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Old 15-12-2009, 20:21   #121
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Originally Posted by s/v Jedi View Post
Yes, sail close to an airport and call their meteo. You can calibrate wind speed and barometer with them. Ask for barometer at sea level and wind speed at 10 meters altitude.

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Ahhh, come on Nick! There you are, sailing near the airport... you ask them what the wind speed is, they say x gusting y knots. You rush out and look at your anemometer readout... hmmm it changes constantly, doesn't it? Just which value do you select as being correct? Add in the fact that different instruments have different time constants -- that is, they respond to changes in velocity faster or slower -- and the uncertainty in using the control tower's value becomes so great as to be useless. And this does not even address the discrepancies between velocity over land and sea surfaces. At the standard 10 metre height (a bit lower than our 20 meter masthead) the turbulance generated when the air flow passes over surface features (trees, airplanes, hangars etc) causes greater local speed variation than when it passes over the relatively flat sea surface.

What one needs to calibrate the damn thing is a source of constant velocity airflow, like in a wind tunnel, and then something like a hotwire anemometer as a standard to compare to. Coulda done it years ago when I was employed, but don't think I will do that again!


My instrument was built by Standard Horizon/Navman some years ago. It, and all the others of its brand that I have encountered read something like 15% high. I reckon that they do it on purpose, just to enhance our tall tales of derring-do at sea. Pisses me off...

Oh well, it doesn't really matter, since the good admiral gave us the B scales, and we all develop some sense of what to do with our boats as we look out at the world around us.

But I detest uncalibrated instruments!

By the way, the met office in Noumea New Caledonia has a website that offers a frequently updated baro reading, which is nice for calibrating that instrument. Wish they all did that!

Cheers,
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Old 16-12-2009, 10:06   #122
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However change the A from 5 kts to 10 kts and we have doubled the energy.
?

Quadrupled ?

b.
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Old 16-12-2009, 14:43   #123
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?

Quadrupled ?

b.
Correct. Kinetic energy goes as velocity squared.
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Old 16-12-2009, 16:33   #124
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Kinetic energy goes as velocity squared.
Sort of E = 1/2 * mass * Velocity(squared)

There is no kenitic energy with no mass. Assming the mass does not change then the energy is actually more than the square of the velocity. The resulting value would be in joules.
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Old 17-12-2009, 07:26   #125
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You are contradicting yourself. If less dense air applies less force then you would read a lower wind speed as well. If you are in fact reading the same windspeed (5 kts) the force is the same as at sea level.
The aenometer is a mechanical measuring device for measuring wind speed which does take some energy to turn so yes, it may read fractionally lower in less dense air. The error in measurement is a result of the instrument used to measure the velocity. You could also measure the airspeed using non-mechanical means in which case you'll get a true velocity reading. And the velocity would remain the same at sea level or 10,000 feet.

The sails on your boat, however, aren't low/no drag measuring instruments. They are turning wind energy (which rest assured is dependent upon air density) into power used to provide kinetic energy to your vessel.

So, I'm not contradicting myself. I actually understand the physics.

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The engine can only produce so much power. In less dense air it takes longer to accelerate. However when the pilot rotates the airplane he will be reading the same airspeed. Because 80kts is 80kts of force regardless of temperature & altitude.

The wings are sails - they need 80kts of force to produce enough lift to fly.
I'm not a pilot but this is wrong. First, velocity isn't force so 80 knots won't lift the same load at 10,000 feet as at sea level. Aircraft leaving high altitude airports often carry lighter cargos because they generate less lift at takeoff. I'm sure there are a few pilots who can clarify the details - if not I'll ask some of the fellows around the club here.
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Old 17-12-2009, 08:13   #126
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The pilot will be reading the same airspeed on takeoff at sea level (high density) or high altitude (low density). That is because the airplane is equipped with a pitot tube airspeed indicator, which measures the pressure of the air hitting the aircraft. It just happens to be labeled in knots, and is calibrated using standard sea level density.

Sailboat cup anemometers do not work this way, they measure wind speed, not wind force. The cups are supposed to rotate at some fixed ratio to the wind speed, regardless of density.
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Old 17-12-2009, 08:28   #127
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The pilot will be reading the same airspeed on takeoff at sea level (high density) or high altitude (low density). That is because the airplane is equipped with a pitot tube airspeed indicator, which measures the pressure of the air hitting the aircraft. It just happens to be labeled in knots, and is calibrated using standard sea level density.
.
Interesting - so the pilot takes off when the pitot reads 80 knots but at higher airports he may in fact be doing 90 knots groundspeed or whatever?

Ergo - to get the same energy out of less dense air you have to go faster (groundspeed) to generate the same force (pitot reading). Conversely, at the same speed less dense air conveys less energy.
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Old 17-12-2009, 13:45   #128
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OK, here is a web site with a nifty calculator for air density. This will give you abetter conversion to energy.

DA Calculator - Density Altitude Calculator - DragTimes.com
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Old 17-12-2009, 20:08   #129
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Originally Posted by zydecotoad View Post
The pilot will be reading the same airspeed on takeoff at sea level (high density) or high altitude (low density). That is because the airplane is equipped with a pitot tube airspeed indicator, which measures the pressure of the air hitting the aircraft. It just happens to be labeled in knots, and is calibrated using standard sea level density.

Sailboat cup anemometers do not work this way, they measure wind speed, not wind force. The cups are supposed to rotate at some fixed ratio to the wind speed, regardless of density.
Both the anemometer and the pitot tube (actually the diaphragm, levers and gears in the instrument) are mechanical devices. To make the devices "display" Force has to be converted to Work.

F=mA Doesn't explain the Work. It is the turning of the anemometer and the displacement of the bellows that allows us to indicate the force being apllied calibrated however it makes sense to us.

If the Force is too small to displace the bellows or turn the anemometer you have to increase m or A.

So to disply 5 kts at altitude with a reduction in m, A would have to increase. F is the same.

Quote:
Interesting - so the pilot takes off when the pitot reads 80 knots but at higher airports he may in fact be doing 90 knots groundspeed or whatever?
Exactly - there are nifty calculators pilots use to calculate true air speed = speed correct for temperature and (pressure) altitude. Most airspeed indicators have this function built in

Both the pitot and the anemometer measure "Nautical miles per hour" - note the time funciton is added.

We are on our boat looking at the wind speed indicator - the boat is stationary - we release a ballon at sea level in 5 knots of wind and we release a baloon at 10,000 feet in 5 knots of wind. After one hour which balloon has travelled farther?
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Old 17-12-2009, 23:02   #130
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Hang on...I'm loosing track here...aeroplanes? taking off at altitute? I do most of my sailing at sea level, if I'm below sea level I'm sinking, if I'm above I'm high and dry on the rocks.

I'm not sure what the original physics question was, but if it was something along the lines of "will my boat go faster or heel more in denser air for a given wind speed?" then the answer is yes, of course it will.

Look at the barometer, it tells you air pressure. The higher the pressure, the more dense the air. Since Density = Mass x Volume, a greater Density of air requires there to be a greater Mass for a given Volume (of air).

Now apply Acceleration to that air and direct it at your sails. This can be expressed as:

Force = Mass x Acceleration.

If Accelleration (in this case windspeed) is constant, and Mass increases as Density increases (as demonstrated above). Then Force must increase with an increase in Density - you will get more power out of your sails.

Increase either Mass (density of air) or Accelleration (windspeed) and you will experience more Force.

Oh, and finally the anometer at the top of the mast measures windspeed (Accelleration) not Force.

If that was the question....

OK, I'll shut up now
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Old 17-12-2009, 23:13   #131
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I saw F10 today for a few seconds...a relaxing 50 mile round trip with my youngest to a soccer practice settled me to a F4 and now 4 good quality beers latter I'm down to a F1..headed for becalmed buy nights end...all is good ....
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Old 18-12-2009, 00:05   #132
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Hang on...I'm loosing track here...aeroplanes? taking off at altitute? I do most of my sailing at sea level, if I'm below sea level I'm sinking, if I'm above I'm high and dry on the rocks.
You have some catching up to do. We are sailing at a lake in the rockies at 10,000 feet...

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I'm not sure what the original physics question was, but if it was something along the lines of "will my boat go faster or heel more in denser air for a given wind speed?" then the answer is yes, of course it will.
This is getting good.

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Look at the barometer, it tells you air pressure.
How?

Look at the windspeed indicator it measures force, calibrated in knots.

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Now apply Acceleration to that air and direct it at your sails. This can be expressed as:

Force = Mass x Acceleration.
Acceleration is a change in velocity over time. No change in velocity = no acceleration

Density is mass per volume

The work required to displace the anemometer is fixed. It is a mechanical device. So if I read 5 knots the Force must be the same at sea level and at the lake.

I know the air is less dense so there is less mass.

Therefore velocity must be higher.

F is the same.


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Oh, and finally the anometer at the top of the mast measures windspeed (Accelleration) not Force.
Ok, if you say so...
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Old 18-12-2009, 02:54   #133
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OK I'm not good at this multiple quote thing:

How does a barometer measure air pressure:
I've no idea, but it does, because that is what it is specifically designed to do.

Look at the windspeed indicator it measures force, calibrated in knots:-
1) - No it doesn't, it measures Accelleration. It cannot measure Force because it has no way of measuring Mass. (F = M x A).
2) Knots are not a unit of force, they are a unit of Accelleration: nautical miles per hour

Acceleration is a change in velocity over time: Thats what I just said, nautical miles per hour ; ) - knots or m/s or km/h or mph or even furlongs per decade if you like.

The work required to displace the anemometer is fixed. It is a mechanical device. So if I read 5 knots the Force must be the same at sea level and at the lake: if you read 5 knots at sea level and 5 knots at 10,000ft, it is because the windspeed is 5 knots in both places - this is because your anemometer measures windspeed - because that is what it is specifically designed to do.

Taking the sea level, 10,000ft and windspeed of 5 knots example (don't worry how windspeed is measured, consider it a given)

The air pressure is most likely* to be less at 10,000ft - if the air pressure is less, the Mass of a given volume of air is less. If the Mass is less and the windspeed constant, the Force must be less (F = M x A)

*but not always - but for 10,000ft I think you can assume it would be.
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Old 18-12-2009, 03:35   #134
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OK I'm not good at this multiple quote thing:

How does a barometer measure air pressure:
I've no idea, but it does, because that is what it is specifically designed to do.

Look at the windspeed indicator it measures force, calibrated in knots:-
1) - No it doesn't, it measures Accelleration. It cannot measure Force because it has no way of measuring Mass. (F = M x A).
2) Knots are not a unit of force, they are a unit of Accelleration: nautical miles per hour

Acceleration is a change in velocity over time: Thats what I just said, nautical miles per hour ; ) - knots or m/s or km/h or mph or even furlongs per decade if you like.

The work required to displace the anemometer is fixed. It is a mechanical device. So if I read 5 knots the Force must be the same at sea level and at the lake: if you read 5 knots at sea level and 5 knots at 10,000ft, it is because the windspeed is 5 knots in both places - this is because your anemometer measures windspeed - because that is what it is specifically designed to do.

Taking the sea level, 10,000ft and windspeed of 5 knots example (don't worry how windspeed is measured, consider it a given)

The air pressure is most likely* to be less at 10,000ft - if the air pressure is less, the Mass of a given volume of air is less. If the Mass is less and the windspeed constant, the Force must be less (F = M x A)

*but not always - but for 10,000ft I think you can assume it would be.
Actually, I got this wrong, when I say Accelleration here, I actually should be saying Velocity - the time element of which needs to be squared (m/s to m/s2) for Accelleration.

OK, now I've confused myself.
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Old 18-12-2009, 05:20   #135
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OK this is finally getting interesting
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