The Jibe2 experiment - revisited

Updated August 3rd, 2020


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The experiment

Back in 2008, Matthieu Scherrer had the opportunity to conduct wind tunnel testing on a small RC racer plane with a wingspan=1370mm.

Jibe2 in the wind tunnel

Note: all images are reproduced here with absolutely no permission from Matthieu whatsoever.

The following links provide details of the experiment:
  1. A video summarizing the experiment can be viewed here: The Jibe 2 experiment (Beware of the unfortunate terrible loud music)
  2. The full story is on Matthieu's blog: Model aerodynamic prediction workshop
  3. Matthieu proposed to people who had analysis tools available to make blind predictions which he then compared to his measurements: Results from the Wind Tunnel

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Analyses with xflr5 v4 - 2008

At the time, predictions were made with xflr5 v4 with mixed results. The test airplane is a difficult test case for xflr5 due to:

jibe2 xflr5_v4

The Jibe2 model in xflr5 v4. Although the body was modeled, it was and still is improperly taken into account in xflr5 due to the lack of connection of the fuselage and wing meshes.

Results v=20m/s
Results v=40m/s

With a more rigorous implementation of the modeling of the fuselage now available in flow5 and with the introduction of the Vortex Particle Wake in flow5 beta12, it seemed worthwhile to revisit this experiment.

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Analyses with flow5 v7.01 beta 12

The model

Analyses were carried out for a model with and without a fuselage in different configurations:
  1. Flat wake panels with and without the viscous loop,
  2. Vortex Particle Wake (VPW) with and without the viscous loop.
The tip chords have been increased slightly to improve the interpolation process of viscous data.

All analyses were performed using the triangular panels with uniform densities. As explained elsewhere, the use of the linear method does not bring any significant benefit but increases the computations times by a huge factor.
In the case of the analyses with the VPW, the vortons' core size was set to 100mm and the wake was extended 30 MAC downstream.

The flow5 model
The project file used to perform the comparisons can be downloaded here: Jibe2_v701_beta_13.fl5.

Model without the fuselage


Comparisons at v=20 m/s

CD_v20m/s CL_v20m/s Cm_v20m/s
The analyses would not converge at a.o.a. higher than 5° due to viscous interpolation errors at the wing tips.
Predictions are reasonably accurate whatever the method used, with an underestimation of the total drag mainly due to the absence of the fuselage in the model. Another reason for the difference of drag between prediction and experiment could be linked to the quality of the airflow in the wind tunnel which was not specified. XFoil viscous data was obtained with NCrit=9.
The use of the VPW reduces the induced drag and increases the lift slightly.

Comparisons at v=40 m/s

CD_v40m/s CL_v40m/s Cm_v40m/s
The conclusions are identical to those made at v=20m/s.

Model with the fuselage

Jibe2 with the VPW

The Jibe2 model with the rolled-up vortex particle wake.

Comparisons at v=20 m/s

CD_v20m/s with fuse CL_v20m/s with fuse Cm_v20m/s with fuse
Drag_polar_v20m/s with fuse
Karman-Schoenherr drag for the fuselage is included in all the analyses.

The main points to be noted are:

Comparisons at v=40 m/s

CD_v40m/s with fuse CL_v40m/s with fuse Cm_v40m/s with fuse
Drag_polar_v40m/s with fuse

The conclusions are identical to those made at v=20m/s.

The fifth and last polar was run with both the VPW and the viscous loops activated. It improves the prediction of drag compared to the VPW alone, and overpredicts the lift slightly.

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Lateral predictions with flow5 v7.01 beta 13

Preliminary considerations

Results at a.o.a. = 2°

CY_a2 Cl_a2 Cn_a2

Results at a.o.a. = 6°

CY_a6 Cl_a6 Cn_a6

The main points to be noted are: Back to top

Conclusions and recommendations

The VPW is a new and relatively untested feature, so more numerical testing and benchmarking needs to be performed before firm conclusions can be drawn. That being said, the comparison to the Jibe2 experimental results seems to show a few trends.
  1. Given the drastic simplifying assumptions made in the potential flow solution, predictions are unexpectedly reliable.
  2. The drag is under-predicted whenever the fuselage is not included in the analysis. However, all things being equal, numerical simulations without the fuselage have the advantage of being fast and can be used to compare solutions before moving on to more accurate but also more complex and lengthy analyses.
  3. The VPW should be preferred to the flat wake panel representation whenever the fuselage is included in the model, especially for the calculation of the moments.
  4. All things being equal, it seems less important to include the viscous loop than the VPW in the analyses.
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