Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Panel Method Solvers - XFoil and XFLR5
XFoil is a panel-based method that uses full potential methods to calculate the bulk flows around two-dimensional sections which can then be corrected for the presence of the boundary layer. The code is aimed at dealing with viscous analysis of airfoils, allowing for forced or free transition, transitional separation bubbles, limited trailing edge separation, and lift and drag predictions just beyond CLmax. Since the corrections used are based on well-established experimental data, very good approximations of lift and drag can be predicted as long as the boundary layer remains attached to the airfoil. Once separation has begun, leading on to stall, panel codes are less reliable, which is a fundamental limitation of such methods. Figure 13.1 shows the kinds of data provided by XFoil, here at a high AoA where separation has begun. To obtain this result, the following XFoil steps are used:
By dividing the lifting surfaces into stream-wise strips, it is possible to take the results from XFoil and compute the behavior of whole wings - this is what XFLR5 does, while also allowing for simple streamlined fuselage elements and multiple combinations of wings. Again, it will not deal so well with heavily separated or stalled flows and, although interference
Figure 13.1 Cp and streamline plot for the NACA0012 foil at 16° angle of attack as computed with XFoil.
effects between widely spaced lifting elements (such as wings and tails) can be dealt with, such methods cannot accurately predict the benefits of slotted flaps and other boundary layer control systems. XFLR5 does, however, readily permit calculation of the aircraft’s dynamic stability, which can be used to check whether elevator and fin sizes and positions are acceptable given the likely mass, inertias, and flight speeds. To analyze a simple wing with XFLR5, the following steps are used:
Figure 13.2 Results of XFoil analysis sweep for the NACA 64-201 foil at Mach 0.17 as computed with XFLR5.
dealt with. Figure 13.3 shows the results of a typical sweep through the available angles of attack. These results can be exported to a text file for subsequent analysis or comparison with other results.
XFLR5 also allows tail fins and elevators to be added and also a second wing (typically for bi-plane configurations). These are all defined as for the main wing and, in the case of fins, can be either single- or double-sided. All the surfaces can have dihedral, sweep, and twist as required, and the code allows for the flow fields around the individual items to impact on each other in terms of downwash but not wake effects. It is also possible to add fuselage-like elements to link the lifting surfaces together but these are not correctly modeled aerodynamically. Dynamic stability analysis can readily be carried out to give the natural frequencies, damping of modes, and so on. To demonstrate what these codes can achieve during design, they are used in the subsequent subsections to predict the behavior of simple 2D airfoil sections, simple wings, and a built-up airframe.
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