Area-51
Joined: 03 May 2021
Posts: 396
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 Posted: Fri May 19, 2023 4:28 am Post subject: The Obsidian Files - Pigs In Space & Other Stuff |
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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 wing’s 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? Let’s 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 0’AMSL 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 Europa’s 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 model’s 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 equal’s 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 Europa’s 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 wing’s incidence on our Model to 2 or less degrees.
As velocity of the Model increases so too does the Acoustic Power being emitted.
The Europa’s 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 wing’s 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 Europa’s 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 model’s 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 left open to forum for people to add or comment upon.
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