For the obvious reasons, I invariably get too busy to blog exactly when things get interesting...
The Dynon D10-A, used for high altitude station keeping
As we discussed before, one of the problems we needed to solve was how to give the pilots enough feedback to allow them to station keep while hovering 4300′ above ground level. Helicopter pilots typically keep the helicopter at a steady hover by either looking sideways at stationary marks like tree-tops, or looking down parallel the lift cable, also called vertical reference.
Since tree-tops are definitely scarce at 4300′, and vertical reference is also tricky given the large distance to the ground, we’ve come up with several alternative methods, and the most elegant one is a compass-GPS combination display cooked up by Keith Mackey of Mackey International, our aviation consultant, and provided by Dynon Avionics, who develops and manufactures “Glass Cockpit” instrumentation for the General Aviation sector.
GPS devices for aviation are easy to come by, but they all fall into the same trap as car GPS devices – they make the implicit assumption that the vehicle is moving face forward. This seems trivial at first, until you realize that an ordinary GPS does not know its own orientation – it relies on the GPS track you leave behind as you move, and assumes you’re facing into the direction of motion.
To illustrate the point, take your car GPS and ask for the directions for the nearest Starbucks. We’ll assume you’ve gotten an arrow pointing to the right. Now put the car in reverse, (look carefully,) and drive straight back. Starbucks is still to the right, but the arrow on the GPS flips, and is now pointing in the opposite direction, to the left. What happened? The GPS assumes you’ve U-turned rather than just reversed – it has no way of telling which way the nose of the car is pointing.
To solve this problem, the GPS unit needs to have a built-in electronic compass. Hand-held GPS devices often have that feature, since people don’t necessarily walk straight forward – it is common to hold the GPS at an angle to the direction you move at. Hand-held GPS devices are too small to be used conveniently by a pilot, however.
Enter Dynon, and Keith.
Dynon provided us with their D10-A EFIS display unit, which contains a complete integrated flight system that includes a magnetic compass, and has an additional remote compass sensor (flux gate) that is particularly useful when used in helicopters in a setting such as ours.
Keith interfaced it with the rest of the instrumentation, and made a helicopter-cockpit worthy GPS-compass kit, which is able to provide the pilot with a big fat arrow pointing directly towards the desired point of hover – the arrow is accurate, responsive, and compensates for any change in attitude of the helicopter, so we don’t get the “Starbucks effect” described above.
During the last test flight we got enough flight minutes to see that the system is working as expected, but did not get to truly fly the tether in position with it – this is one of the left-over tasks for the upcoming second test flight.