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.


i) I'm sure I remember seeing, years ago, a picture of a Noorduyn
Norseman with a single-bladed prop. Since you seem to know what you are
talking about (more than I do, anyway), what factors would drive a
manufacturer to adopt such a radical solution?

In a word, efficiency. Note that many of the model airplanes used in
free-flight competitions, (Escpecially the rubber powered ones, where
the judges issue you your engine (So many strands of Pirelli rubber,
of some particular length) and "fuel" it up for you (So many turns of
the rubber bands)) where getting the absolute most out of the limited
omount of energy you've got means the difference between winning and
losing, use very wide chord single-bladed propellers. The downside is
that you need a fairly large diameter. That's not much of a problem
in a hand-launched model airplane, but it doesn't work so well in Full
Scale stuff.


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. Above about Mach 0.65, the efficiency of
a straight propeller drops off alarmingly. At typical airliner cruise
speeds, (Mach 0.78-0.85) efficiency would be down around 60% at teh
low end of the speed range, and 50% at the high end. That's not very
useful at all - there are some measures that you can do to cut the tip
speed down - for example, the Tu-95 Bear (Russian turboprop transonic
bomber) uses a very high step-down gearing from the engines to the
propellers - the props rotate at 750 RPM, vs, say, 1500 or so for that
of a P-51, and a very clever variable pressure ratio compressor system
in its engines that essentially "supercharges" them to deliver sea
level power at 40,000'. (About 3 times what you'd get from a typical
turboprop). The swept propeller blades supply efficiencies in the
Mach 0.78-0.85 range of between 75% and 70%. Using many blades allows
the diameter to be cut down from, say, 22 ft for our notional
conventional propeller to 13 ft. This gives a lower airspeed at the
propeller tip than a large diameter propeller, thus delaying the
transonic effects.
(Note that the entire propeller doesn't go transonic - the airspeed at
the propeller blade is a product of the propeller's rotational speed,
and teh forward speed of the airplane. The rotational speed of the
propeller in ft/sec or m/sec increases as you move outward along the
propeller blade. So, a propeller will start having supersonic flow
appear at the tips, with the supersonic flow field moving inward as
speed increases. A smaller diameter and a slower rotational speed are
helpful in delaying the formation of these shock waves.
(transonic/supersonic flow). You do lose efficiency in the lower
speed ranges, but you get big gains at what your desired cruise speeds
are.


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.
Can you point me in the direction of some clues?

A contraprop does lose some efficiency by placing one propeller behind
the other, and it requires a more complex drive system. (Which gave
fits on several early U.S. contraprop-equipped aircraft, most notably
the XB-35 Flying Wing, where they never got the contraprops doped out,
and the Hughes XF-11 recon machine (looked like a hyperthyroid
P-38). which was lost on its first flight becasue the aft bladeset in
one of the contraprops went into reverse pitch at low altitude. (This
is the crash that nearly killed Howard Hughes, and led to his drug
addiction (painkillers) and fear of infection.)

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). P-Factor is a Big Deal, with a
high-powered airplane. For example, with a P-51 or a Corsair, you
have to be careful with throttle movement at low speeds, or on
takeoff. If you jam the throttle to it too fast, you'll either swing
off the runway or roll the airplane inverted.

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