Constant speed or constant attitude?

"Martin Gregorie" wrote in message
...
On Mon, 18 Aug 2003 10:53:46 -0700, Eric Greenwell
wrote:
snippage
Rising air comes from beneath the wing, and increases the AOA.

Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.

Martin, you are using the wrong frame of reference. The "sinking speed
velocity" you are using is relative to the ground. Relative to the glider,
the air it is sinking through is rising past it. As the glider flies into
"lift" (relative to the ground) the vertical velocity of the air rising past
it is _increased_ by the velocity of the lift. So the AOA is increased
momentarily.

Tim Ward

Two quick comments on this discussion:

1. There's a timing difference between the main wing and the tail.
Entering lift there is a transition region where the lift is growing
stronger as the glider moves forward. The main wing will be about 15
feet ahead of the tail. As a result the main wing will be at a higher
angle of attack than the tail during the transition into the lift,
creating a pitch-up tendency until the aircraft has gone through the
transition region. This would tend to offset the pitch-down tendency
that's been discussed. I've personally flown aircraft that pitched
down, others that pitched up. Flex-wing hang gliders tend to pitch up
very strongly on entering lift. However, I used to fly a rigid-wing
hang glider (flying wing) that pitched down. I never could explain
the difference.

2. There's been a few mentions of the download on the tail, and Martin
referred to an upload on the tail of a model but said this was
"probably unsuitable for a manned plane." Not at all: at thermaling
speeds, manned sailplanes have a substantial upload on the tail
(contrary to everything my instructors told me!). This was a topic of
discussion some months ago on RAS. Since Martin is familiar with
doing this in models, it may surprise him rather less than it did me -
but I ran the numbers and, indeed, it is so!

On Tue, 19 Aug 2003 07:17:43 -0700, "TIM WARD" wrote:


"Martin Gregorie" wrote in message
.. .
On Mon, 18 Aug 2003 10:53:46 -0700, Eric Greenwell
wrote:
snippage
Rising air comes from beneath the wing, and increases the AOA.

Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.

Martin, you are using the wrong frame of reference. The "sinking speed
velocity" you are using is relative to the ground. Relative to the glider,
the air it is sinking through is rising past it. As the glider flies into
"lift" (relative to the ground) the vertical velocity of the air rising past
it is _increased_ by the velocity of the lift. So the AOA is increased
momentarily.


I'm using the air mass as the reference frame - hence my reference to
a change of reference frame as the glider moves from neutral air into
rising air. If I was using the ground or the glider as reference frame
I would not have mentioned a change of reference frame.

The air mass is the usual one for discussions of stable flight - as in
the periodic circling in a wind discussion.


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

At 15:54 19 August 2003, Finbar wrote:

.....'There's a timing difference between the main
wing and the tail.
Entering lift there is a transition region where the
lift is growing
stronger as the glider moves forward. The main wing
will be about 15
feet ahead of the tail. As a result the main wing
will be at a higher
angle of attack than the tail during the transition
into the lift,
creating a pitch-up tendency until the aircraft has
gone through the
transition region. This would tend to offset the pitch-down
tendency
that's been discussed. I've personally flown aircraft
that pitched
down, others that pitched up. Flex-wing hang gliders
tend to pitch up
very strongly on entering lift. However, I used to
fly a rigid-wing
hang glider (flying wing) that pitched down. I never
could explain
the difference.


I can confirm that, at least in the Discus and Duo
Discus flying with mid to aft C of G , if you simply
cruise with a rigidly fixed elevator position - set
for a reasonable median cruise speed of your choice
- then the glider slowly pitches up and slows under
positive acceleration as you enter regions of lift
and pitches down and speeds up under reduced acceleration
as you enter regions of sink. I have often flown this
way in the last few years since reading about the technique
as an aside in Reichmann (seventh edition in English,
pages 64 and 133). He discusses it in connection with
methods of trying to optimize g loading in transitional
phases of flight between lift and sink and refers to
it as 'the near optimal solution of simply flying with
the controls locked'. I have often wondered why it
works when the Yates effect would at first sight tend
to have the opposite effect so thanks to Finbar for
the obsevation above.

Flying fixed elevator results in very nice gentle speed
variation without the divergence you get flying hands
off but it takes a surprising amount of concentration
to keep the elevator fixed. (Perhaps a little 'dead
man's handle' on the stick that temporarily fixed the
elevator control alone would help.) It works best
when the lift and sink are continually changing because
if there is a long period of steady lift or sink without
vertical acceleration from the airmass then the airspeed
tends to settle back at the cruise speed for the elevator
setting chosen. In those circumstances you need to
depart from the fixed elevator.

John Galloway

Martin Gregorie wrote:
...
Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.
...

But this is near like adding apples an oranges. The vector you are
interested in is the relative wind vector, i.e. the velocity vector
of the airmass seen from the aircraft as reference frame, for what
concerns the AOA. Before entering lift, you can consider it as the
sum of 2 components, a horizontal one, opposite of the horizontal speed
of the glider, and a vertical one, opposite of the sinking speed.
When lift is entered, a 3rd componemt is added, the lift vector. This
3rd component has the same direction as the 2nd one, i.e. both are
upward. So the new relative wind clearly causes a higher AOA. Adding
the sinking speed vector and the rising air vector, while mathematically
possible, has no physical sense.

On Wed, 27 Aug 2003 19:20:48 +0000, root
wrote:

Martin Gregorie wrote:
...
Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.
...

But this is near like adding apples an oranges. The vector you are
interested in is the relative wind vector, i.e. the velocity vector
of the airmass seen from the aircraft as reference frame, for what
concerns the AOA. Before entering lift, you can consider it as the
sum of 2 components, a horizontal one, opposite of the horizontal speed
of the glider, and a vertical one, opposite of the sinking speed.
When lift is entered, a 3rd componemt is added, the lift vector. This
3rd component has the same direction as the 2nd one, i.e. both are
upward. So the new relative wind clearly causes a higher AOA. Adding
the sinking speed vector and the rising air vector, while mathematically
possible, has no physical sense.

That makes sense.

Presumably the pitch down is the aircraft correcting its trimmed AOA,
but where does the commonly seen airspeed increase come from?


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

On Tue, 02 Sep 2003 17:34:17 +0100, Martin Gregorie
wrote:

On Wed, 27 Aug 2003 19:20:48 +0000, root
wrote:

Martin Gregorie wrote:
...
Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.
...

But this is near like adding apples an oranges. The vector you are
interested in is the relative wind vector, i.e. the velocity vector
of the airmass seen from the aircraft as reference frame, for what
concerns the AOA. Before entering lift, you can consider it as the
sum of 2 components, a horizontal one, opposite of the horizontal speed
of the glider, and a vertical one, opposite of the sinking speed.
When lift is entered, a 3rd componemt is added, the lift vector. This
3rd component has the same direction as the 2nd one, i.e. both are
upward. So the new relative wind clearly causes a higher AOA. Adding
the sinking speed vector and the rising air vector, while mathematically
possible, has no physical sense.

That makes sense.

Presumably the pitch down is the aircraft correcting its trimmed AOA,
but where does the commonly seen airspeed increase come from?


The Yates effect as the lift vector tilts forward(and increases in
magnitude).

Mike Borgelt

On Wed, 03 Sep 2003 19:19:13 +1000, Mike Borgelt
wrote:

On Tue, 02 Sep 2003 17:34:17 +0100, Martin Gregorie
wrote:

On Wed, 27 Aug 2003 19:20:48 +0000, root
wrote:

Martin Gregorie wrote:
...
Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.
...

But this is near like adding apples an oranges. The vector you are
interested in is the relative wind vector, i.e. the velocity vector
of the airmass seen from the aircraft as reference frame, for what
concerns the AOA. Before entering lift, you can consider it as the
sum of 2 components, a horizontal one, opposite of the horizontal speed
of the glider, and a vertical one, opposite of the sinking speed.
When lift is entered, a 3rd componemt is added, the lift vector. This
3rd component has the same direction as the 2nd one, i.e. both are
upward. So the new relative wind clearly causes a higher AOA. Adding
the sinking speed vector and the rising air vector, while mathematically
possible, has no physical sense.

That makes sense.

Presumably the pitch down is the aircraft correcting its trimmed AOA,
but where does the commonly seen airspeed increase come from?


The Yates effect as the lift vector tilts forward(and increases in
magnitude).


Thanks

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