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?

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 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 |

test