¨Heavy¨winter winds.

[bilbobaggins]

[ QUOTE ]
(ex Geog teacher, not a met person - apart from 50+ years sailing)

[/ QUOTE ]

That explains a lot..... /forums/images/graemlins/grin.gif

 

[bilbobaggins]

Here's another numpty observation.....

I seem to recall, from somewhere deep inside my library of sailing books, reading an account of one of the earlier Whitbread RTW Races, where the writer describes how each sail was initially 'Solent calibrated' for wind-speed range e.g. No 1 Jib 13-18 knots, No 2 Jib 15-22 knots, etc. ( Pse don't quibble if the numbers don't equate to your own sails ) , and they found it necessary in the Southern Ocean to recalibrate their sails' permissible windspeeds by around 5 knots lower - due, it is surmised, to the considerably greater density of the air and the consequent increased loads on the sail panels at 'reference wind speeds'.

FWIW

 

[JOHNOO]

" Incidentally, comparing a summer day at 30C and a winter day at 0C with the same air pressure, the difference in density is about 10%"

In 1973 the difference was measured at 19%. (at the same wind speed) If you want confermation of the effect of density, ask an airline or a glider pilot or a heating enginer.

 

[DaveS]

Bil,

From your knowledge of high altitude flight, have you anything to offer re. my high altitude anemometer query? Though I suppose you'd use pitots ... same or different?

 

[Pye_End]

[ QUOTE ]
" Incidentally, comparing a summer day at 30C and a winter day at 0C with the same air pressure, the difference in density is about 10%"

In 1973 the difference was measured at 19%. (at the same wind speed) If you want confermation of the effect of density, ask an airline or a glider pilot or a heating enginer.

[/ QUOTE ]

Using 'o'level physics I can understand the 10% figure. Can you explain where you get your 1973 figure from? Can't see why wind speed should have an effect on air density.

 

[bilbobaggins]

Ah, Dave! You're much sharper at this end of the evening than many would give credit for! And at most other times, too..... /forums/images/graemlins/grin.gif

From the dimming - and certainly incomplete - recesses of memory, my turgid and Chilean Cabernet-besotted grey cells call up some concerns about 'non-linearity' and 'compressibility effects'.

Now, those notions are closely related - as I faintly recall - to certain of the limitations of pitot-static ( airspeed indicator ) systems and to compensated aneroid altimeters. Their responses get sluggish and unreliable, due to non-linearity of several contributing parameters. After about Fl250 or thereabouts, I seem to recollect we switched to watching the other dial - the one with 'M' on it. I still have a couple of 'Computer, Air Navigation, Dead Reckoning - Mark MB-4A' thingys sitting above my desk, with the legend 'To obtain TAS from Mach No, Set true air temp ºC against Mach No index.....'

Akshully, that guff-stuff was all kept for the Standardisation visits, and Categorisation rides. Most of the time we operated on just 3 speeds - Fast, Fastest, and Slow. The last one was kept for landings.

I don't recall ever having met an aviator who tried holding a cup anemometer outside a pressurised twin-jet - or four-jet, for that matter. I will concede there was, at times, a degree of ennui, but cracking the crew hatch to shove the Snr Met Forecaster's pet whirly device into 600 KIAS, or Mach 0.95, was not one of the offered remedies. We tended to follow the dictum 'If it ain't broke, don't fix it.'

However, I'm sure there will be a forumeer, holding a current ATPL, along in a minute. This is the sort of question that the CAA just love to build into their theory exams.....

Oh, er, as a matter of interest.... Quite what had you in mind when you mentioned 'high-altitude'? Above 500 hPa? Above 200hPa? Somewhat higher....? When opportunity presented, we would go up as high as the aircraft weight would permit, or one of us chickened out. There was a label saying that the air/oxygen mixer thingy stopped working at 50,000 feet ambient. Now, I know that's not quite correct, but after a bit there wasn't much point on keeping going on up, 'cos there was no-one else up there to talk to.
Some peeps might care to wade through this stuff
Anyway, why the [--word removed--] would anyone want to know the windspeed from an anemometer up there? It is, as the old QFI relates, 'straight ahead, 450 knots'.......


/forums/images/graemlins/smirk.gif


ps Doncha just luvv the new forum 'sweary spell checker'? The [--word removed--] bit was the capitalised name of a UK chain of cheap and nasty clothing shops. These PC Priests are just full of kakk, and I'd certainly like to cut their [--balls--] off.....

 

[rr_123]

difficult to believe that with so many learned peeps subscribing to this forum that no-one has mentioned our old friend Bernoulli

I don't know how to post greek letters on here, but basically the Bernoulli equation states that the square of velocity (V*V/2*g) + H + Pressure(P/density * g) is a constant.

The key here is density. g is a constant, H is a constant (the height above the experimental datum, could be considered to be height above ordnance datum + height to sail if you like), assume that atmospheric pressure is the same in the summer/winter debate, and "ro", the density, determines the force on the sail - increase the divisor on the pressure constant and you have to subsequently increase the numerator on the velocity constant to maintain equilibrium, ie in denser airs the air would flow faster over the sail and generate more propulsion.

Whether this is a noticable effect or not I have absolutely no idea, but I don't doubt the maths.

 

[grumpy_o_g]

I'm gonna dis-agree with that because I reckon there IS more energy in denser air. It's been compressed which takes energy so it it's holding a higher level of energy than less dense air.

I'll back that up with the aeroplane argument ('cos you lot know more about boats than I do). At higher altitudes the atmosphere (or tropos/stratosphere before some one points that out) exerts less force and there's less force required to move the air out of the way.

Indicated Air Speed is measured by the deflection of a membrane or capsule in the Air Speed Indicator. The ASI is calibrated so that the force on the capsule or membrane moves the needle the right amount to show the air speed at sea level. At higher altitudes the force is much less so the IAS is much lower than the True Air Speed. Also you use less fuel to get the same TAS. I reckon that means that a colder wind would produce more force than a warmer one.

Lift produced by the aerofoil changes too - it decreases as shown by the increase in the stalling speed. Some airliners are actually limited in the height they can fly because the stall speed would exceed either the maximum permitted TAS or the max Mach number.

So I reckon that a cold more dense wind will produce a greater force from the sails. It should also allow you to ghost in lighter winds than in summer (at least in countries that have summers).

Awaiting the howls of derision now....

 

[2nd_apprentice]

If I'd take my (handheld) anemometer and measure

a) air moving at x m/s
b) water flowing at x m/s

would it read higher for water?
Don't think so

 

[Superflid]

But.....

Stick one hand in the air and the other in the water, which one feels the most force on it?

I think that was the OP's point.

 

[AngusMcDoon]

[ 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.

 

[Pye_End]

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.

 

[awol]

[ 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?

 

[AngusMcDoon]

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.

 

[KenMcCulloch]

[ 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 twat.)

 

[Pye_End]

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?

 

[AngusMcDoon]

[ 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

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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.