RC Plane, Autopilot Type Stabilizing Circuit

Keep in mind there are two types of commonly used sensors- angular rate gyros and linear accelerometers. Both measure only changes. There's also magnetic compass sensors which can measure heading.

Accelerometers can measure tilt in either axis, simply by looking for the angle of the g-force. There are still difficulties in moving systems. For example, if the craft is accelerating downward at 1 g, the lack of signal in any direction provides no clue as to angle anymore.

The accelerometers and gyros have difficulties with offset errors. Some systems not only have a setup mode to calibrate the offset, but will simply refuse to perform corrections based on a very small signal from the sensor. Other systems actually include a tiny thermostat and heater to keep the sensor at a very constant temp to minimize offset drift.

gyroscope

I do not think you necessarily need a circuit for this task. All you have to do is attach a fast spinning wheel to the plane and it will try to stabilize itself. At least I think it will. Else you could use a stick that is heavier in one end than the other, so it will always point down, use variable resistors to note its position. Then you can send the same analog signal to the servos.
This should work I think.

When I was a kid I made a free-flight model airplane with a pendulum controlling the elevator.
When the nose went up then the pendulum swung back and applied down elevator.
When the nose went down then up elevator was applied.
It worked well.

I'm currently working on a project with multiple sensors and processors to control a UAV.

These things are divided into several modules:

IMU - Inertial Measurement Unit. Which incorporates one ADXL330(XYZ accelerometer), three ADIS16100(Z-axis gyroscope), and one PT100 PTC resistor for thermal compensation, a slave microcontroller is placed in this module to implement Kalman filter and IIC bus interfacing.

GPS - A Progin SR-88 micro GPS module with a passive patch antenna. The GPS module provides standard serial interface with the flight control processor.

Megnetometer module - An HMC1051 and one HMC1052 are placed on a single board for three axis magnetic measurement. A 14-bit ADC and a slave microcontroller is used to digitalize, regulate and format the measurement results, as well as interface through IIC.

An ultrasonic range finder and a laser ranger may be added later on, but right now, there are only a couple of vacant IIC sockets left there.

The flight control porcessor is still to be determined, I'm thinking about a float point DSP from Texas Instrument.