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Re: [GPSL] The mystery of the near constant ascent speed

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  • PAUL VERHAGE
    I tried looking at the drag equation and seeing if I could show the balloon s speed should be constant. But I kept coming up with a speed that was not
    Message 1 of 32 , Apr 1, 2006
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      I tried looking at the drag equation and seeing if I could show the
      balloon's speed should be constant. But I kept coming up with a speed
      that was not constant. I'll see if I can find my notes and show my
      work.

      Paul

      >>> manes@... 3/24/2006 9:57:24 pm >>>
      Hi Hank,

      Well, like so much other technology that's been once well-understood
      and slowly forgotten (like the MIT Series), I'd be not surprised if
      somewhere lurking out there is a paper which precisely describes
      latex balloon ascent behavior.

      Sounds like a great topic for an aero engineer's master's thesis!

      Any takers?

      73 de Mike W5VSI

      Hank Riley wrote:
      >>We were deep in to Reynolds and
      >>Grashof numbers and finally decided that the only way that this
      >>phenomenon can be explained is to allow the shape of the balloon
      >>to deviate from a sphere over the ascent course.
      >
      >
      > Mike,
      >
      > I guess that it just has to remain a mystery with a plausible
      > but not rock solid explanation if you guys didn't solve it.
      >
      > Also my derivation that had the balloon either speeding up
      > or slowing down with increasing height (guessing it was the
      > former) was okay as far as it went, making the usual
      > assumptions. It's just that the basic model is missing
      > something, like the variable shape factor you mention.
      >
      > It still amazes me that the ascent speed seems to adhere as
      > well as it does to a constant. There are a good number of
      > anomalous amateur flight logs, but I believe the vast majority
      > are really pretty steady from liftoff until burst.
      >
      > Hank
      >
      >
      >
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      --
      Mike Manes manes@... Tel: 303-979-4899
      "Things should be made as simple as possible, but not more so." A.
      Einstein





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    • Mike Manes
      Hi Paul, Here s another unabashedly non-high-tech way to get a barometric pressure rate-of-change indication: Use a very sensitive DIFFERENTIAL pressure
      Message 32 of 32 , Apr 5, 2006
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        Hi Paul,

        Here's another unabashedly non-high-tech way to get a barometric
        pressure rate-of-change indication:

        Use a very sensitive DIFFERENTIAL pressure transducer, e.g. inches
        of water full scale. Vent one port directly to ambient, and bond the
        other port to a fixed volume which also has a very small orifice vented
        to ambient. The fixed volume serves as the "memory" of earlier
        pressure, so to speak. If ascent rate is fast, the pressure in the
        volume will be greater than ambient, since the orifice flow must be
        greater.

        The volume could be made from a ping-pong ball. Differential
        sensors are also a lot less expensive than absolute sensors, and
        in this service, they needn't be nearly as a accurate and stable
        as an absolute sensor.

        Now, I suspect that the "gain" of this scheme will vary quite a
        bit with altitude, however. But I have a feeling that it's the
        same method used in aircraft vertical speed indicators and in
        hot air balloon wotchamacallits. Those instruments may have a
        means to adjust orifice size as a function of altitude.

        This is an idea that's been floating around in my head for a few
        years, as yet awaiting that coveted "round tuit". But if this
        interests you, go fer it! Be interested in seeing what you learn
        from it.

        73 de Mike W5VSI (W5 Vertical Speed Indicator)
        EOSS

        PAUL VERHAGE wrote:
        > I had an idea last night.
        >
        > I'd like to develop a barometric climb indicator. Is there a simple
        > way to sample the voltage from a pressure sensor and hold it in a memory
        > for one minute? I'd like to compare the pressure reading from one
        > minute ago with a current pressure reading. The two voltages are the
        > inputs to a comparator. If the pressure has dropped between the two
        > measurements, then the comparator outputs +5V, but if the pressure had
        > increased, then the comparator outputs a 0V. By monitoring the output
        > of the comparator your flight computer can determine if the balloon has
        > burst without a GPS.
        >
        > Paul
        >
        >
        >>>>manes@... 4/3/2006 10:04 pm >>>
        >
        > Hi Bill,
        >
        > Hey, do you have specs and a source for the Vaisala baro sensor?
        > We could use a few more really good ones for payloads and the
        > altitude chamber. Our single Honeywell/Microswitch sensor gets
        > swapped between a payload and the chamber between flights.
        >
        > 73 de Mike W5VSI
        > EOSS
        >
        > wb8elk@... wrote:
        >
        >>I have the raw data from dozens of Ozonesonde flights taken a week
        >>apart....the Vaisala pressure sensor used is more consistent than
        >
        > what
        >
        >>you'd see for GPS altitude and seem to correspond closely with GPS
        >>readings from two calibration flights I flew with them last
        >
        > year....In
        >
        >>the first flight we saw a deviation between the sensor and the GPS
        >>altitude above 90,000 feet but up to that point the pressure altitude
        >
        >
        >>corresponds very closely to the averaged GPS readings....the second
        >>flight was dead on up to 92,000 feet and then we lost the GPS above
        >>that. I'm looking forward to doing one more GPS calibration file and
        >
        > see
        >
        >>how things work above 90k.
        >>
        >>The neat thing with these flights is that the pressure altitude
        >
        > readings
        >
        >>are in 1 second increments and won't show the GPS altitude variances
        >
        >
        >>normally seen with GPS data.
        >>
        >>I'll try to plot a few of them when I return from Ohio to see how
        >
        > they
        >
        >>compare....they are filled to almost the same precise lift, same
        >>balloons and flight train configuration (except for the flights where
        >
        > I
        >
        >>hitchhike with them).
        >>
        >>- Bill WB8ELK
        >>
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        --
        Mike Manes manes@... Tel: 303-979-4899
        "Things should be made as simple as possible, but not more so." A. Einstein
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