Safety of winch launch vrs. aero tow?

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!

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!

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.