On Sunday, 20 December 2020 at 14:01:25 UTC, Martin Gregorie wrote:
On Sat, 19 Dec 2020 20:22:36 -0800, Eric Greenwell wrote:

Doesn't the majority of the wash or downflow from the wing pass under
the glider if it tows at the same altitude as the tug?

Thats definitely the case for a narrow layer containing propwash and
turbulence coming off the tug wing: quite obvious when you hit it, but
there's a general downflow above and below that turbulent sheet and a
matching upflow beyond the tug wingtips which can be seen in both flow
visualizations and, in some cases, in photos of aircraft flying in foggy
conditions which show the upflow extending out beyond the wingtips to at
least half of each wing semi-span. After all, wing lift is essentially
due to momentum transfer: a mass of air with a momentum equivalent to the
aircraft weight is being deflected downward by the wings, so this air
mass must occupy a fairly large volume below and behind the aircraft.

I still have vivid memories of going to Chobham Common for a spot of
model flying on a calm day with a solid, cloud base at 1000-1500 ft. The
road we were on was directly along the Heathrow approach path and we were
heading west, away from Heathrow. Suddenly a 747 dropped out of the
overcast ahead of us with flaps and wheels down. Its wing was scooping
off the bottom of the cloud layer and hurling it downwards, making the
downflow clearly visible under its wing. It must have extended down
20-25% of the wingspan, so was very clearly visible: looking at it was
like seeing the Niagara Falls streaming down below the wing, making it
quite obvious that this downflow was supporting 180 tons of aircraft.
For example, I
used to demonstrate the ease of positioning behind the towplane to
students by banking to left until the glider was way off center line,
and I never noticed any significant difference in the airflow from
center to far out to the left. This was with a 200' long towrope;
perhaps, with a much shorter rope, the experience would be a lot
different.

Yes, but that's in a fairly lightly loaded training glider. Some high
span competition types, e.g a JS-1C when fully ballasted, need a high tow
speed to avoid tip stalling. I've seen an absolute minimum tow speed of
77 kts quoted for a fully ballasted JS-1C. It seems likely that this is
at least partly due to the change in incident airflow along the wingspan
from the downflowing field behind the tug to the upflowing field which
extends much further out than its wingtips and immediate tip vortex. The
effect is to put the glider's tips at a higher AOA than the root, thus
cancelling the effect of any built-in washout in the wing.
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Martin | martin at
Gregorie | gregorie dot org

I agree with Martin's explanation. To my mind the fullest account of the issues towing very high wing-loading long span gliders is by Aldo Cernezzi in the Jan-Feb issue of Gliding International. The article is slightly misleadingly titled "The Creation of a Vortex". As the ex-owner of a JS1c 21m who had just launched in it before a fatal towing accident in another 21m JS1c my mind became very concentrated on this issue. I subsequently never launched it fully ballasted in 21m mode without getting a direct confirmation from the tow pilot that the minimum speed would be 75 knots. Not least because the ASIs in many tug planes over-read in flight as they don't use proper statics.