This Friday we conducted a very successful flight testing session, culminating in a 25-second flight. We came into the day focused on taking our time, and getting each step done correctly before moving onto the next objective, as opposed to the more rushed atmosphere in which summer flight testing was conducted. Our goals going into the day were:
1) Assemble the truss structure for the first time in 4 months, checking for level on the rotor axles and adjusting bracing lines as necessary;
2) Load the truss to full flight weight, and adjust drivetrain/pilot support lines to centre the pilot mass more accurately;
3) Install rotors (without canards) using previously-determined trim settings, conduct short flights to check trim (meant to determine accuracy of new rotor alignment technique);
4) Install canards, conduct further trim flights as necessary, then progress to additional practice flights.
Overall we made excellent progress, but ran out of time in the day after just having integrated the canards again.
Among the day's noteworthy observations:
1) Although reproducing the previously-determined trim settings wasn't perfect on initial setup, it was much closer than ever before. Our new trim process (using foil tape to provide accurate reference markings) is much more precise than our previous methodology. Furthermore, each post-flight trim adjustment gave continuous improvement and shows that we have the process much more refined.
2) Alpha rotor was typically flying low, and Gamma rotor typically high, resulting in a slow forward drift. We suspect this was due to small changes in how the drive lines wind at the bike drive spools (which leads to diverging rotor RPMs after 5-10 seconds of spinning).
3) Flight power averaged about 460 W, which we think is a big improvement (previous testing numbers are a bit fuzzy). The canards will add slightly to that value (we didn't achieve flight with canards installed), but in the end it matches well with our predicted numbers. Also, this is within the requirements necessary for the AHS Sikorsky Prize, with about 10-15% margin.
4) As before it seems that the rotor imbalance progresses with flight duration, rather than achieving a steady-state imbalance. In the future we'll be stiffening the truss structure to compensate.
5) We had a spar failure on the final flight of the day, resulting from a rotor tip ground-strike due to an overlooked adjustment in the control system. We'll be more diligent with pre-flight checklists in the future and starting off more slowly every time we make changes.
One big take-away from this session was that we can assemble the helicopter and achieve well-trimmed flight within a single day, which will be very important for further testing at the Soccer Centre and elsewhere. There will be more fixes and improvements this week, and we'll be testing again on Friday. Stay tuned!