PW-5 longitudinal pitch oscillation

[[email protected]]

Am considering buying a PW-5 as my first single-seater for a variety of reasons, but the one aspect I don't like is that in rough air the tail is easily pitched up in flight from unexpected gusts once I'm off tow. This one's been weighed & balanced recently and its CG is perfectly centered within its designed range.

I'm on the light side for a glider pilot, so in two-seaters I often add extra ballast when my second seat is empty. However, I see these tail-high excursions even with extra ballast in one of the Grob 103s I frequently fly solo in (but not the other Grob 103), so I am wondering whether additional forward ballast would successfully reduce the magnitude or incidence in the PW-5.

Are there any light PW-5 pilots who have resorted to additional ballast or some change to their flight habits to reduce uncommanded tail-up pitching? And if so, how effective has that change been?

Cheers,
Chris

[[email protected]]

On Tuesday, September 5, 2017 at 7:38:43 PM UTC-4, wrote:
> Am considering buying a PW-5 as my first single-seater for a variety of reasons, but the one aspect I don't like is that in rough air the tail is easily pitched up in flight from unexpected gusts once I'm off tow. This one's been weighed & balanced recently and its CG is perfectly centered within its designed range.
>
> I'm on the light side for a glider pilot, so in two-seaters I often add extra ballast when my second seat is empty. However, I see these tail-high excursions even with extra ballast in one of the Grob 103s I frequently fly solo in (but not the other Grob 103), so I am wondering whether additional forward ballast would successfully reduce the magnitude or incidence in the PW-5.
>
> Are there any light PW-5 pilots who have resorted to additional ballast or some change to their flight habits to reduce uncommanded tail-up pitching? And if so, how effective has that change been?
>
> Cheers,
> Chris

A properly trimmed glider will pitch down to some degree when it encounters a vertical gust from below. It is seeking it's natural trimmed condition. Moving the CG forward will increase this tendency to some degree.
I don't recall any glider I have flown reacting to a horizontal gust by pitching down.
You may want to try flying with your forearm resing on your leg to avoid unintentional control inputs.
FWIW
UH

[Dave Nadler]

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 ??

[Andy Blackburn[_3_]]

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

[[email protected]]

Tail up or (nose down) is a normal condition that usually occurs when entering a thermal. It can be a bit more pronounced in a short coupled bird, like the Super Albatross, Genesis-2 and I guess the PW-5. Relax, pull back a bit and roll into the light wing..............you just entered a thermal! Aft CG is best for performance.
😀 JJ

[Dave Nadler]

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...

[Bob Whelan[_3_]]

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.

---
This email has been checked for viruses by AVG.
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[[email protected]]

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.

[[email protected]]

I didn't notice anything unusual on my one PW-5 flight in Invermere years ago - I found it an all around nice handling ship. The relatively light wingloading made for an exciting ride in the strong thermals there but the 1-26's got bounced around even more. A friend of mine who owns her own PW-5 hasn't reported any tail up pitching and she's quite light. In my own ASW-15B I have noticed on entry to the powerful thermals in Invermere that it frequently felt like the tail was being grabbed and shaken - like a terrier would shake a rat. Nothing alarming about it though. In the milder thermals, ridge lift and wave at my home field I've never noticed it.

[[email protected]]

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

[MNLou]

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

[[email protected]]

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

[[email protected]]

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

[[email protected]]

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

[Martin Gregorie[_5_]]

On Fri, 08 Sep 2017 21:08:42 -0700, larsonchristina wrote:

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

All aircraft entering a thermal will do this, some more noticeably than
others.

Reasoning: as the glider flies into the thermal it enters a rising air
mass and the further it moves into this air mass, the faster the vertical
movement becomes. The effect on the glider is that its effective AOA is
reduced by the air's vertical velocity (draw a vector diagram and this
becomes obvious), which in turn means that the wing no longer supports
all the glider's weight. The effect of trim means that the glider will
automatically pitch down and speed up to regain lift.

The combination of this small forward acceleration plus the upward
acceleration as to enters progressively faster air is what you feel as
"the surge" when you enter a thermal.

What you see and feel depends on what you're flying. In my Libelle I
don't particularly notice the pitch down, and anyway it may be masked as
I'm pitching up to slow down from inter-thermal cruise speed or even
faster flight through the sink surrounding the thermal, but the surge
forward and up is quite easy to feel.

The most obvious demonstration of this that I've seen was shown by an A/1
competition model glider of around 4 feet span, even when it was 50-60 m
overhead. When entering a strong thermal it would pitch down quite
rapidly and about as steeply as a helicopter does after takeoff when its
pilot is in a hurry to get moving before returning to its normal trimmed
attitude as it entered the core of the thermal. Like all free flight
models of its era, its trim was set before launch and remained so
throughout the flight, so this strong pitch down was not due to trim
changes.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[[email protected]]

On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie wrote:
>
> Reasoning: as the glider flies into the thermal it enters a rising air
> mass and the further it moves into this air mass, the faster the vertical
> movement becomes. The effect on the glider is that its effective AOA is
> reduced by the air's vertical velocity (draw a vector diagram and this
> becomes obvious)...

Seems the opposite to me. The rising air hits the wings from below, and thus at a higher AOA. This increases the lift on both the wing and the tail, but more so at the tail. This is what positive stability means. The glider then behaves just like after raising the nose momentarily and then letting go of the stick: it pitches nose down. It's not the trimmed speed it is seeking per se, it is the trimmed AOA. Thus in the thermal entry it pitches nose-down despite the speed being normal. Once the glider accelerates upwards due to the rising air, the vertical motion relative to the air returns to normal, and the normal pitch attitude relative to the horizon will be restored on its own. The more pitch-stable the glider (and yes this is called "longitudinal stability" IIRC?), the stronger this pitching effect should be. Maybe that's why I haven't noticed it in my gliders, since I have the CG near the rear end of the range, i.e., weak stability.

[[email protected]]

On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie wrote:
> All aircraft entering a thermal will do this, some more noticeably than
> others.
>
> Reasoning: as the glider flies into the thermal it enters a rising air
> mass and the further it moves into this air mass, the faster the vertical
> movement becomes. The effect on the glider is that its effective AOA is
> reduced by the air's vertical velocity ...

Seems the opposite to me. The rising air hits the wings from below, and thus at a higher AOA. This increases the lift on both the wing and the tail, but more so at the tail. ("More so" in the sense of the strength of the resulting pitching moments around the CG.) This is what positive stability means. The glider then behaves just like after raising the nose momentarily and then letting go of the stick: it pitches nose down. It's not the trimmed speed it is seeking per se, it is the trimmed AOA. Thus in the thermal entry it pitches nose-down despite the speed being normal. Once the glider accelerates upwards due to the rising air, the vertical motion relative to the air returns to normal, and the normal pitch attitude relative to the horizon will be restored on its own. The more pitch-stable the glider (and yes this is called "longitudinal stability" IIRC?), the stronger this pitching effect should be. Maybe that's why I haven't noticed it in my gliders, since I have the CG near the rear end of the range, i.e., weak stability..

[Martin Gregorie[_5_]]

On Sat, 09 Sep 2017 10:25:20 -0700, moshe.braner wrote:

> On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie
> wrote:
>> All aircraft entering a thermal will do this, some more noticeably than
>> others.
>>
>> Reasoning: as the glider flies into the thermal it enters a rising air
>> mass and the further it moves into this air mass, the faster the
>> vertical movement becomes. The effect on the glider is that its
>> effective AOA is reduced by the air's vertical velocity ...
>
> Seems the opposite to me. The rising air hits the wings from below, and
> thus at a higher AOA.
>
If it was a body of air all rising at the same speed I'd agree, but in as
it enters a thermal the vertical velocity will be increasing as the
glider heads for the core, IOW its facing a continous disturbance which
has two components. One is the vertical velocity that the glider has
acquired from the air its just passed through. The second is the effect
of flying into air with a still higher vertical velocity.

> ("More so" in the sense of the strength of the resulting pitching
> moments around the CG.) This is what positive stability means.
>
If it was in a parcel of air with the same vertical velocity I'd agree
with you, but it ain't: its flying onto steadily increasing vertical
velocity as it approaches the thermal core, so if things are as you
describe, you'd expect the glider to be pitching up. As that's not what
we see and feel, there must be something else acting on it, which I think
is the effect of flying into an increasing velocity gradient is to reduce
the effective AOA of the wing. Of course in reality it affects both wing
and tail, but its effect on the wing is a little bigger because the wing
gets to the higher velocity rising air before the tail does. Granted its
a small effect, but I think its enough to cause the nose to drop/tail to
rise.

> It's not the trimmed speed it is seeking per se, it is the trimmed AOA.
>
Yes, you're right there. I should have made that clearer.

> the stronger this pitching effect should be. Maybe
> that's why I haven't noticed it in my gliders, since I have the CG near
> the rear end of the range, i.e., weak stability.
>
Yes, that makes sense. My CG is pretty much in the middle of the
permitted range.

The force set up in the model will also be different: free flight gliders
normally have the CG at 52-55% of the wing chord, use a lifting section
on the tailplane and, yes, they are stable with a lifting tail. Almost
all designs operate with the wing at a Cl of around 1.2 and the tail at a
Cl of around 0.2. This puts the wing at an AOA of about 4 degrees more
than the tail, which keeps this layout stable with fast vertical upset
recovery. The tail is much bigger than on a sailplane - about 15-18% of
the wing area and, as a nice to have, most of the favoured tail sections
give minimum drag at a Cl of 0.2 - a nice bonus, since the tail is also
doing work against gravity as well as keeping the glider stable.



--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[[email protected]]

So I increased the ballast weight in the PW-5 from parachute + 20 pounds to parachute + 40 pounds. The pitch stability increased noticeably, i.e. uncommanded nose-down excursions diminished.

Thanks all; I've enjoyed reading about model gliders and changes upon entering thermals.

[JS[_5_]]

On Wednesday, September 13, 2017 at 6:10:58 PM UTC-7, wrote:
> So I increased the ballast weight in the PW-5 from parachute + 20 pounds to parachute + 40 pounds. The pitch stability increased noticeably, i.e. uncommanded nose-down excursions diminished.
>
> Thanks all; I've enjoyed reading about model gliders and changes upon entering thermals.

In my only flight in the PeeWee had hoped to do spins.
Couldn't get it to stall, even accelerated, so gave up on spins.
That was with no nose weight. Perhaps it could use tail weight?
Like the OP, with way forward C/G, didn't notice any tendency to oscillate in pitch. It still climbed well, don't ask about cruise.
Did notice turbulence perhaps from the large access hole in the horizontal stab. Now there is no World Class and associated rules, that design problem could be fixed with mylar.
Jim

[[email protected]]

On Tuesday, September 5, 2017 at 5:38:43 PM UTC-6, wrote:
> Am considering buying a PW-5 as my first single-seater for a variety of reasons, but the one aspect I don't like is that in rough air the tail is easily pitched up in flight from unexpected gusts once I'm off tow. This one's been weighed & balanced recently and its CG is perfectly centered within its designed range.
>
> I'm on the light side for a glider pilot, so in two-seaters I often add extra ballast when my second seat is empty. However, I see these tail-high excursions even with extra ballast in one of the Grob 103s I frequently fly solo in (but not the other Grob 103), so I am wondering whether additional forward ballast would successfully reduce the magnitude or incidence in the PW-5.
>
> Are there any light PW-5 pilots who have resorted to additional ballast or some change to their flight habits to reduce uncommanded tail-up pitching? And if so, how effective has that change been?
>
> Cheers,
> Chris

Don't buy a PW. Buy a Libelle or Std. Cirrus of LS-1 for the same or a little bit less of cost for much more performance.

Collin

[Squeaky]

;955349']On Wednesday, September 13, 2017 at 6:10:58 PM UTC-7, wrote:
So I increased the ballast weight in the PW-5 from parachute + 20 pounds to parachute + 40 pounds. The pitch stability increased noticeably, i.e. uncommanded nose-down excursions diminished.

Thanks all; I've enjoyed reading about model gliders and changes upon entering thermals.

In my only flight in the PeeWee had hoped to do spins.
Couldn't get it to stall, even accelerated, so gave up on spins.
That was with no nose weight. Perhaps it could use tail weight?
Like the OP, with way forward C/G, didn't notice any tendency to oscillate in pitch. It still climbed well, don't ask about cruise.
Did notice turbulence perhaps from the large access hole in the horizontal stab. Now there is no World Class and associated rules, that design problem could be fixed with mylar.
Jim

Well, the POH is pretty clear at higher pilot weights the aircraft will NOT stall with full aft stick...