Given any propeller rpm a large diameter has a higher tip speed and
therefore is more noisy.
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
> >
>
>
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