It looks like last week’s prediction about the last days of Summer have come true. This week we’re looking at the tail end of a hurricane, so it’s windy with occasional bursts of rain. The forecast is for it to get worse as the day goes on, so no flying this week.
After last week’s flight with the autogyro, I spent some time doing a bit more research into autogyro aerodynamics. This thread on RC groups is very good:
It explains that, because of the rotation of the blades and the lag between where the asymmetric left from the advancing and retreating blades takes effect, insufficient rotor speed will result in a pitch up moment. Only in the case where the blades are held rigidly to the hub will there be a less than 90 degree lag and an additional left roll component. Now, I’m not sure I agree completely with all of this, for two simple reasons: firstly, the ATOM has three blades and secondly, I’ve witnessed a violent left roll first hand, which was the cause of the crash. It does, however, give a much more solid scientific basis for the analysis of how the autogyro flies. Also, when I looked at all the video of my autogyro launches, there is a very definite pitch up exactly as predicted, but coupled with a left roll. This all points to insufficient rotor speed, but I still can’t figure out what the underlying reason is.
What I need to do now is a bit more research and modelling of how the autogyro flies. In the course of this Internet search, I found the following document:
I need to go through this in a bit more detail, as it gives a basic outline of all the forces and moments involved. The references at the end also give some links to other publications which I can follow, plus the same author has a number of other publications held on the NASA archive. This is what I need to make an autogyro flight simulator.
However, this won’t explain what’s wrong with my ATOM. My best guess at the moment is that the launch is tending to put it into a stalled part of the flight envelope, from which I can’t recover because of having no height. I say “tending to”, because the real puzzle is the single 4 minute flight I had a few weeks ago when everything was working perfectly. I’ve taken the trim from what I had in the air then, hence when I look at the head now, it’s tilted over to the left which was to counter-act the right roll I was experiencing in the air. My tests now are going to focus on the power system, just to convince myself that the speed controller and motor are all working properly, and then the head forward angle and blade shims.