flow5 - v7.50

Context

flow5 v7.50 is a major update which among other things introduces deep changes to the handling of trailing edge flaps. This change has been intended for a long time, but its introduction has been delayed regularly because of its impact on existing models, analyses and projects, and also of the need for users to change the way they are using flow5.

With the open-source release date approaching, it seems as good a time as any to take the leap and make the change.

This option should already offer more flexibility to handle T.E. flaps, however it will make full sense when it is associated with XFoil as explained below.

The purpose of this page is

to provide background explanation of the changes,
to provide a quick guide to the use of the new features.

Description

In xflr5 and in flow5 up to v7.25, T.E flap angles are a property of the foil and plane objects. As a consequence each set of T.E. flap angles requires the definition of a specific foil or plane geometry. In turn this requires that a foil and its viscous data be built for each flap angle.

Since in flight, flaps operate more often with non-zero deflection angles than not, the goal is to make the calculation of flapped configurations quicker and simpler, and to remove the need to build a specific object for each.

Starting in v7.50:

The hinge location remains a property of the foil.
The flap angles become a property of the foil and plane analyses.
The management of the leading edge flap is unchanged from the legacy versions and remains a property of the objects.

The change comes with some drawbacks:

Until XFoil is included as a module of flow5, the viscous drag is evaluated on the "base" i.e. non-deflected flapped foil which will likely result in an underestimation of the drag in flap down configurations and a potential overestimation in the case of flap up.
Former existing plane polars of flapped configurations are no longer usable; they are still visible in the polar graphs but will need to be duplicated and recalculated.
Since the influence matrix in 3d depends on the flap configuration and therefore on the active polar, the option which authorized the storage of unit solutions and their re-use across T123578 polars is no longer possible and has been removed.

Notes:

Unflapped foils and planes can be used and analyzed as in former versions of flow5.
It is still possible to make a foil's T.E. flap angle permanent. When this option is selected, the foil is modified to include the deflection and can be used as in former versions fo flow5.
However, the setting of a permanent non-zero T.E. flap angle should be seen as a back-up option. The design intent and the preferred option is to use the flap angle as a variable in the analyses.

Examples

Examples of flap constructions: Flaps_v750.fl5

Foils

Foil initialization

Once loaded, a foil should be initialized with the following operations:

smooth repanelling with ~150 panels, including higher panel densities at the L.E. and T.E.
if the foil is to be used with a T.E. flap, the hinge location should be set before any calculation.
in the special case of a plane configuration where the flapped wing with non-zero flap angle be connected to a fuselage, the T.E. flap angle should be made permament

Step (2) is not necessary if the foil is to be used unflapped. In which case it is handled as in the legacy versions of xflr5 and flow5.

In 3d, the relative locations of the hinge may not be the same on the left and right sides of the flap. In such a case, two foils will be required, each with its specific hinge location.

The T.E. hinge location is set as in former versions using the flap management interface (F10).

A value of T.E. flap angle can be entered in this interface. The goal is to allow testing of min. and max. values of the angle to check that the geometry remains smooth and usable within those limits. The deflection will be discarded when exiting the editor, unless the option "Make deflection permanent" is activated.

If the option "Make deflection permanent" is activated, the foil's geometry is modified to include the deflection and can be used as in former versions of flow5. This however is not the recommended option. Instead, the intent is for the flap angles to be set at the next step when the analysis is defined.

Important: To be consistent with the limitations of the panel methods and of implicit laminar flow, the min. and max. flap angles should remain small.

Foil analysis

Analysis definition

Note: The features will be enabled when XFoil is linked to the future open-source version of flow5.

The T.E. flap angle is denoted θ and is defined in the bottom box of the form.
The other parameters are unchanged.

Type 1-2-3

The T.E. flap angle is a fixed parameter of the analysis.
The Reynolds number is a fixed parameter of the analysis.
The analysis variable is the angle of attack α.

Type 4

The angle of attack α is a fixed parameter of the analysis.
The T.E. flap angle θ is a fixed parameter of the analysis.
The analysis variable is the Reynolds number.

Type 6

The angle of attack α and the Reynolds number are fixed parameters of the analysis.
The analysis variable is the T.E. flap angle θ.

Planes

Procedure

The way to proceed in v7.50 is to define the plane with all T.E. flap angles set to zero, and to define one analysis for each set of flap angles.

As a consequence, if the plane was defined in a legacy version of flow5 with non-zero T.E. flap angles, the associated polars are no longer compatible with the updated plane geometry.

When the analysis is run, the mesh is modified by rotating the flap surfaces around their hinge vector. This implies that the influence matrix and the RHS are built and the linear system is solved at each analysis run. The former option to store the linear solution and use it across all linear analyses is no longer available.

Note that when the flapped surfaces are rotated, the resulting mesh geometry is similar but not identical to the mesh generated in older versions which was built on the flapped foil instead. This means that newly calculated polars will also be similar but not identical to legacy polars.

The viscous drag is evaluated by interpolation of the 2d polar mesh of the non-flapped configuration and should therefore be seen as nothing more than an order of magnitude.
In the future open-source version of flow5, an option will be available to evaluate the viscous drag on the fly by running XFoil on the wing's flapped foils.

The method to conduct a sensitivity analysis with varying flap angles is to use T6 polars. This is unchanged from legacy versions of flow5.

Flap models

The recommended procedure to define flapped wings in xflr5 and legacy versions of flow5 has been to duplicate wing sections at each end of the flaps.

This method is still applicable and is the recommended way if the analyses are to be conducted using thin surface models.

in the case of thick surface methods, it has the inconvenience of leaving volumes opened at the extremities of the flaps when a deflection is applied.

A model with an open volume is theoretically invalid in a boundary element method, and leads to an unknown numerical error.

Numerical tests seem to show that this error is negligible in most cases.

Nonetheless, if no fuselage is included in the analysis, or if the wing is not connected to a fuselage, the recommendation is to use the thin surface model.

Thick surface analyses

The way to build a theoretically sound configuration with deflected flaps in the case of thick surface analyses is to

make the flap deflections permanent in the foil design module;
create an intermediate surface with small width to join unflapped to flapped surfaces.

Thin surface analyses

The way to build a theoretically sound configuration with deflected flaps in the case of thin surface analyses is to duplicate...

Flaps and fuselages

If a fuselage is included in the model, the recommended method is to use a thick surface model, wtih special care given to the connection of the wing to the fuselage.

Unfortunately, no universal method could be found that adresses all issues, which means that some compromise in the construction of the mesh or the definition of the analysis is necessary.

The difficulty is that the mesh should define closed volumes whatever the flap deflection, something which can not be managed automatically by flow5.

A way to do it is to insert a non-flapped surface at the wing's root, so that the wing and fuselage meshes remain connected whatever the flap deflection. This still leaves the drawback of opened volumes.

To solve this last issue, the solution is to make the foil's flap deflection permament, with the drawback that the flap angle can no longer be used as a setting in the analysis.

Flap intersections

Special care should be taken to avoid panel intersections when setting flap angles. These may typically arise in adjacent wing panels with non-identical dihedral angles as illustrated in the image on the right.

Intersections, or parasite numerical interactions may also arise if the flap is adjacent to a fuselage.

Since flow5 cannot detect these interferences, the analyses will run but will yield spurious results.

Legacy models

When projects including flapped configurations are imported, flow5 makes the deflections of flapped foils permament so that the existing analyses can be re-used as is.

User input will be required to rebuild the models to the new standard.

Foil and plane trailing edge flaps

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