Unintentional fully-developed spins...

Convincing story "F1y1n" and I agree with most of it.

However when banking, the loadfactor of the glider increases. E.g. at a bank
angle of 45° the loadfactor is 1.41. The result of this is that the polar
diagram of the glider moves with a factor sqrt(1.41) = 1.19 to the right and
down. So when one likes to stay clear of stalling speeds for what ever part
of the glider, flying speed should go up by 19% when banking 45° compared to
the "normal" flying speed without banking. "Normal" meaning something like
flying with an IAS where sink rate is minimum or may be a little faster to
ease control of the glider.

Karel, NL
Ventus-2cxT


"F1y1n" schreef in bericht
om...
The point I'm replying to is:

I'm convinced that in very long wing gliders at
high angles of bank and slow speeds (and ergo light weights too),
the inner wing is significantly slower than the
outer wing, and tacking on some knots is most
efficient (to keep the length of the inner wing nicely above stall)...

I grant you that the AOA is slightly higher for the inner wing due to
the contribution from the sink, but this is negligible. Consider a
45deg bank, 45 knots. The turn radius (at the fuselage) is about 50
meters, so for a 15-m glider the speed of the outer wingtip is about
50knots, and the speed of the inner wingtip is about 40knots. If the
sink rate in this configuration is 1.5 knots, the difference in AOA
for the two wingtips is about 0.4 degrees. You will notice that (for a
good reason!) this is much less than the typical twist of a wing. You
cannot stall the inner wingtip in a steep turn without stalling both
wing roots first! For the same reason, the inner wingtip is NEVER on
the back side of the polar when thermaling. If it was the wing roots
would already be stalled.

To answer the original question - should one speed up when thermalling
with a steep bank - the answer is no. There are too many factors that
come into play - the twist of the wing as a function of position, the
wing profile as a function of position, the drag produced by the
aileron deflection needed to correct for the overbanking tendency as a
function of speed, and so on. In the end, these effects will tend to
cancel each other: if you speed up a little to bring the wing roots to
the front side of the polar you will a) create more drag on the wing
tips and b) need more aileron input to correct for the overbanking
torque and hence create more drag. I suspect that amount by which one
should speed up or slow down to optimize the sink rate in theory will
be much smaller than the speed of the turbulent currents in the
thermal, and thus utterly irrelevant in practice. Your time will be
better spent flying cleanly and in the core of the thermal rather than
trying to nail the speed to within 0.2 knots.


(Chris OCallaghan) wrote in message
. com...
You are confusing AOA with sink rate. The sink rate is the same across
the airfoil, but AOA is dependent on sink rate and forward speed, so:

If an airfoil has a forward motion of 10 and sink rate of one, then
its angle of attack can be measured -- about 5.7 degrees. If we then
slowed its forward speed to 9 while maintaining a sink rate of 1, the
angle of attack would be higher: 6.3 degrees.

We agree that the angular speed is the same across the span. We agree
"that the inner wing is flying slower." The sink rate is the same
across the span. As you've stated, this is a given: the wings are
fixed to one another. Since AOA is dependent on both sink rate and
forward speed, then the inside wingtip must have a higher AOA.

Inner wing slower, higher AOA. Outer wing faster, lower AOA. Lift is
dependent on both AOA and speed. So even though the outer wing is at a
lower angle of attack, it is moving through the air more rapidly, and
producing slightly more lift than the inner wing. With resulting
overbanking tendency.

Balance this knowledge against the sailplane's response to a turning
stall. Inner wingtip typically drops first. Why? Because it has a
higher AOA. No aggrevation from the aileron required.

I was taking issue with an oversimplification, one you made for the
sake of convenience, I'm sure. Pet peeve of mine.

In an earlier thread on slips, I asked whether anyone had thought
through the notion of using a slip (dihedral rolling moment)to
counteract the overbanking tendency . I didn't note any responses,
though it might prove interesting to develop the idea in the context
of this thread.

Dodging the math, I'll also add that a few extra knots while circling
gives a great deal more aileron and rudder authority. Since smooth,
elevator cores are the exception rather than the rule, the more
effective your controls, the quicker you can correct for or take
advantage of turbulence, then get the controls streamlined (or as
close a practical) to minimize drag. The lower the speed, the greater
the drag for a given control input, and the longer you'll have to
leave it in to achieve the desired change in direction.

Cheers,

Chris

F1y1n wrote:
The point I'm replying to is:

I'm convinced that in very long wing gliders at
high angles of bank and slow speeds (and ergo light weights too),
the inner wing is significantly slower than the
outer wing, and tacking on some knots is most
efficient (to keep the length of the inner wing nicely above stall)...

I grant you that the AOA is slightly higher for the inner wing due to
the contribution from the sink, but this is negligible. Consider a
45deg bank, 45 knots. The turn radius (at the fuselage) is about 50
meters, so for a 15-m glider the speed of the outer wingtip is about
50knots, and the speed of the inner wingtip is about 40knots. If the

snip

thermal, and thus utterly irrelevant in practice. Your time will be
better spent flying cleanly and in the core of the thermal rather than
trying to nail the speed to within 0.2 knots.

I thank you for taking the time to do some math, but
unfortunately I don't have quite the time to verify it,
and I've two references (Carl Herold and Dick Johnson)
who seem to encourage what I've suggested.

And my math must be poor. If a glider has 2 knots of sink
near stall, and stalls at about 40 knots, then straight it has
an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't
be right...

I'll need to look closer at this, but I certainly hope at least
you gained something for yourself from this discussion too...

In article 40289ab7@darkstar, (Mark James Boyd)
wrote:

And my math must be poor. If a glider has 2 knots of sink
near stall, and stalls at about 40 knots, then straight it has
an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't
be right...

No, 3 degrees (down) is its flight path. The AOA could be anything at
all, depending on how far up the nose is (and the rigging incidence).

-- Bruce

Earlier, (F1y1n) wrote:

...The angle of attack of both wings is the
same. The air is impacting both wings from
exactly the same angle - the wings are
connected by the fuselage....

I think I disagree on what might appear to be a technicality: The
wings are connected not only by the fuselage, but also by the control
system that the pilot uses to balance the lift distribution between
the two wings.

Since the inner wing is going substantially slower, and as you point
out later has only a slightly greater angle of attack, something has
to increase the Cl on the inner wing in order to keep from rolling
into the turn (the overbanking tendency). That something is the pilot
applying slight opposite aileron, increasing the inner wing's
effective angle of attack.

That may only be a couple of degrees of deflection down on the inner
wing, but its also the same or more up on the outer wing, decreasing
its effective angle of attack. The deflected surface has a substantial
effect on the pressure distribution of the airfoil in front of it, and
can make a big difference on its stall characteristics.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

You'll want to add the angle of incidence of the wing. Assume it is 15
degrees to the fuselage center line and you're getting close to
typical critical AOA. The math, in fact is a little more tenuous, but
not much. Just assume your airspeed is the path through the air. You
also have vertical speed, so you have the hypotenuse and one side.
Some trig yields horizontal speed, a little more yields AOA at the
fuselage centerline, then add AOI for AOA of the airfoil. A bit psuedo
scientific, but perfectly acceptable for the sake of the discussion.
At least it gets us numbers that look more like the stuff we see in
tables.

Robert Ehrlich wrote:
Mark James Boyd wrote:
...
And my math must be poor. If a glider has 2 knots of sink
near stall, and stalls at about 40 knots, then straight it has
an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't
be right...

call this last one incidence while in France we call this "calage"
because we use "incidence" for what you call AOA :-)

In the US, "calage" is where I would go to learn math and franch,
and how to spell patato.

By the way, I love the bread, fries, and kissing. You can keep
the dressing, horn, doors, and braids. :P

But thanks for the Statue of Liberty!

Mark James Boyd wrote:
...
And my math must be poor. If a glider has 2 knots of sink
near stall, and stalls at about 40 knots, then straight it has
an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't
be right...
...

No, 3 degrees is the angle between the airflow and an horizontal line.
In order to obtain the AOA you have to add the angle of the fuselage
axis with this horizontal line (nose up attitude) and the angle of
the wing chord with the fuselage axis, you english speaking people
call this last one incidence while in France we call this "calage"
because we use "incidence" for what you call AOA :-)

nafod40 wrote:

F1y1n wrote:

This I do not agree with. The angle of attack of both wings is the
same.

It always helps my understanding to look at limiting cases.

If you take the wingspan to an extreme, the inner wing would reach all
of the way to the center of the circle, and its airspeed would be zero.
It would be descending, though, so its AOA would be 90 degrees.
Certainly different from the outer wing.

Yes, but we are far from this limiting case. As someone else pointed,
in the case of a typical glider making a typical turn the difference
is a few tenth of degrees. This explains why it can't counteract the
effect of the speed difference and we all have experienced this
overbanking tendancy that we must counter with outside stick.

So the speed difference make a little difference in AOA but a much more
noticeable difference in lift. We cancel this difference by using
outside stick, i.e. introducing a difference in lift coefficient by
changing airfoil and AOA. But when we are sufficently close to stall AOA,
i.e. maximum lift coefficient, this change is of no help and may even have
the opposite effect if this brings the inner wing tip to a lower lift coefficient.
Then the inner wing will drop, and then, as now both wing have a significative
difference in their sink speed, this introduces a significative difference
in their AOA.

I'm with Bob on this one.

From an aerodynamics manual:

"However, many other parameters influence the lift that a wing produces. The
most basic is the configuration of the wing, specifically the position of
the trailing-edge flaps, leading-edge flaps or slats, and spoilers. As the
trailing-edge flaps are extended, the curvature (or camber) and area of the
wing are increased, and the wing will produce more lift at the same AOA
(fig. 2). Note that although the maximum lift is increased, the AOA at which
stall occurs is actually less because the wing cannot sustain the higher
lift levels up to the same AOA. The airflow separates earlier.
Wing-mounted speed brakes or spoilers have the opposite effect. They reduce
the lift at a given AOA; they also reduce the maximum lift achievable but,
surprisingly, increase the AOA at which stall occurs."

Note that, on a long winged glider with ailerons near the tips, a deflected
aileron on the down wing changes the camber of the wing in that area. With
the overbanking tendency of the down wing, there is inevitably some down
aileron on that wing (to stop the bank) and the above aerodynamics apply.

Allan


"Bob Kuykendall" wrote in message
om...
Earlier, (F1y1n) wrote:

...The angle of attack of both wings is the
same. The air is impacting both wings from
exactly the same angle - the wings are
connected by the fuselage....

I think I disagree on what might appear to be a technicality: The
wings are connected not only by the fuselage, but also by the control
system that the pilot uses to balance the lift distribution between
the two wings.

Since the inner wing is going substantially slower, and as you point
out later has only a slightly greater angle of attack, something has
to increase the Cl on the inner wing in order to keep from rolling
into the turn (the overbanking tendency). That something is the pilot
applying slight opposite aileron, increasing the inner wing's
effective angle of attack.

That may only be a couple of degrees of deflection down on the inner
wing, but its also the same or more up on the outer wing, decreasing
its effective angle of attack. The deflected surface has a substantial
effect on the pressure distribution of the airfoil in front of it, and
can make a big difference on its stall characteristics.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24