Safety of winch launch vrs. aero tow?

You've just described a cross-control stall. I think Eric's point was
that the additional drag at the wingtip wasn't necessary to initiate
auto rotation. The control inputs you've described are counter
intuitive. Is this a peculiarity of the Blanik? I only have a couple
of flights in them.

Typically, shallow banked turns like to roll level, especially if
there is any tendency to slip (dihedral). In most of the models I've
flown, overbanking doesn't become noticeable until you reach 30+
degrees.

Bruce Hoult wrote in message ...
In article ,
(Chris OCallaghan) wrote:

Point of interest: did you let the spin fully develop after the
coordinated turning stall? There is an aerodynamic tipping point --
that is the self-righting tendency of the tail that would typically
favor a spiral over a spin assuming that the only deflected control
surface was the elevator. Of course a wing drops when in a turning
stall, but without aileron deflection generating drag my guess would
be that designed yaw stability would prevent spin development.

Even with the string in the middle, the elevator will *not* be the only
deflected control surface.

The Blanik makes this very obvious. As you slow down in a shallow turn
(10 degrees, say) you need more and more out of turn aileron in order to
prevent the turn from steepening, and you need more and more into turn
rudder to keep the string in the middle. Both controls can get a
significant way towards their limits in what seems like a perfectly
normal turn. When the inner wing eventually stalls everything is
perfectly set up for a rapid departure and spin.

-- Bruce

Your original post suggested a coordinated turn. See my response
regarding crossed controls. It sounds like the Blanik has an odd
tendency to want to overbank from a shallow turn, leading to an
aileron/rudder deflection that establishes the drag profile needed to
enter a fully developed spin.

In a turning stall, there is a tendency to rotate at stall break due
to assymetric drag. However, if there is no aileron input to aggrevate
the situation, the glider will typically drop its nose (lowering angle
of attack) and gain speed. This lowers the drag on the low wing and
envigorates the self-righting tendency of the vertical stabilizer.
This is why the first second or two after stall break scream SPIN, but
in fact the glider has self recovered and is now in a spiral.

Hopefully we'll have enough altitude to play with this today. Dr. Jack
isn't very optimistic. I might even get access to a CAP Blanik over
the next few weeks. I'll be interested to see how it behaves.


Eric Greenwell wrote in message ...
Chris OCallaghan wrote:
Eric,

Point of interest: did you let the spin fully develop after the
coordinated turning stall?

No, but there didn't seem to be any need to, as the inner wing dropped
and rotation began.

There is an aerodynamic tipping point --
that is the self-righting tendency of the tail that would typically
favor a spiral over a spin assuming that the only deflected control
surface was the elevator. Of course a wing drops when in a turning
stall, but without aileron deflection generating drag my guess would
be that designed yaw stability would prevent spin development.


As Mark points out, the ailerons on the Blanik are significantly
deflected with "down" aileron on the inner wing, which is part of the
reason the inner wing stalls first. They are also deflected this way on
other gliders, but perhaps not as much.

There is a significant difference in the assymetric drag profile with
and without aileron deflection. Remember that most modern aircraft
begin their stall at the root.

At least, for a straight ahead stall. I don't think this is true for
many gliders in a turn.

That means less torque and less
disposition to overpower yaw stability and enter a spin. Slapping an
aileron down to pick up the low wing adds significat drag at the tip.
Add some rudder (cross-controls), and now you have a greater
disposition to get the aircraft spinning rather than spiralling.

I'll give this a try over the weekend -- that is, making no recovery
to a coordinated turning stall to see how it develops. My Ventus spins
happily if aggrevated. It should prove a good test bed.

We await your report with interest!

We're taught (in fact, it's hammered into us) to recover immediately
from the insipient phase of any stall. Hanging in there to allow the
condition to fully develop is an exercise that takes practice, and
probably one that you don't want to get too used to.

Given the confusion of your asi, the next best way to differentiate is
by g load. In a spin, the loading quickly stabilizes to 1.5 to 2g. In
a spiral dive it builds quickly beyond this. Visually, the yaw string
goes right over in a spin, but since a spiral dive needn't be
coordinated, this too can be confusing.

Spinning or spiral diving are both unusual maneuvers. Because of that,
each of us will perceive them a little differently, based on our
personal idiosyncracies. For most of us, a canopy full of mother earth
screams acceleration, overpowering any other cues.

I am reminded of an experience I relive at least once every winter:
the first application of brakes on ice. When I step on the brakes, I
expect the reassurance of weight into my shoulder belt. When that
doesn't happen, I get the oddest feeling that instead of decelerating
I am accelerating, which, of course, makes me want to mash the brake
pedal down even harder. It takes a second or two to break through the
misperception and get my foot back up off the brake. I suspect that we
are all subject to varying degrees of a similar effect when we explore
parts of the envelope we don't often visit. Practice makes perfect,
but why do we need to be perfect unless we're aerobatic pilots.

We should focus instead on the insipient phase. Much more subtle, but
much more valuable in gleaning out every last ounce of performance
when it counts most.




(Mark James Boyd) wrote in message news:3fac007b$1@darkstar...
Most pilots instinctively recover long before they can
tell the difference between a stall -- recovery -- spiral dive
scenario and a stall -- spin. This often causes confusion about which
is which.

I personally intentionally tried a spin entry once in a glass
glider and got a surprise and made an immediate spin recovery.

It seems the airspeed indicator rotates all the way around, so
80 knots indicated is the same as 20 knots indicated.

Imagine my surprise when the glider stalls, the nose drops,
and the ASI wobbles and then indicates ???
I tried it a few more times and by god could never
tell the difference, so I was too scared to do
anything but recover immediately (release the cross-controlled
inputs). Whichever it was, the glider sure picked up
speed like lightning when nose down.

I still wonder if this killed the Nimbus4DM pilots in Reno.
Imagine looking at the ASI and not knowing if
you should be doing a spin recovery or a spiral recovery
(two very different things).

On 7 Nov 2003 23:40:26 -0800, (Slingsby)
wrote:

I still wonder if this killed the Nimbus4DM pilots in Reno.
Imagine looking at the ASI and not knowing if
you should be doing a spin recovery or a spiral recovery
(two very different things).
************************************************* *******************************
What really killed them were wings which, by design, are only good for
3.5 g (+50% if the glue holds) when you get into a stall/spin
situation.


The official conclusion sounds a little different:

The maximum maneuvering load factor limits (in units of gravity or
g's) change with variations in glider speed and flap/airbrake
configuration. From a "flaps up" configuration at Va to the condition
of airbrakes and flaps extended at Vne, the maximum maneuvering load
factor limits decrease from positive 5.3 to a positive 3.5.

In other words: If the pilots had not extended the airbrakes, the
Nimbus would not have disintegrated.

This is what NTSB thinks about what killed them:

The National Transportation Safety Board determines that the probable
cause of this accident was the pilot's excessive use of the elevator
control during recovery from an inadvertently entered spin and/or
spiral dive during which the glider exceeded the maximum permissible
speed, which resulted in the overload failure of the wings at loadings
beyond the structure's ultimate design loads.

Note the term "at loadings beyond the structure's ultimate design
loads".




Bye
Andreas

Chris OCallaghan wrote:
You've just described a cross-control stall. I think Eric's point was
that the additional drag at the wingtip wasn't necessary to initiate
auto rotation. The control inputs you've described are counter
intuitive. Is this a peculiarity of the Blanik? I only have a couple
of flights in them.

Bruce's point, and mine, is that the controls ARE crossed in a
coordinated turn, and the crossing becomes greater as you slow down.
This is not a peculiarity of the Blanik, but is true for all the 20+
gliders I've flown.


Typically, shallow banked turns like to roll level, especially if
there is any tendency to slip (dihedral).

Yes, a slip will tend to roll level, but then you are not flying a
coordinated turn. A slipping turn will allow you to use less "top
aileron" (stick opposite the turn).

In most of the models I've
flown, overbanking doesn't become noticeable until you reach 30+
degrees.

Try it again at 15 to 20 degrees, and notice the control position as you
slow down. Top aileron becomes more pronounced as you near stall, but
it's there even at the usual thermalling speeds.


Bruce Hoult wrote in message ...

snip


Even with the string in the middle, the elevator will *not* be the only
deflected control surface.

The Blanik makes this very obvious. As you slow down in a shallow turn
(10 degrees, say) you need more and more out of turn aileron in order to
prevent the turn from steepening, and you need more and more into turn
rudder to keep the string in the middle. Both controls can get a
significant way towards their limits in what seems like a perfectly
normal turn. When the inner wing eventually stalls everything is
perfectly set up for a rapid departure and spin.

-- Bruce

About the Minden accident on 13 July 1999 to a Nimbus 4DM (LAX99MA251
http://www.ntsb.gov/publictn/2002/AAB0206.htm ).

Note that at the time the NTSB report was published there was discussion
about it on RAS. One of the things reported by posters with experience of
the Nimbus 3/4 models (I have none) was that the airbrakes have been known
to deploy uncommanded by the pilot. So the brakes may have deployed
themselves, and it is possible that this is what killed the pilots.

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


"Andreas Maurer" wrote in message
...


On 7 Nov 2003 23:40:26 -0800, (Slingsby)
wrote:

I still wonder if this killed the Nimbus4DM pilots in Reno.
Imagine looking at the ASI and not knowing if
you should be doing a spin recovery or a spiral recovery
(two very different things).


What really killed them were wings which, by design, are only good for
3.5 g (+50% if the glue holds) when you get into a stall/spin
situation.


The official conclusion sounds a little different:

The maximum manoeuvring load factor limits (in units of gravity or
g's) change with variations in glider speed and flap/airbrake
configuration. From a "flaps up" configuration at Va to the condition
of airbrakes and flaps extended at Vne, the maximum manoeuvring load
factor limits decrease from positive 5.3 to a positive 3.5.

In other words: If the pilots had not extended the airbrakes, the
Nimbus would not have disintegrated.

This is what NTSB thinks about what killed them:

The National Transportation Safety Board determines that the probable
cause of this accident was the pilot's excessive use of the elevator
control during recovery from an inadvertently entered spin and/or
spiral dive during which the glider exceeded the maximum permissible
speed, which resulted in the overload failure of the wings at loadings
beyond the structure's ultimate design loads.

Note the term "at loadings beyond the structure's ultimate design
loads".

Bye
Andreas

Chris OCallaghan wrote:

Your original post suggested a coordinated turn. See my response
regarding crossed controls. It sounds like the Blanik has an odd
tendency to want to overbank from a shallow turn, leading to an
aileron/rudder deflection that establishes the drag profile needed to
enter a fully developed spin.

It is not an oddity of the Blanik (in fact, it is an excellent trainer
for spins, because it does them so normally), but a consequence of a
coordinated turn. Because the inner wing is traveling more slowly than
the outer wing, it must have a higher lift coefficent to develop the
same lift as the outer wing. It achieves this with a downward deflected
aileron (and flap in many flapped gliders). This downward deflected
aileron produces a wing tip that stalls at a lower angle of attack than
the outer wingtip, which has an upward deflected wing tip.


In a turning stall, there is a tendency to rotate at stall break due
to assymetric drag.

The asymetric drag is due to a partially stalled inner wing, and
generally in the aileron area.

However, if there is no aileron input to aggrevate
the situation, the glider will typically drop its nose (lowering angle
of attack) and gain speed.

This is true, and is the reason the spin recovery includes centralizing
the ailerons. On some gliders, this is enough to unstall the inner wing
tip, and stop the incipient spin.

This lowers the drag on the low wing and
envigorates the self-righting tendency of the vertical stabilizer.

I don't think "lowers the drag" is the best way to think of this, but
instead, think of it as unstalling the wing tip (of course, a stalled
wing tip does have higher drag than the unstalled one). No stall, no spin.

This is why the first second or two after stall break scream SPIN, but
in fact the glider has self recovered and is now in a spiral.

The situation Bruce and I describe has no discernible stall "break". THe
inner wing begins to drop, and it can't be held up with the aileron. If
the pilot doesn't recognize this is a spin entry, he will continue
adding top aileron, which deepens the stall on the inner wing tip, and
very quickly has a fully developed spin. There never is a "break", as
you get with a straight ahead stall, but a smooth entry into a spin.

November 9, 2003
Turning Stalls and Insipient Spins

As promised, apropos to this discussion on spin entry from coordinated
turning stalls, I took a tow this morning to 5000 feet agl and
performed a series of coordinated and cross control turning stalls.

The aircraft used was a Ventus 2bx, delivered this year. I have
approximately 75 hours in this aircraft and about 525 hours total in
the model. I flew the glider at approximately 70% of the aft cg limit.
Wing loading was 7.8 lbs per square foot. All stalls were entered in
the first positive flap position.

My intention was as follows: to perform a series of turning stalls,
both coordinated and cross controlled, to determine the departure and
post departure characteristics of a modern fiberglass sailplane.
Stalls were entered gently and in a shallow bank (lower wingtip on
horizon). Whether coordinated or cross controlled, I fixed the
controls in the pre-departure position for three full seconds after
departure (that is, no attempt was made to recover immediately after
the stall break).

Once off tow I completed two clearing turns, then stalled the glider
wings level twice to establish attitude. I then entered a coordinated
shallow left turn and gently eased back on the stick. The stall broke
cleanly. The glider initially yawed about 30 degrees to the left,
dropped its nose through the horizon, then began to increase its bank
angle and gain speed. G forces accumulated and I recovered from the
spiral dive at about 80 knots and roughly 70 degrees of bank. (As
noted above, the elevator was held firmly aft and aileron and rudder
neutral until recovery was initiated.

I repeated the same maneuver to the right. The stall break was less
clean (more mushy). Development of the ensuing spiral dive was slower,
but airspeed and bank angle both accumulated until I released the
controls and initiated a recovery.

I repeated this sequence with like results.

I then entered a shallow bank turning stall (left) while skidding
slightly. As the low wing began to drop, I applied about ½ stick
travel to the right, ostensibly to raise the dropping wing. Entry into
the spin was immediate and dramatic. The glider yawed approximately
ninety degrees while dropping it nose to about 60 degrees below the
horizon. I left the controls in this position for a count of three
(one one thousand, two one thousand…) The glider completed
approximately 1.25 rotations before I initiated a recovery (stick
forward, ailerons neutral, opposite rudder, pull up from dive).

I repeated this process to the right. However, this time, I gently
accelerated the stall (achieving a slightly higher nose attitude
before departure). Once again, I skidded the turn (10 to 20 degrees),
and tried to pick up the low wing as it stalled, this time with full
deflection of the aileron. The ensuing spin entry was even more
dramatic. I was unable to measure rotation rate (even roughly) because
the glider's nose went immediately past vertical. As I lost the
horizon I became disoriented, until I looked out at the wingtip and
found the horizon again. I nonetheless fixed the controls for a count
of three. There was no noticeable g build up until I initiated a spin
recovery. Max speed during the dive was just above 120 knots, about 20
knots more than I typically see for a recovery from a fully developed
spin.

It should be noted that my glider has a flap redline of 80 knots. In
all cases, if airspeed exceeded 80 knots, I moved the flap handle to
the first negative position.

My interpretation: while the glider exhibited a yawing motion during
the coordinated turning stall, it did not auto rotate, nor did it show
any such propensity. Some pilots may find the dropping wing, yaw
motion, and reduced g force of a coordinated turning stall
disquieting, but when compared in sequence to an actual autorotation
leading to a fully developed spin, the prior is patently docile.
Height loss after an immediate recovery from a coordinated turning
stall using a release of back pressure and coordinated ailerons and
rudder could be measured in 10s of feet. The spin, however, from entry
to the bottom of the dive recovery was measured in hundreds. Loss of
height for the first spin, from entry, through development, to the
bottom of the recovery dive was 475 feet. The second: 750 feet.

Conclusions: draw your own.

I wish I could say something interesting about the task we flew in the
afternoon (blue, weak, and manky), but what I've described above was
the highlight of my Sunday.

Todd Pattist wrote:

In other words: If the pilots had not extended the airbrakes, the
Nimbus would not have disintegrated.


I don't think this is quite the correct way to look at it.
It implies that opening the brakes was the direct cause. I
don't see it that way.

Each part of the wing can produce a certain lifting force.
The g-force you feel is the result of the total lifting
force produced - applied to the mass of the glider.
Opening the brakes prevents portions of the wing from
producing their share of the lifting force. For structural
reasons, the remaining parts (tips especially) cannot safely
produce any more lift than they were producing before the
brakes were opened, so the total lift force is reduced and
the g-force drops automatically. Thus, it's not so much
opening the brakes that breaks the wings, it's the use of
the elevator to increase the AOA after the brakes are opened
to try to hold the higher g-force.

I think this may not be a correct analysis. In my ASW 20, during steady
straight flight, I could open the spoilers while holding the stick
steady. The glider would maintain it's attitude, but begin sinking. The
G force was reduced very momentarily, then returned to 1 G. The wing
tips would bend upwards, indicating they were producing additional lift.
The additional sink rate increased the angle of attack of the wing, and
this caused the additional loading on the wing tips. In other words, the
lift tends to shift to the wing tips without the pilot doing anything
besides opening the spoilers.

I haven't tried it, but I assume this would also happen in a 3 G turn.
If true, the only way to avoid exceeding the "open spoiler G limit"
would be reduce the G loading before opening the spoilers.

Andreas Maurer wrote in message . ..
On 7 Nov 2003 23:40:26 -0800, (Slingsby)
wrote:

I still wonder if this killed the Nimbus4DM pilots in Reno.
Imagine looking at the ASI and not knowing if
you should be doing a spin recovery or a spiral recovery
(two very different things).
************************************************* *******************************
What really killed them were wings which, by design, are only good for
3.5 g (+50% if the glue holds) when you get into a stall/spin
situation.


The official conclusion sounds a little different:

The maximum maneuvering load factor limits (in units of gravity or
g's) change with variations in glider speed and flap/airbrake
configuration. From a "flaps up" configuration at Va to the condition
of airbrakes and flaps extended at Vne, the maximum maneuvering load
factor limits decrease from positive 5.3 to a positive 3.5.

Right. By design they are ONLY good for 3.5 g. Exceed that amount by a paltry 50% and the wings WILL snap off like toothpicks. Guaranteed. They will, and did, snap off together. Both wings were equally weak by design and construction technique AND, they used enough glue.

In other words: If the pilots had not extended the airbrakes, the
Nimbus would not have disintegrated.

This is what NTSB thinks about what killed them:

The National Transportation Safety Board determines that the probable
cause of this accident was the pilot's excessive use of the elevator
control during recovery from an inadvertently entered spin and/or
spiral dive during which the glider exceeded the maximum permissible
speed, which resulted in the overload failure of the wings at loadings
beyond the structure's ultimate design loads.

Remove the word "excessive" and the description becomes more realistic.

Note the term "at loadings beyond the structure's ultimate design
loads".

Bye
Andreas