1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
|
<html>
<h1>Evaluation of glider model realism in CRRCsim</h1>
<h2>1. Revisions of this document</h2>
<table cellpadding="3" border="1">
<tr>
<td>January 2013</td>
<td>Luca Gasparini</td>
<td>Wrote first version</td>
</tr>
</table>
<h2>2. Introduction</h2>
<p>
CRRCSim provides several glider models to fly with.
</p>
<p>
Some of them are included in the distribution package, others can be download from
<A HREF="http://sourceforge.net/apps/mediawiki/crrcsim/index.php?title=Models">CRRCsim Wiki Models page</A>.<br>
An airplane model is defined by its 3D model, often an <TT>.ac</TT> file in
folder <TT>objects</TT>, which is responsible for the appearance of the airplane, and by
its FDM (flight dynamic model), descibed in a <TT>.xml</TT> file in folder <TT>models</TT>, which is responsible
for the simulation of its (aero)dynamic behaviour.
</p>
<p>
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 <B>December 2012</B> have serious flaws which produce unrealistic simulations.
</p>
<h2>3. A table of FDM parameters</h3>
<p>
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.
</p>
<p>
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. <B>AVL</B> or <B>XFLR5</B> 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.
</p>
<p>
In the table, models from <i>Allegro</i> to <i>Zipper</i> were included in the standard package as of December
2012, except for <i>Pilatus B4 2.6m</i> and <i>Generic F3F</i> which have been added by the
author on January 2013; <i>Pilatus B4 New</i> is just the revised version of the original <i>Pilatus B4</i>.
The other models are representative of most of the glider models available for download on the
<A HREF="http://sourceforge.net/apps/mediawiki/crrcsim/index.php?title=Models">CRRCsim Wiki Models page</A>.
</p>
<img src="xl_01.png"/><br>
<p>
In the table above:<br>
<ul>
<li>
<FONT COLOR="#FF0000">RED</FONT> marks wrong values.<br>
</li>
<li>
<FONT COLOR="#FF8800">ORANGE</FONT> marks likely wrong values.<br>
</li>
<li>
<FONT COLOR="#FFFF00">YELLOW</FONT> marks likely inaccurate values.<br>
</li>
</ul>
</p>
<p>
Note that <i>Crossfire</i>, <i>Erwin</i>, <i>Ceres</i>,
<i>Vampire</i> and <i>Sniper Evo</i> actually share most
of the FDM parameters with the <i>Skorpion</i>, and in fact <u>all</u> the stability & control parameters
(not shown in the table) are just an exact copy of those of the <i>Skorpion</i> while other parameters are
equal or just slightly modified.<br>
This means that apart from looking differently they will all fly exactely the same in CRRCsim.<br>
Actually many more F3F/F3B gliders are available for download (in the <tt>CRRCsim-addon-models</tt> package):
while having detailed and specific 3D models and very nice liveries, they all share the same FDM model of the
<i>Skorpion</i>.<br>
Unfortunately, this means that they all share the flaws of the <i>Skorpion</i> FDM, as will be shown in the following!
</p>
<p>
Similarly, <i>Sovereign</i> and <i>Zipper</i> actually uses the same FDM as the <i>Allegro</i>. In this case, at least,
they share a good FDM although the real glider may behave differently.
</p>
<h3>3.1 Mass and inertias</h3>
<p>
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.
</p>
<img src="xl_02.png"/><br>
<p>
From the chart above we notice that:<br>
<ol>
<li>
All the <i>Skorpion</i>-like models have significantly smaller radius of gyration, hence lower moments of inertia.
Looking at the table above this is mostly due to an excessive mass. In fact, although F3F models
may be ballasted, mass of the order of 5kg up to 11kg (!) seem really excessive to me.
</li>
<li>
The original <i>Pilatus B4</i> (a 4.6m span glider) also has much too low moments of inertia.
In this case the problem is not the mass, which is reasonable, but the values of the inertias which
have been estimated wrong. This fact is promptly recognized when flying the model, which looks
really unrealistically "light" and responds too sharply to control inputs (e.g rudder).
</li>
<li>
The somewhat low inertias of the <i>Flexifly</i> are justified being it a foamie, and are thus
realistic.<br>
</li>
<li>
A new <i>Generic F3F</i> model has been derived from the <i>Skorpion</i>, to be used with <u>any</u> of the
available F3F 3D model. Wrong values have been corrected using reasonable guesses.
Just setting the mass in the range 3-4kg brings the inertias in the expected range suggesting that,
contrary to the mass, they were estimated right.<br>
</li>
<li>
The new <i>Pilatus B4 2.6m</i> model uses the same 3D model (i.e. the <tt>.ac</tt> file) of the original Pilatus B4 (scaled and
slightly revised), but all the FDM properties have been now redefined to reproduce as accurately as possible
the 2.6m wingspan version of this glider made by Tangent (author's latest model :-).<br>
</li>
<li>
Mass and inertias of the <i>Pilatus B4 2.6m</i> have then been scaled-up back to the 4.6m size, and used to correct
the original <i>Pilatus B4</i> data. The <i>Pilatus B4 New</i> model is thus back in line, providing a much more
sensible and realistic simulation.
</li>
</ol>
</p>
<h3>3.2 Drag</h3>
<p>
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).
</p>
<img src="xl_03.png"/><br>
<p>
In CRRCsim (Larcsim FDM) the airplane drag is estimated considering a few contributions, among which:
<ul>
<li>
Minimum airplane drag <tt>CD_prof</tt>, which is the drag at the CL specified by <tt>CL_CD0</tt>.
Note that despite its name <tt>CD_prof</tt> shall include the drag contribution due to fuselage, tails, etc.. not just that
due to wing airfoil.<br>
</li>
<li>
Drag variation with the square of the lift coefficient <tt>CD_CLsq</tt> (mostly due to the variation of airfoil drag
with angle of attack).
</li>
<li>
Effect of the Reynolds number based on the reference <tt>Speed</tt> and <tt>Uexp_CD</tt>: the larger the ratio between
flying speed and the reference <tt>Speed</tt>, i.e. the larger the Reynolds number, the lower the drag; this effect is
stronger for larger negative value of <tt>Uexp_CD</tt>.
<li>
Induced drag, related to CL, aspect ratio (= <tt>Span^2/Area</tt>) and <tt>span_eff</tt> factor.<br>
</li>
</ul>
</p>
<p>
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.
</p>
<p>
From the chart above we notice that:<br>
<ol>
<li>
All the <i>Skorpion</i>-like models have too high efficiency at low CL (0.2) and much lower efficiency
at higher CL: this is not right.
Looking at the table above we see it is caused by: firstly a much too small <tt>CD_prof</tt> (too small even if
it was the airfoil CD alone); secondly a huge wing area (about 3 times the real value!) which results in
an extremely low aspect ratio (about 5, instead of 15!), hence a large induced drag at increasing CL.
</li>
<li>
The original <i>Pilatus B4</i> also has a similar behaviour due again to a much too small <tt>CD_prof</tt>.
But now the aspect ratio is ok, so the glider has an incredibly high efficiency.
</li>
<li>
By comparison, the new <i>Pilatus B4 2.6m</i>, <i>Pilatus B4 New</i> and <i>Generic F3F</i> models with
appropriate estimate of <tt>CD_prof</tt>, <tt>CL_CD0</tt> and <tt>Uexp_CD</tt> have reasonable
and realistic perfomances, with a proper variation of efficiency with CL.
</li>
<li>
Both <i>Fennec</i> and <i>Crobe</i> models have too small <tt>CD_prof</tt>, resulting in too high efficiency
for such small gliders (should be close but lower than respectively <i>Allegro</i> and <i>Apogee</i>).
</li>
<li>
In most cases efficiency at CL=-0.6 (inverted flight) results equal to that at CL=0.6. This is almost always wrong
since cambered airfoils usually have minimum drag at a CL greater than zero, the more cambered the airfoil the
higher the CL at which minimum drag is achieved. Hence, they generate much less drag at CL=0.6 than CL=-0.6
(which in some cases might be even beyond the limit of negative stall).<br>
As we can see from the table for most models <tt>CL_CD0</tt> has just been set to zero (or practically zero),
only exception being:
<i>Pilatus B4 2.6m</i>, <i>Pilatus B4 New</i>, <i>Generic F3F</i>, <i>Zagi-xs</i>, <i>Fennec</i>, <i>Crobe</i> and <i>Riser</i>.
As a consequence only these few models realistically have low efficiency at CL=-0.6.<br>
Thus, I marked in yellow all the models whose value of <tt>CL_CD0</tt> (and of <tt>CD_CLsq</tt>
which is related to it) are most likely inaccurate, causing too good performance in inverted flight (although
they may look realistic in normal flight).
</li>
<li>
The <i>Wasabi</i> model is a justified exception to the previous comment. In fact it has a symmetric airfoil
so that with flap at 0 it flies equally well straight or inverted. However, when flaps are lowered to 4
the CL for minimum drag becomes positive, hence efficiency at large CL improves while efficiency at negative
CL reduces, as it happens in real-life.
</li>
</ol>
</p>
<p>
Although their effect are more subtle and would require a longer explanation, the specification of the reference
<tt>Speed</tt> and of <tt>Uexp_CD</tt> is also likely not right in many cases.<br>
<tt>Speed</tt> is often too low resulting in too low a reference Reynolds number, thus the drag computed
while flying is lower than intended.<br>
<tt>Uexp_CD</tt> is sometimes to large (e.g. -0.6 used for <i>Skorpion</i>-like models), which results in too low
drag at high flying speeds.
In my opinion <tt>Uexp_CD</tt> should be within -0.25 to -0.45
</p>
<h2>4. Conclusion</h2>
<p>
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 <i>Pilatus B4</i> and <i>Fox</i> (this last has not been discussed so far but its FDM is an
exact copy of the flawed Pilatus one..).
</p>
<p>
To provide a reasonable FDM for F3F-like gliders a new <i>Generic F3F</i> model has thus been implemented, replacing
<i>Crossfire</i>, <i>Erwin</i> and <i>Skorpion</i> in the standard distribution package; the various 3D models are now
just different graphical appearence of the same FDM.<br>
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 <tt>.xml</tt> files).
</p>
<p>
The FDM of <i>Pilatus B4</i> has been revised, based on new estimate of aerodynamics, mass and inertias obtained
scaling to the larger size those of a new <i>Pilatus B4 2.6m</i> small scale glider which reproduces the model made by
Tangent.
</p>
<p>
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.
</p>
<br>
Happy flying with CRRCsim !
</html>
|
