motorgliders as towplanes

I put my monies on the elevator authority/AoA ratio. * *We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. *Box the wake, it will tell you
where it is and where it isn't...

That'll tell you where the expanding conical turbulent wake of the
prop is ... there's also the smooth effect of the vortex sheet shed
from the wing to consider. One useful interpretation of the induced
drag is the effect the vortex sheet (modelled as tow tip vortices) has
on the local angle of attack of the wing ... the same will apply to
anything behind the tug ... like a glider ...

Ian

Not as I understand it, I am of the thought that the wake as we know
it IS the sheet of downwash from the wings, and the propwash is quite
insignificant compared with the disturbance from the wings, although
usually residing somewhere within this greater wake although can be
higher up too. If you are in a fabric ship, the propwash can sometimes
be detected sometimes by the pulsation on the skin like a drumbeat,
but in modern glass not so much. I've been wronger before, but my
money is still on the elevator authority/AoA ratio.

-Paul

On Sat, 14 Mar 2009 13:23:37 -0700, The Real Doctor wrote:

On 14 Mar, 17:24, sisu1a wrote:
Agreed. My money is on the towplane wake.

I put my monies on the elevator authority/AoA ratio. Â* Â*We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Â*Box the wake, it will tell you
where it is and where it isn't...

That'll tell you where the expanding conical turbulent wake of the prop
is ... there's also the smooth effect of the vortex sheet shed from the
wing to consider. One useful interpretation of the induced drag is the
effect the vortex sheet (modelled as tow tip vortices) has on the local
angle of attack of the wing ... the same will apply to anything behind
the tug ... like a glider ...

As an approximation the downwash angle behind a wing is 1/3 of its AOA,
so that will add something like 2 to 3 degrees nose-up attitude to the
glider.


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

On 14 Mar, 21:15, Derek Copeland wrote:
Most (sensible) people fly either above the tug's slipstream or below it.

That's the propellor wake. There is also a much larger region of
disturbed, but not turbulent, airflow caused by the action of the
wing. As a rough guide, there are significant effects in a cylinder
two wingspans in diameter (ie twice as wide as the wing) centred on
the tug's flight path and extending back to the point where lift
started being developed. In practice, of course, viscosity in the air
damps it out, but it will still be a significant effect at the other
end of the tow rope.

(The other end of the wing's wake is the starting vortex on the
runway. Nasty thing have happened to light aircraft which flew into a
starting vortex.)

In the first case the tug may be pulling the glider's nose down and in
the second case up. It doesn't seem to make a lot of difference to the
way the glider flies. There is a theory, mostly believed in by the good
folks of Oz, that low tow is slightly more stable.

It makes sense. In low tow, the pull of the towrope is upwards, so it
tends to pitch the glider nose up. If you sink a bit (with controls
held steady) the pitching up moment increases and the glider climbs
back to position. If you rise a bit the pitching up moment decreases
and the control forces you have been using to balance it pitch the
nose down and the glider falls back to position.

High tow is just the opposite: glider rises, pitching up moment
increases, glider rises, pitching up moment increases and so on until
either (a) the glider pilot does something about it or (b) the tug
pilot dies.

OK, it's not quite that simple, or impossible to control (clearly),
but a trimmed glider in low tow will generally be more stable than in
high tow.

I can't somehow
imagine that the downwash from the tug has that much effect on a glider on
the end of a 150ft rope!

See above. The downwash from the wing of a 757 has a considerable
effect several miles from the aircraft.

Ian

On 14 Mar, 21:26, sisu1a wrote:

Not as I understand it, I am of the thought that the wake as we know
it IS the sheet of downwash from the wings, and the propwash is quite
insignificant compared with the disturbance from the wings, although
usually residing somewhere within this greater wake although can be
higher up too.

Hmm. All I need to do is find a tug pilot who is willing to switch the
engine off mid-tow and I can investigate this properly.

Ian

At 11:24 14 March 2009, The Real Doctor wrote:
Summary: in a steady climb or glide, the lift needed from a
glider's wing is as equal to the weight as makes no
difference ...

Hmmm. In a glider with C of G in the correct place the tail plane will be
producing a down thrust, hence the wing will need to generate lift greater
than the weight of the glider when in more or less level flight. The more
back stick, the more down thrust and the harder the wing will need to work
to maintain level flight.

Now my Discus turbo stalls in free flight in the low 40s (kts). With the
engine out and the thrust line well above the fuselage, thus pushing the
nose down, the best climb speed in the POH is given as 49 to 54 kts
depending on weight. If I let if fall to about 45 kts it is still
perfectly controllable - just less efficient. However aerotowing on the
nose hook at that speed would not be a happy experience. I have never
done a slow tow on a belly hook so don't know if the symptoms are the
same.

What I have observed on slow tows, and has been reported by others in this
thread, is that the ailerons are ineffective, the glider tends to wallow,
the stick is a long way back and the nose high - even though the speed is
above the normal 1G stalling speed. These seem to be symptoms of an
accelerated stall or incipient spin - but the rope pulling ahead seems to
stop the glider rotating into a spin with the associated wing drop.

I wish I knew the answer, but if I have the symtoms of an accelerated
stall in more or less straight level flight above the 1g stall speed that
sugests that the wing is generating more lift than the weight of the
glider for some reason; if the stick is near the back-stop at 50 kts there
is more elevator downforce so the wing will have to compenstate for that.
The pull from the rope may be slightly down if I'm in high tow,
especially with a long heavy rope with a slight bow in it, but to raise
the stall speed from say 42 to 50 kts (* 1.2) is equivalent to an increase
in load on the wing from 1g to about 1.4g. At a take-off weight of say 475
kg this is equivalent to an additional 190kg or another 418 lb. Where
could this come from??? The downthrust from the elevator fighting the tow
rope?

On 15 Mar, 00:45, Big Wings wrote:
At 11:24 14 March 2009, The Real Doctor wrote:

Summary: in a steady climb or glide, the lift needed from a
glider's wing is as equal to the weight as makes no
difference ...

Hmmm. *In a glider with C of G in the correct place the tail plane will be
producing a down thrust, hence the wing will need to generate lift greater
than the weight of the glider when in more or less level flight.

If the glider has been well designed,. the tail will be
producingupboard no down- (or up-) thrust at best L/D. The tail load
is still small as the speed varies from best L/D: look at how small
tails generally are and how light the fixing compared to those of the
wings. Very ball park figures: the tail is typically 1/15 of the span
of the wing and 1/2 of the chord. That's 1/30 of the area, so all
other things being equal you wouldn't evry expect the tail to produce
more than about 1/30 of the lift the wing can. So even in the worst
case - a high speed dive or zoom) the tail force probably doesn't
change the wing loading by more than +/- 3%.

The more
back stick, the more down thrust and the harder the wing will need to work
to maintain level flight.

Remember, by the way, that in normal flight the tail produces *down*
force in a dive (stick forward) and *up* force in a zoom. Yes, it's
cambered the wrong way - don't dive or zoom, kids, it ain't efficient.

What I have observed on slow tows, and has been reported by others in this
thread, is that the ailerons are ineffective, the glider tends to wallow,
the stick is a long way back and the nose high - even though the speed is
above the normal 1G stalling speed. *These seem to be symptoms of an
accelerated stall or incipient spin - but the rope pulling ahead seems to
stop the glider rotating into a spin with the associated wing drop.

It would be interesting to know if the glider really *is* near the
stall or if it just exhibits some of the characteristics of being near
the stall. Hmm, there's an interesting experiment there ... I'm not
sure that I'd care to do it.

The behaviour of the Pirat on aerotow is rather different, by the way.
At low speeds it handles very nicely, but if the tuggy is a bit
enthusiastic the ailerons get horribly heavy and ineffective. Quite
the opposite of wallowing, really. At the same speed off tow they are
light and responsive. I'm guessing that the tug downwash affects the
centre of the wing more, effectively increasing the washout.
Meanwhile, I just cajole tuggies into flying with the CHT just below
the red ...

Ian

At 13:43 14 March 2009, The Real Doctor wrote:

Bad example, since tow planes pull - give or take a wee bit -
horizontally, regardless of climb angle.


Well, our tow plane certainly does not pull horizontally!

I figure it pulls at about a 10:1 angle or about 5.5 degrees.

So if you didn't like my last example here is some more for you:

If we took a 900# glider and pulled it up at 5.5 degrees climb angle, the
lift would be 895#......

Just for ****s and giggles, if we had a (lot) more more powerful towplane
we could get the following:

Climb angle 10 degrees......lift 886#
Climb angle 20 degrees......lift 845#
Climb angle 30 degrees......lift 779#
Climb angle 40 degrees......lift 689#
Climb angle 50 degrees......lift 578#
Climb angle 60 degrees......lift 450#
Climb angle 70 degrees......lift 307#
Climb angle 80 degrees......lift 156#
Climb angle 90 degrees......lift 0#

As climb angle increases, lift decreases!

Same for dive angle!

Granted, at "normal" climb and descent angles the reduction in lift is
tiny, but it is a reduction non the less. Just trying to dispell the
rumor that lift somehow must increase in a climb.

Cookie

At 17:24 14 March 2009, sisu1a wrote:

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed,

Not true!

The angle of attack does not have to be high just because the nose is
high. The direction of flight is forward and upward. The angle of
attack is the angle of the wing VS the oncoming airflow, NOT the angle of
the nose relative to the ground.

Remember the towplane is adding power (thrust) to the equation.

With lots of power, a plane and climb rapidly with a relatively low angle
of attack. (so can a glider on tow)

A glider being towed at a relatively high speed will have a relatively low
angle of attack. A glider being towed at a relatively slow speed will
have a relatively high angle of attack. This is independant of the angle
of the glider's nose to the ground and independant of the glider's climb
angle (Direction of flight).


Cookie

I took a good look at the geometry of my tow today. The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. Gurus please explain.

Mike

You are confusing a lack of elevator authority with stalling. The two are
completely different phenomenon.

Mike Schumann

"Mike the Strike" wrote in message
...
I took a good look at the geometry of my tow today. The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. Gurus please explain.

Mike