Hi Daniele,
First thing is to say that you may find some responses here saying these are silly questions - they are not! There are no silly questions! If you don't understand something you should ask. It shows that you are thinking about things and have an enquiring mind - both very good signs at such an early stage of your career in aviation.
Now, to the answer; not such a simple question!
In general you have got it right, with low engines and high wings gravity can do the work of sending fuel to the engines, hence no pump in a Robinson or a C172. With a low fuel tank and a high engine then obviously a pump will be required, in fact a pump and a back up because with just one pump a single faiure would stop the engine - not a good thing!
Once you start to look in more detail there are many many factors which could come into play sch as the routing of the pipes, are there any high points in the pipe runs or any possible air/vapour locks? What is the specific engine's requirement for fuel pressure both during start and during normal running? Is there a need at certain stages of flight (take off and landing perhaps) to anticipate a failure of the single pump and protect against engine stoppage by running the second pump?
Many many questions and as a pilot it's unlikely that you will ever be aware of all the subtle factors which come in to play. The only people who are (or should be!) are the designers, engineers and test pilots who built the thing and wrote the manual so - the only safe answer for you (and all other pilots) is to use the procedres in the manuafacurer's operating manual.
Happy landings
3 Point
Last edited by 3 Point; 5th May 2012 at 15:42.