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Balloon ascent rate calculations

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  • excitontx
    I have been playing with the ascent rate calculators available on the Internet (NSV and CUSF) and comparing them with W0RPK s very nice webpage on the subject.
    Message 1 of 3 , Jan 8, 2012
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      I have been playing with the ascent rate calculators available on the Internet (NSV and CUSF) and comparing them with W0RPK's very nice webpage on the subject. I had a couple questions I wanted to throw out and get some insight on -

      1)Why do the NSV and CUSF calculators use drag coefficients of 0.25-0.3 when the drag of a sphere is closer to 0.5? Given the floppiness of a big balloon with a lesser fill, I would expect the Cd to actually be even higher (rather than lower).

      2) W0RPK's equations, applied at various altitudes (pressures and temperatures), shows a trend of increasing ascent velocity with altitude. Some of our flights have demonstrated that increase, while others have had a relatively constant ascent rate. Any insights on overriding effects that will cause the ascent rate to increase or decrease with altitude? Many of the data sets displayed on W0RPK's ascent webpage show decreasing rates with altitude, though there was quite a bit of variation from flight to flight.

      Thanks,
      Chuck KG5CA
    • Mike Manes
      Hi Chuck, The conventional drag calculations with an assumed constant Cd don t work for high altitude balloons except at low altitude, and they will predict an
      Message 2 of 3 , Jan 8, 2012
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        Hi Chuck,

        The conventional drag calculations with an assumed constant Cd don't work for
        high altitude balloons except at low altitude, and they will predict an
        accelerating ascent rate higher up, which we know doesn't happen in general.
        I think problem lies in two factors:

        - the balloon does change shape from a low-Cd inverted tear drop to a near
        sphere, and

        - the Reynolds number drops like a rock somewhere between 50 and 75K'. Now,
        one would assume that this drop would in fact >reduce< drag, but it doesn't.
        What happens is that the flow over the lower surface of the balloon gradually
        detaches from the skin and creates vortexes up there, and that actually
        slightly increases drag, resulting in the typical inflection in ascent rate
        that we've all seen frequently. Thanks to Nick N0LP for bird-dogging this
        factor in his GPSL pitch a few years ago.

        I have yet to see an aerodynamic model that accurately predicts the ascent
        rate of a latex high-altitude balloon - or at least, one that runs on a PC.
        So we generally have to rely on empirical performance data - and that does
        work out pretty well.

        Another factor that muddies the waters is high-altitude wind rotors which
        show up as oscillatory-looking variations in ascent rate, and modelling
        those might stress out a Cray!

        73 de Mike W5VSI
        CTO EOSS


        On 1/8/2012 13:47, excitontx wrote:
        > I have been playing with the ascent rate calculators available on the Internet (NSV and CUSF) and comparing them with W0RPK's very nice webpage on the subject. I had a couple questions I wanted to throw out and get some insight on -
        >
        > 1)Why do the NSV and CUSF calculators use drag coefficients of 0.25-0.3 when the drag of a sphere is closer to 0.5? Given the floppiness of a big balloon with a lesser fill, I would expect the Cd to actually be even higher (rather than lower).
        >
        > 2) W0RPK's equations, applied at various altitudes (pressures and temperatures), shows a trend of increasing ascent velocity with altitude. Some of our flights have demonstrated that increase, while others have had a relatively constant ascent rate. Any insights on overriding effects that will cause the ascent rate to increase or decrease with altitude? Many of the data sets displayed on W0RPK's ascent webpage show decreasing rates with altitude, though there was quite a bit of variation from flight to flight.
        >
        > Thanks,
        > Chuck KG5CA
        >
        >
        >
        > ------------------------------------
        >
        > Yahoo! Groups Links
        >
        >
        >
        >

        --
        Mike Manes mrmanes@... Tel: 303-979-4899
        "Things should be made as simple as possible, but not more so."
        A. Einstein
      • Steve Randall
        Hi Chuck,        I wrote the ascent rate and burst estimator spreadsheet “burst.xls” on which the CUSF calculator is based - the current version
        Message 3 of 3 , Jan 9, 2012
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          Hi Chuck,
                 I wrote the ascent rate and burst estimator spreadsheet “burst.xls” on which the CUSF calculator is based - the current version (burst3.xls) can be found in the middle of this page:

          http://wiki.ukhas.org.uk/guides:balloon_data

          I'm not sure what the NSV calulator is based on.

          For burst.xls the Cd was just back calculated from Totex data assuming the balloon was a sphere - like you I worried why it was lower than typically published data for a sphere. However (as Mike has pointed out) Cd is not the whole picture when it comes to drag (Reynolds number etc).  Also I found a few examples where a sphere Cd was quoted significantly lower than 0.5. The Cd values have been adjusted since in the light of experience of UK flights.

          The estimator assumes a constant ascent rate – which seems a reasonable model in most situations. The Cd figures used give reasonably accurate ascent rates if you measure fill carefully.

          There are a whole bunch on things that could affect ascent rate - air temperature changes, gas temperature changes (solar gain) , balloon shape changes during the flight, balloon shape due to manufacture, surface texture change during the flight (becoming smoother with stretching), limits of envelope elasticity approaching burst, changes in Reynolds number (with balloon size, air density, air gas makeup), vertical wind flows, the payload+prachute drag (reducing as a % of total drag with altitude).....

          If the purpose is to improve the flight prediction process then one has to question the accuracy of the wind model in comparison with the error introduced by a assuming a constant ascent rate. The predictors are based on weather models (e.g. GFS) – which in turn are based on Met Balloon releases several hours earlier - often many miles from the flight path and top out at about 100,000ft.

          Together with the difficulty of accuratly measuring fill one is left with the impression it might be almost as accurate to stick with a simple constant ascent rate model.

          My own effors have been directed at improving the landing spot prediction following burst based on the ascent data. 

          Steve G8KHW

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