|January 2013||Luca Gasparini||Wrote first version|
CRRCSim provides several glider models to fly with.
Some of them are included in the distribution package, others can be download from
CRRCsim Wiki Models page.
An airplane model is defined by its 3D model, often an .ac file in folder objects, which is responsible for the appearance of the airplane, and by its FDM (flight dynamic model), descibed in a .xml file in folder models, which is responsible for the simulation of its (aero)dynamic behaviour.
This document analyzises most of the glider models looking in detail at a few of the many parameters which are part of the FDM and showing that (to the author's opinion) quite a few of the glider models available as of December 2012 have serious flaws which produce unrealistic simulations.
The table below shows some of the most important parameters of the FDM related to: wing size, airfoil and airplane drag, mass and inertias. The reliability of these parameters can often be judged based on some relatively simple considerations.
While many more parameters are also important, in particular those describing the stability of the airplane and the reaction to the controls, they shall best be derived from an analysis of the airplane through e.g. AVL or XFLR5 codes. In most cases it is not that easy to judge "by eyes" if they look reasonable or not. Thus I will not make any comment about them here, assuming they are all ok.
In the table, models from Allegro to Zipper were included in the standard package as of December 2012, except for Pilatus B4 2.6m and Generic F3F which have been added by the author on January 2013; Pilatus B4 New is just the revised version of the original Pilatus B4. The other models are representative of most of the glider models available for download on the CRRCsim Wiki Models page.
In the table above:
Note that Crossfire, Erwin, Ceres,
Vampire and Sniper Evo actually share most
of the FDM parameters with the Skorpion, and in fact all the stability & control parameters
(not shown in the table) are just an exact copy of those of the Skorpion while other parameters are
equal or just slightly modified.
This means that apart from looking differently they will all fly exactely the same in CRRCsim.
Actually many more F3F/F3B gliders are available for download (in the CRRCsim-addon-models package): while having detailed and specific 3D models and very nice liveries, they all share the same FDM model of the Skorpion.
Unfortunately, this means that they all share the flaws of the Skorpion FDM, as will be shown in the following!
Similarly, Sovereign and Zipper actually uses the same FDM as the Allegro. In this case, at least, they share a good FDM although the real glider may behave differently.
The following picture shows for each model the radius of gyration of the mass moment of inertia Ixx, Iyy, Izz (respectively for roll, pitch and yaw axis) in percentage of the wing span.
From the chart above we notice that:
The following picture shows for each model the efficiency (L/D) resulting from the FDM parameters for a few values of the lift coefficient: -0.6 (inverted flight), 0.2 (speed), 0.6 (often close to max efficiency), 0.9 (often close to max lift).
In CRRCsim (Larcsim FDM) the airplane drag is estimated considering a few contributions, among which:
Additional drag contributions may come from control deflection and stalled condition but these effects are not included in the computation of the efficiency shown in the chart.
From the chart above we notice that:
Although their effect are more subtle and would require a longer explanation, the specification of the reference
Speed and of Uexp_CD is also likely not right in many cases.
Speed is often too low resulting in too low a reference Reynolds number, thus the drag computed while flying is lower than intended.
Uexp_CD is sometimes to large (e.g. -0.6 used for Skorpion-like models), which results in too low drag at high flying speeds. In my opinion Uexp_CD should be within -0.25 to -0.45
Many models are available for CRRCsim but some have serious flaws in their FDM parameters, significantly compromising the realism of the simulation. The worst cases are the various F3F gliders (which are actually all sharing the same problematic FDM) and the two large scale gliders Pilatus B4 and Fox (this last has not been discussed so far but its FDM is an exact copy of the flawed Pilatus one..).
To provide a reasonable FDM for F3F-like gliders a new Generic F3F model has thus been implemented, replacing
Crossfire, Erwin and Skorpion in the standard distribution package; the various 3D models are now
just different graphical appearence of the same FDM.
This FDM could also be applied to the many more 3D models of F3F gliders available as add-on (this requires editing of the relevant .xml files).
The FDM of Pilatus B4 has been revised, based on new estimate of aerodynamics, mass and inertias obtained scaling to the larger size those of a new Pilatus B4 2.6m small scale glider which reproduces the model made by Tangent.
Most other models are reasonably realistic except for what the drag variation with lift is concerned which is appropriately simulated only by a few models.