solution as a starting guess for a new solution and works best if the change from the old to the new solutions is reasonably small For this reason it is best to perform difficult calculations such as past Crmax by gradually increasing alpha The ASEQ command in OPER is convenient for this If the user insists on a large change from one point to another it is best to force a re initialization of the boundary layers with the INIT command from the VPAR menu in OPER before the radical calculation is performed INIT should always be executed whenever the viscous solution blows up but the program doesn t crash The viscous analysis will execute no more Newton iterations than set by the current iteration limit each time an ALFA CL etc command is issued If convergence is not achieved within this limit ALFA or CL can be issued as often as needed most easily with with another set of Newton iterations being performed each time This iteration limit can be changed from its defau
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