Question about spoilers and pitch stability

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


Hi Larry,

There is indeed a simple explanation as to why opening airbrakes increases
stability in pitch. It is to do with drag.

As I assume you are aware, the reason that the airspeed increases when you
'push the nose down' is because a larger component of the lift force
generated by the wings essentially points in the direction of the gliders
horizontal motion. Obviously the opposite happens when you 'raise the
nose', and again obviously the glider stops accelerating when the sum of
the forces acting on the glider is zero (drag increases as the glider
accelerates).
Clearly the total drag at a given airspeed is greater when the airbrakes
are open than when they are closed. If a glider is experiencing a
particular 'amount' of drag at a given airspeed with the brakes open, then
the same glider with the brakes closed will obviously have to fly at a
greater speed in order to experience the same amount of drag.
All of what I've just written is completely and utterly obvious, and I'm
sorry if I've insulted your intelligence.

However what I'm getting at is that, essentially, if you sit in a glider
and move the stick forward by a certain amount, the glider with the
airbrakes open is in some (rather handwavy) sense already 'closer' to being
in equilibrium (or closer to the constant speed at which it will fly given
the amount of forward stick) than the same glider with closed airbrakes.
In other (probably rather clearer) words the glider with airbrakes open
will not accelerate at the same rate, or for as long, as the glider with
closed airbrakes, and will therefore have a lower speed (remember we are
talking about moving the stick forward by a given amount).
Basically what this results in is, again as you know, the glider feeling
less responsive in pitch if the airbrakes are open - you have to push
forward more with the brakes open in order to accelerate at the same rate
as with the brakes closed, and if you want to fly at a given airspeed the
nose will be lower with airbrakes open than if they were closed. This is
why the glider is more stable in pitch, or if you like less responsive in
pitch, when the airbrakes are open.

I'm not entirely sure any of that made sense, but I hope it helps!

Best,
JK.

On Thu, 31 Jan 2013 20:42:53 -0800, 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"

I've not noticed that a Grob G.103a is any less stable in pitch than, say
an ASK.21 or a Puchacz regardless of airbrake setting, but all three do
have different responses to *opening* the brakes. This might cause an ab
initio or a new solo pilot some problems when moving between the types.

- the ASK.21 has almost no attitude change and not much airspeed change
when the brakes are opened

- the Puchacz has very powerful, speed-limiting brakes and a tendency to
pitch up and so will show a noticable drop in airspeed when they are
opened. In consequence you soon learn to push the stick forward as you
open the brakes.

- the Grob G.103a has the opposite reaction: when you open the airbrakes
it has a tendency to drop its nose and accelerate. I had speed control
issues when I first flew one (pre-solo). So, at altitude we trimmed it
to 60 kts hands off and, still hands off, opened the brakes fully. The
glider lowered its nose and accelerated to almost 70 kts. Closing the
brakes caused the nose to rise and the airspeed to settle back to 60
kts. After this demo I took note and found its speed control much
easier to manage.

I do wonder, though, if opening the brakes suddenly[*] on a G.103 after a
too fast approach followed by a not fully held-off landing could tap the
front wheel on the runway. This could easily start a PIO in the hands of
an inexperienced pilot. However if this is the case I'd say the PIO was
due to the fast approach, lack of properly held-off landing and
inexperience on type for the PIO rather than any lack of pitch stability.
[*] not forgetting that the G103 brakes do have a tendency to snatch
open, especially with the airspeed above 60 kts.

My club's fleet contains all three types. My first 20 launches were
equally split between the ASK21 and G103a. I first met the Puchacz on my
21st launch. All these were winch launches in case you're wondering.



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

I witnessed a PIO by a Grob 103 at our club a couple of weeks ago - it resulted from an attempted landing at too high a speed with first contact by the nosewheel and, as far as I remember, either no or very little applied airbrake.

One key point I learned decades ago is to first deploy airbrakes on downwind, checking visually that they are both deployed. If there is a malfunction or sudden deployment, this give you time and altitude to deal with it. Then, you keep them deployed, modulating them as necessary to control glide slope. The same instructor who taught me this also advised that I keep a bit more speed until on final. This approach seems to work for me and I haven't had a seriously bad landing in thirty years - even on land-outs.

Mike

On Saturday, February 2, 2013 11:12:20 AM UTC-6, Mike the Strike wrote:
I witnessed a PIO by a Grob 103 at our club a couple of weeks ago - it resulted from an attempted landing at too high a speed with first contact by the nosewheel and, as far as I remember, either no or very little applied airbrake.

Mike, could it have been a practice "no-spoiler" approach? I know that the instructors at our club teach this in our Grob-103 - which I personally think is dumb as a sack of hammers.

Again, a legacy of 2-33 training? I remember having to demonstrate no-spoiler, slip to a landing in 2-33s, which are really not a big deal - but in a glass ship - a real, no spoiler to touchdown approach is a serious emergency unless you have a really long runway, and not to be taken lightly. Done incorrectly, you run the risk of being too low on final with no way of making the runway, or way too fast over the threshold with no possibility of going around. Manageable if smart abort criteria are briefed, but can result in a broken glider if not done carefully.

And, it's not even necessary in the PTS, which calls for demonstrating slips, not no-spoiler landings, I believe.

Kirk
66

On 2/2/2013 12:26 PM, kirk.stant wrote:

..... I remember having to demonstrate no-spoiler, slip to a landing in 2-33s, which are really not a big deal - but in a glass ship - a real, no spoiler to touchdown approach is a serious emergency unless you have a really long runway, and not to be taken lightly. Done incorrectly, you run the risk of being too low on final with no way of making the runway, or way too fast over the threshold with no possibility of going around. Manageable if smart abort criteria are briefed, but can result in a broken glider if not done carefully.

And, it's not even necessary in the PTS, which calls for demonstrating slips, not no-spoiler landings, I believe.


http://www.faa.gov/training_testing/...-S-8081-22.pdf

R. TASK: SLIPS TO LANDING
REFERENCES: Soaring Flight Manual, Glider Flight Manual.
Objective. To determine that the applicant:
1. Exhibits knowledge of the elements related to forward, side,
and turning slips to landing, with and without the use of drag
devices.
2. Recognizes the situation where a slip should be used to land
in a desired area.

I read the FAA PTS requirement as demonstrating the use of slips to steepen the approach, not as requiring landing without using drag devices.

Kirk
66

On Saturday, February 2, 2013 11:32:46 AM UTC-7, kirk.stant wrote:
I read the FAA PTS requirement as demonstrating the use of slips to steepen the approach, not as requiring landing without using drag devices.



Kirk

66

Yep, Most DPE's have figured out a 'slip to landing' is a 2-33 specific maneuver and it's insane to try it in a slippery glass glider. I know of a DG505 which was totaled attempting it.

On Sat, 02 Feb 2013 09:12:20 -0800, Mike the Strike wrote:

I witnessed a PIO by a Grob 103 at our club a couple of weeks ago - it
resulted from an attempted landing at too high a speed with first
contact by the nosewheel and, as far as I remember, either no or very
little applied airbrake.

Unless I misunderstood you, first contact by the nosewheel as you
described indicates a failure to round out. In which case:

(1) its pilot error pure and simple

(2) you'd expect that it would break something due to either a
PIO or mashing the nosewheel up into the fuselage.

I'd expect the same result regardless of whether the glider was a G.103
or an ASK-21.


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

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


I have flown a G103 II Acro in Belgium and Germany many times and in lots
of different conditions. It is a common 2 seat trainer and cross country
machine in Europe. Its stability is legendary. I have never heard of anyone
having a PIO or experienced one myself. I would assess it as the glider
type that is least likely to have a PIO. (I fly an all flying tail Std
Cirrus). As the article states, a proper approach speed, a well held off
landing and all will be well. I you do have a bounce then hold the
controls steady as you would for any other glider, certainly don't adjust
the air brakes and let the air speed decay until you touch down properly.
Gavin
Std Cirrus, G-SCNN, #173
LSV Viersen, Keiheuvel, Belgium

In Europe standard practice is to teach fully held off landings, touching down just a fraction over stall speed in a 2 point attitude. Do the incidents referred to result from teaching a 'flown on' landing?

The fully held off approach results in far less energy to do damage if you hit a bump in an off field landing, or to generate a bounce. My flight manuals recommend it. I think there is debate about which technique involves greater risk of a PIO.