poor lateral control on a slow tow?

On Thu, 06 Jan 2011 09:09:39 +0000, Doug Greenwell
wrote:


There's a chapter in Eric Brown's book 'Wings of the Weird &
Wonderful' in which he describes flight tests of the GAL 56 flying wing
glider in 1946. This was a 28deg swept wing with an aspect ratio of 5.8
towed by a Spitfire IX* (!!!) to 20000ft (!!).

Coooooooooool.



He describes the opposite effect, with a very strong (often
uncontrollable) nose-up pitch on take-off - this was thought to be due to
ground effect. In this case the tug span was similar (37ft) to the glider
span (45ft), so the wake/wing interaction would be different.

Definitely. I think that the slipstream and the turbulence of that
huge propellor might have an influence, too.


Interestingly he also reports that the GAL56 could be flown hands-free on
the tow - unless the tug slipstream was entered, in which case all lateral
and longitudinal control was lost. Robert Kronfield was later killed
spinning this aircraft.

Seems like some gliders actually stabilize themselves behind a tow
plane.

Here's an example of a free-flight test of a space shuttle model that
flew well in aerotow, but worse in free flight.


Ladies and gents, Great Britains only serious contribution to
spaceflight - the Reliant Shuttle:
http://www.youtube.com/watch?v=pJdrlWR-yFM



Andreas

On Wed, 05 Jan 2011 21:22:53 -0800, Eric Greenwell
wrote:


I'd love to see "3-D" perspective view of the wake behind a towplane, as
I doubt I'm visualizing it well.

Have you seen this?
http://www.centennialofflight.gov/es...tex/TH15G5.htm



BTW: Have you already seen this? (starts at 0:55):
http://www.youtube.com/watch?v=__pyxPb6gMc

Note how long the air behind the plane continues to sink after the
plane has passed... and how the wing tip vortices and the downwash
behind the wing interact.



Andreas

At 16:11 06 January 2011, Andreas Maurer wrote:
On Thu, 06 Jan 2011 09:09:39 +0000, Doug Greenwell
wrote:


There's a chapter in Eric Brown's book 'Wings of the Weird &
Wonderful' in which he describes flight tests of the GAL 56 flying
wing
glider in 1946. This was a 28deg swept wing with an aspect ratio of
5.8
towed by a Spitfire IX* (!!!) to 20000ft (!!).

Coooooooooool.

every tug pilots dream ... wonder what the climb rate was like!




He describes the opposite effect, with a very strong (often
uncontrollable) nose-up pitch on take-off - this was thought to be due
to
ground effect. In this case the tug span was similar (37ft) to the
glider
span (45ft), so the wake/wing interaction would be different.

Definitely. I think that the slipstream and the turbulence of that
huge propellor might have an influence, too.


Possibly - he had trouble getting the nose down on landing too.


Interestingly he also reports that the GAL56 could be flown hands-free
on
the tow - unless the tug slipstream was entered, in which case all
lateral
and longitudinal control was lost. Robert Kronfield was later killed
spinning this aircraft.

Seems like some gliders actually stabilize themselves behind a tow
plane.

Here's an example of a free-flight test of a space shuttle model that
flew well in aerotow, but worse in free flight.


Ladies and gents, Great Britains only serious contribution to
spaceflight - the Reliant Shuttle:
http://www.youtube.com/watch?v=pJdrlWR-yFM



Andreas



That's a bit unfair ... we did manage one satellite into orbit on Black
Arrow

What do you expect from the juvenile mentality of the top gear presenters?
I'll bet they switched the explosive bolts for standard ones.

Derrick.

At 16:40 06 January 2011, Doug Greenwell wrote:
At 16:11 06 January 2011, Andreas Maurer wrote:
On Thu, 06 Jan 2011 09:09:39 +0000, Doug Greenwell
wrote:


There's a chapter in Eric Brown's book 'Wings of the Weird &
Wonderful' in which he describes flight tests of the GAL 56 flying
wing
glider in 1946. This was a 28deg swept wing with an aspect ratio of
5.8
towed by a Spitfire IX* (!!!) to 20000ft (!!).

Coooooooooool.

every tug pilots dream ... wonder what the climb rate was like!




He describes the opposite effect, with a very strong (often
uncontrollable) nose-up pitch on take-off - this was thought to be due
to
ground effect. In this case the tug span was similar (37ft) to the
glider
span (45ft), so the wake/wing interaction would be different.

Definitely. I think that the slipstream and the turbulence of that
huge propellor might have an influence, too.


Possibly - he had trouble getting the nose down on landing too.


Interestingly he also reports that the GAL56 could be flown hands-free
on
the tow - unless the tug slipstream was entered, in which case all
lateral
and longitudinal control was lost. Robert Kronfield was later killed
spinning this aircraft.

Seems like some gliders actually stabilize themselves behind a tow
plane.

Here's an example of a free-flight test of a space shuttle model that
flew well in aerotow, but worse in free flight.


Ladies and gents, Great Britains only serious contribution to
spaceflight - the Reliant Shuttle:
http://www.youtube.com/watch?v=pJdrlWR-yFM



Andreas



That's a bit unfair ... we did manage one satellite into orbit on Black
Arrow

At 16:34 06 January 2011, Andreas Maurer wrote:
On Wed, 05 Jan 2011 21:22:53 -0800, Eric Greenwell
wrote:


I'd love to see "3-D" perspective view of the wake behind a towplane,
as
I doubt I'm visualizing it well.

Have you seen this?
http://www.centennialofflight.gov/es...tex/TH15G5.htm



BTW: Have you already seen this? (starts at 0:55):
http://www.youtube.com/watch?v=__pyxPb6gMc

Note how long the air behind the plane continues to sink after the
plane has passed... and how the wing tip vortices and the downwash
behind the wing interact.



Andreas



nice video - that's an amazing facility ONERA has in Lisle.

Airbus have also been using a ship towing tank to understand the way wake
vortices decay behind an aircraft. There's a really neat effect a long
way downstream which you can see happening in contrails on a good day -
the vortices start oscillating from side-to-side, then merge and split
into rings.

Please think with me -

The argument that the wing has the same AoA for a given speed, only
applies in a homogeneous airmass.

Consider that lift generated is integrated over the wing as a function
of the local AoA, Airspeed, density etc. The geometric angle of the wing
to the flight path is constant (ignoring washout) So - what happens when
the airmass is not homogeneous. According to this explanation - there is
a constantly varying vertical motion that has negative maxima either
side of the tug centreline and positive maxima some distance outboard of
the tug wingtips. This is consistent with the known vortex patterns - so
I think we can accept this is true.

Then we have a constantly varying effective angle of attack on the wing.
Some parts of the wing are at a lower, and others at a higher AoA than
for the "homogeneous airmass" case. So that would mean that on an
untwisted glider wing we are seeing the wing exposed to an angle of
attack varying by 4 or more degrees. The wing load distribution would be
distorted by these local variations in vertical speed of the airmass.

This means that at 1g, over the inboard section of the wing will be
producing less lift than in a homogeneous airmass, and the outboard
parts more. Given the normal load distribution for a glider, it is
reasonable to assume that the inboard section normally accounts for a
disproportionate amount of the lift. So it becomes plausible that the
entire wing may be at a higher aerodynamic AoA for the speed, to produce
the 1g lift required. (More lift coming from low lift sections of the
outboard wing) More importantly the geometric angle to the flight path
will be probably around 3-4 degrees higher than would be the case in
undisturbed air.

Some further thought on possible sources for the need for up elevator.
All the types I have heard mentioned in the thread have polyhedral wings
with an aerodynamic sweep back due to the multi trapezoidal shape. If
the lift distribution is moved outboard then one assumes that the centre
of pressure will also move aft due to geometry of the wing. If so - this
will introduce a nose down moment.
Similarly,if the glider is at a higher AoA and the vertical downwash of
the tug wing passes over the glider tailplane and it will result in a
lower relative AoA for the elevator. So needing more "up" elevator input
to balance.

So it is then possible that local but predictable variation in vertical
air mass movement is responsible for this effect.

So it looks like the wing MAY in fact operate at a higher angle of
attack for some of it's span, and this would be in the aileron portion
of the span, making all sorts of interesting things happen with induced
drag and local stalling etc. Which would in turn make the glider feel
unresponsive and "mushy" - while not being close to a stall inboard.

If that were the case then logic says we should use a little more flap
and unload the outboard part of the wing. Is there any empirical
evidence to support that?

Am I making sense here?

Bruce



On 2011/01/06 3:13 PM, Paula Bold wrote:
On 06.01.2011 12:18, ProfChrisReed wrote:

Is there anyone who has actually stalled on tow unintentionally and
noted the airspeed when the stall occurred? I'd guess not, as the
pilot's attention would probably be elsewhere..

I never stalled a glider unintentionally in tow so far ....


... but I stalled intentionally different gliders in tow behind
aircrafts, TMGs and Microlights in order to find limitations within tow.
And I noted well the differences in behavior and speed.

Doug and Andreas made the right observations with the correct
explanation. You may as well read the studies of Christian Ueckert, DLR
or the studies of DASSU/Stoeckl regarding use of TMGs for towing.

Did you ever look at the main wing of a canard aircraft, like the VariEze?

You may even see the built-in twist in the main wing due to the
downdraft of the canard wing on some pictures.
http://www.aero-auktion.com/angebotd...lectlotid=1786

In tow we have the overall fluid dynamics of a canard aircraft
(neglecting the two stabilisers).

On http://www.desktop.aero/appliedaero/...ardprocon.html
you may find
"Wing twist distribution is strange and CL dependent: The wing
additional load distribution is distorted by the canard wake."
as a inherent disadvantage of all canard aircrafts.

... maybe we should start pushing our gliders into the air instead of
towing ....

PB

--
Bruce Greeff
T59D #1771 & Std Cirrus #57

At 17:45 06 January 2011, BruceGreeff wrote:
Please think with me -

The argument that the wing has the same AoA for a given speed, only
applies in a homogeneous airmass.

Consider that lift generated is integrated over the wing as a function
of the local AoA, Airspeed, density etc. The geometric angle of the wing

to the flight path is constant (ignoring washout) So - what happens when

the airmass is not homogeneous. According to this explanation - there is

a constantly varying vertical motion that has negative maxima either
side of the tug centreline and positive maxima some distance outboard of

the tug wingtips. This is consistent with the known vortex patterns - so

I think we can accept this is true.

Then we have a constantly varying effective angle of attack on the wing.

Some parts of the wing are at a lower, and others at a higher AoA than
for the "homogeneous airmass" case. So that would mean that on an
untwisted glider wing we are seeing the wing exposed to an angle of
attack varying by 4 or more degrees. The wing load distribution would be

distorted by these local variations in vertical speed of the airmass.

This means that at 1g, over the inboard section of the wing will be
producing less lift than in a homogeneous airmass, and the outboard
parts more. Given the normal load distribution for a glider, it is
reasonable to assume that the inboard section normally accounts for a
disproportionate amount of the lift. So it becomes plausible that the
entire wing may be at a higher aerodynamic AoA for the speed, to produce

the 1g lift required. (More lift coming from low lift sections of the
outboard wing) More importantly the geometric angle to the flight path
will be probably around 3-4 degrees higher than would be the case in
undisturbed air.

Some further thought on possible sources for the need for up elevator.
All the types I have heard mentioned in the thread have polyhedral wings

with an aerodynamic sweep back due to the multi trapezoidal shape. If
the lift distribution is moved outboard then one assumes that the centre

of pressure will also move aft due to geometry of the wing. If so - this

will introduce a nose down moment.
Similarly,if the glider is at a higher AoA and the vertical downwash of
the tug wing passes over the glider tailplane and it will result in a
lower relative AoA for the elevator. So needing more "up" elevator
input
to balance.

So it is then possible that local but predictable variation in vertical
air mass movement is responsible for this effect.

So it looks like the wing MAY in fact operate at a higher angle of
attack for some of it's span, and this would be in the aileron portion
of the span, making all sorts of interesting things happen with induced
drag and local stalling etc. Which would in turn make the glider feel
unresponsive and "mushy" - while not being close to a stall inboard.

If that were the case then logic says we should use a little more flap
and unload the outboard part of the wing. Is there any empirical
evidence to support that?

Am I making sense here?

Bruce



I think so - flap should help, unless it's an integrated system - in
which case the ailerons droop as well and load the tips back up.

I can see there being an aft shift in centre of pressure as the tips
become more highly loaded, but the sweep angles are relatively small so
I'm not sure how big this effect would be.

At lower incidences any increased downwash over the tail should actually
help - 'up-elevator' corresponds to a downwards force on the tail, so
the tail is acting as an inverted wing. Downwash would make the tail
iangle of attack more negative and create more downforce, and hence more
nose-up pitching moment. However, if the downwash was large enough it
could possibly stall the tail - at which point you would lose elevator
authority and feel.
Just speculation though - the tug vortex/glider wing interaction is pretty
straightforward to model and predict, but a tug vortex/glider vortex/glider
tail interaction is much harder!

On Jan 6, 9:40*am, Doug Greenwell wrote:
At 16:11 06 January 2011, Andreas Maurer wrote:

On Thu, 06 Jan 2011 09:09:39 +0000, Doug Greenwell
wrote:

There's a chapter in Eric Brown's book 'Wings of the Weird &
Wonderful' in which he describes flight tests of the GAL 56 flying
wing
glider in 1946. *This was a 28deg swept wing with an aspect ratio of
5.8
towed by a Spitfire IX* (!!!) to 20000ft (!!). *

Coooooooooool.

every tug pilots dream ... wonder what the climb rate was like!





He describes the opposite effect, with a very strong (often
uncontrollable) nose-up pitch on take-off - this was thought to be due
to
ground effect. *In this case the tug span was similar (37ft) to the
glider
span (45ft), so the wake/wing interaction would be different.

Definitely. I think that the slipstream and the turbulence of that
huge propellor might have an influence, too.

Possibly - he had trouble getting the nose down on landing too.









Interestingly he also reports that the GAL56 could be flown hands-free
on
the tow - unless the tug slipstream was entered, in which case all
lateral
and longitudinal control was lost. *Robert Kronfield was later killed
spinning this aircraft.

Seems like some gliders actually stabilize themselves behind a tow
plane.

Here's an example of a free-flight test of a space shuttle model that
flew well in aerotow, but worse in free flight.

Ladies and gents, Great Britains only serious contribution to
spaceflight - the Reliant Shuttle:
http://www.youtube.com/watch?v=pJdrlWR-yFM

Andreas

That's a bit unfair ... we did manage one satellite into orbit on Black
Arrow- Hide quoted text -

- Show quoted text -

What about Skynet? I worked the Skynet 4 program.

Andy

What about Skynet?

Talk about thread drift! The Terminator has nothing to do with this
discussion thank you very much.

Have fun Cookie, but until you teach them to draw force diagrams, you
will have frustrating conversations.

I am a practicing aerodyanmicist, but will refrain from speculation on
the tow conditions discussed here. But I will suggest to anyone
trying to figure out what the required lift is, that they draw out the
forces in what is known as a force diagram. Also known as a free body
diagram. http://en.wikipedia.org/wiki/Force_diagram

In un-accelerated flight, which includes aerotow at a constant speed
and climb rate. All the forces, including gravity must add up to zero.