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  1. #31
    AngusMcDoon's Avatar
    AngusMcDoon is offline Registered User
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    Default Re: ®Heavy®winter winds.

    [ QUOTE ]
    ask an airline or a glider pilot

    [/ QUOTE ]

    ...or even your local friendly jellyflopper pilot. I wasn't going to jump in here and start banging on about temperature effects on density and how it affects helicopter performance as it would be fred drift, but seeing that you almost mentioned it!

    Putting in technically... in a small low powered piston jobby, one has to gently inform a potential biffamongous passenger that they may be too lardy to get off the ground on a hot summer day.

    Part of this is to do with engine performance, and part to do with the air being somewhat thinner than on a cold frosty day.

    The pilots' manuals have some wonderful graphs that show it all - plotting air pressure, temperature and passenger excess fat.

    I expect there are similar limitations to the pilots flying the big boys toys in EyeWrack with 30 beefy troopers in the back.
    Andersen 22. The best winch never made.

  2. #32
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    Default Re: ®Heavy®winter winds.

    The trouble with comparrison to the air industry is that you are looking at very big changes in air pressure. Although there is a thoretical comparrison, not convinced there is a practical one.

    For the yacht in the winter/summer situation the difference is much smaller, so whilst there is an effect, the question as to whether it would be noticed by the skipper is another matter.

  3. #33
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    Default Re: ®Heavy®winter winds.

    [ QUOTE ]
    g is a constant,

    [/ QUOTE ]

    Strongest measured gravitational pull in the British Isles has been found at Rum. Does this explain why the wind is always stronger north of Ardnamurchan?

  4. #34
    AngusMcDoon's Avatar
    AngusMcDoon is offline Registered User
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    Default Re: ®Heavy®winter winds.

    I agree, there are big changes in air pressure with height, but I didn't mean to refer to that.

    I was talking about the changes in density caused by summer heat causing problems to heavily loaded small helis at sea level.

    If a wee heli needs 95% of its available power to get off the ground when heavily loaded in cold conditions, it won't make it with the same load if the air density drops by 10% on a hot summer day, even at sea level. (There are also engine power limit issues contributing too, it's not just aerodynamic).

    There have been accidents where pilots have run out of power on hot days when coming in to land in a confined area (which requires more power than an airfield takeoff). In the UK the CAA put air temperature limits on operating some lower powered small helis like Robinsons because of this reason. The limit is approx 30C.
    Andersen 22. The best winch never made.

  5. #35
    KenMcCulloch is offline Registered User
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    Default Re: ®Heavy®winter winds.

    [ QUOTE ]

    Strongest measured gravitational pull in the British Isles has been found at Rum. Does this explain why the wind is always stronger north of Ardnamurchan?

    [/ QUOTE ]
    I think that's a very interesting theory. It might also help explain the well-known 'Sound of Mull anomaly' whereby yachts can often be seen approaching Salen from opposite directions both flying spinnakers.

    (In my capacity as an opinionated ****.)
    Ken McCulloch
    Rival 38 'Cherry Ripe'

  6. #36
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    Default Re: ®Heavy®winter winds.

    Understood.

    Does the windspeed also come into play - havn't thought through the physics, but since the forces are square laws on the windspeed, does the difference in air density have a more pronouned effect at higher wind speeds?

  7. #37
    AngusMcDoon's Avatar
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    Default Re: ®Heavy®winter winds.

    [ QUOTE ]
    Does the windspeed also come into play

    [/ QUOTE ]

    Windspeed is important in heli flying - it is safer and easier when there is a breeze. When short of power because of high air temperature and highly loaded, taking off and landing into wind helps enormously.

    Landing downwind with a small power margin in a confined area is a quick route to an early grave. When landing downwind as forward speed relative to ground decreases air speed can reduce to zero while still out of ground effect. Lift is lost (square law), natural response is to put more pitch on the blades, blades become over-pitched, engine can't cope, rapid sink rate, boink, bent heli.

    In the UK the hottest days are usually calm - not a good combination.

    [ QUOTE ]
    does the difference in air density have a more pronouned effect at higher wind speeds

    [/ QUOTE ]

    If there is a useful breeze on a difficult day, you use it, so low air density becomes much less of a problem. A gotcha of confined area landings is that once you decend closer to the ground, the windpseed may drop suddenly because of the sheltering effect of trees/buildings/tall people standing nearby, and the effect is the same as above...loss of airspeed...loss of lift...overpitch...lack of power...high descent rate...boink...tricky bit of explaining to do to the CAA.

    The standard technique to avoid this when landing in a confined area is to overfly the landing area at the airspeed that requires minimum power (usually about 60 knots) with zero climb rate. Check the power used compared to the power available for that temperature and air pressure from a chart in the cockpit. If the power available is below a certain value, bog off and land somewhere easier. This is why confined area landings are difficult - not just the risk of twatting the nearest tree with you tail rotor, but power isses too. Think of that the next time you see an air ambulance parked in an unfeasibly tight space on a hot day. They are much more skilled than me.
    Andersen 22. The best winch never made.

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