How Low to Spin??

Derek Piggott has written as follows:

"I think lots of people still think that pro-spin controls means having a
lot of rudder or aileron on and don't realise that the important thing is
the stick position. If the stick is well back, spinable machines spin:
without the stick being back they don't spin.

"I don't need to tell you that many other gliders will spin a turn or two if
the stick is kept back on the stop, the c.g. is well aft and a wing drops,
even if the aileron and rudder are still central."

Even if the pilot coordinates perfectly, and string and ball remain exactly
central, a gust or turbulence may cause enough asymmetry to start a wing
drop. Gustiness, gradient, shear and turbulence are particularly likely
close to the ground.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"Chris OCallaghan" wrote in message
om...

snip

BTW, as I noted in another thread, spins are not caused by lack of
airspeed, but uncoordinated use of the controls -- at least in modern
sailplanes. Two things must happen to enter a spin: 1) you must
stall, and 2) you must fail to apply sufficient rudder during your
attempt to pick up the low wing with aileron. That is, the sailplane
is designed with enough rudder to stop autorotation, even with full
deflection of the aileron throughout the stall break.

snip

Bill,

Can't say I agree, but at least from my point of view, you are erring
on the side of safety.

Here is a simple argument that I have backed up with experiment in
many types of gliders. An aircraft that is capable of spinning during
a stall while aileron and rudder are held neutral (and within
published cg limits) is inherently unsafe. This means that such a
glider flown into a strong, turbulent wind gradient 50 feet above the
ground is likely to autorotate. Since recovery from an insipient spin
requires much more altitude than a straight ahead stall, there is a
very good chance that such a glider would see very few flights before
being retired.

I have proven to myself many times that stalling a glider without
abusing the controls results not in a spin but a spiral dive. While we
can all point to experiences of having a wing drop and losing control
in a stall, I doubt very seriously that any of us were holding
coordinated controls throughout the stall break. It takes a very
determined effort not to move the stick throughout the stall and
self-recovery.

Here's another argument. The vertical stabilizer provides a great deal
of yaw stability, even at very low speeds. To start autorotation, you
need a source of drag at the tip greater than the normal differential
to be expected resulting from span effect in a turn. That we don't
kill ourselves everytime the glider approaches stall is testament to
the stability provided by the tail. That we occasionally do screw
gliders into the ground makes me think that the cause lies more in the
way we are applying the controls under stress than any inherent
tendency of the glider snap into a spin at the least external
provocation. Yes, outside factors can influence how the glider flies,
but I think they do more damage by causing pilots to react in
unacceptable ways.

Go back and read through my reports on control use during stall in my
Ventus. What it drives home in my mind is that spins are the result of
control abuse. You're right, don't stall land you won't spin. But it's
just as right to say that a stall needn't develop into a spin so long
as the controls are not abused.

That's nonsense. Spin/autrotation is all about one wing (partially) stalled,
and the other not. It's not about drag.

--
Bert Willing

ASW20 "TW"


"Chris OCallaghan" a écrit dans le message de
m...

Here's another argument. The vertical stabilizer provides a great deal
of yaw stability, even at very low speeds. To start autorotation, you
need a source of drag at the tip greater than the normal differential
to be expected resulting from span effect in a turn. That we don't

Uh, Bert, what happens when a wing stalls? Lift decreases... drag
increases. Something needs to start the spin. Could that be... a
force? I suppose we could call it something other than drag. Gremlins
maybe?

Let's see, perhaps I can offer another explanation. Non-symmetrical,
differential lift across the wingspan produces roll. Non-symmetrical,
differential drag along the wingspan produces yaw (adverse yaw when
actuating the ailerons, for example). The vertical stablizer and
rudder are there to provide stability and yaw authority to counteract
the aileron drag effect (as well as the destabilizing effect of the
fuselage forward of the cg). If the stall (or partial stall) produced
no drag, the glider would simply roll. There would be no yawing
motion. And thus, no spin! (But lots of rolling.) Here's another way
to think about it... if you had an infinitely large vertical
stabilizer (that is, infinite directional stability), would it be
possible to spin? Since the infinitely large tail would produce an
infinitely large counterforce to any adverse yaw, then a spin is not
possible. What's the practical substitute for an infinitely (or very)
large vertical stabilizer? A moveable rudder.

It's all about the flippers, man.

And from a practical standpoint, spins are all about the drag. And
even though a partially stalled wing will display adverse yaw with
neutral control surfaces, so long as you don't move the flippers, the
vertical stabilizer will keep you from spinning. As noted before, I
prove this to myself with every modern model of glider I fly. But if
you move those flippers in an uncoordinated fashion, baby, all bets
are off!

Piggott: "Drag from the badly stalled, falling wing, pulls the glider
down into a steep spiral and the autorotation is speeded up."

There's a graceful way out of your dilemma... we could discuss the
torques brought into play by the rolling motion of a partially stalled
wing. That will introduce a rotation about the yaw axis (the
aerodynamicist's definition of autorotation), but you'll need to prove
to me that it alone is sufficient to overpower the vertical
stabilizer, even at very low airspeeds and relatively high rates of
roll. Since the vast majority of modern aircraft need an additional
yawing moment to enter a spin (pro rudder, counter aileron), it's
going to be a tough sell. But I'd be interested to see you work
through the problem.

Maybe we'll both learn something new.



"Bert Willing" wrote in message ...
That's nonsense. Spin/autrotation is all about one wing (partially) stalled,
and the other not. It's not about drag.

--
Bert Willing

ASW20 "TW"


"Chris OCallaghan" a écrit dans le message de
m...

Here's another argument. The vertical stabilizer provides a great deal
of yaw stability, even at very low speeds. To start autorotation, you
need a source of drag at the tip greater than the normal differential
to be expected resulting from span effect in a turn. That we don't

BTW,

I thought I'd add that "autorotation" is why highly controllable, low
stability aircraft can be spun with ailerons held into the direction
of spin. If you can roll fast enough with stick well back, the
resulting torque about the yaw access is sufficient to overpower a too
small vertical stabilizer (in designs where stability is sacrificed
for greater controlability). But this does not describe a modern,
certified glider. And, after all, we're looking for practical
knowledge we can take into the air. But I remain interested in whether
you can demonstrate that rolling torque alone will make the glider
spin. If it can't, then we can focus on other sources of adverse yaw
that contribute to the autorotation. If it can, then we'd all best be
looking for a new, safer passtime, like freeclimbing solo.

But hey! I'm making your argument for you.

Bert, this could really be fun. Fire away, please!