On Wed, 05 Sep 2018 15:21:11 -0700, Eric Greenwell wrote:

Andy Blackburn wrote on 9/5/2018 9:44 AM:
On Wednesday, September 5, 2018 at 9:20:53 AM UTC-5, krasw wrote:
keskiviikko 5. syyskuuta 2018 8.22.14 UTC+3 2G kirjoitti:

What I know is I don't have a Kalman filter going in my head: but I
do have a butt which feels vertical acceleration. If it doesn't tell
me I am going up, I discount the screaming vario.

Tom

The wing transforms horizontal gust into vertical, and your butt gets
it wrong.

That's true if you only use your butt and not you're inner ear to sense
the pitch rotation. A horizontal gust on the nose excites the phugoid
(dCm/dV) and pitches the nose up. Vertical air movement excites the
short period (dCm/dalpha) and pitches the nose down. A thermal you can
climb in is likely to produce a more prolonged surge than a vertical
gust. The exact magnitude of these effects depend on the specific
aircraft aerodynamics and things like cg location.

Tom, you may not have a Kalman filter in your head, but you are a
neural network - kind of by definition since your brain is made of
connected neurons.. Pattern recognition is how we all interpret the
"feel" of thermals. It helps a little if you can decompose some of the
bigger dynamic effects, but there's a lot going on with lift, gusts and
aircraft dynamics - as UH points out. I think a smart vario ought to be
able to sort out some of these dynamic interactions better than simple
total energy compensation. I figure with cheap gyros and accelerometers
they would be doing a lot of this already, but I don't know how far
it's gotten.

Again, apologies if I didn't completely accurately describe the
engineering of aircraft dynamics. I think this is roughly correct.

I'm thinking a horizontal gust on the nose is similar to a higher
airspeed, and with the glider elevator set for the lower airspeed, a
pitch-up would occur.

Isn't this countered as a glider enters a thermal because it is flying
into air with increasing vertical velocity? This will tend to lower the
effective AOA, causing the glider to accelerate forward as it tries to
return to its trimmed AOA. Hence its pilot 'feeling the surge' forward
and up.

In the past I've seen free flight competition models do this too, some
(the APS Aiglet A/1 design) would sometimes pitch down very obviously
when entering a thermal, while many/most designs can look as if they're
being sucked into a strong thermal, though with not so much visible pitch
change as the old Aiglet used to show.


--
Martin | martin at
Gregorie | gregorie dot org