Glider Handling on Tow

At my club in the Uk we have a Pawnee and a Supercub for towing.
with 80 mph on the Pawnee the glider behind is gettting 60-65 knots...ok
for K21 's and the like but you get distressed LS8's and asg29 pilots
calling asking for more speed...especially if full of water.! This seems to
be caused by the pitot position with the Pawnee in a unatural climb
psositon with a glider on the back. Take the speed to 90mph and they are
happy as they get 70knots on the back!!
Now to the Super Cub( 180hp) With 60 mph on the dial the glider behind is
getting 60-65 Knots( same as Pawnee at indicated 80mph) Once again this I
believe is due to pitot and unnnatural climb angles. speed up to 65 mph and
the guys on the back are happy getting 65-70 knots
So we have 2 different tugs indicating 20 mph differences to give the same
speed to the glider!!!
If you know this , dont shout at the tug pilot but politely call on radio
for 'a few more knots please'
hope that helps
Regards from the Uk


At 18:44 11 November 2013, Andy Melville wrote:
At 13:14 10 November 2013, Papa3 wrote:
I've towed with my LS8 at many sites in many different conditions behind
ma=
ny different towplanes, and I've never had the problem you describe at
an
I=
AS (on my side) of above 65 kts. If the Pawnees (PGC?) were truly doing
80=
mph (70 kts), I can't imagine how this would be described as a "slo
tow".
=
I've had tows that dropped down into the low 60s or even high 50s
(kts)
w=
here it felt squirrely, but that's a full 10kts slower than what you are
de=
scribing.=20

Just a data point.=20

P3



On Saturday, November 9, 2013 10:40:05 AM UTC-5, John Carlyle wrote:
Thanks again for your thoughts, Steve. Sorry for my delay in
responding.=
=20

All of our slow tow incidents have occurred above 500 feet; none of us
ha=
s ever had a problem near the ground. Our airport elevation is 700 feet
wit=
h unobstructed horizons, and the temperatures on the days of th
incidents
=
were mostly in the 70s. The Pawnee pilots claimed that they had been
mainta=
ining 80 mph during the climb to that point; but when they went to 90
mph
t=
he handling problem for the remainder of the tow was gone.=20
=20

We tow with a Pawnee at our club as well. We were also having issues with the tow speeds showing approximately 5-10 knots (10-15 mph) slower on the glider's ASI than on the Pawnee's. This spread worsened when the Pawnee's side window was open. ( we had a window latch that was loose and occasionally would open during takeoff or tow ) There was a very apparent change in the pitch attitude required to maintain the proper indicated but wrong actual airspeed and the glider pilots would request a faster tow speed. I found the static line broken inside the fuselage, hidden between the fuselage fabric and steel tubing next to a bracket near the throttle. This leak obviously induced large errors into the static system especially with the window open. I imagine this could be an issue with many of the Pawnees around. The hard static line is very close to vibrating fabric and can also be damaged by ingress/egress from the cockpit if the fabric is pressed into the line. The line eventually work hardened and broke in two.

On Tuesday, November 12, 2013 6:50:00 AM UTC-5, Greg Delp wrote:
I found the static line broken inside the fuselage,
hidden between the fuselage fabric and steel tubing next
to a bracket near the throttle. This leak obviously
induced large errors into the static system
especially with the window open.

I imagine this could be an issue with many of the
Pawnees around.

Right, the aluminum static line is prone to cracking.
You can also see the ASI jump by opening and closing
the air vent when there's a static leak.

And of course some Pawnees have ASI on cockpit static...

Even without leaks the ASI will seriously over-indicate.

Unfortunately, lots of Pawnee pilots are not trained
about the issues in this thread.

Be careful out there,
Best Regards, Dave

Andy, you say for your gliders to indicate 60-65 kt (70-75 mph) on tow that your Pawnee must indicate 80 mph (70 kt) and that your Super Cub must indicate 60 mph (52 kt)? I figured that there might be some ASI error during climb, but that’s truly remarkable!

Greg, thanks for mentioning your Pawnee’s broken static line problem. If this is common, as Dave says, it may be the source of our problem (which just started this year).

-John, Q3

At 11:50 12 November 2013, Greg Delp wrote:
We tow with a Pawnee at our club as well. We were also having
issues with
t=
he tow speeds showing approximately 5-10 knots (10-15 mph)
slower on the
gl=
ider's ASI than on the Pawnee's. This spread worsened when
the Pawnee's
sid=
e window was open. ( we had a window latch that was loose
and occasionally
=
would open during takeoff or tow ) There was a very apparent
change in the
=
pitch attitude required to maintain the proper indicated but
wrong actual
a=
irspeed and the glider pilots would request a faster tow speed.
I found
th=
e static line broken inside the fuselage, hidden between the
fuselage
fabri=
c and steel tubing next to a bracket near the throttle. This leak
obviousl=
y induced large errors into the static system especially with the
window
op=
en. I imagine this could be an issue with many of the Pawnees
around. The
=
hard static line is very close to vibrating fabric and can also be
damaged
=
by ingress/egress from the cockpit if the fabric is pressed into
the line.
=
The line eventually work hardened and broke in two.

At 11:50 12 November 2013, Greg Delp wrote:
We tow with a Pawnee at our club as well. We were also having
issues with
t=
he tow speeds showing approximately 5-10 knots (10-15 mph)
slower on the
gl=
ider's ASI than on the Pawnee's. This spread worsened when
the Pawnee's
sid=
e window was open. ( we had a window latch that was loose
and occasionally
=
would open during takeoff or tow ) There was a very apparent
change in the
=
pitch attitude required to maintain the proper indicated but
wrong actual
a=
irspeed and the glider pilots would request a faster tow speed.
I found
th=
e static line broken inside the fuselage, hidden between the
fuselage
fabri=
c and steel tubing next to a bracket near the throttle. This leak
obviousl=
y induced large errors into the static system especially with the
window
op=
en. I imagine this could be an issue with many of the Pawnees
around. The
=
hard static line is very close to vibrating fabric and can also be
damaged
=
by ingress/egress from the cockpit if the fabric is pressed into
the line.
=
The line eventually work hardened and broke in two.

Our Pawnee shows an increase in climb rate if the window is
open.

Granted this is best with no wind conditions. But for those who find that
their tow planes continually seem to give slow tow speeds.
The next relatively calm morning put a handheld gps displaying ground speed
in the tow plane where the tow pilot can easily see it while towing. If
his indicated airspeed is notably different than what he sees for gps
ground speed, you have good grounds for a discussion. However if your
indicated airspeed was notably different then his gps ground speed, the
discussion may be go differently.

Another interesting tidbit would be for a glider while on tow at say 60 to
65 kts glider indicated airspeed. Release from tow and maintain that same
deck angle for say 13 seconds to see what indicated air speed the glider
finally stabilizes to. But remember it is critical that you maintain the
same pitch attitude relative to the horizon that you had while on tow.


At 11:50 12 November 2013, Greg Delp wrote:
We tow with a Pawnee at our club as well. We were also having issues with
t=
he tow speeds showing approximately 5-10 knots (10-15 mph) slower on the
gl=
ider's ASI than on the Pawnee's. This spread worsened when the Pawnee's
sid=
e window was open. ( we had a window latch that was loose and
occasionally
=
would open during takeoff or tow ) There was a very apparent change in
the
=
pitch attitude required to maintain the proper indicated but wrong actual
a=
irspeed and the glider pilots would request a faster tow speed. I found
th=
e static line broken inside the fuselage, hidden between the fuselage
fabri=
c and steel tubing next to a bracket near the throttle. This leak
obviousl=
y induced large errors into the static system especially with the window
op=
en. I imagine this could be an issue with many of the Pawnees around.
The
=
hard static line is very close to vibrating fabric and can also be
damaged
=
by ingress/egress from the cockpit if the fabric is pressed into the
line.
=
The line eventually work hardened and broke in two.

Having gone through all the posts on this thread, I'm amazed that no-one
has stated the obvious. the difference in handling between flying at 40
knots on tow and flying at 40 knots off tow.

In the first case you are trying to get the glider to climb so you have a
high angle of attack so you can follow the tow plane. The wings are
pulling more than 1g, you just can't fell it in the cockpit. In the second
you are descending, and you might even be pulling less than 1g.

Or did someone mention it and I missed it?

At 18:10 05 November 2013, John Carlyle wrote:
A question for any aerodynamicists out the why does low aero tow
speed
=
adversely affect the handling of a glider so drastically?

Occasionally I=92ve received an aero tow in my LS-8 (dry) at 55 kt
(minimum=
recommended aero tow speed is 54 kt). The plane is heavy and
unresponsive
=
at that speed; it=92s an extremely stressful experience! But once off tow
t=
he same plane handles like a dream at speeds down into the upper 30 kt
rang=
e.=20

Before anyone asks, yes, I do immediately ask the tow pilot for 10 kt
more
=
speed - right now! I=92d just like to understand what the root
aerodynamic
=
cause of the poor handling might be.=20

-John, Q3

On Tuesday, November 12, 2013 9:47:33 AM UTC-6, Geoff Brown wrote:
Having gone through all the posts on this thread, I'm amazed that no-one

has stated the obvious. the difference in handling between flying at 40

knots on tow and flying at 40 knots off tow.



In the first case you are trying to get the glider to climb so you have a

high angle of attack so you can follow the tow plane. The wings are

pulling more than 1g, you just can't fell it in the cockpit. In the second

you are descending, and you might even be pulling less than 1g.



Or did someone mention it and I missed it?

I don't think anyone has mentioned that. Could you expand a little on why you feel this is true?

On Tuesday, November 12, 2013 9:47:33 AM UTC-6, Geoff Brown wrote:
Having gone through all the posts on this thread, I'm amazed that no-one
has stated the obvious. the difference in handling between flying at 40
knots on tow and flying at 40 knots off tow.



In the first case you are trying to get the glider to climb so you have a
high angle of attack so you can follow the tow plane. The wings are
pulling more than 1g, you just can't fell it in the cockpit. In the second
you are descending, and you might even be pulling less than 1g.

Or did someone mention it and I missed it?

Alas, this is not true. On tow, you are pulling exactly 1 g and flying at exactly the same angle of attack. If anything, in fact, the slight upward pull of the towrope means a slightly lower lift from the wings. Imagine if it were pulling you straight up -- zero angle of attack, zero lift on the wings.

You are flying in an upward trajectory. The wings have a slightly higher angle relative to the horizon, but not relative to the oncoming air.

The discomfort may be because the nose is pointed higher. But since the glider is going up, this higher nose angle does not mean higher angle of attack.

Similarly, an earlier poster suggested releasing from tow and maintaining pitch attitude relative to horizon. This will not work, as now the air is coming at you in a descending direction. In fact, a major fault of many students after a rope break is NOT promptly lowering the pitch attitude so they can maintain the SAME angle of attack and airspeed.

I think we're down to the downwash effect.

The one time you need temporarily more angle of attack is if the towplane climbs suddenly. I think this is why it's particularly annoying to be close to the ground and the towplane takes off, then climbs abruptly while losing speed while the glider is still basically on the ground.

John Cochrane