Where is the LX S80?

On Wednesday, October 29, 2014 1:38:45 PM UTC-7, jfitch wrote:

One problem complicating this is that thermals are normally accompanied by local horizontal "gusts" which are actually sustained flow field (as mentioned in some of the above posts).

Signal to noise can always be a problem - the good news should be that if the air is really going up you ought to be able to pick that out, but I agree if there is turbulence on the thermal entry that has greater velocity than the thermal itself and/or goes on for a long way a human observer won't be able to integrate the net effects for long enough to figure out what is going on. A computer might have a better shot at it.

However, if the phugoid response difference is your only signal, that is going to be a challenge.

I was saying something a bit different. In the case of a horizontal gust the main dynamic reaction from the glider is some modest horizontal deceleration and a slow pitch up followed by a slow pitch down (post-gust) from the phugoid response. In a thermal entry you get mostly a vertical surge plus some downward pitching moment from the short period response if the glider has static stability. Most modern gliders don't benefit from being flown at the aft limit but even if you do the response should be different. If you have a glider that generates a nose-up pitching moment from an increase in angle of attack, that would be a real handful to fly even under benign conditions. This is at least some of the reason why thermals "feel" different. That surge you feel has a different linear acceleration vector and a different (opposite) pitch response.

If the Butterfly uses Kalman filters to separate out the air mass movement that is exactly what I was suggesting (and attempting to explain why) - you ought to be able to pick out the air movement vector IF you have the right onboard sensors AND you have an accurate enough dynamic model for the glider. The better the model represents all the aerodynamic and inertial coefficients the more accurate the answer should be. I could also imagine intelligently looking not just at the instantaneous airmass velocity, but also the profile of thermals for a given day to help identify good ones from bad ones, though that is a much more complex matter.

Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B

On Thursday, October 30, 2014 4:59:22 AM UTC-7, Andy Blackburn wrote:
On Wednesday, October 29, 2014 1:38:45 PM UTC-7, jfitch wrote:

One problem complicating this is that thermals are normally accompanied by local horizontal "gusts" which are actually sustained flow field (as mentioned in some of the above posts).

Signal to noise can always be a problem - the good news should be that if the air is really going up you ought to be able to pick that out, but I agree if there is turbulence on the thermal entry that has greater velocity than the thermal itself and/or goes on for a long way a human observer won't be able to integrate the net effects for long enough to figure out what is going on. A computer might have a better shot at it.

However, if the phugoid response difference is your only signal, that is going to be a challenge.

I was saying something a bit different. In the case of a horizontal gust the main dynamic reaction from the glider is some modest horizontal deceleration and a slow pitch up followed by a slow pitch down (post-gust) from the phugoid response. In a thermal entry you get mostly a vertical surge plus some downward pitching moment from the short period response if the glider has static stability. Most modern gliders don't benefit from being flown at the aft limit but even if you do the response should be different. If you have a glider that generates a nose-up pitching moment from an increase in angle of attack, that would be a real handful to fly even under benign conditions. This is at least some of the reason why thermals "feel" different. That surge you feel has a different linear acceleration vector and a different (opposite) pitch response.

If the Butterfly uses Kalman filters to separate out the air mass movement that is exactly what I was suggesting (and attempting to explain why) - you ought to be able to pick out the air movement vector IF you have the right onboard sensors AND you have an accurate enough dynamic model for the glider. The better the model represents all the aerodynamic and inertial coefficients the more accurate the answer should be. I could also imagine intelligently looking not just at the instantaneous airmass velocity, but also the profile of thermals for a given day to help identify good ones from bad ones, though that is a much more complex matter.

Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B

I agree mostly. A horizontal gust WILL produce a vertical acceleration and this can be quite strong due to the V^2 term in dynamic pressure. This can produce a instantaneous response in the glider that is pure free body effect, superimposed on static aerodynamic stability, and they will normally be opposite. In a vertical gust the effects will in the same direction. In my experience, in strong thermals the dynamic accelerations will be of far greater magnitude than static stability effects, swamping that signal (and eliminating the need, for the most part, to model the glider dynamics closely).. On weak days or with soft well behaved thermals, maybe - but where I fly we are rarely cursed with those conditions. The horizontal air movement in and around thermals is far greater than I thought, until I "instrumented up". Once you know its there, you begin to look critically for confirming evidence and discover that is is there.

On Thursday, October 30, 2014 10:09:13 AM UTC-7, jfitch wrote:
On Thursday, October 30, 2014 4:59:22 AM UTC-7, Andy Blackburn wrote:
On Wednesday, October 29, 2014 1:38:45 PM UTC-7, jfitch wrote:

One problem complicating this is that thermals are normally accompanied by local horizontal "gusts" which are actually sustained flow field (as mentioned in some of the above posts).

Signal to noise can always be a problem - the good news should be that if the air is really going up you ought to be able to pick that out, but I agree if there is turbulence on the thermal entry that has greater velocity than the thermal itself and/or goes on for a long way a human observer won't be able to integrate the net effects for long enough to figure out what is going on. A computer might have a better shot at it.

However, if the phugoid response difference is your only signal, that is going to be a challenge.

I was saying something a bit different. In the case of a horizontal gust the main dynamic reaction from the glider is some modest horizontal deceleration and a slow pitch up followed by a slow pitch down (post-gust) from the phugoid response. In a thermal entry you get mostly a vertical surge plus some downward pitching moment from the short period response if the glider has static stability. Most modern gliders don't benefit from being flown at the aft limit but even if you do the response should be different. If you have a glider that generates a nose-up pitching moment from an increase in angle of attack, that would be a real handful to fly even under benign conditions. This is at least some of the reason why thermals "feel" different. That surge you feel has a different linear acceleration vector and a different (opposite) pitch response.

If the Butterfly uses Kalman filters to separate out the air mass movement that is exactly what I was suggesting (and attempting to explain why) - you ought to be able to pick out the air movement vector IF you have the right onboard sensors AND you have an accurate enough dynamic model for the glider. The better the model represents all the aerodynamic and inertial coefficients the more accurate the answer should be. I could also imagine intelligently looking not just at the instantaneous airmass velocity, but also the profile of thermals for a given day to help identify good ones from bad ones, though that is a much more complex matter.

Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B

I agree mostly. A horizontal gust WILL produce a vertical acceleration and this can be quite strong due to the V^2 term in dynamic pressure. This can produce a instantaneous response in the glider that is pure free body effect, superimposed on static aerodynamic stability, and they will normally be opposite. In a vertical gust the effects will in the same direction. In my experience, in strong thermals the dynamic accelerations will be of far greater magnitude than static stability effects, swamping that signal (and eliminating the need, for the most part, to model the glider dynamics closely). On weak days or with soft well behaved thermals, maybe - but where I fly we are rarely cursed with those conditions. The horizontal air movement in and around thermals is far greater than I thought, until I "instrumented up". Once you know its there, you begin to look critically for confirming evidence and discover that is is there.

I learned something from the discussion I need to find a way to go test. I've always had the sense that a decent thermal gave you a surge that felt a little tail-high. Like you were being shoved upward and a bit forward. Stick thermals have the opposite pitch sensation as do horizontal gusts - though I am less good at recognizing gusts except as random vario readings. I need to confirm whether the sensation comes from the coupling of the thermal's transient vertical air movement through to short-period pitch response via the increase in AOA. A little math and some flying are in order.

9B

On Thursday, October 30, 2014 3:54:54 PM UTC-7, Andy Blackburn wrote:
On Thursday, October 30, 2014 10:09:13 AM UTC-7, jfitch wrote:
On Thursday, October 30, 2014 4:59:22 AM UTC-7, Andy Blackburn wrote:
On Wednesday, October 29, 2014 1:38:45 PM UTC-7, jfitch wrote:

One problem complicating this is that thermals are normally accompanied by local horizontal "gusts" which are actually sustained flow field (as mentioned in some of the above posts).

Signal to noise can always be a problem - the good news should be that if the air is really going up you ought to be able to pick that out, but I agree if there is turbulence on the thermal entry that has greater velocity than the thermal itself and/or goes on for a long way a human observer won't be able to integrate the net effects for long enough to figure out what is going on. A computer might have a better shot at it.

However, if the phugoid response difference is your only signal, that is going to be a challenge.

I was saying something a bit different. In the case of a horizontal gust the main dynamic reaction from the glider is some modest horizontal deceleration and a slow pitch up followed by a slow pitch down (post-gust) from the phugoid response. In a thermal entry you get mostly a vertical surge plus some downward pitching moment from the short period response if the glider has static stability. Most modern gliders don't benefit from being flown at the aft limit but even if you do the response should be different. If you have a glider that generates a nose-up pitching moment from an increase in angle of attack, that would be a real handful to fly even under benign conditions. This is at least some of the reason why thermals "feel" different. That surge you feel has a different linear acceleration vector and a different (opposite) pitch response.

If the Butterfly uses Kalman filters to separate out the air mass movement that is exactly what I was suggesting (and attempting to explain why) - you ought to be able to pick out the air movement vector IF you have the right onboard sensors AND you have an accurate enough dynamic model for the glider. The better the model represents all the aerodynamic and inertial coefficients the more accurate the answer should be. I could also imagine intelligently looking not just at the instantaneous airmass velocity, but also the profile of thermals for a given day to help identify good ones from bad ones, though that is a much more complex matter.

Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B

I agree mostly. A horizontal gust WILL produce a vertical acceleration and this can be quite strong due to the V^2 term in dynamic pressure. This can produce a instantaneous response in the glider that is pure free body effect, superimposed on static aerodynamic stability, and they will normally be opposite. In a vertical gust the effects will in the same direction. In my experience, in strong thermals the dynamic accelerations will be of far greater magnitude than static stability effects, swamping that signal (and eliminating the need, for the most part, to model the glider dynamics closely). On weak days or with soft well behaved thermals, maybe - but where I fly we are rarely cursed with those conditions. The horizontal air movement in and around thermals is far greater than I thought, until I "instrumented up". Once you know its there, you begin to look critically for confirming evidence and discover that is is there.

I learned something from the discussion I need to find a way to go test. I've always had the sense that a decent thermal gave you a surge that felt a little tail-high. Like you were being shoved upward and a bit forward. Stick thermals have the opposite pitch sensation as do horizontal gusts - though I am less good at recognizing gusts except as random vario readings. I need to confirm whether the sensation comes from the coupling of the thermal's transient vertical air movement through to short-period pitch response via the increase in AOA. A little math and some flying are in order.

9B

Or just basic statics: if the CG is ahead of the lifting center of the wing the upward acceleration causes a pitch down moment.

On Friday, October 31, 2014 8:08:06 AM UTC-7, jfitch wrote:
On Thursday, October 30, 2014 3:54:54 PM UTC-7, Andy Blackburn wrote:
On Thursday, October 30, 2014 10:09:13 AM UTC-7, jfitch wrote:
On Thursday, October 30, 2014 4:59:22 AM UTC-7, Andy Blackburn wrote:
On Wednesday, October 29, 2014 1:38:45 PM UTC-7, jfitch wrote:

One problem complicating this is that thermals are normally accompanied by local horizontal "gusts" which are actually sustained flow field (as mentioned in some of the above posts).

Signal to noise can always be a problem - the good news should be that if the air is really going up you ought to be able to pick that out, but I agree if there is turbulence on the thermal entry that has greater velocity than the thermal itself and/or goes on for a long way a human observer won't be able to integrate the net effects for long enough to figure out what is going on. A computer might have a better shot at it.

However, if the phugoid response difference is your only signal, that is going to be a challenge.

I was saying something a bit different. In the case of a horizontal gust the main dynamic reaction from the glider is some modest horizontal deceleration and a slow pitch up followed by a slow pitch down (post-gust) from the phugoid response. In a thermal entry you get mostly a vertical surge plus some downward pitching moment from the short period response if the glider has static stability. Most modern gliders don't benefit from being flown at the aft limit but even if you do the response should be different. If you have a glider that generates a nose-up pitching moment from an increase in angle of attack, that would be a real handful to fly even under benign conditions. This is at least some of the reason why thermals "feel" different. That surge you feel has a different linear acceleration vector and a different (opposite) pitch response.

If the Butterfly uses Kalman filters to separate out the air mass movement that is exactly what I was suggesting (and attempting to explain why) - you ought to be able to pick out the air movement vector IF you have the right onboard sensors AND you have an accurate enough dynamic model for the glider. The better the model represents all the aerodynamic and inertial coefficients the more accurate the answer should be. I could also imagine intelligently looking not just at the instantaneous airmass velocity, but also the profile of thermals for a given day to help identify good ones from bad ones, though that is a much more complex matter.

Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B

I agree mostly. A horizontal gust WILL produce a vertical acceleration and this can be quite strong due to the V^2 term in dynamic pressure. This can produce a instantaneous response in the glider that is pure free body effect, superimposed on static aerodynamic stability, and they will normally be opposite. In a vertical gust the effects will in the same direction. In my experience, in strong thermals the dynamic accelerations will be of far greater magnitude than static stability effects, swamping that signal (and eliminating the need, for the most part, to model the glider dynamics closely). On weak days or with soft well behaved thermals, maybe - but where I fly we are rarely cursed with those conditions. The horizontal air movement in and around thermals is far greater than I thought, until I "instrumented up". Once you know its there, you begin to look critically for confirming evidence and discover that is is there.

I learned something from the discussion I need to find a way to go test.. I've always had the sense that a decent thermal gave you a surge that felt a little tail-high. Like you were being shoved upward and a bit forward. Stick thermals have the opposite pitch sensation as do horizontal gusts - though I am less good at recognizing gusts except as random vario readings. I need to confirm whether the sensation comes from the coupling of the thermal's transient vertical air movement through to short-period pitch response via the increase in AOA. A little math and some flying are in order.

9B

Or just basic statics: if the CG is ahead of the lifting center of the wing the upward acceleration causes a pitch down moment.

True that - I wonder if one effect is reliably larger than the other for a typical glider with a cg midway between forward and aft limits. In any case a solid thermal entry should generate some nose-down pitch. Also seems that if you want to feel what's going on don't mess with the elevator too much..

9B

Think in terms of the relative wind and the trim speed:

When we enter the thermal, the airmass changes, there is a upward component, the nose pitches down relative to the horizon, but the trim speed remains the same. We see a pitch down but the relative wind is the same.

The opposite is true leaving the thermal or in a downdraft: it seems we can't get the nose down far enough as the downdraft pitches the nose up, or at least decreases the AoA.

Like Moffat said (paraphrase) it's easier to accelerate in lift than sink.

On Thursday, 30 October 2014 13:59:22 UTC+2, Andy Blackburn wrote:
Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B


Could you elaborate? I ran the numbers quickly, there is usually temperature gradient of 0,5-2 degrees (centigrade) between thermal core and "free air" around it. If we think normal smallish thermal as couple of hundred meters in radius, we would get average temperature gradient of 0,25-1 degree/100meters between core and surrouding air. Glider with temp sensors at wingtips would probably reguire 0,01-0,05 deg resolution of sensors to give any information. It could be doable, though, relative differences are easier to measure than absolute. It could give you a sort of "left or right" indication when close to thermal.

This was discussed at length in Soaring magazine back in the 80s or,
maybe earlier. A great winter topic which we'll all promptly forget
with the return of thermal soaring season.

Dan Marotta

On 11/2/2014 5:51 AM, krasw wrote:
On Thursday, 30 October 2014 13:59:22 UTC+2, Andy Blackburn wrote:
Thermals also have temperature gradients - though the experiments I participated in recently were not conclusive.

9B

Could you elaborate? I ran the numbers quickly, there is usually temperature gradient of 0,5-2 degrees (centigrade) between thermal core and "free air" around it. If we think normal smallish thermal as couple of hundred meters in radius, we would get average temperature gradient of 0,25-1 degree/100meters between core and surrouding air. Glider with temp sensors at wingtips would probably reguire 0,01-0,05 deg resolution of sensors to give any information. It could be doable, though, relative differences are easier to measure than absolute. It could give you a sort of "left or right" indication when close to thermal.

On Sunday, November 2, 2014 4:51:43 AM UTC-8, krasw wrote:

Could you elaborate? I ran the numbers quickly, there is usually temperature gradient of 0,5-2 degrees (centigrade) between thermal core and "free air" around it. If we think normal smallish thermal as couple of hundred meters in radius, we would get average temperature gradient of 0,25-1 degree/100meters between core and surrouding air. Glider with temp sensors at wingtips would probably reguire 0,01-0,05 deg resolution of sensors to give any information. It could be doable, though, relative differences are easier to measure than absolute. It could give you a sort of "left or right"

That is the basic idea. I know from experimentation that there are measurable temperature variations out there, it's just not obvious to me what they mean with respect to the vertical movement of the airmass.

I'm curious what your running the numbers looks like. I admit I don't have a good mental model for how thermals work from a thermodynamic and aerodynamic perspective. I thought it had something to do with the fact that warm air was less dense and therefore buoyant. How that buoyancy accelerates a volume of air until some form of resistance at the edges progressively resists the acceleration and a steady rate of upward velocity is reached is beyond my understanding at a level detailed enough to relate thermal strength to temperature differences.

As to the temperature gradient across the thermal - I'm not sure it's linear from the edge to the center. Imagine a volume of air rising at 500 FPM. Presumably you have some mixing at the edges but the rest of the heat transfer would mostly be conductive over a period of 10 minutes before the thermal reaches, say, 5000'. I'm not sure what all the coefficients are, but it isn't 100% obvious to me that you'd end up with a linear temperature gradient all the way to the center of the thermal since there is so much new warm air being introduced continuously from the bottom, there isn't much time for heat to transfer to the outside air and air isn't that great a heat conductor in the first place. Have there been studies done?

9B

On Sunday, 2 November 2014 21:03:39 UTC+2, Andy Blackburn wrote:
I'm curious what your running the numbers looks like. I admit I don't have a good mental model for how thermals work from a thermodynamic and aerodynamic perspective. I thought it had something to do with the fact that warm air was less dense and therefore buoyant. How that buoyancy accelerates a volume of air until some form of resistance at the edges progressively resists the acceleration and a steady rate of upward velocity is reached is beyond my understanding at a level detailed enough to relate thermal strength to temperature differences.

As to the temperature gradient across the thermal - I'm not sure it's linear from the edge to the center. Imagine a volume of air rising at 500 FPM. Presumably you have some mixing at the edges but the rest of the heat transfer would mostly be conductive over a period of 10 minutes before the thermal reaches, say, 5000'. I'm not sure what all the coefficients are, but it isn't 100% obvious to me that you'd end up with a linear temperature gradient all the way to the center of the thermal since there is so much new warm air being introduced continuously from the bottom, there isn't much time for heat to transfer to the outside air and air isn't that great a heat conductor in the first place. Have there been studies done?

9B

I can't quote any sources but I bet digging into Google Scholar would result studies (boundary layer physics would be good place to start). I know that warm air in thermal bubble or column is little bit warmer (numbers quoted earlier are realistic) than surround. That temperature difference is maintained with altitude (per hydrostatic equation), and thermal is surprisingly "closed" system, mixing at the edge of ascending air is quite small compared to thermal volume. It's true that average temperature gradient is not realistic figure, the gradient is probable decade higher at edge of thermal than figure I threw out of my sleeve.

I think running constant temp difference analysis would be quite easy with simple and cheap linux computer, temp signals going through DA conversion to digital domain. Sensors could be calibrated over time to match each other, so small differences could be readable, even if absolute accuracy is something like 0,1 degrees. If this has been done in 80's, technology probably was analog electronics. Nowadays it would be more software project.