I am in need of some trigonometric assistance as I am mathematically impaired
I am looking for a number, for each 60 miles that the earth's curvature causes a drop in elevation relative to an 'absolute straight' line.
The reason, you ask? Our aircraft is equipped with an HGS (heads-up guidance system, for the mnemonically dis-advanteged) and I was suggesting to the other pilot that the clearance of a weather cell could be determined by lowering the HGS combiner and referencing whether the cell is above or below the horizon line displayed on the combiner.
My 'theory' is based on the idea that each degree that the cell top is below the horizon, at 60 nautical miles, is a clearance value of approximately 6000'. The old 'rule of 60' states that "at 60 NM, each degree equals about 1 NM (6000') of lateral distance" or something to that effect.......
His response was that, because the earth is curved, the cloud will be cleared by a different value........I say the the amount of curvature of the earth's surface is neglible at the distances we are talking about (up to 300NM) and that the idea is plausible....
Anybody able to provide a little help here? I need a basic rule of thumb for some number (say 60 or a 100 NM) that the earth drops away from an 'absolute' straight line.....
Thanks in advance.........
Jay