Developing a flying R&D Platform
(and an education in the dynamics of multi-rotor flight)

The FW550 was fairly heavily modified, starting with a fairly flexible undercarriage system that had lots of mount points, and the addition of a rudimentary servo-driven gimbal for a GoPro:

Also included were 2 DC buses (in addition to both the ship's main battery bus, and the Flight Controller's avionics bus. I added both 12.5 VDC and 5VDC DC-to-DC converters, which enabled power for a long range video downlink (which need ~12V) and both the GoPro and Gimbal servos, both of which need fairly high current 5VDC.

By separating out these buses, I ensured the flight critical avionics bus was never affected by any payload-related issues, and also isolated the mission payloads from the ships bus (so I could experiment with different cell-count batteries on the ship's bus without having to reconfigure video downlinks, etc).

The key change made to my R&D platform was the addition of an Arducopter Flight Controller and associated telemetry.

My intention at this point was to begin exploring the various flight modes and expanding the mission envelope, but I soon learned I wasn't quite ready yet.

Despite the fact that the FW550 seemed to fly reasonably well, I soon learned all of the modifications I made created a platform that weighed much more than a stock FW550 could _safely_ lift, especially with the associated change in CG of hanging so far underneath the "lift disc" (one of the configurations my custom landing gear enabled were dual, under slung batteries, which create a wonderful endurance profile, but moved the CG well below the plane of the props).

Initially, it appeared I had no problem flying at this weight/CG, and in fact I flew quite a few early tests before gaining a huge lesson in the dynamics of multi-rotor flight.

I learned this lesson "Empirically" via two "incidents." First, I had a hard landing due to "settling with power." After analyzing what happen (perhaps a bit too superficially) via the telemetry logs, I determined that I could avoid similar incidents by carefully limited descent rates.

A key operational change was the institution of a policy where by I never descended under manual control above 20 meters because its too hard to visually judge descent rate that far out. After this incident, above 20 meters, I always used altitude "hold" mode with a descent commanded via either the handheld controller or the GCS (a policy that continues to this day). This eliminated the simple case of settling with power, but actually masked the larger issue I had with this configuration.

What I didn't fully understand, until after the next much more significant incident, (i.e. a fairly spectacular crash) was that I had created a platform with insufficient excess lift margin for the vehicle to maintain controlled flight during a variety of flight conditions.

The second incident was precipitated by a fairly slow descent through downdraft during a wind gust. While the slow descent rate kept the props out of the bulk of the down wash plum, there was still enough turbulence to cause some pitch and roll moment.

During one of these excursions, a wind gust compounded the rolling moment, creating a moment beyond which the platform had enough excess lift to compensate with. As a result, the vehicle reached an pitch/roll angle of 45 degrees, at which point the flight controller shut off all the motors. The rolling moment was strong enough to cause the roll/pitch to continue past 45 degrees, eliminating the possibility of motor restart.

The 'copter flipped on its back and fell to the ground.

The accident cycle happened so quickly it was hard to really see what had happened in real time.

Examination of the video and telemetry logs, however, revealed what had happened in gory detail.

After this second accident (the pictures of which I will spare everyone, not much was left to look at) I spent meaningful time examining the onboard video and logs, as well as video from my ground camera (and indeed I had lots of time to reflect because, unlike the hard landing, this accident left me with nothing to fly for a while).

I wasn't happy about loosing the copter, but gained tons of insight into how to properly engineer the battery-motor-prop combination to produce sufficient lift margins at a given weight.

After calculating the lift I needed, and developing and motor-prop combination that produced enough lift, I revisited my R&D vehicle decision.

I realized I either needed something bigger than my stock FW550, because I needed to use 11in props to get enough lift margin (the arms on a 550 are not long enough for anything more than 10in props). I needed to either find a different R&D platform, or make some significant mods to the 550.

For a bunch of reasons, I decided to stay with an (albeit heavily modified) FW550.

 I added arm extensions, and put on larger motors and props, which created the requisite lift capacity to ensure I could maintain vehicle control throughout the expected flight conditions.

Here's the reborn, heavily modified FW550 that ultimately became my R&D workhorse.

After spending several days (somewhat nervously) probing the flight envelope, I was able to verify I finally had a flying dev platform capably of exploring the expected flight envelope carrying the payloads I needed to carry.