Where is the LX S80?

On Wednesday, October 22, 2014 2:03:48 AM UTC+11, jfitch wrote:
On Monday, October 20, 2014 4:24:46 PM UTC-7, wrote:
On Friday, October 10, 2014 3:50:54 AM UTC+11, wrote:

Excellent questions and observations.



I am focused on the S80 and Butterfly primarily because they have an artificial horizon but also would like a unit that can filter out horizontal gusts that cause me/us to think we have entered a strong thermal. It also would seem that the instantaneous wind readings would be very helpful when flying convergence lines.



You have a built in filter that can eliminate false readings due to gusts. It's called your backside. If you can't feel the acceleration, it's not there.

Acceleration due to a a vertical or horizontal gust are not discernible by the human backside. Both result in an increase in lift: the former due to an increase in the angle of attack and the latter due to an increase in air speed. Both can be used to gain energy, but you will find circling in the latter a pointless exercise. Which is why every variometer manufacturer is trying to differentiate them.

A horizontal gust does NOT produce acceleration. That's why you can't feel it. If you cannot sense vertical acceleration it's not there, despite what your instruments are telling you.
To put it another way: if your vario is telling you that you are accelerating upwards at 5 knots (500 fpm or over 8 feet per second) and you cannot feel it, then you can ignore it as a gust. Why look for a complex technological solution to something so simple?
A vertical gust is a different matter. I presume by vertical gust you are referring to a vertical movement of air. What distinguishes a vertical gust from lift?

The issue with horizontal gusts is that if you fly into a thermal outflow (quite common where they are strong and dry), it temporarily increases your airspeed. The TE probe outputs a signal that interprets this increase in aircraft energy as lift. There is no acceleration, just an increase in energy. The seat of your pants may well be able to sense the lack of acceleration in this circumstance, but sensitive accelerometers will be even better.

Mike

On Wednesday, October 29, 2014 11:07:05 AM UTC+11, Mike the Strike wrote:
The issue with horizontal gusts is that if you fly into a thermal outflow (quite common where they are strong and dry), it temporarily increases your airspeed. The TE probe outputs a signal that interprets this increase in aircraft energy as lift. There is no acceleration, just an increase in energy. The seat of your pants may well be able to sense the lack of acceleration in this circumstance, but sensitive accelerometers will be even better.

On Tuesday, October 28, 2014 5:22:42 PM UTC-7, wrote:
On Wednesday, October 29, 2014 11:07:05 AM UTC+11, Mike the Strike wrote:
The issue with horizontal gusts is that if you fly into a thermal outflow (quite common where they are strong and dry), it temporarily increases your airspeed. The TE probe outputs a signal that interprets this increase in aircraft energy as lift. There is no acceleration, just an increase in energy. The seat of your pants may well be able to sense the lack of acceleration in this circumstance, but sensitive accelerometers will be even better.

Mike

I know what a horizontal gust is, and what causes it,and I am aware that it results in a false reading of lift on the vario.
You are missing my point entirely. Despite what the vario is indicating, if you cannot feel a vertical acceleration, then it is simply not there, and you can ignore the vario. No need for a complex algoriths or hardware. You already have a sensitive accelerometer refined over millions of years: use it.

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.

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

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.

All very interesting but I would bet that Seb Kawa could beat us all with
just a simple winter vario.

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

On Tuesday, October 28, 2014 5:07:05 PM UTC-7, Mike the Strike wrote:
The issue with horizontal gusts is that if you fly into a thermal outflow (quite common where they are strong and dry), it temporarily increases your airspeed. The TE probe outputs a signal that interprets this increase in aircraft energy as lift. There is no acceleration, just an increase in energy. The seat of your pants may well be able to sense the lack of acceleration in this circumstance, but sensitive accelerometers will be even better.

On Tuesday, October 28, 2014 5:24:58 PM UTC-7, jfitch wrote:
On Tuesday, October 28, 2014 5:07:05 PM UTC-7, Mike the Strike wrote:
The issue with horizontal gusts is that if you fly into a thermal outflow (quite common where they are strong and dry), it temporarily increases your airspeed. The TE probe outputs a signal that interprets this increase in aircraft energy as lift. There is no acceleration, just an increase in energy. The seat of your pants may well be able to sense the lack of acceleration in this circumstance, but sensitive accelerometers will be even better.

Mike

The Butterfly vario calculates wind, purely inertially derived from accelerometers, about 20 times a second. If you believe it (and I do mostly, based on independent checks) you learn that the wind it quite dynamic around thermals. 10 or even 15 knot changes in and around western desert thermals are common. This messes with both your backside and your compensated vario. There appear also to be pressure gradients in and around thermals, which confuses things further for barometric based varios.

John,

You can also set the wind filters to a faster number as low as 1 sec. I have mine set a 10 sec.

Richard
www.craggyaero.com