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  #31  
Old September 5th 18, 11:21 PM posted to rec.aviation.soaring
Eric Greenwell[_4_]
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Andy Blackburn wrote on 9/5/2018 9:44 AM:
On Wednesday, September 5, 2018 at 9:20:53 AM UTC-5, krasw wrote:
keskiviikko 5. syyskuuta 2018 8.22.14 UTC+3 2G kirjoitti:

What I know is I don't have a Kalman filter going in my head: but I do have a butt which feels vertical acceleration. If it doesn't tell me I am going up, I discount the screaming vario.

Tom


The wing transforms horizontal gust into vertical, and your butt gets it wrong.


That's true if you only use your butt and not you're inner ear to sense the pitch rotation. A horizontal gust on the nose excites the phugoid (dCm/dV) and pitches the nose up. Vertical air movement excites the short period (dCm/dalpha) and pitches the nose down. A thermal you can climb in is likely to produce a more prolonged surge than a vertical gust. The exact magnitude of these effects depend on the specific aircraft aerodynamics and things like cg location.

Tom, you may not have a Kalman filter in your head, but you are a neural network - kind of by definition since your brain is made of connected neurons.. Pattern recognition is how we all interpret the "feel" of thermals. It helps a little if you can decompose some of the bigger dynamic effects, but there's a lot going on with lift, gusts and aircraft dynamics - as UH points out. I think a smart vario ought to be able to sort out some of these dynamic interactions better than simple total energy compensation. I figure with cheap gyros and accelerometers they would be doing a lot of this already, but I don't know how far it's gotten.

Again, apologies if I didn't completely accurately describe the engineering of aircraft dynamics. I think this is roughly correct.


I'm thinking a horizontal gust on the nose is similar to a higher airspeed, and
with the glider elevator set for the lower airspeed, a pitch-up would occur.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf
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  #32  
Old September 6th 18, 12:35 AM posted to rec.aviation.soaring
Martin Gregorie[_6_]
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On Wed, 05 Sep 2018 15:21:11 -0700, Eric Greenwell wrote:

Andy Blackburn wrote on 9/5/2018 9:44 AM:
On Wednesday, September 5, 2018 at 9:20:53 AM UTC-5, krasw wrote:
keskiviikko 5. syyskuuta 2018 8.22.14 UTC+3 2G kirjoitti:

What I know is I don't have a Kalman filter going in my head: but I
do have a butt which feels vertical acceleration. If it doesn't tell
me I am going up, I discount the screaming vario.

Tom

The wing transforms horizontal gust into vertical, and your butt gets
it wrong.


That's true if you only use your butt and not you're inner ear to sense
the pitch rotation. A horizontal gust on the nose excites the phugoid
(dCm/dV) and pitches the nose up. Vertical air movement excites the
short period (dCm/dalpha) and pitches the nose down. A thermal you can
climb in is likely to produce a more prolonged surge than a vertical
gust. The exact magnitude of these effects depend on the specific
aircraft aerodynamics and things like cg location.

Tom, you may not have a Kalman filter in your head, but you are a
neural network - kind of by definition since your brain is made of
connected neurons.. Pattern recognition is how we all interpret the
"feel" of thermals. It helps a little if you can decompose some of the
bigger dynamic effects, but there's a lot going on with lift, gusts and
aircraft dynamics - as UH points out. I think a smart vario ought to be
able to sort out some of these dynamic interactions better than simple
total energy compensation. I figure with cheap gyros and accelerometers
they would be doing a lot of this already, but I don't know how far
it's gotten.

Again, apologies if I didn't completely accurately describe the
engineering of aircraft dynamics. I think this is roughly correct.


I'm thinking a horizontal gust on the nose is similar to a higher
airspeed, and with the glider elevator set for the lower airspeed, a
pitch-up would occur.


Isn't this countered as a glider enters a thermal because it is flying
into air with increasing vertical velocity? This will tend to lower the
effective AOA, causing the glider to accelerate forward as it tries to
return to its trimmed AOA. Hence its pilot 'feeling the surge' forward
and up.

In the past I've seen free flight competition models do this too, some
(the APS Aiglet A/1 design) would sometimes pitch down very obviously
when entering a thermal, while many/most designs can look as if they're
being sucked into a strong thermal, though with not so much visible pitch
change as the old Aiglet used to show.


--
Martin | martin at
Gregorie | gregorie dot org
  #33  
Old September 6th 18, 04:08 AM posted to rec.aviation.soaring
Eric Greenwell[_4_]
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Martin Gregorie wrote on 9/5/2018 4:35 PM:
On Wed, 05 Sep 2018 15:21:11 -0700, Eric Greenwell wrote:



That's true if you only use your butt and not you're inner ear to sense the
pitch rotation. A horizontal gust on the nose excites the phugoid (dCm/dV)
and pitches the nose up. Vertical air movement excites the short period
(dCm/dalpha) and pitches the nose down. A thermal you can climb in is
likely to produce a more prolonged surge than a vertical gust. The exact
magnitude of these effects depend on the specific aircraft aerodynamics and
things like cg location.

....
I'm thinking a horizontal gust on the nose is similar to a higher airspeed,
and with the glider elevator set for the lower airspeed, a pitch-up would
occur.


Isn't this countered as a glider enters a thermal because it is flying into air
with increasing vertical velocity? This will tend to lower the effective AOA,
causing the glider to accelerate forward as it tries to return to its trimmed
AOA. Hence its pilot 'feeling the surge' forward and up.

In the past I've seen free flight competition models do this too, some (the APS
Aiglet A/1 design) would sometimes pitch down very obviously when entering a
thermal, while many/most designs can look as if they're being sucked into a
strong thermal, though with not so much visible pitch change as the old Aiglet
used to show.


My context, and I think Andy's, was encountering just a horizontal gust. I do
think encountering a vertical gust would cause a momentary nose-down attitude change.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf
  #34  
Old September 6th 18, 05:56 AM posted to rec.aviation.soaring
Andy Blackburn[_3_]
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On Wednesday, September 5, 2018 at 10:09:03 PM UTC-5, Eric Greenwell wrote:
Martin Gregorie wrote on 9/5/2018 4:35 PM:
On Wed, 05 Sep 2018 15:21:11 -0700, Eric Greenwell wrote:



That's true if you only use your butt and not you're inner ear to sense the
pitch rotation. A horizontal gust on the nose excites the phugoid (dCm/dV)
and pitches the nose up. Vertical air movement excites the short period
(dCm/dalpha) and pitches the nose down. A thermal you can climb in is
likely to produce a more prolonged surge than a vertical gust. The exact
magnitude of these effects depend on the specific aircraft aerodynamics and
things like cg location.

...
I'm thinking a horizontal gust on the nose is similar to a higher airspeed,
and with the glider elevator set for the lower airspeed, a pitch-up would
occur.


Isn't this countered as a glider enters a thermal because it is flying into air
with increasing vertical velocity? This will tend to lower the effective AOA,
causing the glider to accelerate forward as it tries to return to its trimmed
AOA. Hence its pilot 'feeling the surge' forward and up.

In the past I've seen free flight competition models do this too, some (the APS
Aiglet A/1 design) would sometimes pitch down very obviously when entering a
thermal, while many/most designs can look as if they're being sucked into a
strong thermal, though with not so much visible pitch change as the old Aiglet
used to show.


My context, and I think Andy's, was encountering just a horizontal gust. I do
think encountering a vertical gust would cause a momentary nose-down attitude change.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf


Yes. That's the difference between a horizontal and vertical gust (reminder: a sustained vertical gust is a thermal). A horizontal gust activates the dCm/dV (phugoid) mode that is nose-up because the center of lift is in front of the center of gravity. A vertical gust activates the dCm/dalpha (short period) mode which is nose down for most airfoils (in addition to vertical acceleration from the air movement itself).

Also, these two modes have different time constants by roughly a factor of 10 (they are also coupled, but over the first few seconds this doesn't come into play). So the vertical acceleration and pitch response together ought to be different for a horizontal gust versus a thermal.

Andy Blackburn
9B
  #35  
Old September 6th 18, 06:11 AM posted to rec.aviation.soaring
Mike Borgelt[_2_]
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On Wednesday, 5 September 2018 15:22:14 UTC+10, 2G wrote:
On Tuesday, September 4, 2018 at 2:40:05 PM UTC-7, Steve Koerner wrote:
On Tuesday, September 4, 2018 at 12:49:36 PM UTC-7, Andy Blackburn wrote:
On Tuesday, September 4, 2018 at 10:59:26 AM UTC-7, Steve Koerner wrote:
On Tuesday, September 4, 2018 at 9:07:09 AM UTC-7, George Haeh wrote:
Lift from horizontal gusts can be filtered out with rates of dynamic pressure and attitude, perhaps with application of polar.

Lots of fun with math.

More holistically, you run a Kalman filter using every sensor you have and then some. You treat the system as having three disturbances: two horizontal wind components plus vertical flow. You continuously calculate a solution yielding least square error for the six DOF system with those disturbances being of primary interest.

Making a trip in the wayback machine to my control theory and aerodynamics classes - apologies if I mess up the details...

If you have a dynamic model for the glider (a Kalman filter would typically require this) you may also be able to use the difference between activating the short period and phugoid dynamic pitch modes with respect to pitch rate. This difference is why a strong thermal has that seat of the pants surge that pitches the nose down instead of up - as you'd get with a gust under normal circumstances.

For all of this you'd get a better result if you also knew the control positions and Cm vs control position - primarily for the elevator. Also, we don't really have dynamic models for gliders though my guess is it would not be that difficult to measure with a reasonably instrumented glider and a couple of test flights. You might be able to get decent results with a generic model for a reasonably current generation racing glider (for instance), though model-specific parameters would of course be better. You're really just trying to filter out the gusts so you may not need anything all that precise to get an improvement, just Cm vs alpha and Cm vs V. Knowing the c.g. and weight will matter as well, but you might be able to calculate these effects.

I wonder if it's easier or harder to use machine learning to do this than a more deterministic least squares model...or if all of the above is overkill.

Andy Blackburn
9B


I agree Andy; to do a Kalman filter you'd want gyros, accelerometers and control position sensors. Position sensors are not hard though and their mapping might be learned for each installation with a smooth air test flight. The problem with Neural network AI is that you have to begin the process with a comprehensive training set. Probably Mike Borgelt has a simpler and better way to get to just the goods that we care about without a plane load of sensors. I'll be looking forward to hearing more about this.


What I know is I don't have a Kalman filter going in my head: but I do have a butt which feels vertical acceleration. If it doesn't tell me I am going up, I discount the screaming vario.

Tom


Actually what you have going on in your head is a pretty good stab at a Kalman filter. You are weighting the vario reading vs your backside and other cues to arrive at what you think is happening. It is workload intensive and all too often fails.

Those talking about about the effects of gusts should read my horizontal gust article on the website.
Very small horizontal velocity gradients cause large signals on a normal TE variometer. I give some examples there. The effect depends on the square of the True Air Speed so in South Africa, Australia and the western US where you may be at high altitude and cruising at 100KIAS + your TAS can be in the 120 to 140 KTAS range.
It is just as well gliders don't cruise at 200KTAS because the normal TE vario would be uselessly and apparently randomly moving between the stops.

When a glider enters a thermal the air coming from below changes the direction of the relative wind which increases the angle of attack which increases the lift and the glider starts going up. On entering strong thermals pitch stability of the glider will tend to maintain the trimmed AoA, hence the glider will tend to pitch nose down. The effect is short lived as the time constant of the response to vertical air changes is short. It depends on airspeed, wing loading and the slope of the lift curve of the wing. With modern gliders it is around 0.4 to 0.5 seconds at low speeds and around 0.2 to 0.25 seconds at high speeds.
I had to derive this and I later found the derivation in a book called "Airplane Response to Atmospheric Turbulence" by John C. Houbolt. Yep, that guy - the one who pushed the Lunar Orbit Rendevous for Apollo.
Now the tendency of a airplane to pitch nose down on entering rising air (which can momentarily stall the airplane if the lift is strong enough) can be a really GREAT way to kill yourself because as nearly everyone has been taught to fly attitude your first reaction is to pull the stick back to maintain the attitude. If the wing was stalled or nearly so you are now stalling or pulling deeper in to the stall. Do this while turning final with what looks like adequate airspeed and you could find yourself on the ground short of the runway wondering what just happened if you live through it. The same of course applies to thermalling at low altitude. Remember the stick controls angle of attack and in very short term vertical velocity changes in the atmosphere also change AoA. Anything else it apparently does is a consequence of the angle of attack change.

Mike Borgelt

Borgelt Instruments
  #36  
Old September 7th 18, 12:23 AM posted to rec.aviation.soaring
Andy Blackburn[_3_]
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Mike,

I understand that the short period response to a step function in vertical air motion would damp out after less than a second, but normally thermal entry would have a positive gradient in vertical air motion as the glider traverses from zero vertical motion to maximum vertical motion towards the center of the thermal. This ought to create a more prolonged nose-down pitch attitude in addition to upward acceleration, both of which ought to be detectable to accelerometers and gyros in a modern vario. This would be quite distinct from an ever-so-slow negative acceleration and nose up pitch rate from a horizontal gust on the nose. I expect having a sense for the distribution of strength and duration for horizontal gusts would also help a bit.

That’s what I tend to sense in moderate to strong thermals versus gusts. Is that that a correct interpretation?

Andy
  #37  
Old September 7th 18, 02:21 AM posted to rec.aviation.soaring
Mike Borgelt[_2_]
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On Friday, 7 September 2018 09:23:31 UTC+10, Andy Blackburn wrote:
Mike,

I understand that the short period response to a step function in vertical air motion would damp out after less than a second, but normally thermal entry would have a positive gradient in vertical air motion as the glider traverses from zero vertical motion to maximum vertical motion towards the center of the thermal. This ought to create a more prolonged nose-down pitch attitude in addition to upward acceleration, both of which ought to be detectable to accelerometers and gyros in a modern vario. This would be quite distinct from an ever-so-slow negative acceleration and nose up pitch rate from a horizontal gust on the nose. I expect having a sense for the distribution of strength and duration for horizontal gusts would also help a bit.

That’s what I tend to sense in moderate to strong thermals versus gusts. Is that that a correct interpretation?

Andy


Andy, that's basically right. The horizontal gust is usually so small though that I've never detected any nose up effect from the horizontal gusts. i.e. a one knot airspeed increase over one second at 100 KTAS will show 5 knots up on the TE vario. My experience has been that a lot of the time the vertical air motion is fairly sharp edged but some times you fly straight in to a strong core over a few seconds and it feels like the tail is being lifted. What is happening is the glider trying to line up with the new relative wind direction and during this time you have increased AoA happening. Trying to hold a constant nose attitude will increase the AoA further.

To digress from the vario topic slightly:
Which is why I said that flying attitude can kill you. I think this is what happened to an ASG-29 pilot at Waikerie about 2 and a half years ago, turning final with what seemed to be plenty of airspeed, into a strong thermal encounter and the next thing he was in the grapevines. Fortunately the wires and vines cushioned the stop enough and the spin wasn't fully established, that he survived. Encounter a fairly sharp 10 knot core at 60 KIAS and the AoA increases by 10 degrees. The AoA may be around 7 degrees before the encounter. What AoA do thin subsonic wings stall at?
This, I believe is one answer to the unexplained spin ins that occur from time to time.
Attitude is fine in equilibrium or very close to equilibrium situations. Doesn't work very well in other situations. How many have died in the winch launch failure scenario where you bury the nose well below the horizon, then roll and pull and the thing flicks in to a spin? Unfortunately all too many merely in practice.
A guy who was Chief Test Pilot for the Royal Australian Air Force during the 1950s and 1960s once told me if the aircraft is wanting to go a certain way and you are trying to force it to go another, just go with the aircraft as it is departing from controlled flight. Good advice IMO.
  #38  
Old September 7th 18, 09:10 AM posted to rec.aviation.soaring
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....and to paraphrase what it says in Stick and Rudder - if *anything* surprising happens in a turn then immediately unload the wing.
  #39  
Old September 7th 18, 02:08 PM posted to rec.aviation.soaring
Nick Kennedy[_3_]
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This has turned into a very interesting thread. It looks like we may be using some newer much better varios in the near future.
Another interesting aspect is Mike Borgelt describing the AOA change and the possible cause of low level, low speed [ in the pattern] accidents.
When were up high cruising at 80-100 knots we fly into 10 knots up all the time and pull hard to slow down and get ready to circle. Up high at speed this is no problem. Down low and slow at say 60 knots that 10 knot thermal you just hit may cause a major problem. I think we may be flying way to slow in the pattern. Looking at the stall/spin rates, in all parts of the arrival pattern, I might be right.
I like using the Knauff idea of having a longer higher final leg, and as my max flap extension speed in my LS3a is 86 knots, I try to keep it over 70 knots the whole way to the deck, works for me. This may sound fast and it is. I don't want to stall close to the ground by some rouge air or a booming thermal.
This fast in the pattern stuff was taught to me by Bob Faris CFIG
So far so good!

  #40  
Old September 7th 18, 02:33 PM posted to rec.aviation.soaring
Tango Eight
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On Friday, September 7, 2018 at 9:08:28 AM UTC-4, Nick Kennedy wrote:
This has turned into a very interesting thread. It looks like we may be using some newer much better varios in the near future.



There are freaking astoundingly good varios available right now. Mike may have made a break through, good for all of us if true. I have an arm's length association with ClearNav and I can tell you that a) we're not out of good ideas and b) nothing I'd like better to have than a fire lit under the development team to get some of these ideas brought to fruition.

Caveat: installation is everything. Bad pneumatic sources, leaks, shared sources with mechanical instruments can/will degrade performance.

best,
Evan Ludeman
 




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