[...] but what kind of sensor could you use to "indicate when the plane is flying straight and level"??
it could be something as simple as two pairs of mercury switches mounted (nearly level) inside the fuselage -- one pair laying in the direction from cockpit to rudder, the other pair laying in the direction from wingtip to wingtip. if switch A1 closes, you're pitching up; if switch A2 closes, you're pitching down. likewise, if switch B1 closes, you're rolling counter-clockwise; if switch B2 closes, you're rolling clockwise.
now, none of that will help you with yaw, however... 