Where is the LX S80?

On Wednesday, October 29, 2014 12:59:01 PM UTC-7, Andy Blackburn wrote:
On Wednesday, October 29, 2014 9:51:00 AM UTC-7, jfitch wrote:
On Tuesday, October 28, 2014 9:24:00 PM UTC-7, Andy Blackburn wrote:
On Tuesday, October 28, 2014 5:28:19 PM UTC-7, jfitch wrote:
You are missing my point entirely. A horizontal gust causes actual, real, measurable, and "feelable" vertical acceleration. Ignoring the vario entirely, how can you differentiate it from that acceleration caused by a vertical gust? You cannot without additional information - vertical acceleration is vertical acceleration.

No.

There are transient versus sustained effects that are different for horizontal versus vertical shears (gust versus thermal).

Saying that because a horizontal gust generates lift that it is the same as a thermal that accelerates the glider's frame of reference in a sustained vertical direction is simply incorrect.

9B

Saying that would be simply incorrect - but that is not what I said. A glider is never accelerated in a sustained way. All accelerations the glider experiences are transient, whether induced by a horizontal or vertical gust (excepting turning flight). Once the glider reaches its new velocity, vertical acceleration is zero, regardless of steady state climb rate. This is high school physics. The transient effect is acceleration, this is what you feel. The sustained effect is climb rate, this is what you hope for. But climb rate cannot be felt, only acceleration. When you feel that acceleration, you have about 2 or 3 seconds to determine its cause and react appropriately.

A transient horizontal gust (say ramping quickly from 0 to 10, then back to zero) will be felt as an upward acceleration, followed by a downward acceleration - a bump. But a sustained gust will be felt as an upward acceleration (and an airspeed increase, and a very slight angle of attack reduction, and a lagging variometer up deflection). In nice smooth well behaved air, you might be able to use the more subtle clues to differentiate that from a vertical gust, which will also cause an upward acceleration (and a smaller airspeed increase, a greater angle of attack increase, and perhaps a small momentary lagging downward variometer deflection). In rougher air (mostly what I fly in) sorting this from the noise is practically impossible most of the time. Remembering also that most gusts are neither perfectly vertical nor horizontal, but some random angle in-between.

Of those transient effects, the angle of attack change is probably the easiest to measure, which makes me wonder why this hasn't been pursued more for variometer use. But that signal has a lot of noise in it too.

That's closer to my understanding though I would quibble about some of the details of how an aircraft responds to a horizontal gust.

Assume, for illustration, the gust is 10 knots, and follows the classic "one minus cosine" profile over a second or two. You would see a 10 knot increase in airspeed and if you kept the controls fixed it would activate a modest phugoid response but then be reversed on the back side of the gust. Presuming the glider is flying with the c.g. forward of the center of pressure you should get some onset of upward pitch, but not a lot of immediate g-force as the phugoid is generally a much longer time constant that the short period (AOA) mode. You should also experience some deceleration against the direction of flight from the higher form drag and induced drag due to the change in airspeed, though I suspect this would be harder to pick up than the airspeed change.

With a thermal entry the glider is entering an airmass with vertical velocity that is altered. Again presume 10 knots and in this case also assume it has a rapid onset like the horizontal gust (my experience is that most thermals actually build over a longer time period and are more sustained than horizontal gusts from turbulence but lets make it as similar as possible to tease out the pure differences). The glider experiences two things - a direct vertical acceleration as its inertial reference changes from still air to rising air and it starts to go up directly - this happens pretty quickly, but in the transition it also experiences an increase in angle of attack which activates the short-period longitudinal mode. Given the geometry you can imagine that a vertical air movement has much more of an effect on AOA than a horizontal gust of similar velocity so the sort-period response should be much more energetic.

The other difference is that horizontal gusts tend to look like a "one minus cosine" profile (ramp up and back down) whereas thermal ramp up but don't really ramp back down until you fly out of them several seconds later.

Of course vertical gusts that are not associated with thermals look more like thermals in everything except this symmetric versus asymmetric profile so if the big surge you feel isn't reversed immediately it's more likely a thermal.

If you are familiar with concepts of aircraft dynamics and control theory this article is somewhat informative:

http://scialert.net/fulltext/?doi=srj.2008.17.28

I think to have a vario filter out horizontal gusts you would need to have a dynamic model for the glider and both accelerometers and angular rate gyros plus air data. A simple Kalman filter could then solve for airmass movement and generate a three-dimensional airmass vector in real time. You're only really interested in the Z component so you'd discard the other info unless you were curious about decoding what your body was telling you.

Whether this is the approach vario designers are taking, whether the varios have the sensors to measure all the linear and angular rates and accelerations and whether the effects are pronounced enough to measure clearly amidst all the noise, control inputs and measurement errors and lags I couldn't really say.

9B

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).

However, if the phugoid response difference is your only signal, that is going to be a challenge. Most gliders are flown near the rear of the CG range, and stability pretty weak, therefore small changes in phugoid response vanishingly small. The g force you experience in either to a horizontal or a vertical gust due to changes in airspeed or AOA will be far greater than that due to stability response of the glider, even disregarding normal control movements - which are going to be happening also.

The Butterfly has full 3 axis accelerometer, rate gyro, and magnetometer (as well as air data) as I understand it, and are using this data with Kalman filters to generate an air mass movement vector in real time at approximately a 20 Hz rate. From this it displays a stable AHRS and fully inertially derived horizontal and vertical air mass movement indications which seem to be at least somewhat accurate. These provide considerably more color on what happens in the air than I was used to seeing on a good variometer. It seems to be both of academic and practical interest. In steady state, the barographicly derived vario and inertially derived VAM match pretty well. But in transient states, they frequently vary quite a bit and it is that variation which is interesting.

Something as yet unexplored is there are probably pressure gradients near thermals accompanying the wind gradients. Your barographic vario must be disturbed by those. I think I see evidence of this but it is a little hard to sort out from all the other effects.

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