Donnell Hewett's original 12 criteria for a good tow system

GROUP 1; Accurate simulation
In order for an artificial gravity to accurately simulate natural 
gravity, it must have the same characteristics as natural gravity,
namely (1) constant in direction, (2) constant in magnitude, 
(3) distributed the same as gravity and (4) acting only through
the center-of-mass of each component. 

#1: CONSTANT DIRECTION
The direction of the towing force must remain essentially constant 
throughout every phase of the towed flight. (Length of the tow line
must be long compared to the transverse motion of the glider...
the faster the glider climbs and maneuvers, the longer tow line
needed... 500 feet or longer.)

#2: CONSTANT TENSION
The tension in the tow line must remain essentially constant 
throughout every phase of the towed flight. (Some tension
regulation device must be used... +/- 15 pounds during every
phase of tow. Settings should reflect a "comfortable" flight and 
"reasonable" climb rate.)

#3: C-M (Center of Mass)
DISTRIBUTION The towing force must be distributed between the 
components of the flying system proportionally to the masses
of the respective components. (Pilot and glider move in relation
to each other and the tow force needs to be evenly distributed.
2:1 bridle is quite effective at this; 2/3 force on the pilot, 
1/3 force on the glider.)

#4: CM ATTACHMENTS
The tow line/bridle must be attached as closely to the effective 
center-of-mass of each of the components and must not be allowed
to touch any other part of the flying system. (Violations of this
produce lockouts, adverse yaw and other loss of control problems.)

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GROUP 2; Safe transition
Any system must be able to handle: deviations from the ideal 
case; pilot release; excessive tow forces; learning.

#5: GRADUAL TRANSITIONS
The transition to and from tow, as well as any variations while 
on tow, must be gradual in nature. (Use of a long line that has 
sufficient stretch; but not too much stretch or driving will be 
difficult. 3/16", 1/4" hollow braided polypropylene, polyethylene
or nylon are used successfully.) 

#6: RELIABLE RELEASES
The release devices and their activation methods must be sturdy, 
rapid and reliable. (Release activation MUST be readily accessible
to the pilot regardless of where his hands are or where his body
has shifted. Only single-point release systems should be used.)

#7: INFALLIBLE WEAK LINK
The system must include a weak link which will infallibly and 
automatically release the glider from tow whenever the tow line
tension exceeds the limit for safe operation. (There is always
the possibility something unexpected can happen. Breaking point
should be appropriate for the weight and experience of the pilot,
not to exceed 1G - sum of all towed parts.)

#8: SAFE LEARNING METHOD
The system must include a method for safely learning to use 
it by gradually advancing from one level of experience to the next. 
(NEVER try 2 new things at once. NEVER allow yourself to be pushed
beyond your personal comfort level. NEVER exceed the limits of your 
equipment and skills. NEVER go more than twice as high, fast, far,
etc., as previously mastered. Learn in smooth air before transitioning
to rougher air.)

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Group 3; Practical Implementation
Perhaps obvious, but safety should never be taken for granted.

#9: ADEQUATE POWER
The system must contain a source of power adequate to maintain 
a safe mode of flight while under tow.

#10: CAPABLE CREW
The system must be operated by a crew which is adequate in number 
and competent in ability to see that it functions properly. 
(Driver, spotter, launch assistant, pilot.)

#11: RELIABLE COMMUNICATIONS
The system must provide a means whereby the pilot can reliably 
communicate to the rest of the crew. (Anything longer that 500 feet
of tow line requires a radio... minimum from pilot to driver... for 
pilot-in-command. Everyone must agree on what signals or commands
that will be used.)

#12: SUITABLE ENVIRONMENT
The system must be operated only within the environment and under
the conditions for which it was designed.