On Jun 20, 12:18*am, Hellman wrote:
On Jun 18, 12:31*pm, bildan wrote:

This calculation must be done in the same domain and in the same
units. *i.e. true airspeed + true rate of climb or indicated airspeed
+ indicated rate of climb in the air-data domain. *TE calculations can
also be done entirely in the inertial domain though no such system
exists to my knowledge. Don't mix units or domains.

I think you've have hit the nail on the head. It's annoying that I
missed that since I'd wondered some time ago about indicated vs. true
rate of climb.

The only remaining question in my mind is if IAS and IROC (Indicated
ROC) change in the right manner. At 18k feet where the atmospheric
density is roughly half that at sea level, TAS = SQRT(2) * IAS and
"true kinetic energy" will be twice "indicated KE." For the two
factors to cancel out, it would seem that the TROC (true ROC) would
have to be twice the IROC, and that sounds right. But it would be nice
if you could say a few words about why that's the case. Here's what I
can see right now:

If, as I seem to remember (and makes intuitive sense), pressure vs.
altitude decays exponential, then the derivative of pressure versus
altitude will be half what it was at sea level. If the vario measures
the rate of change of pressure, then we'd get the factor of 2 we need
and all would be well, but ... the capacity bottle has only half as
many air molecules. If that has any impact on the vario (over and
above the factor of 2 mentioned above), then it would seem like
compensation would be altitude dependent. I suspect that it does not,
but any words on why would be appreciated.

Thanks.

Martin

I'd say it is worthwhile to have an altitude compensated vario. If
you start with that, then you have to use true airspeed to do the TE
calculation.

More interesting to me is the relatively new Inertial Reference Units
(IRU's) that have been developed for UAV's. These are small, light,
use little power, are accurate and are available at a reasonable
cost. In addition to position data, Euler angles, and 3D
acceleration, they provide very accurate 3D velocity data. The
horizontal and vertical velocity can be used for a nearly perfect TE
vario. It would be immune to gusts and would have instantaneous
response. The vario reading should be very smooth since it's
measuring the actual velocity of the glider without any probes or air
data at all. In addition to perfect vario data, together with true
airspeed, could provide highly accurate real time vector winds.