A Tale of Two Takeoffs

On Sun, 18 Jun 2017 06:50:31 -0700, Echo wrote:

Wow that's interesting stuff. I once went to a WW2 glider museum in
Iron Mountain Michigan, but it was more about the manufacturing and
stories and less about the design and flight characteristics. The only
time I've ever noticed any kind of wash from a towplane is on pavement
behind a CAP 182. 100' into the takeoff roll, the right wing always
drops. It's a briefing item when we fly there. Spiraling slipstream of a
tri gear vs taildragger.

It's a shame we can't put a glider and towplane in a wind tunnel
together...or at least some smoke/fluid testing released from the
towplane wingtips.

There seems to have very little research into glider towing, not even in
the Akafliegs, which did surprise me.

Take a look at this:
Wake Turbulence Hazard Analysis For A General Aviation Accident, DLR
2014, DocumentID 340177

You'll need to run a search as I don't have the URL to hand. Its a report
on a crash when a Robin GR400 took off too close behind an Antonov AN-2,
so not directly about glider towing, but there is some good info and
numbers about tip vortexes.

A glider on a 200 ft rope is close enough to the tug to be flying in its
downwash field if it is in the normal tow position, with the glider just
above the turbulent prop wake, which is angled down behind the tug's
flight path at about 1/3 of its AOA. and you can get some idea of the
downwash depth at the glider's distance if you extrapolate from
assumption that the downwash thickness is about half the wing chord at
4-5 chords behind the wing. NOTE this the tug's wing-generated downwash
and has nothing to do with the turbulent prop wake: I don't know how that
is positioned in relation to the wing downwash, how far back it extends
or what its 3D shape might be.


So much of aerodynamic "fact" isn't really known,
but more speculated and generally accepted. Would be a pretty neat
study to actually watch said downwash.

It seems to me that this topic could be the basis of a really nice PhD
thesis for an aerodynamicist.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

IIRC Gliding International magazine had something on towplane/glider interference sometime in the last year.

My limited exposure to aerodynamics textbooks leaves me with the
impression that airflow behind the vehicle is neglected.

When heavy jets came out, there was a spate of light aircraft crashes in
wake
encounters, followed by NASA research and guidance on avoiding.

I have not come across any formulae for air behavior behind aircraft. There

are simply general statements that the air will be descending and vortices

will trail the wing tips.

Perhaps a good analogy for being behind a towplane is that it has
similarities
for approaching a ridge from downwind - a bad place for being slow.

At a flyi that included various flying and landing contests, I was helping with the release of helium balloons for the pilots to try and pop. Harder than some would imagine, as there were a lot more misses than hits (except for ace pilots such as myself). Several of the missed balloons got sucked into the wing tip vortices where they almost stayed in place while rotating at least several hundred rpm. This experience, and other explanations of tip vortices, led me to believe they were of small diameter directly behind the aircraft and expanded in diameter the further back they got while sinking at several hundred feet per minute.

This picture is from "wiki".
https://upload.wikimedia.org/wikiped...ortex_edit.jpg

I was on a photoshoot for a backcountry flying video with my Husky at Nevada's Black Rock Desert. Near the north end of the desert there are a couple of small playas that are protected from the wind, surrounded by small mountains. The playa surface had fine alkali dust that readily showed the air disturbance behind the Husky, which normally takes off full flaps. As soon as the plane started its take off roll, each tip vortice looked to be about 20 feet in diameter, sucking the fine dust up from the ground and rolling it up and over onto the wing reaching almost to the fuselage - it looked impressive.

The Husky has a perhaps undeserved* reputation for "Moose Stalls". So named as the aircraft is typically circling low over game counting animal populations or doing photograph. It is thought the aircraft, while circling tightly, dirty or "slowed up" with flaps extended, flies into its own wake causing a low altitude stall and loss of control. I have flown into my own wake doing this, though at higher altitude with room to recover - it's an eye opener.

*Undeserved, not because it doesn't happen with the Husky, but rather that the Husky has been used by many state Fish and Games for animal surveys, predator control etc. - lots of exposure. There have been Super Cub crashes under similar circumstances.

http://cdn.shopify.com/s/files/1/011...f_jets.pdf?151
Page 2 "Wake turbulence"

At 07:40 19 June 2017, bumper wrote:
At a flyi that included various flying and landing contests, I was
helping
=
with the release of helium balloons for the pilots to try and pop. Harder
t=
han some would imagine, as there were a lot more misses than hits (except
f=
or ace pilots such as myself). Several of the missed balloons got sucked
in=
to the wing tip vortices where they almost stayed in place while rotating
a=
t least several hundred rpm. This experience, and other explanations of
tip=
vortices, led me to believe they were of small diameter directly behind
th=
e aircraft and expanded in diameter the further back they got while
sinking=
at several hundred feet per minute.

This picture is from "wiki".
https://upload.wikimedia.org/wikiped...lane_vortex_e=
dit.jpg/220px-Airplane_vortex_edit.jpg

I was on a photoshoot for a backcountry flying video with my Husky at
Nevad=
a's Black Rock Desert. Near the north end of the desert there are a
couple
=
of small playas that are protected from the wind, surrounded by small
mount=
ains. The playa surface had fine alkali dust that readily showed the air
di=
sturbance behind the Husky, which normally takes off full flaps. As soon
as=
the plane started its take off roll, each tip vortice looked to be about
2=
0 feet in diameter, sucking the fine dust up from the ground and rolling
it=
up and over onto the wing reaching almost to the fuselage - it looked
impr=
essive.

The Husky has a perhaps undeserved* reputation for "Moose Stalls". So
named=
as the aircraft is typically circling low over game counting animal
popula=
tions or doing photograph. It is thought the aircraft, while circling
tight=
ly, dirty or "slowed up" with flaps extended, flies into its own wake
causi=
ng a low altitude stall and loss of control. I have flown into my own
wake
=
doing this, though at higher altitude with room to recover - it's an eye
op=
ener.

*Undeserved, not because it doesn't happen with the Husky, but rather
that
=
the Husky has been used by many state Fish and Games for animal surveys,
pr=
edator control etc. - lots of exposure. There have been Super Cub crashes
u=
nder similar circumstances.

I feel like retitling this from TTTO (Tale of Two Takeoffs) to SSST (Survival Strategies for Slow Tows).

Does this mean that if we're trapped on a slow tow, we should drop down into low tow position...or move up even higher than normal (if that's possible by then)...or move out to the side--all to escape the downwash?

Chip Bearden
"JB"

On Monday, June 19, 2017 at 12:16:35 PM UTC-4, wrote:
I feel like retitling this from TTTO (Tale of Two Takeoffs) to SSST (Survival Strategies for Slow Tows).

Does this mean that if we're trapped on a slow tow, we should drop down into low tow position...or move up even higher than normal (if that's possible by then)...or move out to the side--all to escape the downwash?

Chip Bearden
"JB"

Speaking as a guy who's been the you ain't gonna have the choice. You are going to be heading for deep low tow whether you like it or not. Whether you fall off depends on what your tow pilot does next.

Evan Ludeman / T8

Responding to the slow tow matter...

I've written on this subject here on RAS before as it's one of my pet pive safety concerns. I've had a few seriously slow tows with water. My experience is that you will drop into low tow whether you want to or not - there's no choice to be had. In the extreme case, expect to be hanging lower than "low tow" at a high pitch angle slamming alierons to the stops. When this ocurrs at low altitude, there's no possibility of recovering should you disconnect from the rope.

I now keep printed instructions for towing with water ballast in the side compartment of my glider. If I don't have the chance to talk to the tow pilot immediately before the tow, then I will have the ground crew deliver written instructions to the tuggie as I'm being hooked up. Written instructions are less likely than a radio call to be confused, forgotten or misunderstood.

Here's a pdf of my written instruction sheet:
https://drive.google.com/file/d/0B_X...w?usp=drivesdk

On Mon, 19 Jun 2017 03:58:42 +0000, George Haeh wrote:

My limited exposure to aerodynamics textbooks leaves me with the
impression that airflow behind the vehicle is neglected.

When heavy jets came out, there was a spate of light aircraft crashes in
wake encounters, followed by NASA research and guidance on avoiding.

I have not come across any formulae for air behavior behind aircraft.
There

FWIW the angles and wake thicknesses I quoted work well for free flight
model design, where we know from experience that a model with its
tailplane in the wing wake is not stable in pitch. I've used these values
when designing an F1A with its wing on a low pylon to check that the
tailplane would outside the wing wake. Drawing the diagram said the
tailplane should be in clean air above the wing wake. The model's in-
flight behaviour shows that was a good prediction.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |