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#11
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VNE vs altitude: glider specs vs rules of thumb
Flying a glider at or near Vne is pretty risky given that we fly most
often in turbulent air.Â* Besides, your rate of descent increases more rapidly the faster you go.Â* Better to do better final glide planning. My 2 cents... On 6/10/2020 6:51 PM, Mike N. wrote: So the S80 does have an alert for VNE.I assume at calculated TAS. So I'm looking to put in the VNE alert with a safety factor of 85 to 90% of POH specified VNE -- Dan, 5J |
#12
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VNE vs altitude: glider specs vs rules of thumb
Agreed completely.
My primary reason for adding the VNE alert is when flying through areas of heavy sink. Flying faster in sink is of course SOP in most cases. So obviously as I fly faster through sink I will be seriously watching my airspeed, but having a secondary alert to my instrument scan while flying fast through sink, or even final glide, is an additional safety measure to help mitigate human errors. |
#13
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VNE vs altitude: glider specs vs rules of thumb
If you're interested in knowing more about the science behind the protocol, a very good paper is "Critical Flutter Speed of Sailplanes Calculated
for High Altitude: Examples of computation" by Wojciech Celcstyn, 1993, a featured article in Technical Soaring: https://journals.sfu.ca/ts/index.php...e/view/216/200. Fig 1 top half shows shaded flutter region for one oscillation mode and the dashed line how one would enter it flying fast enough IAS at 2000m (EAS and IAS are the same for glider speeds). If you flew at higher altitudes you would enter it at a lower speed, given by the parabolic looking curve going upward to the left, which gives the true altitude change of IAS-VNE flutter. The real change is NOT given by constant TAS, but the TAS line is shown for comparison. It's simply a convenient line that has enough slope so it will keep you out of danger in most cases. The other figures show other modes. If you read the conclusion, it's clear that the standard (German and everyone else I guess) VNE protocol is not based on knowing where flutter is, either by testing or calculation. It's based on testing to a certain altitude and finding no flutter. Then IAS-VNE is chosen to drop off as TAS is held constant to stay away from unknown flutter at higher altitudes. |
#14
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VNE vs altitude: glider specs vs rules of thumb
I don't fly above VNE and don't plan to. But I hear that the most aggressive racing pilots do, often as their average speed! This makes some sense now because VNE is simply based on the testing altitude chosen, not knowledge of flutter (see my post just above this).
So you aggressive racing pilots, you are basically test pilots. Do you share the velocity and height space you've explored without flutter? Or do you just see how fast others with your same model fly? We could define new faster IAS-VNE plots for your model based on your experience. |
#15
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VNE vs altitude: glider specs vs rules of thumb
On Tuesday, June 16, 2020 at 3:23:59 PM UTC-7, Bret Hess wrote:
I don't fly above VNE and don't plan to. But I hear that the most aggressive racing pilots do, often as their average speed! This makes some sense now because VNE is simply based on the testing altitude chosen, not knowledge of flutter (see my post just above this). So you aggressive racing pilots, you are basically test pilots. Do you share the velocity and height space you've explored without flutter? Or do you just see how fast others with your same model fly? We could define new faster IAS-VNE plots for your model based on your experience. Not so. Racing pilots have no competitive motive to fly above Vne. Decades ago there was a potential motive to do so when optical start gates were in use. That all ended when GPS came on the scene. |
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