poor lateral control on a slow tow?

On Jan 1, 3:00*am, Doug Greenwell wrote:
At 23:25 31 December 2010, Andy wrote:





On Dec 31, 1:47=A0pm, Martin Gregorie
wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
On Dec 31, 6:19=A0pm, bildan *wrote:
On Dec 31, 4:40=A0am, "Doug" *wrote:

As an aerodynamicist/flight dynamicist recently re-soloed after
25
years off, people keep asking me hard questions. =A0One that has
com=
e
up recently is why a heavy glider on tow feels horrible, but
thermalling in the same glider at lower speeds is fine? (see also
Mike Fox's article on aerotowing in the October issue of S&G).

I did some calculations, and I reckon it's probably due to the
tug
wing wake (tip vortices generating a downwash inboard, upwash
outboard) changing the lift distribution on the glider wing -
with
a=
n
increased angle of attack out at the tips reducing aileron
effectiveness. =A0There's possibly an interesting academic
research
project here, but it's always best to get a reality check first
..

Is poor handling at low speed on tow a common experience? =A0I'd
appreciate any thoughts/comments/war stories ... particularly bad
tug/glider/speed combinations, incidents of wing drop during a
tow
etc etc?

Doug Greenwell

I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons.
=A0Overuse
o=
f
ailerons is very common and it makes aero tow 'wobbly'. =A0If you
consciously use rudder to aim the nose at the tug's tail and just
keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right
height
relative to the tug. =A0Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed
to
do so, will drift the glider to the center position and help keep
it
there. =A0I haven't noticed any tendency for them to yaw a glider
towa=
rds
a tugs wing tip.- Hide quoted text -

- Show quoted text -

There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being
asked
t=
o
climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a
greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.

If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will
make
the glider look quite nose-high to its pilot. =A0

I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not
what
its angle might be at the end of a tow rope.

--
martin@ =A0 | Martin Gregorie
gregorie. | Essex, UK
org =A0 =A0 =A0 |

I'd be surprised if the flow field from the towplane wake is
significant for gliders in normal high tow position. I do wonder if
the "sluggish controls" effect is to some extent psychological because
flying formation requires much more precision than normal slow flight
off tow. I'm most uncomfortable when I find myself slow and below the
towplane and need to climb up.

Unless the glider is accelerating vertically, I'm pretty sure that
steady climb requires the same amount of lift as steady glide. Steady
climb is not the same as accelerating climb. (F=3DMxA so if the lifting
force exceeds the glider's weight by definition it accelerates
vertically).

The towplane provides thrust to overcome the frictional and lift-
related drag losses, but unless you are well below the towplane the
force on the rope is, for all practical purposes, horizontal. If you
have a cg hook you will get a modest nose-up pitching moment from the
rope, but this is a trim issue more than an AOA issue I believe. The
tension on the rope could also provide some counter-force to rudder
and elevator inputs, but I don't think you'd feel much for small
angular displacements.

9B

It is surprising, but part of the problem is the word 'wake' ... in
order to generate lift a wing has to move a fair amount of air around
(although let's not start the bernoulli argument now!), so its influence
on the surrounding atmosphere extends a surprising distance away from it.
Tip vortices are also a very stable flow structure, so don't begin to
break up or decay for a very very long way downstream.

The climb angles are too small to make a significant difference to the
lift required from the glider wing (assuming the tow rope is straight),
since the effect on lift goes with the cosine of the angle

On the other hand, if the tow rope is not straight then there could be a
significant lift component from the tension force (going with the sine of
the tow rope angle) ... but you would have to be quite a long way above
the tug to make a big difference.



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’t 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