PW-5 longitudinal pitch oscillation

On Tuesday, September 5, 2017 at 6:04:49 PM UTC-7, Dave Nadler wrote:
On Tuesday, September 5, 2017 at 8:23:23 PM UTC-4, wrote:
I don't recall any glider I have flown reacting to a horizontal gust
by pitching down.

IIRC Genesis does this a tiny bit - initially disconcerting but of
no consequence as it stabilizes promptly (it's a nice flying bird).
JJ ??

IIRC vertical gusts activate the short period (AOA) mode and pitch the aircraft down while horizontal gusts activate the phugoid (airspeed) mode and pitch up (if the gust is on the nose). These are kind of fundamental aspects aircraft stability. Anything else might lead one to check the c.g. People sometimes confuse the type of gust they are experiencing.

Andy
9B

On Tuesday, September 5, 2017 at 9:24:25 PM UTC-4, Andy Blackburn wrote:
On Tuesday, September 5, 2017 at 6:04:49 PM UTC-7, Dave Nadler wrote:
On Tuesday, September 5, 2017 at 8:23:23 PM UTC-4, wrote:
I don't recall any glider I have flown reacting to a horizontal gust
by pitching down.

IIRC Genesis does this a tiny bit - initially disconcerting but of
no consequence as it stabilizes promptly (it's a nice flying bird).
JJ ??

IIRC vertical gusts activate the short period (AOA) mode and pitch the
aircraft down while horizontal gusts activate the phugoid (airspeed) mode
and pitch up (if the gust is on the nose). These are kind of fundamental
aspects aircraft stability. Anything else might lead one to check the
c.g. People sometimes confuse the type of gust they are experiencing.

Andy
9B

Absolutely, I was thinking of vertical gust response,
no idea why the OP mentioned horizontal gust response as that is
generally not an issue...

On Wednesday, September 6, 2017 at 9:46:16 AM UTC-4, Dave Nadler wrote:
On Tuesday, September 5, 2017 at 9:24:25 PM UTC-4, Andy Blackburn wrote:
On Tuesday, September 5, 2017 at 6:04:49 PM UTC-7, Dave Nadler wrote:
On Tuesday, September 5, 2017 at 8:23:23 PM UTC-4, wrote:
I don't recall any glider I have flown reacting to a horizontal gust
by pitching down.

IIRC Genesis does this a tiny bit - initially disconcerting but of
no consequence as it stabilizes promptly (it's a nice flying bird).
JJ ??

IIRC vertical gusts activate the short period (AOA) mode and pitch the
aircraft down while horizontal gusts activate the phugoid (airspeed) mode
and pitch up (if the gust is on the nose). These are kind of fundamental
aspects aircraft stability. Anything else might lead one to check the
c.g. People sometimes confuse the type of gust they are experiencing.

Andy
9B

Absolutely, I was thinking of vertical gust response,
no idea why the OP mentioned horizontal gust response as that is
generally not an issue...

Possibly because the original poster was not clear about what kind of gust he was talking about. Both Andy and I chosed to address both possibilities.
UH

Hi Chris -

61 flights and 96 hours in my PW-5 - mostly in MN. I can only remember once that the tail lifted in a thermal due to a vertical gust.

There were no issues at all. The PW-5 just flew itself out of the gust and continued circling.

The PW-5 is the easiest theramlling glider I've flown.

Lou

I didn't mention gust direction. I have been assuming it was a vertical gust lifting the tail, but I have no direct evidence for that.

In a nutshell, the comments offered seem to boil down to the following:
a) check flight habits for unintended movements
b) pull stick back to reduce tail-up pitching and roll into the thermal so indicated
c) ignore it
d) haven't seen/experienced it
e) moving CG forward would exacerbate the rocking effect of the tail-up pitching

I think about the rocking of a sailboat-- the retired coach says to move everyone toward the keel when dealing with waves so that the boat can rock fore and aft more easily (and thus lose less speed by bobbing a little more with the waves instead of slamming hard through the waves). From there I extrapolate that a wider weight spread toward the edges (fore and aft) can increase inertia and slow down oscillation about a central point.

Seems the thing to do is to add ballast on the next flight, hope for strong thermal activity, and report back to the thread. More weight will inevitably move the CG forward; can't move myself backward or I wouldn't be able to reach the controls!

Thanks,
Chris

Chris,
Some posters were confused over the title you used......believe you made a common mistake!
An aircraft pitches around its lateral axis, rolls around its longitudinal axis and yaws around its vertical axis.
Hope this helps,
JJ

Perhaps! What I describe is a pitch of the nose down and the tail up. Longitudinal seesawing (pitching) at the lateral axis.

On 9/5/2017 7:24 PM, Andy Blackburn wrote:
On Tuesday, September 5, 2017 at 6:04:49 PM UTC-7, Dave Nadler wrote:
On Tuesday, September 5, 2017 at 8:23:23 PM UTC-4, wrote:
I don't recall any glider I have flown reacting to a horizontal gust
by pitching down.

IIRC Genesis does this a tiny bit - initially disconcerting but of
no consequence as it stabilizes promptly (it's a nice flying bird).
JJ ??

IIRC vertical gusts activate the short period (AOA) mode and pitch the aircraft down while horizontal gusts activate the phugoid (airspeed) mode and pitch up (if the gust is on the nose). These are kind of fundamental aspects aircraft stability. Anything else might lead one to check the c.g. People sometimes confuse the type of gust they are experiencing.

Years ago I owned a V-tailed HP-14 which enthusiastically pointed its tail to
the sky when in the presence of thermals. First time it did so it distinctly
alarmed/disconcerted me; second time it triggered my "I wonder?" brain cell;
soon thereafter I accepted it as normal-to-the-ship, useful thermal-detection
behavior of no otherwise remarkable note beyond "all the requisite/normal
stick movements" required by uncommanded ship excursions.

YMMV,
Bob W.

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I wonder why my V-tailed HP-14 did not behave like that. (I had the CG far aft within the range.) Or why the original poster noticed it in one G103 and not in another G103.