Where is the LX S80?

On Tuesday, November 4, 2014 5:39:22 PM UTC-8, Evan Ludeman wrote:
On Tuesday, November 4, 2014 8:10:44 PM UTC-5, jfitch wrote:
On Tuesday, November 4, 2014 4:24:00 PM UTC-8, Andy Blackburn wrote:
On Tuesday, November 4, 2014 3:04:41 PM UTC-8, Evan Ludeman wrote:
On Tuesday, November 4, 2014 4:38:33 PM UTC-5, Andy Blackburn wrote:
I did the simple physics on this. A volume of air rising at 600 fpm with no other forces acting on it other than gravity has enough momentum to rise an additional 1.6 feet before its upward velocity reaches zero.

I am now officially at a loss as to why thermals go up if there is no temperature difference versus the surrounding air.

9B

That's for an object in a vacuum. The thermal isn't surrounded by a vacuum, it's surrounded by air at almost precisely the same density.

T8

Wouldn't that make it go up even less due to the frictional resistance?

Just doesn't weigh as much as you assumed

Buoyancy.

T8

Added that in. Buoyancy works opposite to gravity but momentum still based on total mass of the air.

The math seems to indicate that the momentum of a bubble of air is actually small compared to the buoyancy effects of even slight changes in temperature. I also looked at air density versus temperature and humidity. The humidity effects are real but generally small as well. Even a giant swing in humidity from 80 percent to 30 percent generates an increase in density equivalent to just 2 degrees F. I can see why it pays to look for thermals downwind of cattle tanks in the desert, and why you might get a kick in the climb rate at several thousand feet, but it doesn't explain for me why thermals go up in the first place.

I suppose this confirms why a Skew-T plot generally predicts thermal heights based on temperature differences - the height where e a parcel of air at the surface, when adiabatically lifted, reaches the ambient temperature in the sounding. There's no adjustment for mixing of the thermal down low and no adjustment for momentum of the rising air or the humidity of the air - the thermal goes to the height where the core of the thermal adiabatically cools to the surrounding air temperature and it stops.

9B

On Wednesday, November 5, 2014 1:01:27 PM UTC-8, Andy Blackburn wrote:
On Tuesday, November 4, 2014 5:39:22 PM UTC-8, Evan Ludeman wrote:
On Tuesday, November 4, 2014 8:10:44 PM UTC-5, jfitch wrote:
On Tuesday, November 4, 2014 4:24:00 PM UTC-8, Andy Blackburn wrote:
On Tuesday, November 4, 2014 3:04:41 PM UTC-8, Evan Ludeman wrote:
On Tuesday, November 4, 2014 4:38:33 PM UTC-5, Andy Blackburn wrote:
I did the simple physics on this. A volume of air rising at 600 fpm with no other forces acting on it other than gravity has enough momentum to rise an additional 1.6 feet before its upward velocity reaches zero.

I am now officially at a loss as to why thermals go up if there is no temperature difference versus the surrounding air.

9B

That's for an object in a vacuum. The thermal isn't surrounded by a vacuum, it's surrounded by air at almost precisely the same density.

T8

Wouldn't that make it go up even less due to the frictional resistance?

Just doesn't weigh as much as you assumed

Buoyancy.

T8

Added that in. Buoyancy works opposite to gravity but momentum still based on total mass of the air.

The math seems to indicate that the momentum of a bubble of air is actually small compared to the buoyancy effects of even slight changes in temperature. I also looked at air density versus temperature and humidity. The humidity effects are real but generally small as well. Even a giant swing in humidity from 80 percent to 30 percent generates an increase in density equivalent to just 2 degrees F. I can see why it pays to look for thermals downwind of cattle tanks in the desert, and why you might get a kick in the climb rate at several thousand feet, but it doesn't explain for me why thermals go up in the first place.

I suppose this confirms why a Skew-T plot generally predicts thermal heights based on temperature differences - the height where e a parcel of air at the surface, when adiabatically lifted, reaches the ambient temperature in the sounding. There's no adjustment for mixing of the thermal down low and no adjustment for momentum of the rising air or the humidity of the air - the thermal goes to the height where the core of the thermal adiabatically cools to the surrounding air temperature and it stops.

9B

I think you need to recheck your math. 600 fpm is 10 fps. That would be around 20 ft at 1G with no other retarding forces and mass = weight. But in this case mass weight depending on the buoyancy, i.e., much less than 1G deceleration acting on it. When the density matches the surrounding air, the air bubble is weightless (A due to G = 0), and therefore will carry on forever absent another retarding force. Of course as it rises through an inversion of less dense air, it will lose buoyancy and G will begin to have some effect, also there are frictional effects of the surrounding atmosphere which I imagine are hard to calculate. Much for that 1.6 feet though.

On Wednesday, November 5, 2014 3:10:29 PM UTC-8, jfitch wrote:

I think you need to recheck your math. 600 fpm is 10 fps. That would be around 20 ft at 1G with no other retarding forces and mass = weight. But in this case mass weight depending on the buoyancy, i.e., much less than 1G deceleration acting on it. When the density matches the surrounding air, the air bubble is weightless (A due to G = 0), and therefore will carry on forever absent another retarding force. Of course as it rises through an inversion of less dense air, it will lose buoyancy and G will begin to have some effect, also there are frictional effects of the surrounding atmosphere which I imagine are hard to calculate. Much for that 1.6 feet though.

Must've missed a decimal place, but even with that corrected, the inertial effects seem to be a lot less than the buoyancy effects, which is mostly driven by temperature profile in the atmosphere versus a dry adiabat. Humidity matters a little, but is the equivalent of a a fraction of a degree to maybe a couple of degrees of temperature variation - which is way less than the variations you see in a typical sounding. Temperature seems like more than an order of magnitude more important than humidity, which itself is roughly an order of magnitude more important than inertia effects.

Where's Dr Jack when I need him?

9B

On Wednesday, November 5, 2014 3:22:46 PM UTC-8, Andy Blackburn wrote:
On Wednesday, November 5, 2014 3:10:29 PM UTC-8, jfitch wrote:

I think you need to recheck your math. 600 fpm is 10 fps. That would be around 20 ft at 1G with no other retarding forces and mass = weight. But in this case mass weight depending on the buoyancy, i.e., much less than 1G deceleration acting on it. When the density matches the surrounding air, the air bubble is weightless (A due to G = 0), and therefore will carry on forever absent another retarding force. Of course as it rises through an inversion of less dense air, it will lose buoyancy and G will begin to have some effect, also there are frictional effects of the surrounding atmosphere which I imagine are hard to calculate. Much for that 1.6 feet though.

Must've missed a decimal place, but even with that corrected, the inertial effects seem to be a lot less than the buoyancy effects, which is mostly driven by temperature profile in the atmosphere versus a dry adiabat. Humidity matters a little, but is the equivalent of a a fraction of a degree to maybe a couple of degrees of temperature variation - which is way less than the variations you see in a typical sounding. Temperature seems like more than an order of magnitude more important than humidity, which itself is roughly an order of magnitude more important than inertia effects.

Where's Dr Jack when I need him?

9B

The inertial effects are inversely proportional to the thread drift coefficient...

Cheers,
Craig

There are a couple of nice videos of the S80 in flights has been posted by a Danish soaring club: http://herningsvaeveflyveklub.dk/nyh...-far-lxnav-s80

The installation is in the back seat of ask ASK-21 -- not sure if it is showing an S80 back seat repeater in use or a primary S80.

On Wednesday, November 5, 2014 8:31:07 PM UTC-5, Craig Funston wrote:
On Wednesday, November 5, 2014 3:22:46 PM UTC-8, Andy Blackburn wrote:
On Wednesday, November 5, 2014 3:10:29 PM UTC-8, jfitch wrote:

I think you need to recheck your math. 600 fpm is 10 fps. That would be around 20 ft at 1G with no other retarding forces and mass = weight. But in this case mass weight depending on the buoyancy, i.e., much less than 1G deceleration acting on it. When the density matches the surrounding air, the air bubble is weightless (A due to G = 0), and therefore will carry on forever absent another retarding force. Of course as it rises through an inversion of less dense air, it will lose buoyancy and G will begin to have some effect, also there are frictional effects of the surrounding atmosphere which I imagine are hard to calculate. Much for that 1.6 feet though.

Must've missed a decimal place, but even with that corrected, the inertial effects seem to be a lot less than the buoyancy effects, which is mostly driven by temperature profile in the atmosphere versus a dry adiabat. Humidity matters a little, but is the equivalent of a a fraction of a degree to maybe a couple of degrees of temperature variation - which is way less than the variations you see in a typical sounding. Temperature seems like more than an order of magnitude more important than humidity, which itself is roughly an order of magnitude more important than inertia effects.

Where's Dr Jack when I need him?

9B

The inertial effects are inversely proportional to the thread drift coefficient...

Cheers,
Craig

Length of the thread is also inversely proportional to relevance to the real world.
UH

THanks.
Would be nice to see Flarm, map function, and horizon displays.
Soon