Arduino Based Soaring Related Devices

On Sat, 19 Dec 2020 16:24:01 -0800, Eric Greenwell wrote:

As we all know, tow ropes do not break in steady flight! It's dynamic
loads from turbulence and piloting that put the peak loads on the rope;
nonetheless, the average load (say, over 1 minute) will be close to the
simple physics of lifting the weight of the glider at the rate of climb.
That number doesn't have much value in our operational choices, I think.

Yep. I only made an attempt at calculating it a while back because I was
curious about the tension in the tow rope under during normal operating
conditions.

I think there are other towing factors that are probably more important
to understand. For instance, the aerodynamics of towing our gliders with
our typical tow planes are quite different from those of the majority of
military glider tows because almost for virtually all military towing the
tow plane has a bigger wingspan than the glider. This was the case for
all British and US operations in WW2 and for most German towing too.

In fact, the only cases I've found where the military glider was bigger
span than the tug was the ME 321 Gigant (the Gigant was bigger than its
He-111Z towplane) and the DFS 230 when it was being towed by a BF-109 or
Bf-110.

Conversely the only civilian gliders I'm aware of that are smaller than
their towplane are Perlan 2 when the Grob G520 Egrett is towing it
and an SGS 1-26 behind a Piper Cub.

This can matter, because if the glider is smaller than its tug, its
entire wing is operating in the downwash from the tug's wing, while if
the glider is bigger than its tug, then, while the inner part of its wing
is in the downwash behind the tug's wing, the outer parts of its wing
project through the tug's tip turbulence and into the upwash created by
the outer parts of the tug's tip vortex and may well give an tendency for
the glider to tip stall if the tow speed is too slow.


--
--
Martin | martin at
Gregorie | gregorie dot org

Martin Gregorie wrote on 12/19/2020 6:52 PM:
On Sat, 19 Dec 2020 16:24:01 -0800, Eric Greenwell wrote:

As we all know, tow ropes do not break in steady flight! It's dynamic
loads from turbulence and piloting that put the peak loads on the rope;
nonetheless, the average load (say, over 1 minute) will be close to the
simple physics of lifting the weight of the glider at the rate of climb.
That number doesn't have much value in our operational choices, I think.

Yep. I only made an attempt at calculating it a while back because I was
curious about the tension in the tow rope under during normal operating
conditions.

I think there are other towing factors that are probably more important
to understand. For instance, the aerodynamics of towing our gliders with
our typical tow planes are quite different from those of the majority of
military glider tows because almost for virtually all military towing the
tow plane has a bigger wingspan than the glider. This was the case for
all British and US operations in WW2 and for most German towing too.

In fact, the only cases I've found where the military glider was bigger
span than the tug was the ME 321 Gigant (the Gigant was bigger than its
He-111Z towplane) and the DFS 230 when it was being towed by a BF-109 or
Bf-110.

Conversely the only civilian gliders I'm aware of that are smaller than
their towplane are Perlan 2 when the Grob G520 Egrett is towing it
and an SGS 1-26 behind a Piper Cub.

This can matter, because if the glider is smaller than its tug, its
entire wing is operating in the downwash from the tug's wing, while if
the glider is bigger than its tug, then, while the inner part of its wing
is in the downwash behind the tug's wing, the outer parts of its wing
project through the tug's tip turbulence and into the upwash created by
the outer parts of the tug's tip vortex and may well give an tendency for
the glider to tip stall if the tow speed is too slow.

Doesn't the majority of the wash or downflow from the wing pass under the glider if it tows at
the same altitude as the tug? For example, I used to demonstrate the ease of positioning behind
the towplane to students by banking to left until the glider was way off center line, and I
never noticed any significant difference in the airflow from center to far out to the left.
This was with a 200' long towrope; perhaps, with a much shorter rope, the experience would be a
lot different.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

Its all vary admirable that you are trying to calulate the nominal load while under tow, but it is not needed for the task at hand. You just need to know when it is non zero. A simple push button switch such that when the cable is under load it pushes on the switch. Design a link that come in contact under load. All the load goes to the link, and the switch detects that it is closed. A light spring seperates the link to open the contact with an empty rope. The monitor sees that the link is open and if open long enough (longest conceivable slack rope duration) and records the hight when it first went slack.

All that said, Eric's human factors solution (don't do soft releases, or say "thanks" on the radio) will avoid over charge.

On Sat, 19 Dec 2020 20:22:36 -0800, Eric Greenwell wrote:

Doesn't the majority of the wash or downflow from the wing pass under
the glider if it tows at the same altitude as the tug?

Thats definitely the case for a narrow layer containing propwash and
turbulence coming off the tug wing: quite obvious when you hit it, but
there's a general downflow above and below that turbulent sheet and a
matching upflow beyond the tug wingtips which can be seen in both flow
visualizations and, in some cases, in photos of aircraft flying in foggy
conditions which show the upflow extending out beyond the wingtips to at
least half of each wing semi-span. After all, wing lift is essentially
due to momentum transfer: a mass of air with a momentum equivalent to the
aircraft weight is being deflected downward by the wings, so this air
mass must occupy a fairly large volume below and behind the aircraft.

I still have vivid memories of going to Chobham Common for a spot of
model flying on a calm day with a solid, cloud base at 1000-1500 ft. The
road we were on was directly along the Heathrow approach path and we were
heading west, away from Heathrow. Suddenly a 747 dropped out of the
overcast ahead of us with flaps and wheels down. Its wing was scooping
off the bottom of the cloud layer and hurling it downwards, making the
downflow clearly visible under its wing. It must have extended down
20-25% of the wingspan, so was very clearly visible: looking at it was
like seeing the Niagara Falls streaming down below the wing, making it
quite obvious that this downflow was supporting 180 tons of aircraft.

For example, I
used to demonstrate the ease of positioning behind the towplane to
students by banking to left until the glider was way off center line,
and I never noticed any significant difference in the airflow from
center to far out to the left. This was with a 200' long towrope;
perhaps, with a much shorter rope, the experience would be a lot
different.

Yes, but that's in a fairly lightly loaded training glider. Some high
span competition types, e.g a JS-1C when fully ballasted, need a high tow
speed to avoid tip stalling. I've seen an absolute minimum tow speed of
77 kts quoted for a fully ballasted JS-1C. It seems likely that this is
at least partly due to the change in incident airflow along the wingspan
from the downflowing field behind the tug to the upflowing field which
extends much further out than its wingtips and immediate tip vortex. The
effect is to put the glider's tips at a higher AOA than the root, thus
cancelling the effect of any built-in washout in the wing.


--
--
Martin | martin at
Gregorie | gregorie dot org

On Sunday, 20 December 2020 at 14:01:25 UTC, Martin Gregorie wrote:
On Sat, 19 Dec 2020 20:22:36 -0800, Eric Greenwell wrote:

Doesn't the majority of the wash or downflow from the wing pass under
the glider if it tows at the same altitude as the tug?

Thats definitely the case for a narrow layer containing propwash and
turbulence coming off the tug wing: quite obvious when you hit it, but
there's a general downflow above and below that turbulent sheet and a
matching upflow beyond the tug wingtips which can be seen in both flow
visualizations and, in some cases, in photos of aircraft flying in foggy
conditions which show the upflow extending out beyond the wingtips to at
least half of each wing semi-span. After all, wing lift is essentially
due to momentum transfer: a mass of air with a momentum equivalent to the
aircraft weight is being deflected downward by the wings, so this air
mass must occupy a fairly large volume below and behind the aircraft.

I still have vivid memories of going to Chobham Common for a spot of
model flying on a calm day with a solid, cloud base at 1000-1500 ft. The
road we were on was directly along the Heathrow approach path and we were
heading west, away from Heathrow. Suddenly a 747 dropped out of the
overcast ahead of us with flaps and wheels down. Its wing was scooping
off the bottom of the cloud layer and hurling it downwards, making the
downflow clearly visible under its wing. It must have extended down
20-25% of the wingspan, so was very clearly visible: looking at it was
like seeing the Niagara Falls streaming down below the wing, making it
quite obvious that this downflow was supporting 180 tons of aircraft.
For example, I
used to demonstrate the ease of positioning behind the towplane to
students by banking to left until the glider was way off center line,
and I never noticed any significant difference in the airflow from
center to far out to the left. This was with a 200' long towrope;
perhaps, with a much shorter rope, the experience would be a lot
different.

Yes, but that's in a fairly lightly loaded training glider. Some high
span competition types, e.g a JS-1C when fully ballasted, need a high tow
speed to avoid tip stalling. I've seen an absolute minimum tow speed of
77 kts quoted for a fully ballasted JS-1C. It seems likely that this is
at least partly due to the change in incident airflow along the wingspan
from the downflowing field behind the tug to the upflowing field which
extends much further out than its wingtips and immediate tip vortex. The
effect is to put the glider's tips at a higher AOA than the root, thus
cancelling the effect of any built-in washout in the wing.
--
--
Martin | martin at
Gregorie | gregorie dot org

I agree with Martin's explanation. To my mind the fullest account of the issues towing very high wing-loading long span gliders is by Aldo Cernezzi in the Jan-Feb issue of Gliding International. The article is slightly misleadingly titled "The Creation of a Vortex". As the ex-owner of a JS1c 21m who had just launched in it before a fatal towing accident in another 21m JS1c my mind became very concentrated on this issue. I subsequently never launched it fully ballasted in 21m mode without getting a direct confirmation from the tow pilot that the minimum speed would be 75 knots. Not least because the ASIs in many tug planes over-read in flight as they don't use proper statics.

On Sunday, 20 December 2020 at 16:52:39 UTC, John Galloway wrote:
On Sunday, 20 December 2020 at 14:01:25 UTC, Martin Gregorie wrote:
On Sat, 19 Dec 2020 20:22:36 -0800, Eric Greenwell wrote:

Doesn't the majority of the wash or downflow from the wing pass under
the glider if it tows at the same altitude as the tug?

Thats definitely the case for a narrow layer containing propwash and
turbulence coming off the tug wing: quite obvious when you hit it, but
there's a general downflow above and below that turbulent sheet and a
matching upflow beyond the tug wingtips which can be seen in both flow
visualizations and, in some cases, in photos of aircraft flying in foggy
conditions which show the upflow extending out beyond the wingtips to at
least half of each wing semi-span. After all, wing lift is essentially
due to momentum transfer: a mass of air with a momentum equivalent to the
aircraft weight is being deflected downward by the wings, so this air
mass must occupy a fairly large volume below and behind the aircraft.

I still have vivid memories of going to Chobham Common for a spot of
model flying on a calm day with a solid, cloud base at 1000-1500 ft. The
road we were on was directly along the Heathrow approach path and we were
heading west, away from Heathrow. Suddenly a 747 dropped out of the
overcast ahead of us with flaps and wheels down. Its wing was scooping
off the bottom of the cloud layer and hurling it downwards, making the
downflow clearly visible under its wing. It must have extended down
20-25% of the wingspan, so was very clearly visible: looking at it was
like seeing the Niagara Falls streaming down below the wing, making it
quite obvious that this downflow was supporting 180 tons of aircraft.
For example, I
used to demonstrate the ease of positioning behind the towplane to
students by banking to left until the glider was way off center line,
and I never noticed any significant difference in the airflow from
center to far out to the left. This was with a 200' long towrope;
perhaps, with a much shorter rope, the experience would be a lot
different.

Yes, but that's in a fairly lightly loaded training glider. Some high
span competition types, e.g a JS-1C when fully ballasted, need a high tow
speed to avoid tip stalling. I've seen an absolute minimum tow speed of
77 kts quoted for a fully ballasted JS-1C. It seems likely that this is
at least partly due to the change in incident airflow along the wingspan
from the downflowing field behind the tug to the upflowing field which
extends much further out than its wingtips and immediate tip vortex. The
effect is to put the glider's tips at a higher AOA than the root, thus
cancelling the effect of any built-in washout in the wing.
--
--
Martin | martin at
Gregorie | gregorie dot org
I agree with Martin's explanation. To my mind the fullest account of the issues towing very high wing-loading long span gliders is by Aldo Cernezzi in the Jan-Feb issue of Gliding International. The article is slightly misleadingly titled "The Creation of a Vortex". As the ex-owner of a JS1c 21m who had just launched in it before a fatal towing accident in another 21m JS1c my mind became very concentrated on this issue. I subsequently never launched it fully ballasted in 21m mode without getting a direct confirmation from the tow pilot that the minimum speed would be 75 knots. Not least because the ASIs in many tug planes over-read in flight as they don't use proper statics.

To correct myself. The main article is called Poor Handling on Tow. The above mentioned article follows on from it.

John Galloway wrote on 12/20/2020 11:01 AM:
On Sunday, 20 December 2020 at 16:52:39 UTC, John Galloway wrote:
On Sunday, 20 December 2020 at 14:01:25 UTC, Martin Gregorie wrote:
On Sat, 19 Dec 2020 20:22:36 -0800, Eric Greenwell wrote:

Doesn't the majority of the wash or downflow from the wing pass under
the glider if it tows at the same altitude as the tug?

Thats definitely the case for a narrow layer containing propwash and
turbulence coming off the tug wing: quite obvious when you hit it, but
there's a general downflow above and below that turbulent sheet and a
matching upflow beyond the tug wingtips which can be seen in both flow
visualizations and, in some cases, in photos of aircraft flying in foggy
conditions which show the upflow extending out beyond the wingtips to at
least half of each wing semi-span. After all, wing lift is essentially
due to momentum transfer: a mass of air with a momentum equivalent to the
aircraft weight is being deflected downward by the wings, so this air
mass must occupy a fairly large volume below and behind the aircraft.

I still have vivid memories of going to Chobham Common for a spot of
model flying on a calm day with a solid, cloud base at 1000-1500 ft. The
road we were on was directly along the Heathrow approach path and we were
heading west, away from Heathrow. Suddenly a 747 dropped out of the
overcast ahead of us with flaps and wheels down. Its wing was scooping
off the bottom of the cloud layer and hurling it downwards, making the
downflow clearly visible under its wing. It must have extended down
20-25% of the wingspan, so was very clearly visible: looking at it was
like seeing the Niagara Falls streaming down below the wing, making it
quite obvious that this downflow was supporting 180 tons of aircraft.
For example, I
used to demonstrate the ease of positioning behind the towplane to
students by banking to left until the glider was way off center line,
and I never noticed any significant difference in the airflow from
center to far out to the left. This was with a 200' long towrope;
perhaps, with a much shorter rope, the experience would be a lot
different.

Yes, but that's in a fairly lightly loaded training glider. Some high
span competition types, e.g a JS-1C when fully ballasted, need a high tow
speed to avoid tip stalling. I've seen an absolute minimum tow speed of
77 kts quoted for a fully ballasted JS-1C. It seems likely that this is
at least partly due to the change in incident airflow along the wingspan
from the downflowing field behind the tug to the upflowing field which
extends much further out than its wingtips and immediate tip vortex. The
effect is to put the glider's tips at a higher AOA than the root, thus
cancelling the effect of any built-in washout in the wing.
--
--
Martin | martin at
Gregorie | gregorie dot org
I agree with Martin's explanation. To my mind the fullest account of the issues towing very high wing-loading long span gliders is by Aldo Cernezzi in the Jan-Feb issue of Gliding International. The article is slightly misleadingly titled "The Creation of a Vortex". As the ex-owner of a JS1c 21m who had just launched in it before a fatal towing accident in another 21m JS1c my mind became very concentrated on this issue. I subsequently never launched it fully ballasted in 21m mode without getting a direct confirmation from the tow pilot that the minimum speed would be 75 knots. Not least because the ASIs in many tug planes over-read in flight as they don't use proper statics.

To correct myself. The main article is called Poor Handling on Tow. The above mentioned article follows on from it.

Did all these accidents/incidents occur while the glider was at the altitude as the tow plane?
What was the length of the tow rope in use?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On 12/20/2020 9:01 AM, Martin Gregorie wrote:
On Sat, 19 Dec 2020 20:22:36 -0800, Eric Greenwell wrote:

Doesn't the majority of the wash or downflow from the wing pass under
the glider if it tows at the same altitude as the tug?

Thats definitely the case for a narrow layer containing propwash and
turbulence coming off the tug wing: quite obvious when you hit it, but
there's a general downflow above and below that turbulent sheet and a
matching upflow beyond the tug wingtips which can be seen in both flow
visualizations and, in some cases, in photos of aircraft flying in foggy
conditions which show the upflow extending out beyond the wingtips to at
least half of each wing semi-span. After all, wing lift is essentially
due to momentum transfer: a mass of air with a momentum equivalent to the
aircraft weight is being deflected downward by the wings, so this air
mass must occupy a fairly large volume below and behind the aircraft.

I still have vivid memories of going to Chobham Common for a spot of
model flying on a calm day with a solid, cloud base at 1000-1500 ft. The
road we were on was directly along the Heathrow approach path and we were
heading west, away from Heathrow. Suddenly a 747 dropped out of the
overcast ahead of us with flaps and wheels down. Its wing was scooping
off the bottom of the cloud layer and hurling it downwards, making the
downflow clearly visible under its wing. It must have extended down
20-25% of the wingspan, so was very clearly visible: looking at it was
like seeing the Niagara Falls streaming down below the wing, making it
quite obvious that this downflow was supporting 180 tons of aircraft.

For example, I
used to demonstrate the ease of positioning behind the towplane to
students by banking to left until the glider was way off center line,
and I never noticed any significant difference in the airflow from
center to far out to the left. This was with a 200' long towrope;
perhaps, with a much shorter rope, the experience would be a lot
different.

Yes, but that's in a fairly lightly loaded training glider. Some high
span competition types, e.g a JS-1C when fully ballasted, need a high tow
speed to avoid tip stalling. I've seen an absolute minimum tow speed of
77 kts quoted for a fully ballasted JS-1C. It seems likely that this is
at least partly due to the change in incident airflow along the wingspan
from the downflowing field behind the tug to the upflowing field which
extends much further out than its wingtips and immediate tip vortex. The
effect is to put the glider's tips at a higher AOA than the root, thus
cancelling the effect of any built-in washout in the wing.

See: https://www.youtube.com/watch?v=WIZWzvMu1dM from time 13:40
Hope that's clear,
Best Regards, Dave

""If anyone is interested in building any of these devices I can provide a parts list, schematics, controller code, STL files for the enclosures, and more operational details. ""


Upload them to GitHub or Hackaday.io