Arduino Based Soaring Related Devices

On Sat, 19 Dec 2020 08:55:02 -0700, Dan Marotta wrote:

On 12/19/20 6:49 AM, Martin Gregorie wrote:
sin(glider weight)

How do you convert pounds, kilograms, stone, etc. to radians,
degrees...?

OK... glider_flying_weight * sin(climb angle)

This factor is needed because the tow rope is supporting a part of the
glider's flying weight - somewhere between zero in level flight to all of
it when a (jet-powered) tug is climbing vertically. At typical climb rate
on tow this amounts to a non-negligible fraction of the tension in the
rope.


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

OK guys, let's get this sorted out - and Yes, I will have donned my flame-proof suit by the time you're reading this!

A workable figure for the 'underlying' tension, (D + U), can be arrived at as follows: work out ...
D = (total weight of glider + pilot, etc.) / best glide ratio of that glider, and
U = (total weight of glider + pilot, etc.) / 10
Then add D and U to get the total 'underlying' tension (the steady-state tension, if you prefer).

The theory:
Tension in the aerotow rope in flight comprises 3 components:
D: the drag of the towed glider, dependent on its weight and its glide angle at towing speed
U: the "pulling Uphill" force, which is the weight of the glider x the sine of the angle of climb through the air (normally written as sin(angle))
J: the highly unpredictable and dynamically variable Jerk or "impulse" forces, resulting from bumps in the air, wiggles by the tug pilot, and proper (or otherwise) following behaviour of the glider pilot. These will also be scaled by other factors, such as elasticity and mass per unit length of the tow rope and (to a small extent in practical cases) by the respective total masses of tug and glider.

Note for D: normal towing speed is rarely far from best glide speed
Note for U: typical figures, in UK units:
- for a single-seater: 7kts climb at 70kts airspeed, giving the sine as 7/70 = 1/10
- for a two-seater: 6kts climb at 60kts airspeed, giving the sine as 6/60 = 1/10
- for better two-seaters: surprisingly little difference in climb angle, but ...
Note for J: this is potentially so variable that imprecision in assessment of D and U is unlikely to be of any concern.

Of course, a really powerful tug towing a really light glider will climb more steeply, so 10 may not be appropriate as the divisor in U.
If determining the sine in your own case, you must use identical units for airspeed and rate of climb - any density correction you apply to airspeed must also be applied to rate of climb. Some varios may give you true rates of climb, but the ASI will not give you true airspeed!
Note that extra climb rate caused by flying through lift does not affect the geometry, so does not affect the 'underlying' tension. It may well have an effect on variability of J - particularly in gusty thermals.

Martin's explanation was about right, in principle, but wrong in that the 'Uphill' component is significantly larger that the 'Drag' component for any practical glider (as opposed to hang-glider).

Eric wrote "No trigonometry required." But that *is* trigonometry - just without the frightening name!

Some may think that I have been random in my use of "mass" and "weight" - but no, barring slip-ups, and bearing in mind that it's late here!

More detailed explanation could be given, but are you still awake? Do you still have the will to live?
Happy Christmas, J.

Several years ago, one of our more "scientifically" inclined tow pilots rigged up a strain gauge at the tow hook and a laptop in the Pawnee cockpit to try to record loads and forces on the rope. I don't recall the actual numbers for my Pegasus, but the "scatter" in the data rendered the information virtually unusable. The "J" factor referenced above as the glider and tow plane encountered turbulence, reactions to control movements and out-of-position and/or uncoordinated flight made the load on the rope go from zero (slack line) to the weight of the glider. And it did it in such random steps that you couldn't make any sense of it. Maybe a more sophisticated system with more controlled conditions (i.e., smooth air) might reveal relevant results, but based on that one test, (as well as the data from several other tows with other gliders), I doubt any really usable information can be gleaned. The tow plane, tow rope and glider are in a very dynamic situation that is tough to quantify accurately. Good luck, as it would be interesting to see tests with better results.

Mark Mocho wrote on 12/19/2020 3:20 PM:

Several years ago, one of our more "scientifically" inclined tow pilots rigged up a strain gauge at the tow hook and a laptop in the Pawnee cockpit to try to record loads and forces on the rope. I don't recall the actual numbers for my Pegasus, but the "scatter" in the data rendered the information virtually unusable. The "J" factor referenced above as the glider and tow plane encountered turbulence, reactions to control movements and out-of-position and/or uncoordinated flight made the load on the rope go from zero (slack line) to the weight of the glider. And it did it in such random steps that you couldn't make any sense of it. Maybe a more sophisticated system with more controlled conditions (i.e., smooth air) might reveal relevant results, but based on that one test, (as well as the data from several other tows with other gliders), I doubt any really usable information can be gleaned. The tow plane, tow rope and glider are in a very dynamic situation that is tough to quantify accurately. Good luck, as it would be interesting to see tests with better results.


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.

--
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 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