Circling for rodents?

Vultures are amazing *pilots*. The ones around the Magalies gliding club in
SA are known to range as far as the Kruger National Park, over 200 km away.
I've flown with them at cloudbase as high as 14 000', had them formate on a
wingtip and once over the shoulder of a Blanik, peering into the rear
cockpit from less than a metre away.

Flying in the Drakensburg one can often end up with 30 or more birds in a
big thermal or fly in formation with a group of them along the cliff faces.

Ian





"Martin Gregorie" wrote in message
...
On 04 Jul 2004 18:48:04 GMT, ospam (Frostowits)
wrote:

Pardon this input from an uninformed intruder to this subject, but why
couldn't
birds simply sense how strong the lift is by the amount of stress it puts
on
their "airframe". When I pump iron, I'm all too aware of the amount of
effort
required. Surely birds can do the same.

Some seem to do just that. Kites in particular. I used to watch them a
lot in India and discovered that you can tell how strong the lift is
by looking at them - the stronger it is the more dihedral they use. If
its really strong they just bomb round with a steep V-form and their
tip feathers closed. Weaker lift gets more care and attention, less
dihedral and more open tip feathers. When they're really scratching
their wings are flat or even a little anhedralled and the tip feathers
are fully spread and up to give tip dihedral. They initiate a turn
with a big dab of negative in the inner tip and then control the turn
on tail tilt - the outer tail tip is raised, so you can tell that
they fly like we do with down force on the tail. Kites are easy to
read because they often work low altitude lift where you can see
exactly what they are doing and have big, long tails that are easy to
observe.

I wondered about how vultures fly but they were so seldom low enough
to really watch that I couldn't work out very much. Also, with much
shorter tails than kites its difficult to see whether they use tail
tilt at all or which way its applied.

The above is about all I know about soaring birds: I'm no
ornithologist or naturalist. My background is chemistry, competitive
free flight model flying and, latterly, soaring.

I've heard a number of theories about how birds detect thermals
including that they hear them. I'd well believe that, with a nerve on
each feather, they must *really* feel the air and all its
micro-turbulence. Maybe they can hear it too. However, that tells
something about how they work 'normal' thermals but not a lot about
how they can find and work the very weak, smooth lift you get early
and late in the day. We know that migratory birds have a excellent
directional sense so why shouldn't a soaring bird have a built-in
vario too? I'd love to know how it works.

If you haven't read it, find a copy of Philip Wills' "On Being A
Bird". There's a chapter about flying with vultures in South Africa
and how he worked just how vultures operate - altitudes, spacing, food
finding strategy etc. The whole book is a good read too.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Martin Gregorie wrote:
On 04 Jul 2004 18:48:04 GMT, ospam (Frostowits)
wrote:
Pardon this input from an uninformed intruder to this subject, but why couldn't
birds simply sense how strong the lift is by the amount of stress it puts on
their "airframe". When I pump iron, I'm all too aware of the amount of effort
required. Surely birds can do the same.

Well, the stress only occurs briefly on entry into the thermal,
when you have upward acceleration. Once established in the climb,
assuming it is more or less smooth, you no longer have acceleration
nor any acceleration generated stress forces so you need some other
means of knowing whether you are still climbing, such as rate of
pressure change, which is what the vario measures.

cheers CV

Martin Gregorie wrote:

On 04 Jul 2004 18:48:04 GMT, ospam (Frostowits)
wrote:

Pardon this input from an uninformed intruder to this subject, but why couldn't
birds simply sense how strong the lift is by the amount of stress it puts on
their "airframe". When I pump iron, I'm all too aware of the amount of effort
required. Surely birds can do the same.

Some seem to do just that.

This doesn't work, as well as the earlier mentionnned feeling of vertical
accelaration. This is because both only give just a "differential"
information, i.e. not the rate of climb, but the change in the rate of
climb. The real rate of climb can only been obtained from this information
by a (mental) integration, but every such value in any domain obtained
by integration from a raw differential input suffers a major deficiency:
errors accumulate in time during this process so that if no other
absolute information is available to determine the correct value at
regular time intervals, the integrated value becomes meaningless.
We all know that the feeling of accelaration is a good hint for
a thermal, but we also check the vario to be sure it is not due
to a high sink becoming very quickly a lower sink not deserving
any circling. And while circling we check with the vario that
the lift is not very slowly vanishing, at a rate at which no
acceleration perception would warn us, and leave the thermal if this
happens.


... They initiate a turn
with a big dab of negative in the inner tip and then control the turn
on tail tilt - the outer tail tip is raised, so you can tell that
they fly like we do with down force on the tail.
...

A raised tail (or elevator) doesn't mean there is a down force on it.
On our gliders the opposite would rather be true : modern gliders are
tuned so that that there is a zero force on the tail near the best glide
angle of attack, in order to minimize the (induced ) drag due to this
force, and as the airfoils used are unstable, at higher angles of attack
we should have an upward force and a downward force at lower angles
of attack, but the elevator is raised for higher angles of attack
and lowered for lower ones. This is not a contradiction, it just means
than without the move of the elevator, the upward or downward force
on the tail plane would even be higher, so bringing back the angle
of attack to its previous value, i.e. the tail plane fulfils its
stabilizing duty.

Pardon this input from an uninformed intruder to this subject, but why
couldn't
birds simply sense how strong the lift is by the amount of stress it puts
on
their "airframe". When I pump iron, I'm all too aware of the amount of
effort
required. Surely birds can do the same.

Ted Frost
Soaring Society of Boulder

The stress would only change when acceleration was present (as in entering a
thermal, we use our bums for for that - transitting from an area of intense
sink to and area of lesser sink: e.g. no thermal). In a steady climb the
stress wouldn't change and would be the same as in normal turning flight. I
suspect that Martin's observation about the birds choosing a more extreme
dihedral has more to do with lack of effort than sensing the lift intensity
- a high dihedral result and a high value of lateral stability, but poor
efficiency of the wing.

Rgds,

Derrick Steed

Can birds sense pressure changes? Maybe they have bio-varios.
I've noticed they seem to chirp, even on gloomy mornings,
when I have noticed pressure rising on the
altimeter or baro. It wouldn't surprise me if
this was true, but I haven't heard of any studies...

So I'm guessing maybe they know they are
climbing from vario alone...

In article ,
Derrick Steed wrote:
Pardon this input from an uninformed intruder to this subject, but why
couldn't
birds simply sense how strong the lift is by the amount of stress it puts
on
their "airframe". When I pump iron, I'm all too aware of the amount of
effort
required. Surely birds can do the same.

Ted Frost
Soaring Society of Boulder

The stress would only change when acceleration was present (as in entering a
thermal, we use our bums for for that - transitting from an area of intense
sink to and area of lesser sink: e.g. no thermal). In a steady climb the
stress wouldn't change and would be the same as in normal turning flight. I
suspect that Martin's observation about the birds choosing a more extreme
dihedral has more to do with lack of effort than sensing the lift intensity
- a high dihedral result and a high value of lateral stability, but poor
efficiency of the wing.

Rgds,

Derrick Steed






--

------------+
Mark Boyd
Avenal, California, USA

Stress, or G-loading, is a measure of acceleration.
As such it allows birds (or pilots!) to sense changes
in the rate of climb, but not the climb rate itself
(which we all know is a velocity, not an acceleration).
I would imagine that birds can use these changes in
acceleration to help find the center of a small thermal
in some cases, but it might be less helpful in larger,
more uniform bands of lift where the ability to integrate
the cumulative acceleration effects over time is more
difficult.

A falconer at the Parowan regionals last week told
us that soaring birds have sensory organs that are
able to measure the pressure differential between the
outside air and inside their hollow bones. I have not
been able to confirm this, but it would seem to make
sense - think of the bones as capacity bottles.

Any bird experts out there?

9B



At 19:48 04 July 2004, Martin Gregorie wrote:
On 04 Jul 2004 18:48:04 GMT, ospam
(Frostowits)
wrote:

Pardon this input from an uninformed intruder to this
subject, but why couldn't
birds simply sense how strong the lift is by the amount
of stress it puts on
their 'airframe'. When I pump iron, I'm all too aware
of the amount of effort
required. Surely birds can do the same.

Some seem to do just that. Kites in particular. I used
to watch them a
lot in India and discovered that you can tell how strong
the lift is
by looking at them - the stronger it is the more dihedral
they use. If
its really strong they just bomb round with a steep
V-form and their
tip feathers closed. Weaker lift gets more care and
attention, less
dihedral and more open tip feathers. When they're really
scratching
their wings are flat or even a little anhedralled and
the tip feathers
are fully spread and up to give tip dihedral. They
initiate a turn
with a big dab of negative in the inner tip and then
control the turn
on tail tilt - the outer tail tip is raised, so you
can tell that
they fly like we do with down force on the tail. Kites
are easy to
read because they often work low altitude lift where
you can see
exactly what they are doing and have big, long tails
that are easy to
observe.

I wondered about how vultures fly but they were so
seldom low enough
to really watch that I couldn't work out very much.
Also, with much
shorter tails than kites its difficult to see whether
they use tail
tilt at all or which way its applied.

The above is about all I know about soaring birds:
I'm no
ornithologist or naturalist. My background is chemistry,
competitive
free flight model flying and, latterly, soaring.

I've heard a number of theories about how birds detect
thermals
including that they hear them. I'd well believe that,
with a nerve on
each feather, they must *really* feel the air and all
its
micro-turbulence. Maybe they can hear it too. However,
that tells
something about how they work 'normal' thermals but
not a lot about
how they can find and work the very weak, smooth lift
you get early
and late in the day. We know that migratory birds have
a excellent
directional sense so why shouldn't a soaring bird have
a built-in
vario too? I'd love to know how it works.

If you haven't read it, find a copy of Philip Wills'
'On Being A
Bird'. There's a chapter about flying with vultures
in South Africa
and how he worked just how vultures operate - altitudes,
spacing, food
finding strategy etc. The whole book is a good read
too.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

On 4 Jul 2004 20:39:04 GMT, Andy Blackburn
wrote:

Any bird experts out there?

While still in India I met a German lass who was an ornithologist and
in India do a PhD on vultures. Naturally, I asked her about their
flight performance. She just looked at me like I was a dinosaur: she
knew nothing and cared less about their flight performance or
operating methods. All she was interested in was stuff like population
densities, diet and their behaviour when not flying. I found her
attitude most odd.

That was 25 years ago so things, hopefully, might have improved in
ornithological circles.

So, pick your bird expert carefully before asking about how birds fly.

BTW, a good book about flight in general (literally from insects to
747s) is 'The Simple Science Of Flight' by Henk Tennekes. It won't
tell you how raptors find lift but has a good analysis of how flying
creatures size and weight affect their way of making a living and vice
versa. Besides, any book on flight that can sensibly show everything
from a Monarch Butterfly to a 747-400 on the same graph can't be all
bad!



--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

I'm no bird expert, but did 20 years competing hang
gliding around the world, before really getting into
gliding.
Hang gliders mixed with (soaring) birds far more than
sailplanes. Probably a performance thing.

The birds seem to do different things depending upon
the task at hand.
IE: want to gain height quick - they'll be in the best
core or thermal within the vicinity. Want to go somewhere-
they'll be heading towards the next thermal that optimises
their route. Want to 'hang out' they'll be in any old
lift.
I witnessed the whole USA hang gliding team joining
a flock of circling Orubu in Brazil. They all landed
next to the dead cow.
Larger raptors can be more territorial and therefore
don't have the same choice on area, thus might put
up with inferior lift.
Storks just seem to follow the leader.
Swifts and Swallows are almost always in good cores,
but I haven't a clue how they transit between thermals
so quickly.

The soaring birds not only find the best cores, but
seem to very quickly know where the next thermal is.
Many times I've watched their transiting direction
- always dead straight - and altered my course to intercept
or get ahead. It usually works.

When we fly a lot, we pick up those extra sensory inputs
- the micro turbulence, the twitch of the wing - it
gives us huge amounts of info if we're open to it.
By the end of the soaring season, I've sometimes just
known where the core was by feel - which way to turn,
etc. I don't know how - just intuitive. Birds fly all
the time, so should be totally tuned in.
Lastly, I'm sure they plot sink. We tend to focus on
lift, whereas, of course, it's just a part of the equation.
If you know where the sink is, then happy days, avoid
it!
Pete Harvey

A falconer at the Parowan regionals last week told
us that soaring birds have sensory organs that are
able to measure the pressure differential between the
outside air and inside their hollow bones. I have not
been able to confirm this, but it would seem to make
sense - think of the bones as capacity bottles.

Any bird experts out there?

Take a look at Darryl Stinton's book "The design of the aeroplane" - there
is a captioned picture in the front of it showing a gull (an Albatross, I
think) soaring a cliff in front of the camera. The caption points out
aerodynamic function of various parts of the birds anatomy in a most
enlightening manner! I don't have the book in my possession right now (I
loaned it to the CFI of a local club, I should get it back I suppose) or I
would post the picture and it's caption.

Rgds,

Derrick Steed
On 4 Jul 2004 20:39:04 GMT, Andy Blackburn
wrote:

Any bird experts out there?

While still in India I met a German lass who was an ornithologist and
in India do a PhD on vultures. Naturally, I asked her about their
flight performance. She just looked at me like I was a dinosaur: she
knew nothing and cared less about their flight performance or
operating methods. All she was interested in was stuff like population
densities, diet and their behaviour when not flying. I found her
attitude most odd.

That was 25 years ago so things, hopefully, might have improved in
ornithological circles.

So, pick your bird expert carefully before asking about how birds fly.

BTW, a good book about flight in general (literally from insects to
747s) is 'The Simple Science Of Flight' by Henk Tennekes. It won't
tell you how raptors find lift but has a good analysis of how flying
creatures size and weight affect their way of making a living and vice
versa. Besides, any book on flight that can sensibly show everything
from a Monarch Butterfly to a 747-400 on the same graph can't be all
bad!

Our understanding of the reason/s for a bird’s excellent
soaring ability continues to be severely hampered by
the lack of progress in establishing verbal communications.
Perhaps a species that can fly through a thicket and
sleep on a branch do not consider it worth their time.



At 20:54 04 July 2004, Andy Blackburn wrote:
Stress, or G-loading, is a measure of acceleration.
As such it allows birds (or pilots!) to sense changes
in the rate of climb, but not the climb rate itself
(which we all know is a velocity, not an acceleration).
I would imagine that birds can use these changes in
acceleration to help find the center of a small thermal
in some cases, but it might be less helpful in larger,
more uniform bands of lift where the ability to integrate
the cumulative acceleration effects over time is more
difficult.

A falconer at the Parowan regionals last week told
us that soaring birds have sensory organs that are
able to measure the pressure differential between the
outside air and inside their hollow bones. I have not
been able to confirm this, but it would seem to make
sense - think of the bones as capacity bottles.

Any bird experts out there?

9B



At 19:48 04 July 2004, Martin Gregorie wrote:
On 04 Jul 2004 18:48:04 GMT, ospam
(Frostowits)
wrote:

Pardon this input from an uninformed intruder to this
subject, but why couldn't
birds simply sense how strong the lift is by the amount
of stress it puts on
their 'airframe'. When I pump iron, I'm all too aware
of the amount of effort
required. Surely birds can do the same.

Some seem to do just that. Kites in particular. I used
to watch them a
lot in India and discovered that you can tell how strong
the lift is
by looking at them - the stronger it is the more dihedral
they use. If
its really strong they just bomb round with a steep
V-form and their
tip feathers closed. Weaker lift gets more care and
attention, less
dihedral and more open tip feathers. When they're really
scratching
their wings are flat or even a little anhedralled and
the tip feathers
are fully spread and up to give tip dihedral. They
initiate a turn
with a big dab of negative in the inner tip and then
control the turn
on tail tilt - the outer tail tip is raised, so you
can tell that
they fly like we do with down force on the tail. Kites
are easy to
read because they often work low altitude lift where
you can see
exactly what they are doing and have big, long tails
that are easy to
observe.

I wondered about how vultures fly but they were so
seldom low enough
to really watch that I couldn't work out very much.
Also, with much
shorter tails than kites its difficult to see whether
they use tail
tilt at all or which way its applied.

The above is about all I know about soaring birds:
I'm no
ornithologist or naturalist. My background is chemistry,
competitive
free flight model flying and, latterly, soaring.

I've heard a number of theories about how birds detect
thermals
including that they hear them. I'd well believe that,
with a nerve on
each feather, they must *really* feel the air and all
its
micro-turbulence. Maybe they can hear it too. However,
that tells
something about how they work 'normal' thermals but
not a lot about
how they can find and work the very weak, smooth lift
you get early
and late in the day. We know that migratory birds have
a excellent
directional sense so why shouldn't a soaring bird have
a built-in
vario too? I'd love to know how it works.

If you haven't read it, find a copy of Philip Wills'
'On Being A
Bird'. There's a chapter about flying with vultures
in South Africa
and how he worked just how vultures operate - altitudes,
spacing, food
finding strategy etc. The whole book is a good read
too.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :