Jeff; I've read the article and am very concerned. I'm a CFI so what I teach
determines pilot's behavior in practice. I want to read more before answering,
but I have four observations: 1. The Mooney turbocharged models do not vary Vne
(IAS) with altitude. 2. sailplane manufacturers lower Vne with decreasing
density out of an abundance of caution:
"
The real answer is that flutter charactistics for most gliders are not
evaluated throughout the full altitude/airspeed range during flight testing.
Aerodynamic forces are proportional to indicated airspeed but this is not
the only factor to consider. Traditionally lowering the IAS VNE at altitude
to give a TAS corresponding to TAS for VNE at sea level is considered safe
practise. This is what your sailplane manufacturer has done.
Recent OSTIV papers on flutter speeds at high altitude have appeared in
"Technical Soaring", well worth looking up for anyone interested in the
flying high and fast ( essential reading for wave flyers ).
A suggestion for using a VNE of the mean value between equivalent airspeed
(almost the same as IAS) and TAS was offered in one of these articles.
TAS (T+E)/2 EAS
.
30k \ |
\ . |
\ |
20k \ . |
\ |
\ . |
10k \ |
\ . |
\ |
\ .|
sea level \ |
------------------------------------------------------------------
VNE (IAS) speed --->
a reduced IAS for VNE at altitude is still used but the reduction is not
as great as the reduction required for keeping TAS below sea level VNE.
The absence of flutter when using these speeds is not guaranteed, infact
the same applies to the "keep TAS below sea level VNE" speeds.
Speed and altitude are not the only factors. Add OAT,wing loading, flap
setting to the list for starters."
3. We have no way of directly measuring TAS except in reference to the ground.
Some ASIs on high-performance aircraft are adjusted for pressure effects, but
this is a small effect. Even the heavy aircraft, operating at 40,000ft at just
above stall speed, use pitot/static measurements to measure speed through the
atmosphere and dynamic pressure.(and crash if they freeze up.)
4. In the same publication you cited, the charts for Vne are straight vertical
lines, not sloped with pressure altitude.
I'll go read my Kershner (my bible) and get back to you. How fun! Peter
