Evolution and Radio Control Aircraft Design Development
Myself and others often design a model and take what we get with no
intention of further developing the design. It's not the way to create
a design that is as good as it can be, but if the model is for our own
personal use and we're happy with it then that's all that matters.
With enough experience and a relatively standard design it isn't too
difficult to achieve fair success with an initial prototype.
Unfortunately, many who publish their plans as well as manufacturers appear
to take this approach with their designs. It's obvious the model
hasn't been properly developed when the flight trimming phase reveals that
the wing incidence, decalage, dihedral, thrust line and other items need to
be significantly changed for the model to trim properly.
95% of all sport kits on the market can be built by the average builder on
their bench with no plans. They are simple, standard designs.
That's why you don't see many of those kits in my
gallery.
Why pay for a kit that I can build from better materials without financially
driven compromises imposed by manufacturers?
As soon as you begin building you should be looking at what you could do
better the next time. My designs are usually better developed when I draw
even minimal plans rather than making things up as I go along.
Either way I often find that in practice I could have improved some
things by doing them differently. This aspect mainly involves
engineering, contruction and control systems.
From an aerodynamics standpoint, design development begins with the first
flight of each prototype. It is important that you take the time to
properly
trim your new model. Most guys get the plane flying straight and
level, make an adjustment or two to the CG and then consider their
aircraft trimmed. It is all done in a weekend or two. Talk to
any aerobatic pilot and he'll tell you that it takes gallons of fuel to
get an aircraft in trim.
Get
a trim chart and use it. If you fail to
trim your model then
anything else you do to correct it may be an improvement to your model,
but not to the design because you are basically treating the symptoms and
not the disease.
One thing that always bugs me is when I
hear the comment, "<Insert aircraft here> is the best model I have ever
flown" - especially when what I am looking at is basically a rehash of a
Stik model. My first thought is, 'It is probably the only one you
ever built that was close to straight and close to being in trim.'
The fact is that most models will fly very well if built and trimmed
properly. After the model is trimmed fly the model as much as
possible to learn about it. Specifically, perform sets of maneuvers you want
it to be able to do to determine which characteristics need improvement.
Dedicate entire flights or even flying days to learning about one particular characteristic of
your model.
Always use the previous prototype as a starting
point
If you want to build a 3D aircraft that compliments your flying style,
then the wrong way to go about it is to design a model, decide it does not
do what you want and then design a whole new model.
Work on
improving the prototype and you will be more successful than you will be if
you keep starting from scratch. Make one change at a time to improve the
prototype. If that means cutting off the tail feathers and building
new ones of different proportions then do it. If the ailerons need
to be a different shape, then make new ones. Try different
propellers, changes to the CG, thrust line, wing incidence, control
throws/differential, etc.
Trim charts do not apply only to aerobatic
models. Use one for reference to help you determine why a model
flies the way it does and what to do about it.
Build the next prototype only when changes to the previous prototype
are no longer a significant improvement or changes would be impossible or
impractical to make. Normally it is a poor practice to make more
than one change at one time. However, if there are several flight
characteristics you want to improve then you should make all the changes
you think will improve the design unless you want to build one hundred
prototypes (which would probably be better in theory, but a nightmare in
practice). Rule out — do not guess
— as many
potential reasons for a performance problem as possible. For example, the prototype may have a
sluggish roll-rate. This could indicate one or more of the
following:
Don't give up on the prototype even if the problem is severe.
Several of the above possibilities can be ruled out without having to
build a new model:
-
Seal the hinge line
- Adjust the linkages for more or less control surface throw
- Strengthen the linkages
- Replace the servo with one that is stronger or faster
- Replace the hinges with hinges having less play
- Build new control surfaces
If the problem still exists, then the only way to
resolve it may be building another prototype. Before you begin
building the next prototype you
should continue ruling out process for each flight quality you want
to improve. Then when you build the next mark you can
significantly improve the design.
Now that you have ruled out as many potential reasons for undesirable
flight qualities as possible you need to consider remaining possibilities
and choose which one(s) may be the cause. This is where experience
is helpful because you might have to guess, but if you eliminated items
then the choices should be very limited which increases your chances of
picking the correct fix. No matter what you choose you don't want to
make the problem worse but that is always a possibility.
In the above example of an aircraft having poor roll response, you may
have eliminated all linkage/servo possibilities leaving only aerodynamic
causes such as airfoil or control surface size and proportions. If
you are stumped at this point then look at other models similar to yours
that have the response you want yours to have. Adjust the control
surface design to more closely match that of the other designs.
Model aircraft design is not as complicated as it may seem, but it does
require some experience with different aircraft to learn how each
parameter affects an airplane's performance.
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