Vario Comparison

On Friday, 7 September 2018 09:23:31 UTC+10, Andy Blackburn wrote:
Mike,

I understand that the short period response to a step function in vertical air motion would damp out after less than a second, but normally thermal entry would have a positive gradient in vertical air motion as the glider traverses from zero vertical motion to maximum vertical motion towards the center of the thermal. This ought to create a more prolonged nose-down pitch attitude in addition to upward acceleration, both of which ought to be detectable to accelerometers and gyros in a modern vario. This would be quite distinct from an ever-so-slow negative acceleration and nose up pitch rate from a horizontal gust on the nose. I expect having a sense for the distribution of strength and duration for horizontal gusts would also help a bit.

That’s what I tend to sense in moderate to strong thermals versus gusts. Is that that a correct interpretation?

Andy

Andy, that's basically right. The horizontal gust is usually so small though that I've never detected any nose up effect from the horizontal gusts. i.e. a one knot airspeed increase over one second at 100 KTAS will show 5 knots up on the TE vario. My experience has been that a lot of the time the vertical air motion is fairly sharp edged but some times you fly straight in to a strong core over a few seconds and it feels like the tail is being lifted. What is happening is the glider trying to line up with the new relative wind direction and during this time you have increased AoA happening. Trying to hold a constant nose attitude will increase the AoA further.

To digress from the vario topic slightly:
Which is why I said that flying attitude can kill you. I think this is what happened to an ASG-29 pilot at Waikerie about 2 and a half years ago, turning final with what seemed to be plenty of airspeed, into a strong thermal encounter and the next thing he was in the grapevines. Fortunately the wires and vines cushioned the stop enough and the spin wasn't fully established, that he survived. Encounter a fairly sharp 10 knot core at 60 KIAS and the AoA increases by 10 degrees. The AoA may be around 7 degrees before the encounter. What AoA do thin subsonic wings stall at?
This, I believe is one answer to the unexplained spin ins that occur from time to time.
Attitude is fine in equilibrium or very close to equilibrium situations. Doesn't work very well in other situations. How many have died in the winch launch failure scenario where you bury the nose well below the horizon, then roll and pull and the thing flicks in to a spin? Unfortunately all too many merely in practice.
A guy who was Chief Test Pilot for the Royal Australian Air Force during the 1950s and 1960s once told me if the aircraft is wanting to go a certain way and you are trying to force it to go another, just go with the aircraft as it is departing from controlled flight. Good advice IMO.