At 19:29 01 January 2011, Gary Osoba wrote:


Span differential, the nature of wake roll-ups, and effects in the
larger free stream.

An airfoil moving through a viscous media makes quite a disturbance.
Among other things, it results in relative upwash upstream in the flow
field, downwash aft in the flow field, and effects which are
vertically displaced in the flow field as well.

However, lateral influence in the flow field- outside the wake rollup
at the tips- is of special interest here. Wake rollups with vorticity
do not spread the energy spent in achieving pressure equilibrium very
efficiently. That is why displacing the event over a larger area such
as laterally (as in more span) or vertically (as in the case of
winglets) makes the wing more efficient. Since the wing doesn=92t do a
very good job of inducing lift beyond the tip on the other side of the
wake rollup, the downwash immediately aft of the wing is significantly
greater than the downwash aft of the wing and a meter or two outboard
of the tips.

This lateral downwash differential is preserved in the aft flow field,
albeit to lesser degrees with increasing distance until the free
stream reaches unity. However, when being towed slow and heavy it
doesn't take much to create a noticeable effect. In the case of a tow
plane with 10-11 meter span towing a glider of 15 meter span, the
downwash aft of the towplane and inboard on the glider span is greater
than the free stream field meeting the tips and the ailerons. The
effect is that a glider under tow can behave more like a design with
wings geometrically twisted in the wrong direction- with ailerons
operating near the stall. The effect increases with increasing
downwash required of the towplane.

One way to check this effect would be tow behind a motorglider of
greater span than your glider. This should provide for a better match
of downwash angles across your span.Get all the climb you can for a
given airspeed. Time your roll rates. Then tow behind a conventional
towplane at the same speed and same climb rate as the first case. Time
and compare roll rates.

You can also check numerically by calculating the rolling moments and
taking into account the assymetrical lift distributions using the
methods of Multhopp and Redeker. However, arriving at the effective
angles of attack across the span, in a modified and vertically
displaced flow field 200 feet aft of the tow plane might be rather
difficult. Several angle of attack probes positioned in front of your
wing and distributed along the span would likely be the better
approach.

If a towplane could push rather than pull a glider, the effect would
be reversed and the aileron authority would increase.

Best Regards,

Gary Osoba


Absolutely - I used a simple vortex lattice method (AVL) to come to the
same conclusion. For the relatively short spacing between glider and
towplane the lack of a wake roll-up model in this code probbaly doesn't
affect the adverse change in spanwise lift distribution on the glider wing
- however, modelling any more complex interactions between tug and glider
vortices would need a proper CFD study. The other interesting element is
the effect of bank - with 15m+ wing spans it doesn't take much of a roll
angle to put one tip right into the tug wake, leading to yet more
asymmetric effects.

Most experimental wake vortex interaction studies (eg recent Airbus A380
studies) have used models in big ship tow tanks to get long vortices, but
because glider and tug are so close, we could probably use a large wind
tunnel. Another possibility would be to fly a motor glider behind a tug
aircraft, in order to take out any effect of rope angle.

It would probably be difficult to get someone to fund it though, since the
solution is simple - tow faster :-)

basic conclusions towthis doesm