"Hands off" decent from altitude with spoilers?

In article ,
(MHende6388) wrote:

I was taught during wave training that all sailplanes, properly trimmed, when
you let go of the controls, will seek a gentle banked circle to one direction
and maintain that attitude, even in light turbulence.

There must be great variation from one a/c to the next with varied CG and trim
settings.

Pretty much *any* aircraft should do this, not only gliders. The only
requirements are that a) the airframe not be *too* out of whack, and b)
there being sufficient drag available to allow a reasonable nose-down
attitude without a large speed increase.

The key is that the hands-off roll rate is probably non-zero, and that
would eventually turn you upside down. A significant nose-down attitude
allows roll rate to be coupled into turn rate and the whole thing
stabilizes when the degrees per second of turn matches the degrees per
second of roll.

So trim for a slowish speed (trim right back unless that is going to
stall you) and put out all the drag you can manage: spoilers, wheel,
flaps.

Preferably try it in the aircraft you fly before you actually need it.

-- Bruce

Bruce Hoult wrote:

In article ,
(MHende6388) wrote:

I was taught during wave training that all sailplanes, properly trimmed, when
you let go of the controls, will seek a gentle banked circle to one direction
and maintain that attitude, even in light turbulence.

There must be great variation from one a/c to the next with varied CG and trim
settings.

Pretty much *any* aircraft should do this, not only gliders. The only
requirements are that a) the airframe not be *too* out of whack, and b)
there being sufficient drag available to allow a reasonable nose-down
attitude without a large speed increase.

The key is that the hands-off roll rate is probably non-zero, and that
would eventually turn you upside down. A significant nose-down attitude
allows roll rate to be coupled into turn rate and the whole thing
stabilizes when the degrees per second of turn matches the degrees per
second of roll.

So trim for a slowish speed (trim right back unless that is going to
stall you) and put out all the drag you can manage: spoilers, wheel,
flaps.

Preferably try it in the aircraft you fly before you actually need it.

-- Bruce

Beside providing drag, spoilers or airbrakes, as opposite to some others drag
sources like the wheel, as they are far from the longitudinal axis, provide
a mean of countering and damping yaw movements, as soon as there is some yaw,
the difference of drag on both sides tend to cancel it. With some dihedral,
which converts slip into roll, this will cancel the spiral instability that
most gliders exhibit in sleek configuration. The best ones for this should
be gliders with air brakes extending far from fuselage and appreciable dihedral.
The LS4 comes in my mind as such an example.

BTW I wonder why the standard procedure, as it was also taught to me, is to
let go the controls rather than block them in the approriate position, i.e
neutral for ailerons and rudder, trimmed position for elevator. This should
provide a better damping of oscillations than free controls. Maybe the idea
is that it would be to difficult to resist to do some blind corrections, which
would probably be in the wrong direction. Any opinion on the subject?

GDay,
Have been in that position in an L13 (Blanik) in lee wave, climbed to
top of the roll cloud 10000 amsl, foehn gap about 150 metres wide. Wind
stopped. Foehn gap closed. Cloud base 2000 agl. Had sighted ground and
knew position relative to known hills. Extended brakes fully, trimmed
for 30 deg down and popped out the bottom with no fuss.
Very forgiving aeroplane the Blanik. Can verify previous reference to
speed of closure of gap.
Cheers
Henell


--
henell
------------------------------------------------------------------------
Posted via OziPilots Online [ http://www.OziPilotsOnline.com.au ]
- A website for Australian Pilots regardless of when, why, or what they fly -