Definition(s) of pitch sensitivity? All-flying tails

[Bret]

Hi, I hear a lot about pitch sensitivity. Can that mean more than one
thing?

For example, it seems it could mean:

1. A glider in which smaller than typical stick movements are needed
for pitch control

2. A glider in which the pitch stability is less, in that when
perturbed from trimmed level flight the number of pitch oscillations
is large before returning to level flight (or not return at all).

3. A glider in which the pitch stability is fine, but the time to
respond to correction is long, so the pilot overcorrects to cause
PIO.

.... or does it always mean #2?

Are all-flying tails always pitch sensitive (definition #2), or is it
a combination of the all-flying tail with something else?

Does even an experienced pilot have to live with more risk in a pitch
sensitive glider, or is the process of getting used to it that is more
risky?

Thanks

[Chris Nicholas[_2_]]

I think it always means 1, at least when I use it and have heard
others.

By the way, a single model can have widely differing sensitivity
depending on cockpit load/CofG position. A well-known 2- seater with a
permissibly light solo pilot (think 16-year old girl on 1st solo) can
be 4x more sensitive than with 2 up, one being an overweight
instructor. She finds herself flying a glider totally different in
feel to anything she has flown before. Can lead to PIO.

No heavy instructor knows what she is going to experience.

So I am told.

Chris N.

[Basil]

For the Janus A I had a share in it was none of the above. The two
less than ideal features were,

1. The elevator loads didn't increase with speed so that at high
speed you had to be very careful to move the stick gently or you might
pull alot of G (in either direction).

2. The elevator didn't seem to be mass balanced so in turbulence you
would get alot of feedback through the stick.




On Fri, 26 Jun 2009 11:01:39 -0700 (PDT), Bret >
wrote:

>Hi, I hear a lot about pitch sensitivity. Can that mean more than one
>thing?
>
>For example, it seems it could mean:
>
>1. A glider in which smaller than typical stick movements are needed
>for pitch control
>
>2. A glider in which the pitch stability is less, in that when
>perturbed from trimmed level flight the number of pitch oscillations
>is large before returning to level flight (or not return at all).
>
>3. A glider in which the pitch stability is fine, but the time to
>respond to correction is long, so the pilot overcorrects to cause
>PIO.
>
>... or does it always mean #2?
>
>Are all-flying tails always pitch sensitive (definition #2), or is it
>a combination of the all-flying tail with something else?
>
>Does even an experienced pilot have to live with more risk in a pitch
>sensitive glider, or is the process of getting used to it that is more
>risky?
>
>Thanks

[Martin Gregorie[_5_]]

On Fri, 26 Jun 2009 11:01:39 -0700, Bret wrote:

> 2. A glider in which the pitch stability is less, in that when
> perturbed from trimmed level flight the number of pitch oscillations is
> large before returning to level flight (or not return at all).
>
Both the gliders I've tried straight flight, hands off stick with
(ASW-20, Std. Libelle) have gradually built up a definite, but stable
phugoid over 5 cycles or so. The '20 stabilized with a +/- 5 kt speed
excursion over a 25 second period. I don't remember the Libelle numbers
except that both speed excursion and period were less than for the '20.

As I'm told that all aircraft show this behavior and that it becomes more
pronounced as the airframe drag is reduced, I question whether counting
oscillations is relevant to stability estimates.


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

[n7ly]

On Jun 26, 1:01*pm, Bret > wrote:
> Hi, I hear a lot about pitch sensitivity. *Can that mean more than one
> thing?
>
> For example, it seems it could mean:
>
> 1. *A glider in which smaller than typical stick movements are needed
> for pitch control
>
> 2. *A glider in which the pitch stability is less, in that when
> perturbed from trimmed level flight the number of pitch oscillations
> is large before returning to level flight (or not return at all).
>
> 3. A glider in which the pitch stability is fine, but the time to
> respond to correction is long, so the pilot overcorrects to cause
> PIO.
>
> ... or does it always mean #2?
>
> Are all-flying tails always pitch sensitive (definition #2), or is it
> a combination of the all-flying tail with something else?
>
> Does even an experienced pilot have to live with more risk in a pitch
> sensitive glider, or is the process of getting used to it that is more
> risky?
>
> Thanks

Look up the phrase stick force per g. Also the phrase phugoid.

Generally, stick force is what we notice the most. Stick motion
also, but the real touchy feel is lack of stick force.

Phugoid is a hands off situation. Short term phugoid should be
damped. Long term phugoid should be at least neutral. We don't
spend much time hands off. If you have your hands on (stick fixed)
the phugoid will be more damped.

Like you said, all flying tails often have less well behaved stick
force and phugoid characteristics.

The problems can be worked via pivot points, balance, contour
changes, springs and bobweights. Or you can design the thing
with good stability characteristics in the first place and forget
about making everything as small as possible.

[[email protected]]

Bret wrote:

<Thoughtful questions about pitch sensitivity massaged below...>

> Hi, I hear a lot about pitch sensitivity. Can that mean more than one
> thing?

"Almost certainly...to different people of various backgrounds." So
it'll pay dividends to keep asking your questions until the answers
make sense to you. FWIW, you're probably asking (at least partially)
about a topic postgraduate engineers could (and do) specialize in,
i.e. longitudinal ('pitch') stability.

From Joe Pilot's usual perspective though, 'pitch sensitivity' tends
to have a broader meaning. Typically, ships having a low ratio of
stick-force/G response are deemed 'more sensitive' than those with a
high(er) ratio response. Examples of the latter are any Schweizer -
you have to pull 'really hard' to induce (say) 4G on them, while a
lighter pull force (from the same speed) will induce 4G on (say) a
G-103. The actual force numbers are what they pay test pilots to
measure.

Another general measure of pitch sensitivity is the starting force
('breakout force' in test pilot terms) required to induce a pitch
change from trimmed flight. And since so much glider trimmed flight
occurs around thermalling speed, the pilot community tends to dial in
'normal stick forces' at these speeds. Painting with a broad brush,
glass ships tend to have lower breakout forces than Schweizers and are
thus thought of as 'more pitch sensitive.'

In the interest of communication, I recommend expending effort
defining the terms of any conversation about this sort of stuff...
- - - - - -

> For example, it seems it could mean:
>
> 1. A glider in which smaller than typical stick movements are needed
> for pitch control

Yup...
- - - - - -

> 2. A glider in which the pitch stability is less, in that when
> perturbed from trimmed level flight the number of pitch oscillations
> is large before returning to level flight (or not return at all).

Yup...more below.
- - - - - -

> 3. A glider in which the pitch stability is fine, but the time to
> respond to correction is long, so the pilot overcorrects to cause
> PIO.

A *VERY* thoughtful realization...and one that seems applicable in
some circumstances to the Zuni I fly. It actually took me a number of
years to realize the validity of your surmise, and where I think I
tend to see it is in two situations: 1) turbulent air aloft, and 2)
landing flare.

Aloft, in turbulent air (all my Zuni flight has been in the U.S.
intermountain west) there - sometimes, not always - seems to be a
distinct delay between making a pitch input and that input's taking
effect. I usually notice this after the effect happens (& I forget or
get impatient), I make a(nother) pitch input, and both seem to take
effect 'at once.' It's more a nuisance than anything else, but I don't
think it's entirely in my imagination. Some pitch inputs simply don't
seen to take effect in the same time constant 'most others' do. (It's
not control slop...)

Where it can be more problematic (to me, anyway) is in the landing
flare. I years ago gave up trying to execute a smooth, continuous
stick motion as part of transitioning from the approach position to
the 2-point position, because I simply can't do it. (It was trivially
easy in the HP-14 from which I transitioned.) Whether my failing comes
about from a time-constant lag due to the all-flying stabilator, pitch
system friction, pitch system geometry, pilot ineptitude or whatever,
I have no real idea. (I evolved a work around, 2-step flare
methodology that works well enough.)
- - - - - -

> ... or does it always mean #2?

"Yes," to aeronautical engineers. "Pitch stability" has a distinct
engineering meaning when applied to airplanes/gliders, and even at the
undergraduate level the topic may occupy a full-semester course; your
'#2' directly addresses the engineering aspect of 'pitch stability.'
Quite math intensive the course is, but the comprehensive gist of the
course is easily (more or less) summarizable: as the CG moves aft,
pitch stability lessens. If you messed about with flying models as a
kid, you already understand this.

Engineers talk about 'stick-free' and 'stick-fixed' pitch stability,
and as you might guess one is a measure of the plane's pitch reaction
to a perturbation (stick movement, gust, etc.) while the pilot is
holding the stick, and the other the plane's reaction when s/he is
not. An example of the former might be doing a high-speed pass and
initiating a pitch change, while an example of the second might be
flying through turbulence while you're trying to offload personal
water ballast (aka 'take a leak') with neither hand on the stick.
Mathematically, the situations are different, but to Joe Average Pilot
an *absence* (or even the reduction) of pitch stability (of either
type) is (may be any or all): noticeable, alarming, backward,
different, dangerous.

Typically, aft CG limits on a glider are chosen to provide
'sufficient' pitch stability at full-aft CG. The glider definitely
does NOT suddenly go from 'stable' to 'unstable' with the shift
occurring exactly AT the aft CG limit. Nor will it go mathematically
unstable until the CG is 'some ways *aft*' of the aft limit. (How far
aft? Ah! Here's one of the black arts in glider design!!!) Rather, the
glider's ability to return to its trimmed speed (in the absence of
pilot input) diminishes as the CG moves aft. It is becoming 'less
stable' and takes longer to return to its trimmed speed if flown stick
free. In $10 word terms, the less stable a glider is, immediately
following a stick-free pitch perturbation its 'phugoid oscillations'
*tend* to increase in amplitude for a given magnitude perturbation,
and, similarly increase in number before the plane again achieves
trimmed speed after the perturbation.
- - - - - -

> Are all-flying tails always pitch sensitive (definition #2), or is it
> a combination of the all-flying tail with something else?

Another question worthy of books! My personal answer to the second
part of your question is, "I don't know!" while my response to the
first part is, "They sure seem to be!" If it happens, it must be
possible.

The way I tend to think of it is it's much more difficult for a
designer to end up with 'acceptable pitch characteristics' designing
around an all-flying tail than around a conventional (fixed horizontal
stab w. trailing elevator). True whether we're talking power plane or
glider. Obviously, designers of both have designed w. all-flying
horizontal stabs (e.g. Cessna Cardinal, various Pipers, Zuni, St'd
Cirrus, early Mini-Nimbus, LS-1, etc.). I find it interesting - and
perhaps telling - that glider designers' attraction to all-flying
horizontal stabilizers tended to be short-lived/early-ish in the glass
revolution; both LS and Schempp-Hirth ultimately moved away from them.

In the case of the Zuni, my ship (S/N 3, intended to be the production
prototype) shows in its log a series of 7 flights over 7 months,
logged as follows: 1) "Test" (initial flight); 2)/3) "Stall Tests"; 4)
"Stall & High Speed"; 5) "Stability Tests"; 6) "Test New Stabilator";
7) "Test Stabilator Modification". Unfortunately I don't know the
details of the change(s?) implied by 6) & 7), but it's my
understanding a hinge point change is in there somewhere...the
distance of the change being considerably under an inch.
- - - - - -

> Does even an experienced pilot have to live with more risk in a pitch
> sensitive glider,

Short form answer is, "Yes." The glider just 'is' & it's up to the
pilot to always adapt its characteristics. Without intending to be
dramatic or scaremongering, here's a couple of illustrative stories
about Standard Cirrii told me by good friends experienced in all
flying stabilizer examples...

One - a flight instructor/experienced XC gliderpilot of sober
judgment, and mechanical engineer by training - said he was distinctly
surprised to learn that above ~90 knots (as I recall from ~20 years
ago) his 'stick force per G' began to noticeably lessen. In other
words, instead of needing more force to apply more - in this case pull-
up G-load - it required *less.* In response to the direct question,
"Did you have to *push* to keep from pulling more G?" his answer was,
"No...but I *did* have to lessen my pull force." Because this isn't a
'natural' response, designers generally try to avoid playing in this
particular sandbox; as you might imagine, PIO - or worse - could
easily result should Joe Pilot be surprised by the change. In
engineering terms, the slope of the stick force per G line decreased,
for this guy in this ship. Had the slope gone negative, he would have
been flying a definitionally 'pitch unstable' glider.

Two other good friends who owned/flew different St'd Cirrii for over
20 years each - both of whom weighed >210 pounds - independently told
me that they 'always' flew with their hand at the base of the stick,
forearm resting on thigh, when cruising above 70 knots or so, because
it was just too easy to induce inadvertent high-G in turbulent air,
flying faster while grasping the stick's hand grip. Both also said
they routinely experienced 'unusual attitudes' while taking leaks.

All 3 of these pilots loved their Cirrii...but gave their ships'
limitations due respect.

Now the above-mentioned things just 'are.' Simply being aware of them
beforehand is a huge weapon in a pilot's arsenal. I don't consider any
of them outright 'bad' or 'alarming'...they're just
'different' ('characteristics,' if you will). To pretend they're not
there is to indulge in fantasy, but to suggest they're mountainous is
to indulge in unwarranted hyperbole, in my view. Nevertheless, they're
the sort of stuff you'll hear in post-flying bull sessions, often
beer, ignorance and distance enlarged.
- - - - - -

or is the process of getting used to it that is more
> risky?

Short form answer is, "Yes." To me, anything new is definitionally
'risky.' That noted, having a grasp beforehand of what you're likely
to encounter considerably alters the picture. Think 'new experience'
as opposed to 'risk of the unknown.'

Regards,
Bob - tries to avoid unintentional test piloting - W.

[[email protected][_2_]]

Pitch "sensitivity" is most likely a result of an aft CG, IMHO.
Flying tail may feel "sensitive", but in gliders that is most likely
due to lack of feel in the older all-flying tail gliders.

An aft CG, up to a point, makes most gliders fly a lot sweeter. But
get too far aft, and things get squirrily real fast!

The big Schweizer 2-32, which has a all-flying tail, is neither pitch
sensitive or lacking in feel - on the contrary it is a nice flying, if
somewhat heavy, glider. The slab tail is big, and has a bit anti-
servo tab on it, which probably contributes to it's nice feel.

Spins nice too - and requires a manly push on the stick to recover!

Kirk

[Galen Fisher]

I agree with Kirk that the 2-32 feels pleasant due to the anti-servo
tab. I remember it was the same with the single place Lark.

I'm sure that part of the "problem" with all-flying tails is that the
mass in the system, which is higher than you would expect due to the
need for mass balancing, imparts a little more momentum to the control
system. If you correct one way or the other the surface will keep
moving a little after you stop the input. I suppose we automatically
learn to stop the input with different timing or physically stop the
stick from overshooting the desired displacement. You basically
have to stop the control from overshooting, whether you realize it or
not, which I've never noticed with normal elevators or rudder-vators,
mass balanced or not..

Again as Kirk suggested, this was most noticeable in an old, light
glider that suddenly was an unpleasant handful when we flew through
wake turbulence on the takeoff roll.

Cheers, Galen (remove 'xxx' to reply)

[rlovinggood]

Bret,

I don't know the definition of "pitch sensitivity, but I do know about
flying with an all-flying stabilator.

I've been flying my LS1-d for about 15 years (has it been that long?)
and it has the all-flying stabilator. Is it "pitch sensitive?" I
don't think so, but I rarely fly anything else, thus I don't have
anything to compare against. The glider will trim at my thermalling
speeds (48-52 knots) and will trim at cruising speeds (70 - 75 knots)
and will fly "hands off" for the short length of time I need to be
hands off to do a small task as fold or unfold a chart, open a
sandwich bag, etc. I haven't really explored how long it will go
hands off. No need to. If I wanted to fly "hands off", I would sell
the glider and get an airplane that has an autopilot.

My good friend flies a Standard Cirrus, also with an all-flying
stabilator. From what he says, his glider is a bit more "twitchy" in
pitch than mine. He has his CG around 70 or 75 % rearward on the CG
range and according the Weight and Balance I did this past Friday,
mine's at 68%.

From my experience, if a glider is supposedly "pitch sensitive", the
pilot might get accustomed to it over time.

And for the record, the sexy little stabilator on the LS1-c, LS1-d has
it ALL OVER the big, fat, homely stabilator found on the Std. Cirrus.

:-)

Ray Lovinggood
Carrboro, North Carolina, USA

[Bruce]

rlovinggood wrote:
> Bret,
>
> I don't know the definition of "pitch sensitivity, but I do know about
> flying with an all-flying stabilator.
>
> I've been flying my LS1-d for about 15 years (has it been that long?)
> and it has the all-flying stabilator. Is it "pitch sensitive?" I
> don't think so, but I rarely fly anything else, thus I don't have
> anything to compare against. The glider will trim at my thermalling
> speeds (48-52 knots) and will trim at cruising speeds (70 - 75 knots)
> and will fly "hands off" for the short length of time I need to be
> hands off to do a small task as fold or unfold a chart, open a
> sandwich bag, etc. I haven't really explored how long it will go
> hands off. No need to. If I wanted to fly "hands off", I would sell
> the glider and get an airplane that has an autopilot.
>
> My good friend flies a Standard Cirrus, also with an all-flying
> stabilator. From what he says, his glider is a bit more "twitchy" in
> pitch than mine. He has his CG around 70 or 75 % rearward on the CG
> range and according the Weight and Balance I did this past Friday,
> mine's at 68%.
>
> From my experience, if a glider is supposedly "pitch sensitive", the
> pilot might get accustomed to it over time.
>
> And for the record, the sexy little stabilator on the LS1-c, LS1-d has
> it ALL OVER the big, fat, homely stabilator found on the Std. Cirrus.
>
> :-)
>
> Ray Lovinggood
> Carrboro, North Carolina, USA
Well the Std Cirrus may have a homely stabilator. And it is indeed
pretty "responsive" on the elevator. Compared to something like a
DuoDiscus it is downright twitchy.

Some points -
Mine is at >90% aft CG - that makes it very twitchy. It got that way
from taking heavy old instruments out of the front, that appear to have
been balanced by adding gel to the tail boom around the wheel. - She's
going on diet soon to get the CG to ~65%.
You learn to love it - especially in not having to counter high forces
at high speed.
You learn to be very circumspect with control inputs at high speed...
My Cirrus is an early model with the .75 degree washout wings, hands off
is not really an option except for very brief moments. Trimmed to just
about any speed she enters a slowly diverging phugoid if you leave the
stick.


Again - it seems to depend on the aircraft a little.

Cheers
Bruce

Big homely Std Cirrus Driver. (both the glider and the pilot)