Heavy wind

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Discussion had on Flexifoil Forum: - Needs unravelling:

Its also to due with cold air is more dense, not just with water. Cold air compresses because the particles are alot less excited so can be in closer vicinity to each other. This means more air particles per square meter. If you say blew 15mph of hydrogen onto a kite not alot would happen because its so thin. 15mph of a heavy gas, say CO2 would feel more than hydrogen because there are more particles hitting the kite.

Its how fast and how many particles are hitting the kite that determine the power, not just how fast the particles are. Cold air = more particles due to density!

The density of gases changes significantly with temperature, hence the pressure (hence force) that they can exert for the same volume (and it is a volume of airflow, not a mass or mass flow rate that a wind speed meter measures).

The same is not the case for solids, whose mass does changes very little with temperature, so the peanut agument is useless.

An anemometer measures flow rate, not power.

It would make no difference to an anemometer if it was measuring liquid flow at 10mph or air flow at 10mph - The same measuring cups would both measure 10mph.

However the kinetic energy available in the water would be considerably more than that of the air, and it can't measure that.

force = [constant] * [Area] * [Speed^2] == [m^2] * [(m/s)^2] == [m^2] * [m^2 * s^-2] == m^4 s^-2 which are not units of force.

multiply by density (kg/m^3) and you will get: kg m s^-2 = Newton = Force

http://wahiduddin.net/calc/density_altitude.htm

for: temp = 25 degC air pressure = 1013.25 mb dew point = 18 degC (about 45% humidity) you get density = 1.1748 kg/m3

and for: temp = 0 degC air pressure = 1013.25 mb dew point = 10 degC (about 30% humidity) you get density = 1.2864 kg/m3

so that is a 9.5% increase for not unreasonable hot summer vs. cold winter conditions and that is at a constant air pressure

So the relationship between cold dry weather and additional kite power seems proven, which was the point in question .

I have to confess though that I was looking for a figure of more like 20% to make a real difference to the way that a kite feels from one day to another. Is it possible to get a sensible variation in weather conditions that would give this sort of difference?

The reason that I ask is that if I am flying a 7M kite in 10mph wind, a 10% increase in power would be almost identical to me flying the same kite in 10.49 mph wind, assuming that the power generated is proportional to the square of the windspeed . That doesn't seem like so much does it?

you are not wrong, but it is proportional to speed squared, so at 20mph a 10% increase in power is 1mph

or to think of it another way, at 30mph a 1mph increase in wind speed gives only a 6.8% increase in power; and at 40mph you get about a 5% increase in power for a 1mph increase in wind speed.

edit: in fact you can say that for a given %age increase in wind speed, you get approximately double that %'age increase in the power. - e.g.: the increase from 25-26mph is 4% in speed, and the increase in power is 8.16%

Yes. The lifting / pulling force is directly proportional to the force exerted by the wind, which is in turn directly proportional to the air density.

So 10% more density should result in 10% more lift / pull.

However lift / pull is proportional to the square of the windspeed, so like I pointed out above, a 10% increase in lift / pull would be generated by a wind speed increase from 10mph to 10.49mph .

<LOKI_79> Assuming that the kite behaves according to the standard equation for lift: Lift = [constant] * Fluid Density * Area * Wind Speed^2

then a 10% increase in density == a 10% increase in power.

unless there is a part of the 'constant' term that somehow varies as a function of density, then there is a direct correlation.

edit: just incase there are any physicists reading this, i'd better correct the fact that power is not the same as force, but 'power' in the sense talked about by kiters is really just the force generated anyway

<Sand Yeti> I'm looking for someone to tell me where I went wrong please.

I'd like to quote acouple of practical example to demonstarte that cold air is denser than warm air .

1) I fly kites in ambient conditions of 50+°C during our summer. For the same wind speeds in the winter (say 20°C), I use a smaller kite (one size down). The wind speed measures the same but the pulling power under cold condition is a lot more.

2) Ever wondered about turbocharged truck engines?. They are there simply to pack more air into the engine. The air with the right ratio of fuel optimizes combustion to the pump out more power than a naturally aspirated engine. The turbo is simply an air compressor & when you compress air it heats up i.e less dense. (bicycle pump trick). This is contrary to what we like to do in the engine's combustion chamber as we are trying to pack as much dense air into it as possible. So to fix that problem most modern truck engines are fitted with intercoolers (sometimes referred to as after-coolers). These are simply heat exchangers either air or water cooled that transfer the hot induction air after the turbo to allow cooler more dense air into the engine. Forced cooled air will allow more fuel to be pumped in so that even more power is obtainable than from a simple turbochargd engine. Engine manufacturers figured out over 50 yeras ago that cold air is denser and hence took advantage of this fact by cooling the air after turbocharging so as to pack more of it into the combustion chamber.

No sums but two simple practical examples showing why cold air is denser than warm air.

<jlmusic> doesnt really show WHY cold air is more dense, only shows things that are of a consequence of that

to explain WHY cold air is more dense, is elementary particle physics, the sort of thing we learn at primary school in 'science' lessons

if we take the coldest something can be to be a solid, and the hottest it can be to be gaseous, in a solid, the molecules are tightly packed, and have very little kinetic energy and exist in a regular lattice arrangement

when a gas, the molecules are far more spacious, the increased heat makes increased kinetic energy and the molecules move around a lot and collide with one another. for this reason theres are less molecules per volume unit thus a low relative density

every chemical state is not an absolute, each has varying degrees, eg a gas can be more or less spacious between molecules. as the gas cools, the molecules move less, and the gaps between them lessen also. this in turn means there are more molecules per unit area thus a more dense gas.