On Wednesday, September 5, 2018 at 10:09:03 PM UTC-5, Eric Greenwell wrote:
Martin Gregorie wrote on 9/5/2018 4:35 PM:
On Wed, 05 Sep 2018 15:21:11 -0700, Eric Greenwell wrote:



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.
...
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.

My context, and I think Andy's, was encountering just a horizontal gust. I do
think encountering a vertical gust would cause a momentary nose-down attitude change.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

Yes. That's the difference between a horizontal and vertical gust (reminder: a sustained vertical gust is a thermal). A horizontal gust activates the dCm/dV (phugoid) mode that is nose-up because the center of lift is in front of the center of gravity. A vertical gust activates the dCm/dalpha (short period) mode which is nose down for most airfoils (in addition to vertical acceleration from the air movement itself).

Also, these two modes have different time constants by roughly a factor of 10 (they are also coupled, but over the first few seconds this doesn't come into play). So the vertical acceleration and pitch response together ought to be different for a horizontal gust versus a thermal.

Andy Blackburn
9B