CAD Tool For Design Tiny Aircraft

On Tue, 2 Oct 2007 13:14:34 -0700, "Jerry Steiger" wrote in :

"Le Chaud Lapin" wrote in message
oups.com...
1. One dimension of fuel tank depends on required fuel capacity.
2. Fuel capacity depends on mass of certain parts.
3. Mass of parts depend on geometry and density of material of those
parts and load requirements, etc..
4. Load requirements depend on configuration of other structures.

And it would seem that there is a right way and a wrong way, and
again, finding the right way is more art than science. "Reaching" too
deep into model to extract parameters to be used elsewhere might be a
bad idea. Deliberate indirection and hierarchy would be important.
There would also be opportunity for circular references.

This is going to be very tricky. There HAVE to be circular references in
your optimization. When you change the weight of the fuel tank, you have to
reevaluate the size and weight of all of your other components to account
for the new load. But now you have changed the weight of the rest of the
components, so the fuel tank needs to change again. If you are lucky, the
solution converges and you end up with a design that works. If you start
from the wrong spot, it might never converge.

The good news is that you seem to have the type of mind set that would allow
you to work through this type of problem. The bad news is that it is an
extremely complex problem that requires a lot of deep knowledge in many
areas of design.

Speaking only from practical experience with RC models (~14 years total),
the envelope for small aircraft is extremely forgiving for ordinary
flight regimes. Most RC aircraft can double or triple their fuel load
without noticeably affecting flight performance.

Look at the college competitions for evidence. A few years back,
a weight-lifting competition was won at around 19 pounds of payload
for an aircraft powered by a plain-bearing 0.40 ci engine. Engines
like that are usually used to fly ~5 pound trainers with a wingspan
between 40" to 48" or so.

Drat. The American Institute of Aeronautics and Astronautics
website is down right now ... Here's a report on the 2006
competition:

http://mae.eng.uci.edu/aiaa/DBF2006.pdf

If you're trying to fly an RC aircraft across the Atlantic with
a gross weight of 5 kg (11 pounds), then you DO need to consider
a multitude of tradeoffs such as you describe:

http://www.progressiveengineer.com/profiles/maynardHill.htm

An engineer friend of mine likes to say, "One observation is worth
ten thousand expert opinions." Regardless of what the design
software predicts, the product needs to be tested in flight to
see whether the theories work. (I'm assuming that the initial
post in this thread was about a flying model.)

Something funny happens as you go down in scale. It has something
to do with Reynolds Numbers and the volumetrics of small aircraft
(volume decreases far more quickly than area). This means that
the power-to-weight ratio favors the model aircraft and that,
as a general rule, the ratio of the strength of materials to
G-forces increases. A 1/4 scale model (using 1/4 of the linear
dimensions) has 1/16 the area of the prototype and just
1/64th of the volume.

One way to get in the ballpark when designing a new model is
to select dimensions from aircraft that are already known as
good flying designs. The airfoils that work well for full scale
do not work well on small aircraft (as a general rule--Clark Y
airfoils probably scale OK; fighters and bombers from WW II on
generally do not scale well).

So a lot depends on the kind of aircraft the other poster
wants to build, whether it is supposed to fly, and what kind
of performance is to be optimized.

Marty
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