physics question about pull ups

John Cochrane wrote:
No, Gary means it. In theory, we can gain a lot by strong pull ups and
pushovers in thermal entries and exits. In fact, in theory, you can
stay up when there is only sink. You push to strong negative g's in
the sink, then strong positive gs when you are out of the sink. Huh?
Think of a basketball; your hand is sink and the ground is still air.
When you push hard negative g's in the sink, the glider exits the sink
with more airspeed than it entered, just like the basketball as it
hits your hand. The opposite happens when you pull hard for the first
second or two after entering lift.

I _think_ I get what you are saying: you basically propose extracting the
kinetic energy that is available due to the different fluid speeds. It
doesn't matter which direction the fluid streams flow - merely that one
part of the fluid is moving relative to another part and you can move your
aircraft from one to the other.

We're so used to getting energy out of upward fluid flows that we overlook
the fact that in a fundamental sense it doesn't matter (to a first
approximation) which direction the stream is going.

So what you all seem to be saying is that there is energy available for
extraction in wind shear, sinks, and thermals. If the whole mass of fluid
is moving then you are out of luck because you need a difference in fluid
speeds - with the exception that upward flows always make energy available
due to conversion of the fluid kinetic energy to gravitational potential
energy. (Hence the "first approximation" caveat.)

Is all that about right?

On Jun 6, 10:33*am, John Cochrane
wrote:
Hi John:

Precisely what Taras Keceniuck, Paul MacCready and I were doing in a
DARPA funded study when Paul passed away.

Best Regards,
Gary Osoba

Is the study finished and any publication done? I want the pitch
controller for the worlds!

Hmm. Come to think of it, I don't recall seeing anything in the rules
that prevent use of an autopilot.

Do you even have to have a human on board?

John Cochrane wrote:
Hi John:

Precisely what Taras Keceniuck, Paul MacCready and I were doing in a
DARPA funded study when Paul passed away.

Best Regards,
Gary Osoba

Is the study finished and any publication done? I want the pitch
controller for the worlds!
John Cochrane

Hmmm..I see the ArduIMU runs about $300. That might make a suitable
start on it?

Brian W

On Jun 5, 4:08*pm, Jim Logajan wrote:
John Cochrane wrote:
No, Gary means it. In theory, we can gain a lot by strong pull ups and
pushovers in thermal entries and exits. In fact, in theory, you can
stay up when there is only sink. You push to strong negative g's in
the sink, then strong positive gs when you are out of the sink. Huh?
Think of a basketball; your hand is sink and the ground is still air.
When you push hard negative g's in the sink, the glider exits the sink
with more airspeed than it entered, just like the basketball as it
hits your hand. The opposite happens when you pull hard for the first
second or two after entering lift.

I _think_ I get what you are saying: you basically propose extracting the
kinetic energy that is available due to the different fluid speeds. It
doesn't matter which direction the fluid streams flow - merely that one
part of the fluid is moving relative to another part and you can move your
aircraft from one to the other.

We're so used to getting energy out of upward fluid flows that we overlook
the fact that in a fundamental sense it doesn't matter (to a first
approximation) which direction the stream is going.

So what you all seem to be saying is that there is energy available for
extraction in wind shear, sinks, and thermals. If the whole mass of fluid
is moving then you are out of luck because you need a difference in fluid
speeds - with the exception that upward flows always make energy available
due to conversion of the fluid kinetic energy to gravitational potential
energy. (Hence the "first approximation" caveat.)

Is all that about right?

Yes. Your wing is a machine, and the work it performs imparts a
downward flow to the air it moves through. When that downward force is
aligned in a direction that opposes the movement of the air, it gains
energy. The air movement can be from the side, from above, or below-
the most efficient case since this vector opposes gravity. The
transfer of energy from air motion can be increased by manipulating
the inertial field of the glider, and there is an optimal g loading or
unloading for each case. Although physicists define such inertial
forces as "psuedo", the wing does not know this and must develop twice
the lift to sustain 2g flight as 1g flight, three times the lift for
3g flight,etc. The power transferred from the air to the wing
increases linearly with g force increases, while the the losses
associated with the increased g loadings are fractional and therefore
nonlinear, yielding excess power. This excess power can be carried by
the glider into a differential airmass with relative sink by a coupled
acceleration and a portion of it can be transferred to this airmass.
The case of 0g accelerations (freefall) is special in that
theoretically the wing doesn't produce induced drag. Theoretically
only, because the lift distribution will never be perfect- especially
in the unsteady flows which punctuate a soaring environment. In
practice, I have found 0g to be the best target for accelerations
since most of our wing sections are not designed to fly efficiently
upside down and everything is happening so quickly you lose less if
you guess wrong on the strength of the relative downdraft.

Much of this is counterintuitive. For example, here's something
presented in a 2001 lecture on the subject. It is stated as
exclusionary to emphasize how flight through a discontinuous
atmosphere can up-end long held conventions.

"For any body of mass moving through or in contact with a medium that
is not uniform, the most efficient path(s) for a given power input
will never be defined by a straight line or a constant speed." -
Osoba's Theorem of Dynamic Locomotion

The concise statement of this is "...never be defined by a constant
velocity..." since velocity incorporates both speed and direction but
most pilots don't understand the term that way.

Best Regards,

Gary Osoba

Gary Osoba wrote:
/snip/
"For any body of mass moving through or in contact with a medium that
is not uniform, the most efficient path(s) for a given power input
will never be defined by a straight line or a constant speed." -
Osoba's Theorem of Dynamic Locomotion
/snip/
Gary Osoba


Darn! I was following along nicely with this note, until I got to the
conjunction of a heading which included the word "physics"
and a person citing his own name for a physics construct.

That's usually a warning about the level of information....

:-)

Brian W

On Jun 5, 3:07*pm, Gary Osoba wrote:
On Jun 5, 2:41*pm, Nine Bravo Ground wrote:





On Jun 5, 2:30*pm, Gary Osoba wrote:

On Apr 25, 8:21*am, Andy wrote:

As an aside - the strong G-effect on induced drag is the main reason
why you should try to avoid hardpullupsinto thermals - you give away
a bunch of altitude.

9B

Yes, if you both accelerated and are now pulling up in a constant
velocity of transportation field. But by mentioning the thermal, this
is not likely. With discontinuous fluid fields, coupled pullups and
pushovers which are properly timed within a shifting frame of
reference have the potential to gain much more energy than is ever
lost to induced and friction drag- dry or fully loaded. The fully
loaded case has more potential in typical soaring environments because
more time is available to apply the technique and the events can be
further apart.

For most gliders, the optimized multiplier is so substantial that you
run out of positive g maneuvering envelope (based on JAR standards)
with a mere 2-3 knots of lift.

Best Regards,

Gary Osoba

If you mean dynamic soaring then the airmass velocity gradient needs
to be horizontal, not vertical as is the case with thermals - plus the
magnitude of the gradient in a thermal is way too low to be useful,
even if it were in the correct orientation.

If you aren't referring to dynamic soaring then all I can say is
"huh"?

9B

9B:

The physics apply in all directions, but the potential is greatest
with positive vertical velocity gradient since that vector directly
opposes gravity- *and that's our job if we're going to stay up. The
reason the horizontal gradients are more readily recognized is that
they are often sustainable in a cycle, witness the Albatross. However,
I'm not wanting to argue about it. I know the physics and the math and
have been using them effectively for about 15 years now.

Best Regards,
Gary Osoba

Got it - sounds a bit uncomfortable since moving the velocity vector
around in the vertical axis takes a lot more aggressiveness then
horizontally. I assume it also helps to know where the boundaries of
the gradients are before you reach them. If you miss you just mush and
lose altitude fro all the induced drag.

It's the exact opposite technique from what I see and hear from most
top racing pilots who advise flying slower than McCready theory and
maintaining laminar flow over the wing with only modest maneuvering.
How do you decide when to use which technique when you are cruising
along at 15,000 feet and 85 knots and run into a 6 knot thermal?

9B

On Jun 6, 10:06*am, Andy wrote:

Got it - sounds a bit uncomfortable since moving the velocity vector
around in the vertical axis takes a lot more aggressiveness then
horizontally. I assume it also helps to know where the boundaries of
the gradients are before you reach them. If you miss you just mush and
lose altitude fro all the induced drag.

Yes, as you have properly shown in the still air case- only now the
penalties are even higher than in still air.


It's the exact opposite technique from what I see and hear from most
top racing pilots who advise flying slower than McCready theory and
maintaining laminar flow over the wing with only modest maneuvering.
How do you decide when to use which technique when you are cruising
along at 15,000 feet and 85 knots and run into a 6 knot thermal?

9B

Well. that's the trick, isn't it? I would say that if you're at
15,000', full of water, but only going 85 knots, it must be pretty
spotty overall and would recommend sticking to the conventional
approach. For one thing, you only have a little over a second of
deceleration time at that speed. When the conditions allow, it is much
better to have more maneuvering time through higher velocities.
However, it has also been shown that chasing MacCready through a
thermal will usually yield poorer results than stick-fixed excursions
(Braunschweig Tech. University, 1982). Chasing any of this with the
vario is futile due to lag times.

In any event, much of this does run counter to the normal "racing"
protocol. E.g., Moffat's final turn at the top of a climb when it is
tightened and you accelerate across the thermal core before exiting.
Exactly opposite to the best total energy/dynamic maneuvering
scenario, apart from tightening the turn in order to be right at the
center of the core for the straight line flight.

I only entered a contest once as an individual, and chose to fly it
without a computer (or even a speed ring). I did effectively use these
techniques, and lateral dynamic maneuvering as well.

Best Regards,

Gary Osoba

However, it has also been shown that chasing MacCready through a
thermal will usually yield poorer results than stick-fixed excursions
(Braunschweig Tech. University, 1982). Chasing any of this with the
vario is futile due to lag times.

As I think of Gary's maneuvers, they are more characterized by sharp
pull ups and pushovers when lift changes, as opposed to classic
"chasing the needle" which we know doesn't work. I suspect that when
we get this right, G and airspeed will be a much more important input
than varios. We're trying to pull when we get the increased G from
entering a thermal, "bouncing" off the change in vertical speed, and
vice versa. Similarly, the time when this works is during the rush of
positive airspeed as you enter a vertical gust.

The instrument or pitch controller that gets this right may be
essentially one that tells us what the G reading is subtracting the
effects of controls -- a "total energy g meter" if you will. Then you
can pull when "total energy" g is positive and push when it's
negative, subtracting the "stick g"


In any event, much of this does run counter to the normal "racing"
protocol. E.g., Moffat's final turn at the top of a climb when it is
tightened and you accelerate across the thermal core before exiting.
Exactly opposite to the best total energy/dynamic maneuvering
scenario, apart from tightening the turn in order to be right at the
center of the core for the straight line flight.

Moffat's technique was great in the 70s, but most pilots don't use it
now. Especially in wind or under clouds, there is often not sink
surrounding a thermal, but a long stretch of buoyant air. They didn't
know that in the 70s because they didn't have netto or speed to fly
varios, so when they sped up to 90 knots they were in fact sinking
like stones. Most of the time the key to thermal exit is to leave
gently in such a way as to milk the surrounding up air while cruising
relatively slowly for a few miles

John Cochrane

On Jun 7, 5:42*am, Gary Osoba wrote:
In any event, much of this does run counter to the normal "racing"
protocol. E.g., Moffat's final turn at the top of a climb when it is
tightened and you accelerate across the thermal core before exiting.

I've never understood how you are supposed to do that. I'm *already*
circling as tightly as I can at the speed I'm flying!

Unless all you're doing is increasing the bank angle while maintaining
the same elevator setting, which will make you turn in a bit more and
enter a dive.

On Jun 6, 1:14*pm, John Cochrane
wrote:
However, it has also been shown that chasing MacCready through a
thermal will usually yield poorer results than stick-fixed excursions
(Braunschweig Tech. University, 1982). Chasing any of this with the
vario is futile due to lag times.

As I think of Gary's maneuvers, they are more characterized by sharp
pull ups and pushovers when lift changes, as opposed to classic
"chasing the needle" which we know doesn't work. I suspect that when
we get this right, G and airspeed will be a much more important input
than varios. We're trying to pull when we get the increased G from
entering a thermal, "bouncing" off the change in vertical speed, and
vice versa. Similarly, the time when this works is during the rush of
positive airspeed as you enter a vertical gust.

The instrument or pitch controller that gets this right may be
essentially one that tells us what the G reading is subtracting the
effects of controls -- a "total energy g meter" if you will. Then you
can pull when "total energy" g is positive and push when it's
negative, subtracting the "stick g"



In any event, much of this does run counter to the normal "racing"
protocol. E.g., Moffat's final turn at the top of a climb when it is
tightened and you accelerate across the thermal core before exiting.
Exactly opposite to the best total energy/dynamic maneuvering
scenario, apart from tightening the turn in order to be right at the
center of the core for the straight line flight.

Moffat's technique was great in the 70s, but most pilots don't use it
now. Especially in wind or under clouds, there is often not sink
surrounding a thermal, but a long stretch of buoyant air. They didn't
know that in the 70s because they didn't have netto or speed to fly
varios, so when they sped up to 90 knots they were in fact sinking
like stones. Most of the time the key to thermal exit is to leave
gently in such a way as to milk the surrounding up air while cruising
relatively slowly for a few miles

John Cochrane


Okay, this is going to totally mess up my next contest.

I admit that I substantially rely on Gs to decide whether to turn in
lift - the vario only tells you if you made a good choice 1/4 turn
later. I honestly don't find that many thermals with a very strong
gradient, so I am wondering how much benefit I'll get from the extra
push and pull, especially if the optimal strategy emphasizes search
range over theoretical McCready optimum cruise speed.

As to flying 85 knots - that's pretty common for me when I am dry -
maybe 95 knots wet on a good day. If the thermals are closely placed
and consistent and the lift band is deep enough I'll bump it up,
otherwise I like the extra search range.

9B