motorgliders as towplanes

Mike I don't think the effect has anything to do with standard vs
flapped gliders. I've owned three flapped ships and this phenomenon
has perplexed me on each.

I had a tow in my ballasted ASG-29 yesterday and I'd told the tow
pilot to maintain minimum 80 mph. Several times he got down to 70 mph
(60 knots IAS) and I was fighting to maintain control of the glider,
at a speed that would be just a couple knots below best L/D were I in
free glide.

I've learned to ask for a speed that will keep the glider happy at
neutral flap setting (think standard class) and then lower the flaps
when the tug is flying to slow, and I have to do this at some point on
darn near every tow.

2NO

On Mar 12, 4:02*pm, Tuno wrote:
Mike I don't think the effect has anything to do with standard vs
flapped gliders. I've owned three flapped ships and this phenomenon
has perplexed me on each.

Region 12 contest at Cal City maybe 20 years ago, I was flying my
ASW-20B. Fully ballasted, behind a Maule the takeoff and first 4-500'
were OK, then inexplicably the tow pilot decides to slow down. I
start asking, then screaming for more speed, but to no avail - the
IDIOT decided to fly with the radio off for some reason.

I forget what the indicated airspeed was, but it was a little above
stall, perhaps 50-55 knots. With the stick at the aft stop, the
glider still settled slowly into low tow and that's where I stayed up
to release altitude. The ailerons had almost no authority - the stick
felt as sloppy as it would on the ground with no air loads on the
ailerons.

My left hand was holding the release as I waited for indications of a
spin entry. Had I been lower when this started to happen, the only
safe option would have been to release and land in the desert scrub.
But I was comfortable enough with the ship's stall / spin
characteristics (exciting to say the least, but manageable) that I
felt it was safer to just hang on to 2K or a thermal - again I don't
recall that detail. But I do remember that it took a good 15 - 20
minutes of thermalling away from the other contestants before I
stopped shaking from both fear and anger.

-Tom

I can't imagine why the stall speed would change on tow. The controls may
feel different because the tow rope is pulling on the nose, so any attempt
to turn or change the angle of attack will face an increased counter force,
but that's different than a change in the stall speed of the glider.

Mike Schumann

"Derek Copeland" wrote in message
...
Bruce,

So can you explain why the stalling speed definitely seems to increase
during an aerotow? Either the wing must be flying at a greater angle of
attack, i.e. producing more lift for a given airspeed, or the wing loading
must increase in some way.

As I said before, gliders that will quite happy fly at 40 knots in free
flight seem to need at least 50 knots on aerotow, even in smooth air. If
you aerotow behind a slow, low powered tug such as a motorglider, it often
seems to be quite difficult to keep up with its rate of climb, even though
it is very low. If you pull back the stick enough to do this, the glider
will start to buffet and the controls become rather ineffective. Both
symptoms of being close to the stall I believe! This is staying high
enough to avoid the tug's slipstream BTW, which could also produce
similar effects. If you do drop into the slipstream, it is often very
difficult to climb out of it again.

I agree that the accepted theory of flight says that in steady flight, the
vector of lift plus thrust must equal weight plus drag. I suppose that if
you had a tug powerful enough to produce enough thrust to more than equal
it's own weight plus the weight of the glider, then you could go
vertically up without the wings producing any lift.

Discuss!

Derek C


At 02:34 11 March 2009, Bruce Hoult wrote:
On Mar 11, 3:15=A0am, Derek Copeland wrote:
Remember that a glider has to produce more lift when climbing.


Er .. say what?

Any constant rate of climb (including flying level or constant
descent) requires exactly the same amount of upward force -- identical
to the weight of the aircraft.

In a powered aircraft flying level the weight and lift balance, and
the thrust and drag balance.

In a glider gliding, the lift from the wings is slightly less than the
weight (it is multiplied by the cosine of the glide angle), and the
balance of the upward force comes from drag (multiplied by the sine of
the glide angle).

In a glider being towed upwards, the lift from the wings is also less
than the weight (by the cosine of the climb angle), with the balance
of the upward force coming from the difference of the tow rope force
and the drag (multiplied by the sine of the climb angle).

If you're climbing at only a few hundred feet per minute while being
towed at 60 or 70 knots (6000 or 7000 fpm) then these angles are tiny
and the lift is essentially equal to the weight, but if a powerful
towplane could climb at, say, 45 degrees, then (far from having to
generate more lift than usual) your wings would only have to generate
lift equal to 70% of your weight.

On Mar 11, 6:33*pm, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
I can't imagine why the stall speed would change on tow. *The controls may
feel different because the tow rope is pulling on the nose, so any attempt
to turn or change the angle of attack will face an increased counter force,
but that's different than a change in the stall speed of the glider.

Read my earlier post!

The tow rope in some gliders (especially those standard class racing
gliders with a shallow angle of incidence) acts to pull the nose down,
reducing the angle of attack of the wing and tailplane. The stall
speed depends not only on speed, but angle of attack - if you reduce
it by pulling down on the nose, lift will be reduced. As I mentioned
earlier, the Discus 2 runs out of elevator authority somewhere below
60 knots and descends into low tow, even though its free-flight stall
speed is less than 40 knots. It's not just a difference of feel - the
glider wallows and almost becomes uncontrollable.

Mike

Running out of elevator authority is very different then stalling. A glider
stalls when the angle of attack increases past a critical point. Reducing
the angle of attack, increases your stall margin.

Mike Schumann

"Mike the Strike" wrote in message
...
On Mar 11, 6:33 pm, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
I can't imagine why the stall speed would change on tow. The controls may
feel different because the tow rope is pulling on the nose, so any attempt
to turn or change the angle of attack will face an increased counter
force,
but that's different than a change in the stall speed of the glider.

Read my earlier post!

The tow rope in some gliders (especially those standard class racing
gliders with a shallow angle of incidence) acts to pull the nose down,
reducing the angle of attack of the wing and tailplane. The stall
speed depends not only on speed, but angle of attack - if you reduce
it by pulling down on the nose, lift will be reduced. As I mentioned
earlier, the Discus 2 runs out of elevator authority somewhere below
60 knots and descends into low tow, even though its free-flight stall
speed is less than 40 knots. It's not just a difference of feel - the
glider wallows and almost becomes uncontrollable.

Mike

At moderate climb angles, and as long as you are not totally dangling on
the end a bit or rope (as in a helicopter lift), the glider is essentially
still in free flight during an aerotow. To produce enough lift to climb,
the glider's wing must be at a higher angle of attack for a given
airspeed than it would be in normal gliding flight. Therefore the airspeed
at which the wing stalls must be higher.

Or looking at it another way, the wing has to support the normal weight of
the glider and its occupants, plus the component of the weight acting
backwards and downwards, so an effective increase in wing loading.

Quite a few pilots who have aerotowed behind motorgliders and other slow,
low powered, tugs have commented on how the glider feels uncomfortably
close to the stall, even when the airspeed is a least 10 knots above the
normal stalling speed, so I am reasonably sure that this is a real effect.


Derek Copeland


At 18:19 13 March 2009, Mike Schumann wrote:
Running out of elevator authority is very different then stalling. A
glider
stalls when the angle of attack increases past a critical point.
Reducing

the angle of attack, increases your stall margin.

Mike Schumann

"Mike the Strike" wrote in message
...
On Mar 11, 6:33 pm, "Mike Schumann" wrote:
I can't imagine why the stall speed would change on tow. The controls
may
feel different because the tow rope is pulling on the nose, so any
attempt
to turn or change the angle of attack will face an increased counter
force,
but that's different than a change in the stall speed of the glider.

Read my earlier post!

The tow rope in some gliders (especially those standard class racing
gliders with a shallow angle of incidence) acts to pull the nose down,
reducing the angle of attack of the wing and tailplane. The stall
speed depends not only on speed, but angle of attack - if you reduce
it by pulling down on the nose, lift will be reduced. As I mentioned
earlier, the Discus 2 runs out of elevator authority somewhere below
60 knots and descends into low tow, even though its free-flight stall
speed is less than 40 knots. It's not just a difference of feel - the
glider wallows and almost becomes uncontrollable.

Mike

On 13 Mar, 20:15, Derek Copeland wrote:
At moderate climb angles, and as long as you are not totally dangling on
the end a bit or rope (as in a helicopter lift), the glider is essentially
still in free flight during an aerotow. To produce enough lift to climb,
the glider's wing must be at a higher angle of attack for a given
airspeed than it would be in normal gliding flight.

How much extra lift do you think is required to climb?

Or looking at it another way, the wing has to support the normal weight of
the glider and its occupants, plus the component of the weight acting
backwards and downwards, so an effective increase in wing loading.

I'm sorry, but that just doesn't make sense. Lift has to balance all
the other forces acting at right angles to the direction of flight.
You can't count the weight twice.

Ian