Everything begins at Zero
So now that we have dealt with the preamble and have some familiarity with what
all the rainbow images and basic terminology used is about it should be a good
moment to introduce our science pig, sorry plug I call it Pig because that is
what it is; a happy jovial highly intelligent pig that does not mind being
pushed
and prodded around the yard.
Pig-01 is used as a Baseline measure that all upstream tests can be referred
back
to and compared against. It is a Clean Skin 3D model, meaning there are no
extra bits added or protruding from the basic aerodynamic shape; it is as
slippery
as a Europa Classic can possibly be without further modification. And contrary
to the mention of two Pigs in the Preamble there can only be space for one
Pig in this story of shock and awe.
The model lends itself to set criteria of variance in order to identify our
Primary
Baseline. This criterion consists of the following:
Wings 6DOF
Horizontal Stab 6DOF
Fuselage Fixed
With only three pieces making up our pig the third piece being the fuselage is
fixed at 0deg attitude; the lower surface line of the hatches set to the
horizontal
of the XY plane with the longitudinal axis being in the XZ direction. This
allows complete augmentation of both Wing Incidence and Pitch of the Horizontal
Stab. Height of both pieces can also be altered relative to the fixed fuselage.
Dihedral of the main wing is set at 4deg.
In order to Zero the model Incidence of the Main Wing is set at 0deg for the
primary
round of CFD analysis, using the wings chord line at the longitudinal centreline
of the fuselage as the identifier.
Alpha is also set at 0deg.
Now that we have set our model parameters we can go about creating the Solution
Domain parameters.
The Solution Domain is a large area measuring some 20mx20mx20m around the model.
This area is required to mitigate any interference that may occur should the
free stream fluid impact the outer perimeter wall and feedback against the model
during solution, resulting in false readings this is a big area and when we
turn this into a 3D mathematical mesh our mesh sizing will also affect the time
the solution requires to solve along with the resolution and clarity of results.
So we need to firstly ask what is it that we want to know, and how critical
do we need the answer to be If its just for guidance and we are searching
for direction to develop a concept in then we can use a very loose large cell
mesh however if we require more definitive answers we would opt for a finer mesh
and go do some gardening or shopping for an hour or two or just go to bed
and come back to the machine the next morning with a hot cup of coffee in hand
and sit down to read the note on the screen Meshing Failed Fortunately that has
not happened for a while, but it does happen
So we are at the point of what is it that we want to know? Well how about some
Lift and Drag numbers? Lets look at the way the fluid Velocity is flowing around
the model; is there any Turbulence or Cavitation evident? Where are the Pressure
curves happening? What level of Acoustic Power is being emitted? ... These
aspects are the primary criteria used in most of all the contrast studies We
are primarily looking for Divergence and Convergence of data in order to
identify
areas of aerodynamic efficiency.
How great it is to be able to play with what ifs?! And not kill ourselves doing
it!
With that out of the way we can chuck it all out across the table and record our
ground zero baseline, which on this occasion, based upon the above, is:
Velocity (m/s) Alpha (deg) Inc (deg) Drag (kg) Lift (kg)
38.50 0 0 18 78
51.44 0 0 32 137
77.16 0 0 76 312
102.88 0 0 137 544
128.61 0 0 215 887
154.33 0 0 324 1331
205.77 0 0 658 2677
Yes that really is 400kt TAS at 0AMSL 15C Dorothy
Next we will look at a few of these above results with the wing incidence
adjusted
Velocity (m/s) Alpha (deg) Inc (deg) Drag (kg) Lift (kg)
38.50 0 1 18 128
51.44 0 1 32 209
77.16 0 1 76 512
51.44 0 2 36 330
77.16 0 2 80 729
What can we assume and theorise from these above results?
The Europas aerofoil generates Lift! It actually generates more lift than
required
at certain velocities
What else can we assume from these numbers?
With a Rotax 912UL/ULS we can aerodynamically Vmax at 200kt 230kt
With a Rotax 916is we can aerodynamically Vmax somewhere between 250kt 300kt
With a Rotax 914 we can aerodynamically Vmax somewhere between 230kt 250kt
Not too shabby But what is happening to the air while it skirts over the models
skin? Is there a lot of turbulence? Is there a lot of noise? Well Yes and No
At first Velocity prints were looked at across all the velocity vectors, and it
was surprising to observe very little alteration in the velocity pattern over
the model. Other than the velocity band increasing in magnitude, at Alpha 0 or
1 or 2 the patterns remained fairly unaltered.
Then Acoustic Power prints were looked at in the same manner as with Velocity,
and it revealed a slightly different picture.
At 38.50m/s (75kt) to 51.44m/s (100kt) there is very little audible noise being
generated by our Pig; 35db 55db max respectfully. In other words, this Pig is
silent while flying through the sky
With increases in velocity our pig starts to murmur a bit, and as velocity
increases
our pig is squealing at the top of its capacity at 110db at 400kt 747 on
T/O equivalent its Loud
The other thing that was picked up by the Acoustic prints was Cavitation over
the
Boundary Layer; as velocity increases so does cavitation. This being the cause
of the rise in db power; and more will be written about this later, but for
now all we need to know is that cavitation equals noise
So what have we discovered so far in this primary study that we can take into
stuff
going on further down the wire?
Well, it appears we know that:
The Europas wing generates too much Lift to aerodynamically fly the
Model beyond
say 230kt; unless we want to keep climbing up to FL25 where the air is thin
enough to absorb the extra Lift.
In order to attain the above data, aerodynamically, we need to reduce
the wings
incidence on our Model to 2 or less degrees.
As velocity of the Model increases so too does the Acoustic Power being
emitted.
The Europas aerofoil profile is able to reach 400kt at Alpha-0 and
remain efficient.
It is probably a good moment to review our model with the wing incidence set as
per factory and compare some results. These results have been mashed through
the Solver with an Alpha of -3deg and the Horizontal Stab adjusted to be
parallel
of the wings Incidence; So we are simulating pitching the nose of the Model
slightly down on step The results received were:
Velocity (m/s) Alpha (deg) Inc (deg) Drag (kg) Lift (kg)
51.44 -3 factory 42 129
102.88 -3 factory 175 518
128.61 -3 factory 280 837
205.77 -3 factory 842 2469
With these above numbers, with a 912ULS, aerodynamically, we are just able to
reach
200kt; still not too shabby
The Europas form is very slippery, but the CFD studies are revealing areas
around
the Empennage aft of the cabin where cavitation begins and continues aft; so
we are seeing the effects of boundary layer separation that is taking place
just above the models skin The Velocity prints are not detecting this as they
are focused upon a very shallow layer, where as the Acoustic Power prints reveal
the affects more of what is going on around the selected layer band; so this
is why we can use them to tell another story other than just noise, and more
so read them as an indication of what may be the cause of the noise; which, in
this environment, can only be cavitation.
Cavitation also is evident with the wing. At 0deg Incidence the cavitation is
minimal
and is seen along the last inch of the upper trailing edge and along the
last 2 or so inches of the lower trailing edge. Longitudinal cavitation streaks
are also evident on the lower surface of the wing at Alpha-0. As Velocity
increases these areas of cavitation extend further in from the trailing edge of
the wing and increase in amplitude. However this study is not focused upon the
wing so we will leave this story for another episode.
As far as actual noise is concerned even at 150kt the Acoustic Power being
generated
by this above cavitation is only at 70db max So we are still, from 100m
or so away, flying silently. At 200kt things are just starting to become
detectible
over the murmur of daytime nature.
So what about this cavitation and how might it be mitigated? At this point no
definitive
idea, however looking at the shape of the empennage, being slightly
concave, changing it to a slightly convex shape may return favourable numbers.
Looking toward the Liberty XL may provide some direction. Regarding cavitation
around areas of the wing, this tends to diminish as Alpha is increased. Again
this is another bedtime story to be told another night
This concludes this session upon flying pigs and baseline setting, and the
subject
is now open to forum for people to add or comment upon.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510821#510821
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