>>Think about it, you are still using 10 pounds of pull from the rubber, but
>>now it has to lift up 2.5 times as much rubber 2.5 times higher (center of
>>mass of rubber). That's where a lot of the lifting power is being lost.
>... In fact, the
>fractional difference in calculated launch height is the same independent
>of the length of the high start.
I agree with this also, percentage of energy comsumed by raising tubing to
vertical, versus potential energy of the tubing is length independent.
It's also a small effect. For example, I launch a 2.25 pound DAW 1-26 2m
using 60 feet of tubing that weighs 1.275 pounds, to an altitude of 270
feet. 38.25 pounds of work done lifting tubing to vertical, 607.5 ft lbs
of work done lifting glider, and my guestimate is that over 300 ft lbs
of energy is lost to drag (300 ft lbs if 180 foot pull is required,
580 ft lbs was lost if 210 foot pull is required).
>I have calculated the energy that goes into lifting the high start rubber
>into the air. This does make a minor difference in launch height, but not
>enough to explain what Angel and Jeff Reid have observed.
I need to amend my "observations". In still air, my revised "guestimate"
for hi-start efficiency is in the 50% to 70% range, which includes
the efficiency numbers you've observed. Variables include
glider's lift to drag ratio, speed, chute drag, and line drag.
Headwinds will make a huge difference in this number. I've seen a
floater launched with 25 feet of small tubing, 200 feet of line in
a mild headwind, seemed like it took 1/2 minute for it to reach
altitude, efficiency in this case would be well over 100% (if wind
energy ignored).
My "ideal" formula for hi-starts is 3.5 to 1 line to elastic ratio,
with 4x tension to glider weight ratio at 350% strain. (For slow
speed floaters, maybe 3x tension to weight ratio and lower ratio.)
I think this is about what Dick Williamson states also.
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