Peter Stickney wrote:
In article ,
Alan Dicey writes:

Peter Stickney wrote:


It's all a balancing act - but in ggeneral, you're best off going with
the largest diameter propeller with the fewest number of blades that
you can practically manage.


a Noorduyn Norseman with a single-bladed prop: what factors would drive a
manufacturer to adopt such a radical solution?

In a word, efficiency.

Hmm. Efficiency in the sense of translating engine power to thrust? I
can't see it being aimed at top speed, so I guess it would give more
range for a given fuel load?


ii) Radical solutions such as the Unducted Fan proposals mooted a few
years ago, had many curved blades - any idea what gain they were seeking
that justified the loss in efficiency?


In tha case, what they're trying to do is reduce the effects of the
shockwaves that form on the propeller blades as they fly further and
firther into the transonic region. It's not unlike sweeping a wing
back to delay the Mach Number that the drag rise occurs at, and the
magnitude of the drag rise. [...] You do lose efficiency in the lower
speed ranges, but you get big gains at what your desired cruise speeds
are.

Of course - tip speed and transonic drag rise. To get more airscrew in
the airflow /and/ keep the tip speed suitably subsonic,the only answer
is more blades - with sweepback to delay the drag rise. I should have
remebered that from the discussions at the time. None of the Unducted
Fan experiments seem to have made it into a production implementation. I
guess the aim was a cheaper powerplant - propellors being cheaper than
ducted fans - but the loss of efficiency was too great.


iii) How does this work with contraprops? On the face of it they must
interfere with each other horribly, but they seem to fly quite well.


What you gain is a greater ability for a propeller of a particular
diameter to absorb power, adn the elimination of torque and P-factor
(destabilization of the airframe due to the rotating airflow from the
propeller affecting the airframe).


So, for an increase in power turned into thrust there's an improvement
in flyability and the ability to make the airframe lighter because it
doesn't have to absorb the stresses - they're balanced out at the
source. That explains to me how the Fairey Gannet was able to shut off
one half of the Double Mamba powerplant, feather one half of the
contraprop and achieve better endurance at patrol speed.

Thanks very much for taking the time to give me some pointers.

Do you do this for a living? :-)