Counter rotating propellers

[Buy_Sell]

I was explaining to a friend that I read an article about these vanes
that were positioned behind a turbine blade on a jet engine to try to
straighten out the airflow which is supposed to help increase the
thrust. I have seen the same sort of thing sold for hovercraft ducted
fans. My friend wanted to know why they didn't use two propellers.
One spinning clockwise and one spinning counter-clockwise. I remember
seeing a Russian helicopter that was imported into Canada for the
purpose of selective harvesting of trees from hard to reach areas. The
advantage of the dual counter rotating blades was that these
helicopters had much shorter blades and twice the thrust. Plus they
did not require a tail rotor and could operate in very windy conditions
and face any direction regardless of wind direction. So, that got me
thinking. <By now, you probably have noted that I do a lot of
thinking> Has anyone ever seen such a setup on a light aircraft or
hovercraft? It might actually be an interesting idea to try. I am not
too sure how to design it but it would certainly be an interesting test.

[Ken Roberts]

We seem to be having a personal chat here.

The vanes you're talking about are called stators.

The reason we don't tend to use counterrotating props is that it's generally not
worth it. There's only a little bit of twisting force that is related to
thrust. It generally shows up when you're changing RPMS rapidly, so it's sort
of like a muscle car revving up at a stop light. That part is mostly related to
the mass of the prop and engine and whatever else spins.

There's also a swirl component to the air that is significant, but most of us
don't bother dealing with it. Stators fix it for exactly one set of conditions,
which generally means flat out and full throttle because that's the only time
you really want to optimize your thrust.

Anyway, the stators and the counterrotating props are doing two different
things:
The stators straighten all the air coming out of the duct as best they can, so
that the engine does more "effective" work. In other words, they convert some
of the energy that started the air to swirl back into straight-back thrust.
There are a significant number of hovercraft folks who go out of their way to
make good stators. Generally, these are for racing. There are also some
commercial machines that have stators, both good and bad.

Bad stators, by the way, are worse than no stators. Good stators produce good
results at one power setting, and are worse than no stators most of the rest of
the time. Some brands of commercially made recreational hovercrafts put stators
in that were not just bad, they were terrible. For that matter, they put them
into a terrible duct as well. Both duct and stators were either there for
cosmetics or for a nifty-looking guard, I can't decide which. Maybe both?


The counter-rotating props are simply trying to reduce the overall effect of the
roll motion induced by changes of thrust. But they really can't do that because
you'd need counter-rotating engines too. To eliminate an effect that doesn't
really bother too many people, it just gets to be too much work.

Before we get into a net brawl, counter-rotating props in other applications can
make all kinds of sense. Twin-engine aircraft, for example, can set things up
so an excessive angle of attack doesn't cause one prop to pull more than the
other. We don't run at an excessive angle of attack, we run at the same one
pretty much all the time.

Don't apologize for thinking these things. Most of us (myself included) thought
the same sorts of things when we got into it.

Post by Buy_Sell
I was explaining to a friend that I read an article about these vanes
that were positioned behind a turbine blade on a jet engine to try to
straighten out the airflow which is supposed to help increase the
thrust. I have seen the same sort of thing sold for hovercraft ducted
fans. My friend wanted to know why they didn't use two propellers.
One spinning clockwise and one spinning counter-clockwise. I remember
seeing a Russian helicopter that was imported into Canada for the
purpose of selective harvesting of trees from hard to reach areas. The
advantage of the dual counter rotating blades was that these
helicopters had much shorter blades and twice the thrust. Plus they
did not require a tail rotor and could operate in very windy conditions
and face any direction regardless of wind direction. So, that got me
thinking. <By now, you probably have noted that I do a lot of
thinking> Has anyone ever seen such a setup on a light aircraft or
hovercraft? It might actually be an interesting idea to try. I am not
too sure how to design it but it would certainly be an interesting test.

[Barry Palmer]

An improvement in thrust can be obtained, especially in the case that
the thrust rotor is too small in diameter for the power used on the
craft, which makes the flow waste energy by swirling out the rotor
exit. With contra rotating rotors, torque is balanced out, and the
rotor exit flow is straightened out for higher efficiency, both very
minor factors.

However, the complexity adds cost and weight, the extra weight alone
may negate any advantage obtained with counter-rotation.

Some Russian aircraft use contra-rotating propellers due to high
propeller disc loadings, (horsepower per square foot of propeller disc
area. ) (The same problem could have been solved by using twice the
number of blades, but the final drive ratio, engine to propeller might
be unacceptible.)

Two side by side rotors are a realistic proposition, contra-rotating or
not, however, aiding efficiency through lower disc loading than if the
rotors discharged through a single rotor diameter, and providing a new
area of steering a craft by differential power, even reverse, to the
rotors.

Barry Palmer, for Sevtec