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On 09/09/2014 05:30 PM, Roland wrote:
> I found out, that it makes quite a difference for measuring
> performance and fuel consumption, if I'm solo on board or at MTOM.
I always fly (at least) at MTOW. However what I found out is that the
distribution of the weight has quite an impact on performance: the
more aft, the less drag, and the better the performance.
Hence when Ilona and I are going on vacation with a lot of lugage with
us, the airplane is actually faster than when we are flying with just
the seats occupied.
The reason is simple: with the CoG more aft the tailplane has less
"inverted lift" to produce, and this also reduces the load on the main
wings. So this reduces two times induced drag.
Indeed, while cruising in stable weather we often shift a few heavy
things more aft in the bagage compartment, which is immediately
rewarded by the need to trim a bit more forward, followed by a higher
cruise speed.
Another thing you need to be aware off is that it makes a difference
whether you climbed up to that speed, or came down to that speed. So
we often overshoot our intended altitude a bit, and after reaching a
stabilized speed, we push her down to the desired altitude, and a
higher speed is kept if you be cautious not to let that speed to be
"climbed away" again.
I know, we consider it some kind of sport to get the most out of our
flying machine (or out of our precious fuel). It is worth a grin if
you get a consistent 120 knots for 16 l/h.
> Plus it's often not mentioned at which altitude the speed is
> indicated. At least this (from my point of view) is important to
> have comparable figures.
IAS is IAS regardless of altitude as far as I can recall. Of course
altitude has influence on ground speed, but that should never be used
for performance comparisons.
Frans
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