Question about spoilers and pitch stability

At 15:12 02 February 2013, Martin Gregorie wrote:
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 has this right. The problem with opening the brakes on a Grob 103 is
that doing so will cause the nose to pitch down, closing the brakes will
cause the nose to pitch up, try it. You can actually fly a Grob 103 using
the spoilers to control pitch, not very well but it works. The apparent PIO
is different to the nosewheel contact and is caused by over correction,
this is especially true if the stick and airbrake lever are moved in
different directions, magnifying the effectiveness of the elevator.

The 103 airbrakes are linear in operation as regards airspeed. At 55 knots
the airbrakes will sit at roughly half deployment with no pressure on the
lever, closing if the speed is reduced and opening if it is increased. They
sit fully open with no pressure on the lever at about 60-65 knots.
On of the requirements of an airtest following service on the military
G103s in the UK was to accelerate to VNE and also operate the airbrakes at
about 70 kts in the recovery, at 70 kts the airbrakes will suck out very
quickly and over that speed the rate of uncontrolled deployment is very
rapid indeed.
As far as I am aware the airbrakes are rated for full deployment up to VNE,
probably break your wrist though when you unlock them.