Contestants in the competitive sport of radio-controlled model helicopter precision flight are scored on flying high-speed precision aerobatic sequences and performing complex hovering maneuvers exactly as prescribed by the rules of competition. Here's a world champion pilot, Japan's Hiroki Ito, flying one such sequence of maneuvers: https://www.youtube.com/watch?v=ejHUSnjzc8A
In precision hovering maneuvers, the pilot must briefly hover the model directly over a 0.5m flag planted on the ground, at heights 2m-7m above ground depending on the maneuver. In actuality there are two flags, one at each end of a 10m straight line ("centerline") chalked or painted on the ground. The pilot stands 9m behind the center point of the centerline ("pilot box"), with the flags visible to the left and right in front of the pilot. Correct hover placement axially centers the main rotorshaft (powered vertical shaft driving the main rotor system) on the imaginary line projecting vertically from the flag location ("reference line.")
Even expert pilots admit it is very difficult to establish and hold a stationary hover position precisely over a flag, especially in windy conditions. Many pilots struggle with depth perception while learning to establish correct hover placement. Also, the pilot generally must concentrate on maintaining the model's flight attitude, and must rely mainly on peripheral vision to gauge position over a flag while gazing up at the model. Eventually, with enough practice and with corrective assistance from a helper ("spotter") situated directly in front a flag where a contest judge would sit, one memorizes what correct placement looks like at various hovering heights. ("In front of a flag" means at a location along an imaginary line perpendicular to, and incident upon an endpoint of, the centerline, behind the pilot, 15m from the flag.)
Note: While difficult, it is less challenging to visualize the correct height over a flag for a particular hovering maneuver, typically 2m, 4m, 6m or 7m, as markings painted at several known points along the centerline can be used to visualize a right triangle originating at or near one of those markings, with the triangle base along the ground, and with the triangle side along the reference line.
When practicing without a spotter present, a hover training aid is desired to notify the pilot when the model is accurately positioned over a flag up to 7m above ground. A poor solution is to video record a practice flight and study the recording after the fact, as this offline analysis deprives the pilot of crucial real time feedback. Furthermore, the video camera should be positioned directly in front of a flag as above, and it is difficult after the fact to determine the in-flight correction required for the difference in perspective between the camera and pilot positions. Other problematic solutions involve putting the model (powered down, of course) up on a pole, or somehow statically suspending it at the right height, so the pilot can stare at it from the pilot box in order to memorize the perspective relative to the flag; or tying to the model a string of the desired length with a brightly colored weight at one end (rotorwash whips the string around.) A Japan company once custom built a fairly expensive training aid comprising electronic flag stands/bases that could sense the model above them and beep when the model was detected directly overhead. I know nothing about the sensor technology used for this defunct system but its users, who are few, claim it was accurate.
A tiny number of people around the world compete in RC helicopter precision flying, hence there is no market opportunity for hover training technology. Not yet a competitive pilot, and as someone who practices alone, I just need a hover training aid for myself. I have a software background, I can solder and I'm pretty handy but I have no electrical design knowledge. Do off-the-shelf sensing/positioning solutions exist for this problem, or could one be adapted to solve it? If not, what would it take, and how would it work? The ideal solution will be small, portable, battery-operated, and will not add any equipment to the model helicopter owing to power, weight and CG concerns. No pilot notification shall be present when the main rotorshaft is (axially) more than 3cm off the reference line in any direction. The solution must work in sunny and cloudy conditions. Audible notification is preferred over visual cues, e.g., flashing lights or motion signals.
Thanks and best regards.
Due to the narrow width tolerance for your application, consider a laser rangefinder. Possibly something like that shown here: https://www.parallax.com/product/28043 could work.