Wes,
I may be wrong, so check me out but I believe Ed Tradup used a Kevlar
towline a few years back and had failures with it because it lacked the
shock absorbing qualities of a PROPER nylon tow line. I had chute problems
at Speedweek this year and Jim Diest spent quite a bit of time working with
me on it. He has a computer program that considers speed, weight, altitude,
etc. and determines fabric porosity and strength, area of canopy, and tow
line size requirements based on a 150 percent safety factor. Some of these
cars are going so fast that anything less than that kind of information is
dangerous.
This is not a paid commercial for Jim and I hope I haven't said anything I
shouldn't, but I am following his advise from now on. I took two trips to
the 8 1/2 or 9 this year and did some damage to the car. I don't like that
very much.
Howard
----- Original Message -----
From: "Wester S Potter" <wspotter@jps.net>
To: "Nafzger" <nafzger@vtc.net>; <land-speed-digest@autox.team.net>
Sent: Sunday, November 05, 2000 9:13 PM
Subject: Re: Parachute
> Howard, List,
>
> One of the things that has disturbed me has been seeing the main tether
snap
> and leave the canopy fully deployed but just staying there. In that case
> the size of the chute and vehicle speed determine the pull on the tether.
> If the canopy can come out and stay deployed it can handle the pressure,
> just makes me wonder what type of tensile strength is needed to hold it to
> the car. Having done some fiber tensile strength testing at much lower
> pressures I know that there are some fibers and yarns that are very much
> stronger than others. In this case the flexibility is a factor as is the
> way the fibers are twisted into a yarn/thread. When the tethers snap as
> cleanly as they did on Burklands's car or on the Hoffman/Markley car a few
> years ago there has to be some way to strengthen the fiber in the tether
and
> prevent it. The fibers were only shredded over a 3/4" length, a pretty
> clean snap. Obviously they were the same strength across the width of the
> tether. If there were some other fibers such as a carbon fiber that were
> core spun with the nylon in the tether, that would result in some stretch
> and a tear at different places along the line, not all in the same exact
> area. This might help to save the line and keep the chute on the car.
> Nylon is stronger on that kind of a tug than a steel wire of the same size
> and I'm wondering if carbon fiber could be strong and flexible enough to
do
> the job.
>
> I'm guessing Jim Diest will suggest two things about the canopy, either a
> way to allow less air to be captured at the speed in question, a smaller
> canopy probably, or a ribbon canopy that will have slots to allow the air
> through. The ribbon will be more easily damaged and the smaller canopy
> won't slow the car as well. Dragging a chute over the salt can't be good
> for the fabric either way. The "slow" speed chute on Burklands car was in
> ribbons, obviously the tensile strength of the fabric was not up to the
> task. The military contractors developed ripstop nylon for that
originally
> but the weight of the canopy as used in military applications is a much
> higher tensile strength and a heavier fabric. A jeep also has a different
> problem with canopy depoyment coming out of an airplane and only freefalls
> at around 147 mph anyway. Even the space shuttle isn't as hot as these
> liners. Burkland is using compressed air to kick the chute out of the
tube
> so I'd guess deployment isn't the problem. He is using the clamshell air
> brake then three chutes in sequence, Perhaps the triggers are set at too
> high a speed for the canopy size.
>
> Maybe if Rick Vesco is right about the salt, we'll be back on the long
> course and stopping won't be such a major problem next year. A three mile
> shut down is much better than what we have had the last few years.
>
> Wes
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