Vne in gliders - TAS or IAS?

From: Scott Schiffres <scottns@xxxxxxxxx>
To: Harris Hill <hhsc1@xxxxxxxxxxxxx>
Date: Thu, 16 May 2019 00:01:05 -0700

Hi Folks,

I was flying out near Phoenix today and got quite high (11,900').  My
instructor here said that we have to be careful because the Vne is actually
lower at higher altitude.  If the 2-33's Vne is 98MPH at SL, then it would
be 81MPH at 12k'. From the manual for the 2-33, it is not clear whether it
is IAS or TAS.  For the C-172, the POH I checked lists Vne is IAS.  One
instructor was saying that Vne of the glider reduces for the same reason
that jets have decreasing margins between stall speed and Vne at altitude.
But I don't think that is relevant to gliders because that Vne has to do
with keeping the jet subsonic, which has to do with the speed of sound
going with the squareroot of temperature, which reduces with altitude.

If the g vs v diagram tells the full story of the failure modes, then I
think the aerodynamic loads are all going to be a function of the dynamic
pressure (0.5 Rho*v^2), so it would seem the Vne should be IAS.  However,
if aerodynamic flutter is the failure mode, I am less sure.  I think it
would depend the lowest speed that triggers a flutter mode and the damping
of that mode vs altitude.

Any thoughts on the Vne for 2-33 vs altitude or in Vne vs altitude in
general?

Thanks,
Scott

I found these two articles relevant:
http://journals.sfu.ca/ts/index.php/ts/article/viewFile/607/570
http://www.australianflying.com.au/news/vne-and-flutter-explained

Vne in gliders - TAS or IAS?

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