Air density, temp and lift

edited October 7 in Lessons Learned
I'm having trouble getting lift in the tropics. Air temp is 86f humidity 86% windspeed: 11mph. At home 11mph is a 'go' speed, in cooler dryer air my 20' parafoil will lift 500g happily. Here on Barbados it's touch and go, the kite feels 'soft' and even thermal lift is weak.

I had the option of packing an ff30 but thought the forecast wind-speeds were more suited to the smaller kite, but I ended up under powered.

Anyone had experience of this sort of thing?

image

B

Comments

  • edited October 8
    Hi Bill,

    I've always had the impression that the low wind performance of my large rok is better on cold winter days that on hot summer days...

    There have been messages in this forum about the effect of altitude on kite flying. But Temperature and to a lesser extent humidity have an effect as well.

    Al forces involving wind are proportional to the air density. According the the nice graph here the difference in lift could amount to 15%

    The NET effect (lifting power after discounting the weight of the kite and line) is much larger!

    Example Lift
    winter: Lift kite: 1500gr - weight kite+line: 800gr = net lift 700gr
    summer: Lift kite: 1275gr - weight kite+line: 800gr = net lift 475gr

    Difference 32%

    Furthermore: wet air has lower density, better wait for dry cold sunny weather :-)

    image
  • The chart is very educative. Thanks Hans :-)

    BKT
  • edited October 8
    Many thanks Hans. I guessed as much. I wonder if a proportional increase in surface area would do the trick. Given the flowform/parafoil has to lift its own weight and that of the air it contains some experiment is needed.

    I flew at the Savannah which is the traditional site for the Easter kite flying at Bridgetown. By that tIme of year the wind direction changes and the humidity decreases (so I'm told) so perhaps a big framed kite and better wind at Easter time would work better.

    Kite flying on Barbados was popular but interest is dwindling now. The Savannah festival is still an event and the site is perfect, provided it's not a race day.

    B
  • This page is good for UK users: it can be switched to imperial units.
    https://gribble.org/cycling/air_density.html
    Cheers, Hans.
  • Useful. I should point out that in the UK we are metric (SI) since 1971 (Brexit or no) with the exception of road distances. Barbados is on the US unit system for most things.

    B
  • Bill,
    Most of my flights in Tahiti are in 25-35 degrees C with a 85 to 100 humidity and just like in tbe Barbados we enjoy 10 ro 25 knots tradewinds which are very stable and never had that feeling... going to more "continental" weather confitions, I never had the feeling you describe, the only major difference was the altitude...
    Nebertheless I always travel with a R 8, R 11 and flow form 2 which cover any type of wind and my modular rig allows me to "lose wait on the go" and still KAP.
  • edited October 12
    Pierre,

    I think the huge diference between what I fly in at home (temperate, cool, dry air) and what I found in Bridgetown is what threw me. A previous attempt on St Vincent in drier air ( in Febuary) behaved better but I had to contend with katabatic flow. I wish I'd had my R8 with me but it's not practical to travel with. The lesson learned is to be prepared. I could have taken an ff30 which might have saved the day.
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