Scientific Challenges

Just to be clear, is this a design thing, or, is this something you plan
to implement as a prototype/demo or is it a planned "fully operational
system"? This will help me with understanding the depth you want for
answers. If fully operational, the project is significant...

==================== the next couple of paragraphs are a scientific aside
which goes into some challenges =================================
this may seem scary, and if it is, ignore it I think is the best
advice...as these are scientific problems the bat research group must deal
with eventually and your vis. environment may provide us a nice way to do
this....
=====================================================

Not to scare you, but to make you aware, for "real" implementation of a
system like this there are some major challenges which remain ....some of
what you are trying is unexplored territory from a high fidelity/accurate
scientific viewpoint(I'll highlight them here quickly, just so you are
aware, please don't get concerned on this front...these are scientific
quesitons/concerns which need addressing either in the future to carry
this project to fruition). Again, these are only questions which in the
future need to be addressed...for your info:

a) When changing the wing shape and size, are the bone motions modified?
Prescribed? Interpolated from other bats? or are new "optimal" bone
motions solved for? Wing motions can drastically affect all your desired
outputs and can wash out the effect of the changes in wing shape if the
motions aren't appropriately set. I don't know the best way to do
this...and am not sure if anyone else does at this point.

b) When the wings change size, the aerodynamic forces also change (there
is a heavy force dependenc on wing area, and a slightly smaller
dependenthce on aspect ratio in fixed wing flight) . If this is the case
and lift == weight, then we need to adjust the wing beat motions and wing
shape in order to make sure there is a balance of forces -- if the wing
size and lift increases, then the bat will climb unless we trim out the
bat wing motions -- hope this makes sense (we must make sure make sure the
vehicle is trimmed if we are comparing things OR...we could look at
dimensionless coefficients....I can talk more about this...but this would
erase the effects of area changes but would not erase the effects of
aspect ratio changes (shape changes)--as an analogy, consider and
aircraft...if we change the wingarea (or in some cases the shape), then
there may be some adjustment in the trim (angle the aircraft makes with
the relative air velocity) of the aircraft inorder for it to fly level
without limbing/sinking). Also as the wing sizes change, do we require a
different bat weight to be "lifted" into the air? On a similar note, the
thrust has to equal the drag in non-accelerating forward flight (This
allows you to compare bats from an apples to apples viewpoint). One must
adjust the flight parameters to make the bat fly so that the resulting
flow structure from the "newly" shaped bat makes sense and is an apples to
apples comparison. This could be averted by somehow interpolating the
results from data collexted from different bat species and that way you
can get something close to a decent representation. Although this would be
a possible solution, it isn't likely to be the most thorough one.

c) One of the biggest shallenges is the different wing sizes need to have
a specified motion. The shape of the batwing is crucial to the lift
generation. A change in the curvature of the wing of 5% can drastically
alter the lift generation and give you a completely different result. The
question is, when we vary the wing size, what are we using to determine
the motions. Finally on this note, membrane deflections are heavily
dependent on the distances between the supporting skeleton....this means
a change in size of the skeleton, may mean the membrane deflections are
vastly different that the baseline bat...again altering the force
properties.

That said, and if I didn't scare you too much....As you can see there are
MANY unanswered problems....It is a challenge to simulate a bat given the
actual wing shapes, let alone figure how a scaled bat will change the
flight properties (since we need to ensure the scaled bat is something
representative of a bat)....The thing I would tend to do from a scientific
viewpoint is try to boil the problem down a little. I would scientifically
look at how shape shanges affect stationary wings (a much better
understood problem) and then try to build on that to a simple flapping
wing, then to a more refined multi-jointed wing....

One of the things I am working on is a model which does not require the
bat shape details. Again, if interested I can talk more about this.

================= ANSWERS TO YOUR QUESTIONS =================

I'm not clear I am sure what you mean by features....I'll make some
assumptiosn and list some thoughts (if you have something else in mind,
lemme know, I am sure there are plenty of things i'd like to see):

--- *Some* Things I'd like to mess around with to do with the bat (the
list would grow given more time):

o The effect of forwrd bat velocity on the bat (each bat may have a
different forwrd speed and perhaps a best forward speed for each of the
lift, energy and manouvrability metrics)
o The effects of flexibility: A more flexible large span wing will have
significant bending...
o the effect of flapping frequency
o The effect of the wing shape
o the effects of camber variations (either through passive membrane
deflections of active skeletal wing shaping).

--- Visually: Features I'd (personally) like to see (again the list may
grow with more time):
o What is the vortex structure behind the flapping wing
o What is the flow structure(s) areound the wing. Is there a leading edge
vortex?
o How does the fluid structure interaction affect the problem?
o General streamline and flow traces (body fixed and flow fixed reference
frames).
o Velocities, pressures, etc in the flow field.

======================= Q2 =======================

This is tough to do and one of the big challenges I face daily :-) .....
There is some hope :-)....

Lift, energy and drag: Computational fluid dynamics (CFD) for "accurate"
computations...again a poorly defined bat shape in....means a poor result
out (aerodynamics is heavily dependent on the shape, hence, this is one of
the big difficulties). Panel method/vortex lattice models for modelling
and approximate answers. These are all relatively costly to run (timewise:
CFD ==> a month or two, Modeling methods: 10 minutes to an hour per bat
flight per couple of flapping cycles) and hard to develop.

The other approach is to calculate very approximate values from
near-realtime simulations...something like the ASWING or AVL (Lookup Drela
at MIT for these codes....though they aren't really setup for flapping
wings in their released form....not sure if it is freeware/something
else)...If you are interested further in this, talking in person would be
best (I'll be down at Brown on Tuesday next week).

A very approximate way ti to use quasi steady 2-D theory and extrapolate
it to 3-d....this will be ok for some simple computations, but not to be
heavily trusted as a fully complete "theory".

If you wanted to actually compute the values, let me know and we can look
into ways to do this. there may be a way to use a "batless" model to do
some approximations....

Manouvrability Capability: I'm not sure how we calculate this...I think
the first step is to understand the feature(s) that make bats so
manouvrable....and then figure how this can be computed based on that. The
motions are heavily dependent....

======================= Q 4 ===========================