Counter rotating propellers

Raoul wrote:

What advantages were being sought through the counter rotating
propeller and, if there were indeed advantages, why aren't they seen on
production propeller driven aircraft today?


Contraprops allow large amounts of shaft-horsepower to be turned into
thrust without making a single-engined aircraft unhandlable through
torque effects. At takeoff, full throttle can be applied with no
unbalanced effect on the aircraft. The extra number of blades also
allows the diameter to be reduced, helping to keep the ends off the
ground and tip speeds lower. They aren't seen today because nobody is
trying to put that much power through propellors.

Peter Stickney gave me some very good answers to a similar question a
little while ago. Here is an extract from our conversation:

------------------------

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

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 because 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 because 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 the Avro Lincoln into
the 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.

In article ,
Raoul writes:
I've had a questions I'd like to foist upon the collective knowledge
here...

I have a few of those "World's Worst Airplane" books and enjoy reading
about the creations of those in the old days who were basically working
by the seat of their pants.

I've noticed that their were many planes during the prop-to-jet
transition years from about 45 to about 55 that used counter rotating
propellers. I'm wondering what the perceived advantage was?

Seems to me that the added complexity and cost would be a disadvantage.
It's pretty simple with one propeller: Take engine. Fasten propeller to
flange on front. Put on airplane, Fly into the wild blue yonder, All
those gears and driveline parts were mighty complex and, in my reading,
the added complexity was usually the thing that put the plane into the
'world's worst' catagory.

I notice that notable post war military aircraft such as the B-36 and
the C-130 (plus more) used one propeller per shaft. If there were an
advantage, you'd think you'd find 'em on a military plane. Yet, if my
reading is correct, the Soviet long-range Bear bomber had
counterrotating propellers.

So, that leaves me again with my initial question:

What advantages were being sought through the counter rotating
propeller and, if there were indeed advantages, why aren't they seen on
production propeller driven aircraft today?

Other folks have been covering aspects of this well, so I'll leave out
the long treatise.
There's one things that is being messed. Since a contraprop allows
more power to be absorbed by a smaller siameter propeller, the tip
speed of the propeller is lower. This is important, since teh
efficiency of the propeller drops sharply as teh flow over the
propeller goes transonic and supersonic. Since the propeller tip
speed is the vector sum of teh propeller's rotational speed adn its
forward airspeed, it allows better overall efficiency at higher
speeds.
The Tu-95 uses this in two ways. Not only does the contraprop cut
down on the propeller diameter, but the props are geared to turn at
about 760 RPM. This allows that big meatgrinder to churn along at
Mach 0.85. (Which allows it to outpace a Tornado in dry (No reheat)
thrust.)

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

Even by 1940, the world's air forces had discovered that there was a
problem with the more-powerful engines that were coming on line:
they'd drive the propeller too fast. Once the tips go trans-sonic, the
prop loses efficiency. So they went from two-bladed to three-bladed
props, and then to four-bladed. And they made the props longer. But
there are limits to both these solutions. Thus the notion of having
two sets of propellers, rotating in different directions.

On prop-jets these days, you routinely see multi-bladed propellers.I'm
not sure why airframe manufacturers didn't go in this direction for
warplanes, but perhaps it has to do with the power output of a plane
under combat conditions--that is, a seven-bladed prop will work on a
transport but not on a fighter. Dunno.

You're certainly right about the complexity of the counter- or
contra-rotating propellers.

all the best -- Dan Ford
email: (put Cubdriver in subject line)

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