Question about spoilers and pitch stability

In his article on avoiding PIO in Grob 103's,

http://www.soaringsafety.org/pilots/ic8.htm

Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"

Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.

And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....

Thanks,
Larry

It doesn't really say if the PIO did involve those 4 touchdowns. In
that case it sounds to me like the classic problem of the tail going
down due to inertia, increasing AOA again and starting to fly because
of the increased lift.
In this cased using airbrakes will stop the PIO more quickly because
it is less likely that the increased AOA will be enough to make the
plane fly again. Also, the number of times this happens will probably
be less since you bleed speed more quickly, which also reduces the
amount of lift you're able to produce at the max AOA.

On a side-note: I am interested if anyone has ever performed Cl-Alpha
measurements on a glider with airbrakes applied?

Maybe unrelated to G 103, but I do know that the SGS 1-34 will pitch nose
up when spoiler is deployed, especially if done at relatively high speed.
(I got a big surprise once) So the lift must move forward? ('34 spoilers
are way back on the wing)

Cookie






At 04:42 01 February 2013, Larry Suter wrote:
In his article on avoiding PIO in Grob 103's,

http://www.soaringsafety.org/pilots/ic8.htm

Dean Carswell writes, "the more the airbrakes are closed, the less pitch
st=
ability the Grob will have, making a PIO more likely if otherwise
mishandle=
d"

Is there a simple explanation why opening the spoilers increases the
pitch
=
stability? Does it somehow move the center of lift further aft? I believe
i=
ncreasing the separation between the cg and center of lift is the classic
w=
ay to increase pitch stability.=20

And if that's how it works, why does the center of lift move aft? I would
g=
uess spoilers destroy the lift downwind of their location, causing the
cent=
er of lift to move forward.....

Thanks,
Larry

On Thursday, January 31, 2013 11:42:53 PM UTC-5, Larry Suter wrote:
In his article on avoiding PIO in Grob 103's, http://www.soaringsafety.org/pilots/ic8.htm Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled" Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability. And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward..... Thanks, Larry

The event described is indicative of a failure to recognize that the attitude of the glider is the most important element of not knocking the wheels off the end of the glider.
When watching one of these events, it is common to see pitch changes and spoiler changes all mixed into a confused over correction event.
I would expect that application of spoilers may change the trimmed condition that the glider will tend to seek on it's own, but I doubt there is a significant change in the stability of the glider.
I teach the principle that once the glider is in the round out and pre touchdown
position, the spoilers/brakes change from rate of descent control to rate of deceleration control.Given that, the message is leave the left hand fixed unless the glider is going to land short.
FWIW
UH

There are two effects in play. A wing entering ground effect will see the center of pressure move forward which tends to destabilize the glider. A pilots feels this as increased elevator "twitchiness" when near the ground. This effect is particularly noticeable in G103's.

Any increase in drag will tend to damp pitch oscillations. A free flight demonstration is easy. Just set up a stick-free Phugoid oscillation then open the spoilers and watch it damp out.

The only issue I have with Carswell calling the G103 issue a PIO is that term generally refers to a free flight phenomena not involving ground contact.. Perhaps the G103 specific problem should be called a PIB or Pilot Induced Bounce.


On Thursday, January 31, 2013 9:42:53 PM UTC-7, Larry Suter wrote:
In his article on avoiding PIO in Grob 103's,



http://www.soaringsafety.org/pilots/ic8.htm



Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"



Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.



And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....



Thanks,

Larry

In checking out pilots in the G-103, I refer to the nose wheel/tail
wheel oscillation as a "tire PIO" to clearly differentiate it from the
more classic PIO associated with the excessive landing speeds and ground
effect destabilization. The correction for the latter is also
classic...FREEZE THE STICK.

The correction for the tire PIO is a bit different and is the more
difficult to perform. This oscillation, as pointed out in another post,
is divergent and VIOLENT! I have seen these result in 3-4 cycles in
about a second. Avoidance is the best cure but if it happens, there is
not enough time left to think of a plan so this is best thought out in
advance.

This is my plan...since it is the ground that is going to smite me, I
get away from it. RE-FLY AND RE-LAND the aircraft. (Here is how I know
this works...once while rolling on the main gear in an ASK-21, carrying
enough energy to taxi to parking, an airplane pulled out in front of me.
I hit the wheel brake hard, the nose pitched down and when the nose
wheel contacted the runway a tire PIO ensued, despite having full
spoilers out. I stowed the spoilers briefly and froze the stick.The
glider resumed flying and I then re-landed). Don't plan on your logical,
rational mind helping much...the motion is too rapid and violent. In my
case, I was acting instinctively. Be spring-loaded to use that excessive
energy that led to the problem in the first place, to get away from the
ground.

Some may disagree with these methods but so far, they have worked for me
and the pilots that I have instructed.

Paul
ZZ


On 02/01/2013 08:18 AM, Bill D wrote:
There are two effects in play. A wing entering ground effect will see the center of pressure move forward which tends to destabilize the glider. A pilots feels this as increased elevator "twitchiness" when near the ground. This effect is particularly noticeable in G103's.

Any increase in drag will tend to damp pitch oscillations. A free flight demonstration is easy. Just set up a stick-free Phugoid oscillation then open the spoilers and watch it damp out.

The only issue I have with Carswell calling the G103 issue a PIO is that term generally refers to a free flight phenomena not involving ground contact. Perhaps the G103 specific problem should be called a PIB or Pilot Induced Bounce.


On Thursday, January 31, 2013 9:42:53 PM UTC-7, Larry Suter wrote:
In his article on avoiding PIO in Grob 103's,



http://www.soaringsafety.org/pilots/ic8.htm



Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"



Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.



And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....



Thanks,

Larry

On Thursday, January 31, 2013 11:42:53 PM UTC-5, Larry Suter wrote:
In his article on avoiding PIO in Grob 103's,



http://www.soaringsafety.org/pilots/ic8.htm



Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"



Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.



And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....



Thanks,

Larry

The classic problem here isn't a PIO the way we normally understand it.

The incident starts with trying to "land" on the nosewheel. The normal setup involves the pilot trying to land at 70 kts or so with spoilers closed. Landing long as in Dean's article, or perhaps getting sink in the pattern and getting a little panicky and diving for the runway with brakes closed. The nose wheel touches first and the AOA increases as the rest of the glider momentarily descends, usually resulting in some main wheel contact (usually modest on first contact). The glider, now at about 69.9 kts, with increased AOA, takes flight again. This happens *really* fast. Inertia has probably caused the control stick to move forward during this rotation, so the glider launches, noses over and hits much harder on nose / main / tail the 2nd time and it will lather rinse and repeat until the process is interrupted. If you see this happen, you will not soon forget it!

I'm familiar with a couple such incidents. In all cases I know of, it involved trying to land hot with spoilers closed. In all cases, it starts with nose wheel contact. If it ends well, it's because the pilot managed to stabilize the glider in flight, open the spoilers, achieve a better landing attitude and complete the landing normally.

Opening the spoilers increases the AOA necessary to fly at any given speed, so it decreases the chances of making first contact on the nose wheel, which is obviously to be prevented.

Evan Ludeman / T8

This may border on "scab picking", but I wonder how many pilots who have been involved in G-103 PIBs had initial training in 2-33s.

I know of one incident of a pilot trained in 2-33s and 1-26s, transitioned to our club G-102 (retract gear taildragger version), and proceeded to drag the nose via full forward stick after a hot touchdown.

Law of Primacy?

Just sayin'...

Kirk
66

On Friday, February 1, 2013 12:57:09 PM UTC-5, kirk.stant wrote:
This may border on "scab picking", but I wonder how many pilots who have been involved in G-103 PIBs had initial training in 2-33s. I know of one incident of a pilot trained in 2-33s and 1-26s, transitioned to our club G-102 (retract gear taildragger version), and proceeded to drag the nose via full forward stick after a hot touchdown. Law of Primacy? Just sayin'... Kirk 66

I teach the same principles descibed above when teaching in the 2-33. Transition to the ASK-21 is a non issue because the habits started right.
UH

On 2/1/2013 11:02 AM, wrote:
On Friday, February 1, 2013 12:57:09 PM UTC-5, kirk.stant wrote:
This may border on "scab picking", but I wonder how many pilots who have
been involved in G-103 PIBs had initial training in 2-33s. I know of one
incident of a pilot trained in 2-33s and 1-26s, transitioned to our club
G-102 (retract gear taildragger version), and proceeded to drag the nose
via full forward stick after a hot touchdown. Law of Primacy? Just
sayin'... Kirk 66

I teach the same principles descibed above when teaching in the 2-33.
Transition to the ASK-21 is a non issue because the habits started right.
UH

Apologizing for contributing to thread drift, but Kirk's mental exercise seems
a good entry for a "related safety bull session"...

The particular pitfall of "landing hot" was the only thing I ever thought my
basic training - done entirely in a 2-33 - might've been lacking, as in,
perhaps off-the-mark/incomplete. Natcherly I'd been taught to dial in "proper
approach speed/target point" and land accordionly...and doing so worked well
in both the 2-33 and subsequently in 1-26s.

But when I "advanced" to practicing short-field landings in the 2-33 on grass
- with its widely varying drag wet/dry - I soon encountered a "slightly
snaking" rollout when I used full forward stick as part of braking to a halt
as rapidly as possible (which of course was part of the exercise). My
instructor not being there that day, I sussed out the reason on my own,
concluding that - depending on the magnitude of the drag produced by the
forward-of-the-main-wheel skid - my nose-dragging 2-33 could in effect become
(briefly) a tail-dragger in ground stability terms...i.e. CG behind the
primary rotational drag point (skid), should I somehow get sideways.
Definitely food for thought for this newbie...

The next summer, probably having grown somewhat ignorantly complacent in my
1-26, I recall being distinctly surprised when the first sound I heard during
an "only slightly hot" paved runway landing, was the sound of metal scraping.
My first "thought" was I'd a flat tire, but the reality was I was fast enough
to put the skid plate below the level of the tire. (Duh!)

Those two "minor learning experiences" convinced me of the soundness of
landing as slowly as safe under existing conditions being a generally good
pattern policy...regardless of ship type/configuration. Never found any
reason(s) to change that thinking in the 30+ years since.

Different ships/configurations will react differently when landed
gracelessly/thoughtlessly fast/etc. And it's usually not good for either ship
or pilot to be surprised under such circumstances...

Bob W.