Very interesting, Hans. Wing aerodynamics could play a role also, as
in a laminar flow wing there is a "bucket" of low drag at low lift
coefficients, ideally at design cruise speed. The laminar bucket on
Mike Arnold's record-holding AR-5 is so pronounced that the pilot can
feel her going in, or dropping out, such as in a steep bank
(reportedly like "hitting a wall of feathers"). However, it
accelerates into the bucket on its own, without need to dive into it.
But on Rutan's Voyager as described in Rutan & Yaeger's book, there's
frequent discussion of the "bucket," and the struggle it was early in
the flight, due to weight of fuel, to stay in that region. I can't
recall from the book now, but I suspect they tried diving into a lot.
Their fuel computations demanded they stay in the bucket. They did
not have fuel quantity gauges -- tank design would have rendered them
useless -- and fuel remaining was a serious worry throughout the
round-the-world flight.
Regards,
Fred F.
Hans Jrgen Danielsen wrote:
>
> To Fred and all you other speed-freeks...
>
> On all the aircraft I have flown, there is a net gain in speed when you fly
> it "on the step". The results are more pronounced with "cleaner" aircraft.
> In the old days when we flew DC-6, C-46 and so forth - although not
> standard operating procedure - we used to climb slightly higher (100-200ft.)
> than cleared FL, maintaining climb power throughout until cruise speeds
> where achieved, then slowly descending to correct altitude while reducing to
> cruise power. A net gain of 5-6 kts where achieved. (NB: This was before
> Mode C times - not at all tolerable today!!)
>
> On the B 737 -200 we used to maintain climb thrust to a slightly higher
> speed than chart values - after which cruise EPR was set. Speed gain: Mach
> 0.005 to M 0.01 with fuel flow about the same as best economy (M.0.72). The
> interesting thing is that fuel used was thus lower throughout while cruising
> a wee bit faster than best economy. Sheer magic!
>
> On the -400 and -500, - aircraft with approximately similar drag curves,
> the Flight Management Computer found out just the same: Best economy speed
> turned out to be M 0.73/M 0.74 - instead of the more "draggy" 0.72. (Of
> course the picture is quite complicated with all the various inputs to the
> computer, of which Cost Index plays a major part).
>
> The lessons learned from this is that of Body Angle while in the cruise. The
> faster you go - the less is your angle of attack. Which again spells less
> down pull on the stabilizer (less drag from stabilizer "lift") and less
> wetted area from aircraft structure. As others have pointed out, a more
> rearward placement of C of G also plays a major part in obtaining the
> flattest - or optimum - angle. For the pure reason of easing off the load on
> the Stabilizer. All this is of value to the Europa with its all flying tail.
> Its really a little Boeing!!
>
> With Christmas greetings to you all!
> Hans.
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