"Andrei B." wrote:
> I don't want to discourage you, but there are loads of problems...

Not really, any standard aircraft dynamics textbook has most everything you need
to do this sort of this. Mine even has a section(s) devoted to the topic. As for
GPS...autoland systems existed long before GPS.

>  Another thing, an autopilot can be built and used ONLY, I repeat, ONLY
> when you know very well all the handling characteristics of the plane
> in all conditions. Research and flight testing of real airplanes takes
> years to gather most of the data for the flight manual and for the
> autopilot. Without this, an autopilot is next to useless.
>  The autopilot needs to know exactly in the given conditions, and the
> given orders how to control the plane. Changing the propeller, adding
> 200grams of weight somewhere, another type of engine, etc, would throw
> off completely the autopilot and the planes characteristics will have
> to be retested and the autopilot refitted.
>  A human pilot learns very easily and adapts very quickly to a
> situation, but a computer-driven pilot who has a limited knowledge of
> the plane and no learning capability (you're not putting an array of
> multiprocessor systems with complex neural nets in your model
> aircraft...), it won't be able to cope.

Human pilots learn through trial and error and so can the computer. All of the
systems I've worked with for model airplanes ignore the modeling problem and
just use PID or PIDD controllers. The gains for which are determined through
trial and error.

>  Also take into account that for such a small scale, even small,
> apparently insignificant changes can dramatically affect the plane's
> handling. As a human pilot, one can adapt quickly, sometimes
> instinctively, maybe barely noticing it...
>  Example: your plane, fully equiped, at 20kph stalls at 14deg. Let's
> say that you put a new battery pack with more capacity, but it also
> weighs 100 grams more. This will shift the center of gravity of the
> plane and now it will stall at 12 deg at 20kph. The difference may
> seems small, but it is actually huge!
>  The autopilot WILL stall this plane, because it assumes that at 20kph,
> it is safe to fly at 13deg inclination! Unless ofcourse, you go through
> the whole process of getting the new flight characteristics.

Stall is not a function of pitch angle, its a function of angle of attack and
airspeed, which are related. As long as the autopilot has a control law wrapped
around the airspeed and knows not to go below the stall speed, it should be
fine. Adding an angle of attack sensor will help to.

>  Actually, making such an autopilot would be much easier for model
> helicopter then for a plane. Due to the nature of the helicopter, and
> the way it flies, (excluding acrobatics, ofcourse), many things are
> simpler to handle.
>  The advantage of model helicopters is the fact that they can reach a
> dynamic equilibrium in which they appear static and can hold this. Then
> it's a simple matter of slightly lowering the collective...
> (simplified)
>
>  The plane, although by it's nature tends to fly, while the helicopter
> does the opposite, dynamicly balancing the parameters on a plane could
> be more challenging then for a helicopter.

Achieving steady state flight with a stable airplane is easier, since it will
tend to stay at equilbibrium (hence "stable"). The difference is that for an
airplane the equilibrium state involves forward velocity. Helicopters are
inherently unstable and require constant attention to stay at the steady state
point (excluding autogyro). There's a reason why papers airplanes fly better
than paper helicopters.

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