"Jim" wrote in message
...
Well, the relationship of flutter to IAS and TAS is certainly a
puzzle to me.

Somewhere I got the understanding that IAS, in a sense, indicates the
impact rate (pressure) of molecules on the aircraft, and thus in
thinner air an aircraft will "actually" (TAS) be flying faster to
receive the same air molecule impact rate (pressure).

True AFAIK, and the effectiveness of controls responds to this pressure.
Control flutter limitations are a function of IAS. Some sailplanes have
been designed with and even retrofitted with dampers. Bear in mind that
age, wear, repair, compromised mass balances, and paint can impact this.
(Not mutually exclusive changes)

TAS, on the other hand, indicates, in a sense, indicates the speed
at which the air molecules are moving past the aircraft - something
quite independent of just how MANY air molecules are passing by
the aircraft in a given amount of time.

Yes, and the center of pressure that generates lift shifts as a result and
may twist (maybe better un-twist) the wing. IIRC, the FL500 Grob had an
extra lamination or two of glass in the wings so it could fly faster than
stall speed at extreme altitude. In my DG-100 at speeds 120kts under
3000m, the amount of downward deflection at the tips was really impressive
and a bit unnerving. I don't recall similar deflection at 8500m at similar
TAS, but, like most everyone else, I have little empirical evidence.

Further, I have had the impression that flutter is a consequence of
the speed of the aircraft through the air (molecules) (TAS) rather
than the number of air molecules that happen to be impacting the
aircraft in a given amount of time (IAS).

Flutter in an elastic mode and is dependent on wing design. As I understand
it, a Lear Jet's Vne is based on IAS with Mach limits. The wings are quite
short and stiff compared to a sailplane, and have greater torsional
resistence by design. The twist in glider wings is there to provide more
benign handling, however, as in the OSTIV paper I've referenced previously,
sailplane design is a compromise of performance and engineering. The
elastic mode may be the limiting factor and engineering a sailplane to
perform at altitude as a Lear Jet would increase both weight and cost
unacceptably (unless your name is Fossett maybe). Since the sailplane
spends 99% of it's service life 6000m and a Lear Jet spends 90% of its
service life 8000m, each is designed appropriately.


So, I have always considered it prudent to view VNE due to FLUTTER
to be a TAS airspeed, not an IAS airspeed.

Perfectly safe as a conservative view.

Have I been wrong about this?

The conjecture in the OSTIV paper was that (IAS+TAS)/2 was safe and that
this envelope might extend to 0.8 * TAS. However, there are a couple of
Nimbus 4 incidents that might suggest adoption of the prudent view. There
are also some 15m designs with little twist and stiff wings that might be
real rocket rides. I have a little time in a Jantar Std 2 and found it
nimble, a bit stiff, and honest in performance. I also found that things
like the aileron hinges wear a bit more quickly than some other gliders, so
pressure effects might be of interest. Too many factors and not enough
evidence to say who's right or wrong or taking unacceptable risks. The
third mode of flutter is pilot induced. Control inputs at high altitude and
speed could potentially induce either of the other two modes, I suppose.

Anyway, that's my take,

Frank Whiteley
Colorado