Once again, I have to agree with what you are saying. However, there
must be a way to achieve this without inducing excessive drag. If you
watch enough science-fiction, you might come to the conclusion that
mankind is doing it all wrong. We should be using "magnetics".
There must be a way of reducing the drag created by the prop itself.
The prop itself is the generator of the noise. If you feather a prop,
it doesn't make much noise but once the prop is pitched to move the
air, the associated friction is somewhat responsible for the noise
being generated. This is of course, when the tip speed is below
mach. As discussed in an earlier post, the owl is one of the most
stealthy birds because of it's tattered feathers. I can just imagine
that either a hairy prop or dimpled prop would reduce the noise
factor. Efficient thrust aside, the desire is to make an absolute
stealthy hovercraft.
On Jun 6, 1:32=A0pm, Ken Roberts <for...@[EMAIL PROTECTED]
> wrote:
> On Jun 6, 2:20 pm, Buy_Sell <werksp...@[EMAIL PROTECTED]
> wrote:
>
> > My thinking on this matter, is that of 'equal and opposite reactions'.
>
> > If I were to direct airflow 90 degrees to where I wanted to go, I
> > wouldn't get there. =A0...but if I directed airflow exactly opposite
of
> > where I wanted to go, the reaction would be, to push me in the
> > direction that I wanted to go. =A0The analogy is that, if turbulent
air
> > is exiting the aft of the hovercraft, it is not all going in the exact
> > opposite direction of where I want to go. =A0I figure that the more
> > streamlined the thrust, the more quiet and efficiently I can move
> > forward. =A0i.e. randomly directed thrust will probably not give me
> > efficient momentum. =A0 Noise =3D Inefficiency. =A0 Any thoughts?
>
> I agree so far as you said. =A0The thing is, there is a cost for any of
> the things you do with an air stream.
>
> Take the simple, common case of what you're talking about: =A0Stators.
> Stators in a well-designed system only improve thrust in one case:
> One small RPM band, one relatively narrow set of conditions. =A0In all
> other cases they cost energy even though they make more air flow in
> the desired direction. =A0In a system where the stators are not well-
> designed, the loss is always greater than the benefit.
>
> Stators have surface area and a shape. =A0That surface area and shape
> cause drag. =A0That drag lowers the total energy exiting the duct.
=A0The
> benefit of the stator can, in some cases, be greater than the drag
> loss induced by the stator.
>
> Your laminar flow idea is just like stators, only more so. =A0You have
> more surface, you are trying to make the air flow more coherent, and
> you will have more drag.
>
> You may also have less noise, such that the engine noise is the only
> significantly audible thing. =A0Or at least, the noise exiting the back
> of the duct might be lower, or more directed straight back. =A0The noise
> coming out the front of the duct would probably be close to the same.
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