A Tale of Two Takeoffs

I must admit the downwash discussion is one I'm not fluent on. I've been around a while, but it's certainly an interesting concept I don't know much about. Makes you wonder how high a position you'd have to be for it not to matter...especially when you consider actual climb gradient, compared to AoA and angular fluid dynamics from the tug wing back to a glider 200' back. I haven't seen it in any book, but the books have been updated a few times and it doesn't mean it doesn't exist.

On Sat, 17 Jun 2017 19:57:22 -0700, Echo wrote:

I must admit the downwash discussion is one I'm not fluent on. I've been
around a while, but it's certainly an interesting concept I don't know
much about. Makes you wonder how high a position you'd have to be for it
not to matter...especially when you consider actual climb gradient,
compared to AoA and angular fluid dynamics from the tug wing back to a
glider 200' back. I haven't seen it in any book, but the books have been
updated a few times and it doesn't mean it doesn't exist.

Its even a bit worse than has been described: your outer wings and
ailerons are in the tow plane's wingtip vortex, which adds a still
further to the AOA difference due to the upward flaw in that part of the
vortex.

Low tow may be a better answer simply because it puts the whole glider
below the tugs downwash and tip vortices. It certainly feels smoother,
but doesn't sort out takeoff and initial climbout problems.

Side comment: the issue of wingtips extending beyond the tug's wing
downwash in pretty much unique to our sailplanes: all of the Allied troop
carrying gliders during WW2 had lower spans than their tugs (a Waco CG4
is smaller than a C47 - look it up if you don't believe me). The only
exceptions were the British Hamilcar (about 6 feet bigger span than the
Halifax tow plane and one or two of the German and Russian troop carriers
when towed by single engine aircraft and, of, course the Me 321 Gigant.


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

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. 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. I still have never noticed it in flight; It was explained differently to me many years ago, and it has always seemed to prove to be passing below me. If the glider keeps the towplane referenced on the horizon from high tow position, they're essentially level horizontal with each other. That means that the towplane is not dragging the glider through it's path, but rather the two are climbing together, a level horizontal plane going up. (Most gliders' drag in lbs at a tow speed is probably only 50ish pounds...so the tension on the rope wouldn't be much more in order to yield a climb at constant speed. Similar to the discussion on excess power required for climbs in the book Aerodynamics for Naval Aviators). If applying that theory of the horizontal plane climbing evenly, and towing at say 60kts and 500ft/min (for simple math), that's a climb gradient of 500ft/nm, or 5 degrees. So the relative wind is essentially from 5 degrees above the horizon, and downwash results in 5 degrees off below to the rear, or more depending on angle of incidence, etc. That, at 200, back, puts the theoretical downwash line a little over 15 feet below the level horizontal plane that the glider exists in with the towplane. So clearly in that theory, a weak towplane and a heavy glider has many issues beyond clearing the trees, but a strong towplane makes everything less of a factor.

Again that's just a theory, just like the perceived downwash one. My only point here is to say that all this stuff is theory, something from voodoo black magic aerodynamics. It would be truly interesting to hook up some smoke canisters across the wing of the tug and see where it all goes. Same with the tips.

I would love to hear the perspective of military guys here who have done many hours of aerial refueling...what they think about the presence of downwash and how much it affected their aircraft when sitting behind a tanker (with a much higher wing loading).

Jordan

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.


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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.

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.


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