How Low to Spin??

Kirk Stant wrote:
(Mark James Boyd) wrote in message news:412b9c97

Think about it for a minute. If you're going 50 knots in one direction,
and then one-half second later the nose of the glider is 180 degrees
pointed the other way, does this mean you are doing 50 knots in the other
direction? That's some G's, and I don't feel them in a spin.

And what do you mean by doing "a
complete 180 during the spin ...and is now to some degree flying
backwards." That isn't a spin, it's a frisbee! As far as G's, if you
enter a spin at low speed (not accelerated), you can't pull any G's -
as the plane unloads and goes down there is actually a decrease in
G's!

Some aircraft have a very nose low spin (Blanik), others have a much
flatter spin (Katana). The Katana, which spins very flat on the horizon,
is going North at 30 knots. I stall it and spin. Over the course of
a second, the nose is now pointed South. Is the Katana moving
South with an airspeed of 30 knots? No, it is not. This is
part of the reason why, during the first turn or two, the
pitch oscillates more violently than in a fully developed spin.

Because of momentum, the airspeed from front to back of the
wings is less during the South pointed nose part of the spin than
during the entry of North.

And yes, this is a frisbee. At least for the first 180 anyway...

This is why aircraft oscillate pitch up and down for a few turns before
stabilizing in a spin. For the first few turns, the aircraft momentum
is still slogging through the air.

I don't have a spin text handy, but I would think the oscillation is
more due to angular momentum and changing AOA as the glider rotates
around it's pitch and roll axes than from "flying backwards".

Yes, and part of this changing AOA is due to momentum in the
Northerly direction.

Even with an aft CG, any glider is fully controllable up to the spin -
it's recovering that would be interesting.

True, true. The more aft the CG, the more controllability.

It's pretty hard to get the CG that far aft (it can be done,
especially if you are light, but any sort of preflight should find it)

Ms. Campbell is the Hawaii state altitude record holder. She
worked at Dillingham as a CFI. She told me during a ground session
she was in an uncontrolled spin for more than 5,000 feet at one point,
with a passenger, before recovering.
She said after landing, she weighed the glider and the CG
was well aft of what was on the 10+ year old "official" form.
And her new calculated CG for that flight was well aft of limits.

In my experience, the older the calculation, the further back
the actual CG is from it. Dirt and crap on the long lever arm
of the tail do a lot more than crap in the short nose.

and if discovered the plane is still fully controllable - unlike a too
far forward CG that can lead to a heavy landing. Just my opinion, but
I bet there have been very few spin accidents caused by aft CGs (CG
out of the aft limit, not just at the aft limit).

Except for that 1 in 10 case, I'd guess aft CG is just a contributing
factor, not a cause. But I'd like to see data. When I hear of
a winch launch by an experienced pilot during the first
flight of the season, ending in a fatality, I have to wonder if
he took something out of the nose, or put something in the
tail, and so his stick pressure feel and initial trim setting
were off... Of the stall spin fatalities on record, I'd
bet most, if not all, had CG further back than the 60-70%
forward that Eric described...

Sounds like trying to turn via ground references down low - a big
no-no

This is required to fly a rectangular pattern with wind correction,
and still part of the PTS...

and probably the real reason for low altitude "stall-spin"
accidents.

Clearly true. If one weren't trying to land on a particular
bit of ground, and the world was just one big flat runway,
I'm certain landing accidents would be more rare.

The classic spin entry from a shallow bank is uninteresting.
I won't be jamming in the rudder for a skid at some obviously
low speed close to the ground. I think the focus on the classic case
is niave and dangerous. Yes, it's easy to teach and demonstrate,
but it ignores too much. The more complex, less discussed
spin entry is the one in the accident reports: tight pattern,
higher speed, steep bank, lots of inside rudder, pilot focussed on
keeping the yaw string straight, quite a bit of opposite aileron
in the steep bank, in vertigo, pulling stick back to tighten up the
turn, and then wham! I'll look back through the accident reports, but
the ones I recall, and the B-52 and the DG spin I saw on video, involved
stabilized, 30-45 deg bank turns before each of the spins. In each,
it looked like the craft was overbanking, and the pilot put in more
opposite aileron and more elevator and WHAM! Instant spin...

Again, you are describing a pilot who has no clue how to fly his
glider.

Hard to quiz them, the dead are VERY quiet...

A stabilized steep turn doesn't call for a lot of inside
rudder.

Many of the 10 reports seem to indicate the spirals/spins happened
during the roll, not the turn. High roll rates require a lot of
rudder (and then rudder release), used quite precisely.

And I am a bit confused by your reference to vertigo -
again, this is avoidable (don't stare at the ground, no rapid head
movements, etc) and should be taught.

I commonly induce vertigo in students to demonstrate unusual attitude
recovery. Although easiest to induce by rapid head movements,
I can also induce it with nothing more than a rapid, perfectly
coordinated roll into a steep bank, and then a rapid coordinated
roll to level flight. I've done this with pilots from 10-30,000 hours.
In all of them, if I cover all the instruments on a nice dark night
with foggles on, they get vertigo. Not staring at the ground and
avoiding rapid head movements is a start, but is an incomplete
solution...rapid roll rates and dramatic G changes are another
factor.

When I fly gliders, I have to remind myself to fly at least a 1/4 mile
out pattern. I normally fly a power plane (day VFR only) with a 5:1
glide ratio, and a tight pattern, with steeper banks and
faster roll rates at higher airspeed. I don't do this when
in a glider approaching an unmarked landout field with mountains and
no horizon around.

If a pilot continually gets
vertigo in steep turns (and I have some really good friends who do,
unfortunately) they need to seriously consider the ramifications of it
and fly accordingly!

I see we are agreeing again
--

------------+
Mark Boyd
Avenal, California, USA