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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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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 |
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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 |
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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 |
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