On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin Gregorie wrote:
On Mon, 15 Feb 2016 14:48:52 -0800, Bruce Hoult wrote:

On Monday, February 15, 2016 at 7:13:43 PM UTC+3, Tango Whisky wrote:
Le lundi 15 février 2016 16:32:27 UTC+1, Bruce Hoult a écrit*:

I don't agree.

As I've already said once in this thread, you're going to get a
certain amount of drag from the fuselage anyway. If the presentation
to the airflow for minimum drag generates zero lift then, by the
calculus definition of "minimum" of a continuous function, the first
little bit of lift will not add any drag. The optimum thing to do is
to use it. It might be *very* little, and a very small AoA, but it's
nonzero.

If the presentation to the airflow for minimum drag generates
non-zero lift ... then of course you'll take it!

Yes, the fuse has a low L/D. But that's better than the 0.0 L/D if
you don't take what lift you can from it...

Why do you assume that the fuselage has zero lift in a perfect airflow?

I don't. I present the argument for both cases: zero and non-zero lift
at minimum drag.


Why do you assume that the increase of drag is zero for small slip
angles?

Follows directly from the definition of "minimum" for a continuous
function.

The minimum is, by definition, at the point at which the function (the
drag) has zero change for small changes in the input (the AoA or slip
angle).

But, as soon as the fuselage generates any side-force, its drag will
increase,

Incorrect. Please study your Calculus 101 books a little more closely.

and by more than the energy needed to generate the side-force.
This is due to two things:

1) The energy conservation law would be violated if the energy taken from
the moving aircraft as drag is less than that needed to generate the side
force.

Incorrect. Force is not energy.

If you were correct then wings would not work at all.

For maximum efficiency, every part should (as much as possible) be operated at its angle of attack for best L/D, not at minimum drag.

An L/D of 5 on some part is better than an L/D of 0 on that part. Even if some other part has an L/D of 50. (provided you don't make that other part perform worse, of course)