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I learned that glider's vario reading lags behind the vertical speed of the airmass. For example, when flying a steady circle and having a thermal not quite centered yet, the vario is reading a maximum well after the core has been left. See BGA Webinar "Thermal Centring".vario lag

The implications for thermal centering are explained e.g. in the BGA Instructor Manual Ch. 24. "The average mechanical variometer is about three seconds too late". lagging vario reading in a thermal.

What is the nature of this lag? Wouldn't a gentle blow into the static port make the needle move immediately (certainly without a delay of a few seconds between strongest blow and strongest needle deflection)?

  • Has it to do with the the traditional vario's capacity flask and capillary hole or does it apply to newer electric variometers operating on the transducer principle as well? (A tiny vacuum cavity on a circuit board is sealed with a flexible membrane with embedded strin gauges.) See FAA-H-8083-13A, Glider Flying Handbook, Fig 4.21A variometer diaphragm anatomy
  • Or is it the glider's mass (inertia)? A glider can't change its vertical speed instantly.
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  • $\begingroup$ I'd always thought of vario lag as being around 1-2 sec. The whole point about them vs conventional VSIs is they are nearly instantaneous. 3-5 secs is more like a conventional VSI. $\endgroup$
    – John K
    Aug 17 at 16:16
  • $\begingroup$ @JohnK -- I've always understood-- must have read it somewhere-- that conventional VSI's were intentionally constructed to average the vertical speed over a period of about 7 seconds. $\endgroup$ Aug 17 at 22:51

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The vario is essentially a high-sensitivity vertical speed indicator, operating on the same principle: the air flow rate in or out through the orifice is measured as a pressure difference between the inside of the instrument and the outside of the instrument.

Hence, the sensing apparatus responds as a resistance (the orifice) in series with a capacitor (the capacity flask) and this mathematically yields a system response time constant which is the root cause of the lag.

The time response of a vertical speed indicator can be improved by adding an "accelerator pump" in the form of a small spring-loaded piston that can rise or fall in its cylinder in the vertical axis. The cylinder is connected to the inside of the instrument case so if the plane suddenly rises, the piston on its spring slides down and pumps a little air into the case and thus forces the gauge needle to get with the program quicker. The opposite happens when the plane suddenly descends: the piston on the spring rises and pulls a little air out of the case and urges the needle to more quickly indicate a descent.

The sprung piston only responds to sudden movements; for slow movements it stays out of the picture.

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  • $\begingroup$ OK, so re content in question "Has it to do with the the traditional vario's capacity flask and capillary hole or does it apply to newer electric variometers operating on the transducer principle as well?", you are saying "yes" to the former and "no" to the latter? (That would tend to be my opinion, although my flaskless Brauniger IQ Comp hang gliding variometer still has at least .5 seconds of lag I believe-- ) $\endgroup$ Aug 17 at 22:50
  • $\begingroup$ And how much role do you think this plays into it: "Or is it the glider's mass (inertia)? A glider can't change its vertical speed instantly."? Which is honestly a point that I had never given much thought to before-- $\endgroup$ Aug 17 at 22:54
  • $\begingroup$ Re " although my flaskless Brauniger IQ Comp hang gliding variometer still has at least .5 seconds of lag I believe-" -- this observation is based on holding the vario in my hand and abruptly moving it upwards as far as my arm can reach, so the inertia of the aircraft in coming into equilibrium with the updraft plays no role in this observation! $\endgroup$ Aug 17 at 23:04
  • $\begingroup$ One more observation-- nowadays, of course, the most advanced variometers for sailplanes (and now for hang gliders/ paragliders too) use peizoelectric accelerometers, not mechanical "accelerator pumps". Which naturally has led us down the path to full AHRS systems incorporated into modern sailplane variometers, so that one or more of the display screens includes an artificial horizon display-- $\endgroup$ Aug 17 at 23:08
  • $\begingroup$ I wonder if there is a feasible way to find out or to measure the lag. Motivation: misjudging the lag by two seconds leads to performing straightening up & turning again about thirty degrees away from what would be the best point of the circle for doing this. $\endgroup$
    – Xpector
    Aug 18 at 8:56

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