by KaiserJeep » Sat 11 Mar 2017, 18:16:18
The pilot - or more often today, the autopilot - is continuously adjusting the attitude of the plane to maintain "level" flight (remember that in this context "level" is not a straight line, but defined as a 90 degree angle from a line connecting the plane and the Earth's center of gravity) and constant altitude above mean sea level. In spite of Newton's first law of motion, the airplane will not maintain a straight line, due to winds, air pressures, and air currents - all which are larger forces that overcome the inertia of straight line travel. The "altitude" is really a constant distance from the Earth's center of gravity - a "constant altitude of 20 feet" above your perfect sphere would be a perfect globe-shaped shell 40 feet greater in diameter than the Earth itself, and located as to have the same center of gravity as the planet.
I am reasonably familiar with gyroscopes which are used in SINS (ships inertial navigation systems). The gyroscopes you speak of, which used to be precision high speed rotating instruments made by Sperry among others, are today much more precise "ring laser gyros" where light travels in a circle at "the speed of light". These can be entirely contained in one component soldered into a circuit board. Gyros are "rotation sensors", and if you took one entirely around the earth, it would point one degree in a different direction as you moved North-South or South-North, for every "degree of longitude" you travelled on the Earth's surface, a distance of about 69 miles, in the 24,840 "statute miles" of the Earth's circumference. Then 1/60th of that 69 mile distance is a "minute" of longitude, or a distance of "one nautical mile" which is about 1.15078 "statute miles".
Gyroscopes are in fact not directly usable for navigation, and the term "gyrocompass" is nonsensical, because as you pointed out, the axis of the compass is always pointing in the same direction. A "gyrocompass" actually measures how far the axis of the gyro deviates from the direction of travel, using the axis "zero" when the gyroscope was spun up, then calculates the direction of North, and points an electric compass needle in that direction. Movement is measured by another instrument called an "accelerometer". Today the ring laser gyro, the accelerometer, a 12-channel GPS receiver, and a computer to calculate your position on the Earth's surface, can all be built into your cell phone.
That's right, your phone is continuously calculating your position, both with GPS and with inertial navigation when it doesn't have GPS signals. This is done "for your own good" in case you are in distress and call 911 here in America, or for "the good of the USA" if you are a terrorist in Afghanistan, so we can rain a missile down on you from a UAV.
Last edited by
KaiserJeep on Sat 11 Mar 2017, 18:23:30, edited 1 time in total.
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