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
>
|