Question about spoilers and pitch stability

On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote:

Opening the spoilers reduces the coefficient of lift. After the first
'bounce', which results in an increase in the angle of attack, the
reduced CL dampens the lift vector and amplitude of the PIO.

It also pushes the Cd up, which always makes things less twitchy because
increased drag tends to damp out pitch changes.


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

On Monday, 4 February 2013 21:22:23 UTC, Martin Gregorie wrote:
On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote:



Opening the spoilers reduces the coefficient of lift. After the first

'bounce', which results in an increase in the angle of attack, the

reduced CL dampens the lift vector and amplitude of the PIO.



It also pushes the Cd up, which always makes things less twitchy because

increased drag tends to damp out pitch changes.





--

martin@ | Martin Gregorie

gregorie. | Essex, UK

org |

Touching down much above stall speed requires a very smooth landing area, otherwise you get thrown back up into the air by a bump. It would not work on my bumpy home field. What happens in a field landing if you have been trained to land at flying speed?

On Monday, February 4, 2013 4:22:23 PM UTC-5, Martin Gregorie wrote:
On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote: Opening the spoilers reduces the coefficient of lift. After the first 'bounce', which results in an increase in the angle of attack, the reduced CL dampens the lift vector and amplitude of the PIO. It also pushes the Cd up, which always makes things less twitchy because increased drag tends to damp out pitch changes. -- martin@ | Martin Gregorie gregorie. | Essex, UK org |

Please explain the physics behind your statement.
UH

On Mon, 04 Feb 2013 15:57:12 -0800, unclhank wrote:

On Monday, February 4, 2013 4:22:23 PM UTC-5, Martin Gregorie wrote:
On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote: Opening the
spoilers reduces the coefficient of lift. After the first 'bounce',
which results in an increase in the angle of attack, the reduced CL
dampens the lift vector and amplitude of the PIO. It also pushes the Cd
up, which always makes things less twitchy because increased drag tends
to damp out pitch changes. -- martin@ | Martin Gregorie gregorie. |
Essex, UK org |

Please explain the physics behind your statement.
UH

The effect of drag in damping a phugoid is described in this PDF:
http://www.flightlab.net/Flightlab.net/
Download_Course_Notes_files/7_LongitudinalDynami%232BA157.pdf

....sorry about the URL wrapping. That's the best reference I can find,
which is annoying because I know I've seen better explanations than that
one.

The math is here - scropp down to 'Phugoid':
https://en.wikipedia.org/wiki/
Aircraft_attitude#Dynamic_stability_and_control

though its badly presented (again, apologies for the wrapped URL). This
makes the point that: "Since the lift is very much greater than the drag,
the phugoid is at best lightly damped." With the unstated implication
that reducing the L/D ratio will make the phugoid more heavily damped.

The reference I was looking for, and I'm pretty certain it was talking
about gliders, mentioned the opposite effect by pointing out that as the
L/D ratio increases, the phugoid becomes progressively less damped,
making the aircraft less dynamically stable.

Hence lowering the wheel or opening the brakes will increase dynamic
pitch stability because the deceased L/D damps the phugoid more heavily.
This is what I was getting at: I accept that my initial comment wasn't
clear on this point and plead lateness of the hour and tiredness, m'lud.


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

This whole discussion is about the difference between (in taildragger terms) three point (low energy, stalled) landings and wheel (flown-on) landings. In power, both are taught; when winds are gusty or strong, a flown on wheel landing at higher airspeed (but, critically, lower groundspeed) is much more controllable and safer, compared to a held off 3-point landing. In really strong crosswinds, it's even preferable to land on one wheel, banked, then slowly lower the other main wheel, followed by the tailwheel, as speed decreases. No big deal, it's all part of good airmanship, and applies just as well to tailwheel gliders.

As far as the concern about flown-on landings on a rough field - remember this is used when there is a strong and/or gusty wind, so the groundspeed/energy is not equal to the airspeed, and the additional control lets the pilot accurately place the plane on the ground. Trying a low-energy tail down landing in strong, gusty crosswinds is asking for a slammed in landing and groundloop!

Of course, do not confuse this with "flying onto the runway" at high speed. Forcing the plane to land is ALWAYS

On Tuesday, February 5, 2013 10:16:51 AM UTC+1, kirk.stant wrote:
This whole discussion is about the difference between (in taildragger terms) three point (low energy, stalled) landings and wheel (flown-on) landings. In power, both are taught; when winds are gusty or strong, a flown on wheel landing at higher airspeed (but, critically, lower groundspeed) is much more controllable and safer, compared to a held off 3-point landing. In really strong crosswinds, it's even preferable to land on one wheel, banked, then slowly lower the other main wheel, followed by the tailwheel, as speed decreases. No big deal, it's all part of good airmanship, and applies just as well to tailwheel gliders.



As far as the concern about flown-on landings on a rough field - remember this is used when there is a strong and/or gusty wind, so the groundspeed/energy is not equal to the airspeed, and the additional control lets the pilot accurately place the plane on the ground. Trying a low-energy tail down landing in strong, gusty crosswinds is asking for a slammed in landing and groundloop!



Of course, do not confuse this with "flying onto the runway" at high speed. Forcing the plane to land is ALWAYS

Oops, fat fingers, here is the rest:

ALWAYS a bad thing, especially with the current "nosewheel" trainers. But a student should be able to land not only with min energy (main and tail or slightly tail first) or carefully wheeled on (slightly tail low) in strong winds.

If you can get a copy of "Big Rocks & Long Props", you will see plenty of examples of taildraggers being landed in "interesting" terrain with the tail up - precisely because of the additional controllability.

Kirk
66

On Tuesday, February 5, 2013 4:30:51 AM UTC-5, kirk.stant wrote:
On Tuesday, February 5, 2013 10:16:51 AM UTC+1, kirk.stant wrote: This whole discussion is about the difference between (in taildragger terms) three point (low energy, stalled) landings and wheel (flown-on) landings. In power, both are taught; when winds are gusty or strong, a flown on wheel landing at higher airspeed (but, critically, lower groundspeed) is much more controllable and safer, compared to a held off 3-point landing. In really strong crosswinds, it's even preferable to land on one wheel, banked, then slowly lower the other main wheel, followed by the tailwheel, as speed decreases. No big deal, it's all part of good airmanship, and applies just as well to tailwheel gliders. As far as the concern about flown-on landings on a rough field - remember this is used when there is a strong and/or gusty wind, so the groundspeed/energy is not equal to the airspeed, and the additional control lets the pilot accurately place the plane on the ground. Trying a low-energy tail down landing in strong, gusty crosswinds is asking for a slammed in landing and groundloop! Of course, do not confuse this with "flying onto the runway" at high speed. Forcing the plane to land is ALWAYS Oops, fat fingers, here is the rest: ALWAYS a bad thing, especially with the current "nosewheel" trainers. But a student should be able to land not only with min energy (main and tail or slightly tail first) or carefully wheeled on (slightly tail low) in strong winds. If you can get a copy of "Big Rocks & Long Props", you will see plenty of examples of taildraggers being landed in "interesting" terrain with the tail up - precisely because of the additional controllability. Kirk 66

The other reason the tail is up is so they don't knock the tailhweel off on the big rocks. The mains are much stronger than the tailwheel.
UH

test

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 Sun, 3 Feb 2013 11:54:33 -0800 (PST), Bill D
wrote:

I hope our European friends jump in too. The following is based on years of experience at Boulder in wildly turbulent west wind rotors.

I don't think a "hot" landing has any value. It just means you'll float down the runway in ground effect while you are vulnerable to those gusts and with enough energy to seriously damage the glider. Better to get it down and stopped and that can't happen until the glider slows.

With a little practice, you'll find a glider can be slowed sharply with a few seconds of full airbrake while raising the nose to maintain the glide path just before entering ground effect. This can be done on short final to hit the recommended approach speed allowing the pattern itself to be flown at any speed the pilot deems safe for the gustiness.

This discussion is about nose wheel gliders where "hot" landings put the glider on the nose wheel first.


From a European (German) point of view, I can only agree.

Even when it's pretty turbulent it is not necessary to fly faster than
60 kts with a G103. Even if one is faster, bleeding off excessive
speed with full airbrakes during the flare is very simple without any
danger of a PIO.

I am absolutely sure that these PIOs on the G103 (which is one of the
most forgiving basic trainers available) are a result of lacking
training.
This kind of accident is completely unheard of in Germany where nearly
*any* student pilot is trained from the beginning that any landing
where the tail wheel does not touch down first is a bad landing.



Andreas