Perhaps a summary introduction in control theory could help you. From a control point of view, your aircraft is a dynamic system, which can be described by a state and a dynamic model.
The state is simply a collection of variables of where the aircraft is in each point of time. For a full description, you would have the position and attitude in space, as well as their first derivatives. Any other inertia may also have their own state variable; for example, the propeller speed.
The state equations describe how the state evolves in time. From these equations (especially in linearized form), we can distill a number of (eigen)modes. These modes describe a certain behaviour of the system, and come in two forms: stable and unstable modes. A stable mode is a system dynamic that, for a finite input (perturbation), does not lead to any state variable going to infinity. Note that this does not mean that the state variables go back to some constant value: a bounded oscillation can still be considered stable! An unstable mode is a dynamic that for finite input leads to unbounded growth of some state variable.
To see how many sensors you need, you will want a few things. First of all, you want your system to be stable. Obviously, this means that you want to have a set of sensors that can measure any unstable mode (detectability), and a set of actuators that can control unstable modes (stabilizability). There are a number of (possibly) unstable modes, the most important of wich is spiral divergence. Many fixed-wing aircraft have no inherent roll stability, so you will want to be able to sense your roll angle (a yaw rate or heading sensor could work too, due to the roll/yaw coupling). Furthermore, there is simply your linear motion, which is also unstable (this may seem counter-intuitive, but all it means that you can get infinitely far from your initial point by travelling in a straight line). For this, you will want some kind of position sensor (most likely, a GPS).
All other dynamic modes are not inherently unstable in a fixed-wing aircraft, although your particular design may have some additional instabilities, like an unstable phugoid motion, for which you want to be able to measure and control the pitch motion.
So, the absolute minimum is two to three sensors (latitude and longitude and probably roll or yaw), and use feed-forward for all other variables (set trim and power based on desired altitude and speed, and just go for it). In practice, nobody would build an UAV with just three sensors. There are two reasons for this,
- Sensor inaccuracy. Measuring attitude is hard. A gyroscope will inevitably drift over time, and as such, you will need some other sensors to correct for this.
- Performance. You ideally want to travel in a straight line to the target, not swaying left and right in lazy semicircles while bobbing up and down a phugoid, praying to a suitable deity that the trim- and power settings leave enough margin to go over that mountain on a hot day.
For a hobby UAV, it typically turns out that sensors are relatively cheap, thanks to MEMS. You would typically use a 3-axis accelerometer, 3-axis (roll rate) gyroscope and 3-axis magnetometer (compass), and why not have a barometer and thermometer as well? And since you use a GPS, you might as well use its altitude data. If you use a brushless DC motor, it should be no effort to measure the prop speed. I think you will find it a lot easier to make a working UAV with these off-the-shelf components (and open-source software that is typically written for these sensors) than finding a single-axis gyroscope sensor and writing your own software.
Furthermore, using more sensors significantly simplifies your observability. This is the notion of being able to infer the state of the system from the outputs. In theory, if your model is sufficiently complex, you need very few sensors to estimate the current state (dead reckoning). However, if you can actually measure your states, your model is no longer as important, and your UAV will be able to respond much better and be less sensitive to changes in the environment or the UAV itself.