Rope Tension Meter for Auto Tow

On Dec 29, 3:26*pm, Grider Pirate wrote:
On Dec 24, 9:03*pm, bildan wrote:



On Dec 24, 8:12*pm, " wrote:

On Dec 23, 6:37*am, STEVE SARGEANT wrote:

On Dec 19, 5:52*pm, " wrote:

We at the Fault Line Flyers Glider Club in Briggs Texas want to build
or buy a tension meter for our auto tow operation.

Does anyone have experience that might be helpful.

Bill Snead
6W

LCM Systems can offer various solutions, depending on what the force
range is that you want to measure.

Try this link for the sensorhttp://www.lcmsystems.com/sta-4

and for a handheld display, we would recommend:-

http://www.lcmsystems.com/TR150

You could but the complete kit, calibrated an ready to go for less
than $550

Could someone describe how you use the tension meter when auto
towing? *Once you are towing at a constant tension, does it change how
you fly the glider on tow? *What is the typical tension maintained
when towing a 2-33? *Also some pictures how the electronic load cell
is rigged with the release hitch would be helpful. *Thank for all the
good information

Bill Snead

With constant tension, the pilot controls airspeed with pitch attitude
(Nose up, slow down - nose down, speed up) just as in free flight. *In
a thermal, the car will back off to maintain constant tension as the
thermal provides a fraction of the energy. *In a downdraft, the tow
car will add power to maintain tension. *It's a very pleasant way to
launch.

A 2-33 is a special case since the elevator is so small and the hook,
even the "compromise hook", is so far forward. *The limited elevator
authority makes it difficult to control airspeed at any tension
value. *Also, Schweizer didn't provide information on weak link or
best climb speed for ground launch. *At any tension greater than about
75% of the gross weight, a 2-33 will accelerate even with full up
elevator. * Most people find 55 MPH works as well as anything.

Note that winch or auto towing puts stress on the wing roughly equal
to a loop. (~3.5G) *I seem to remember a letter from Schweizer around
1980 strongly discouraging loops due to the age of the 2-33. *30 years
on, I'm not comfortable ground launching one.

The tension load cell fits into the tow line at the tow car right at
the attach point. *Usually in the rope just aft of the tow ring.
However, while a bit more complicated, it's safer for the load cell to
be part of the cars tow release mechanism. * The full towing force
must pass through the load cell to get a correct reading.- Hide quoted text -

- Show quoted text -

I'm a little confused. *If the weak link is less than 200% of the max
AUW of the glider, how is is possible to generate ~3.5G?

Your question has implied two parts. One is the approved weak link
for winch launch and the other is the "G equivalent" wing spar bending
moment.

First, the approved weak link. The well known 80-200% rule (FAR
91.309 a (3) ii) applies only to aero tow, not winch launch, and then
only to those gliders whose Approved Flight Manual did not otherwise
specify a weak link since an AFM trumps 91.309 a (3) ii. AFAIK, only
Schweizer failed to specify weak links. Expect this to be clarified
in future updates of the FAR's.

All JAR-22 gliders will have a weak link specification in their AFM
for both aero tow and winch launch. Sometimes the aero tow and winch
launch weak links are the same, sometimes different. Almost
universally, a plus or minus 10% tolerance will also be specified.
FAR's require compliance with an AFM and therefore the use of weak
links with these specifications including the tolerance specification.

An example is an ASK-21 whose AFM specifies a 1000 daN +or- 100 daN
weak link (Tost Black Link) for winch launch and 600 daN +or- 60 daN
for aero tow. (1 daN = 1Kg-force)

The second part of your question involves the concept of "equivalent
G". Since the force vector of a winch rope passes through the
longitudinal axis and is not distributed along the span like the
weight of the wing structure, the spar bending moment is higher than
the force alone would indicate. The "3.5G" is only an estimate and
first order calculations suggest it could be slightly less but a winch
launch is indeed roughly equivalent to a loop in terms of wing stress.