OOPS - Should have said I think for this bit as this is supposition only on
my part.
> High rotational speeds means the air has to be accelerated quickly to a
high
> speed. That acceleration and high speed leads so losses so if the
propeller
> diameter gets too large in my opinion it becomes more efficient to use
> another blade. If it were more efficient to use single blades (or even two
> blades) we would see a lot more of them.
>
> Don't get me wrong sometimes two blades are best. I would just about
always
> recommend a two blade propeller on a small direct drive engine like the 80
> hp Jabiru for example.
>
> Jerry
>
> LTS@avnet.co.uk
> http://www.avnet.co.uk/touchdown
> ----- Original Message -----
> From: Peter van Schoonhoven <pvans@pacifier.com>
> Subject: Re: Engine selection
>
>
> > Now wait a minute.
> >
> > A one blade prop is the most efficient ( yes, there have been some!)
> > It has the least tip losses and has only one intersection with the
> spinner.
> >
> > And a long blade or large prop diameter is the most efficient and
> > quietest. Prop diameter is usually dictated by the prop turning speed.
If
> > the engine is geared, or like the old radials, designed to turn low RPM,
> > then the diameter can be big. The prop tips must not get to supersonic
> > velocity, and the lower the better. Prop diameter is also dictated by
> > ground clearance.
> >
> > Consider the Osprey tiltrotor or any helicopter. If a small diameter
was
> > most efficient, why would they use giant diameters?
> >
> > Optimum choice is a large diameter two blade prop turning slowly since a
> > one blade is too odd and harder to balance. Rotax has a gearbox to
reduce
> > propspeed specifically to allow a larger diameter prop. If it were not
> > geared down, it would take a smaller diameter prop with maybe 12 blades
to
> > absorb the horsepower, and that is not efficient at all. Otherwise the
> tips
> > would go supersonic , get really noisy ( like a Cessna 185 floatplane on
> > takeoff) and get real inefficient.
> >
> >
> >
> >
> > >I am not sure why people think three blades are less efficient. Like
> > >everything in aircraft design propeller selection is a compromise. The
> most
> > >important feature is probably that the propeller is well matched. The
> blade
> > >size needs to be matched to the engine hp. Large diameter props are
more
> > >noisy and having a large diameter prop may limit the tops speed of the
> > >aircraft. In my experience a 64" three blade wide chord (130 mm) is a
> good
> > >choice on aircraft with the 912S at speeds up or near 160 kts. If you
> think
> > >your A/C can go faster it might be advantageous to have a smaller prop
> but
> > >at the loss of take off and climb performance.
> > >
> > >Jerry
> > >
> > > LTS@avnet.co.uk
> > > http://www.avnet.co.uk/touchdown
> > >----- Original Message -----
> > From: Nigel Charles <72016.3721@compuserve.com>
> > >Subject: Engine selection
> > >
> > >
> > > > Message text written by INTERNET:clevelee@cswebmail.com
> > > > >
> > > > In addition to wondering why Europas use 3 blade less efficient
(than
> two
> > > > blade wood) carbon fibre props, he offered the best advice yet
(which
> has
> > > > been rattling in the back of my mind) on engine selection: The
Europa
> is
> > > > an expensive aircraft (from his VW aircraft perspective) and
> installing
> > > > anything other than the factory recommended engine will severely
> > > > deteriorate the resale value of the aircraft.
> > > > <
> > > >
> > > > The loss of efficiency for the 3 bladed prop doesn't seem to be as
> much as
> > > > the theory would predict. The main reason for the 3 bladed version
is
> to
> > > > improve ground clearance - very relevant for the mono wheel. The
> necessary
> > > > increase in diameter for a 2 bladed prop would increase the risk of
> prop
> > > > strike. It may be practical to look at 2 bladed props with the
> tri-gear.
> > > >
> > > > Nigel Charles
> > > >
> > > > The Europa List is supported by Aviators Network UK -
info@avnet.co.uk
> > >
> >
> >
>
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