In article ,
Alan Dicey writes:
Peter Stickney wrote:
In article ,
Alan Dicey writes:

Peter Stickney wrote:

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?

Efficiency in the sense of power to thrust. You're right - it won't
work so well at higher speeds.

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.

They're also complicated and heavy. UDFs, like propellers on
high-powered aircraft, have to be variable pitch to operate halfway
decently across their speed & altitude range. The pitch change
mechanism and, for that matter, the structure of the blade itself
aren't simple problems. For teh lower end of teh airliner cruise
range they may be somewhat more efficient - but they'll also have a
shorter Time Between Overhauls, and overhaul costs aren't going to be
cheap. Fuel prices would have to go up a _lot_ more than they have to
make it worth the extra overall cost.


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.

Right. Another example would be the Griffon engined Seafires. With a
single rotation prop, the Griffon Seafires had 5-bladed single
rotation propellers, and were limited to roughly 66% power on takeoff.
This was becasue of 2 reasons - the Torque/P-Factor would drag the
airplane right into the carrier's island. (A bad idea), and trying to
hold it straight was overstressing the tire sidewalls, forcing tire
changes after only a couple of flights. It's tough when you've got to
explain that you need to pull your ship out of the battle becasue you
ran out of tires, rather than gas, bullets, or bombs. The contraprop
used on the later Seafire 47s (6 blades, 3 per bank) allowed more
power to be used without the swing, and better propeller clearance.

The same basic engine allowed the development of teh Avro Lincoln into
teh Shackleton - you could hang Griffons with contraprops in the same
wing center section without changing the location of the engine
mounts. That's basically a Lancaster wing, so they got a lot of
stretch out of it.

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

Do you do this for a living? :-)

Sometimes. I'm an Engineering Mathemetician/Computer Scientist type,
and a Certified Wing Nut and Gearhead.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster