This Sunday at 5pm, after preparing all weekend long, we did the first set of FPV flights. Luckily, this was also the first weekend that could be described as offering fall weather in Austin, with pleasant temperatures. The flights were successful, despite some wind that was beyond the airframe’s comfort zone.
The airframe is an old EasyStar electric glider, which is not particularly performant, but very reliable, predictable and forgiving. It is propelled by an old Zagi brushless power system, offering more then enough thrust, even with all the extra FPV payload.
The FPV equipment is some of the better stuff available, mostly bought gently used, relying on brand names that use absence of subtlety to convey amazing technological offerings. The video frequency is 5.8gHz, to work with the 2.4gHz radio system. Choosing a video frequency is a bit of a religious decision.
Long-range FPV fliers and purists advise against 2.4gHz radio usage, because the digital nature of the 2.4gHz system offers little predictability of gradual signal loss, but for our not-long-range-at-all-so-far-flying, it’s just fine, especially with DSM2 and dual receivers.
The FPV system was using a Team Black Sheep camera and Core (power supply and on-screen-display), components that are described as operating with low noise. Electrical noise is the enemy of range. The video transmitter is a proven ImmersionRC 600mw unit, and it talks to a ground station with an ImmersionRC diversity receiver. The diversity receiver actually consists of two receivers, and always switches to the one that offers the stronger video signal, between video frames. The two receivers are hooked up to two different antennas: a low-gain circular-polarized one, matching the transmitter’s, and a higher-gain linear-polarized one.
Circular-polarized antennas offer less overall theoretical range then linear-polarized ones, but they reject multipath interference. Multipath interference occurs when the 5.8gHz signal, with its crisp, bouncy short waves, gets reflected by buildings and other structures. The reflected signal, when picked up by the receiving antenna, messes up the video signal. Since a circular-polarized signal changes direction (clockwise-counterclockwise) when reflected, it is mostly ignored by the circular-polarized receiver antenna.
Linear-polarized antennas are prone to multipathing. The linear antennas that I have available offer some higher gain, though, meaning they can pick up a signal from further away… but only of the signal source is within the “beam” of the antenna, which is, for example, a sixty-degree cone. Outside, no signal: higher gain does not mean that an antenna is “stronger”, it just means that it focuses its attention into a specific area. The smaller the area, the better the reception in that area.
By having these two different receiver antenna characteristics available on a diversity setup, the system gets a lot of redundancy.
The video, once received by the ground station, is fed into a set of video goggles – Fatshark Dominator goggles. The goggles are mildly uncomfortable, but bearable. With glass optics, they offer a decent picture quality, compared to plastic optics on lower-end goggles.
Supporting the goggled pilot, a spotter keeps track of the real world, and provides guidance. I am lucky to have a wife who is actually interested in helping out with this.
When flying, the in-goggle live video from the plane is very immersive. I found myself tilting my head to try correcting the plane’s attitude when it got blown around by high winds. The in-plane perspective looks much higher up in the air then the on-ground perspective, which is cool but creates the risk of unintended ground contact. Seen from in-plane, the world also looks very different then from the ground. This is a trivial observation, but in practice it means that orientation is challenging even in very familiar surroundings. In other words, it’s great fun.