Good explanantion, Pete... For those interested in these topics I suggest
they start with Alex Strojniks three books on laminar flow aircraft, and
delve into Martin Hollmans series on Modern Aircraft Design... Then they
can google on Kent Phaser, Barnaby Wainfan, and Harry Riblett, for more
information..
denny
"Peter Duniho" wrote in message
...
"Stealth Pilot" wrote in message
...
[...] it seems to take the same amount of energy to move the aircraft
the
distance. all you vary with rpm is the rate of energy conversion and
the air speed.

I don't know how you did your test, or what the specific airspeeds are for
the Tailwind, but generally speaking, Bob's statement was exactly correct.

It does not take the same amount of energy to move the aircraft a given
distance. The amount of energy depends on a variety of things, but the
biggest variable in the equation is airspeed (most other factors are
constant). The least amount of fuel will be used at the airspeed that
corresponds to maximum lift-to-drag ratio, and for most aircraft, this
airspeed is well below the normal cruising airspeed.

This is because in level cruise flight, lift is constant, so at L/Dmax,
drag
as at a minimum, and drag is what the fuel you're burning is working
against. Less drag means less fuel required, even for the same distance.

I doubt that the Tailwind's L/Dmax is at or above 114 knots (though,
there's
no theoretical reason it couldn't be, I guess), which means that there IS
a
speed below 114 knots at which fuel consumption would have been less. Of
course, if 70 knots is below *that* speed, then what you found was the
increase in drag that occurs as you slow down further below L/Dmax.

Another possibility, of course, is that you simply forgot to lean the
engine
properly during your test.

Bottom line: maximum range is found at L/Dmax, and this airspeed is almost
always significantly slower than normal cruise speed. In most aircraft,
you
can significantly increase your range simply by flying slower.

(The above is all only valid in no-wind conditions...add a tailwind or
headwind and it becomes more complicated, since you need to speed up in a
headwind and slow down in a tailwind in order to achieve best range).

Pete