On 16 Mar 2004 13:28:47 -0800, (Mark James Boyd)
wrote:


I've been thinking about how to measure strain on the
rope during auto-tow, but this time from the auto side, not the
glider side.

No need to measure anything - maths are completely sufficient to
calculate this.

In level flight the drag of a glider is

(1) drag = weight / (L/D)

example: 500lbs glider, L/D is 40, drag will be 12.5 lbs. This will be
also the strain on the tow rope in level flight. Adjusting this
formula for a climb is simple.


I'm planning to tow using a 50# spring scale, and never exceed
40#, and see how long (far) it takes to accelerate to rotation
speed. For 500#, I seem to get calculations near to 1000ft or so.
I'd like to see if a 40# thrust AMT turbine would accelerate
a 500# glider to rotation in a reasonable runway length
(say 2000 ft or less).

Aircraft weight: 500 lbs
Engine thrust: 40 lbs
Liftoff speed: 34 kts

(2) F = m * a. F = engine thrust
m = glider weight
a = acceleration

(3) a = F/m F = 177.6 N = 40 lbs
m = 226 kg = 500 lbs
- a = 0.79 m/s^2

(4) v = a * t v = liftoff speed (22.2 meters/sec = 34 kts)
t = time to reach v

(5) t = v / a

- t = 28.1 seconds

(s) s = 0.5 * a * t^2 s = distance covered (during t = 28.1 seconds)

- s = 307 meters = 1010 ft.



As a second issue, I'm wondering if anyone has measurements of
the typical strain on a winch or auto tow rope. I'm
guessing .5 to .9 times the glider weight (probably near the
highest altitude right before release, right?).

Strain during winch launch can easily be calculated by the weak link:
If a Ka-8 (weight 600 lbs) manages to break a weak link of 1.000 lbs,
maximum strain is 1.000 lbs. Maximum strain happens during the initial
acceleration of a heavy glider when still on the ground and in the
phase of maximum climb angle (usually between 300 and 900 ft).


Bye
Andreas