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
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