Peter,
I agree with you. When I was a young lad I races control line models, we all
used 1 blade props and they were measurably faster than a 2 blade or a 3
blade prop. Add a blade to the prop and your model went slower every time.
Rob
>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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