

Rolls-Royce brings propeller engines back in vogue - pg
http://www.guardian.co.uk/environment/2008/oct/20/travelandtransport-rollsroyce

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pg
This type of engine is quite dramatic looking:

[http://pic60.picturetrail.com/VOL1689/10590113/19957526/3259...](http://pic60.picturetrail.com/VOL1689/10590113/19957526/325959229.jpg)

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gcv
I thought the article said the engine doesn't have a casing, but in the
picture, it does. Kind of makes sense though: if I understand jet engines,
they must compress air, so the air must be compressed in some kind of
container. This design just uses smaller containers.

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ram1024
they were saying there's no casing around the propeller part of the engine, in
a turbofan, the casing encloses the blades.

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rms
During the time I worked for US Airways, a Boeing 757 with a Rolls Royce
engine lost a ball bearing and all the oil dumped out of the plane. This is
known as a "significant event." The pilot kept the engine on so it was
destroyed by the time the plane landed.

The investigation concluded that a mechanic had probably made some type of
mistake in maintenance. This was the most likely cause because there was no
evidence that strongly pointed to any other cause. It very well could have
been a mechanic's fault, but there was something fishy about the whole thing.

I'm not nearly as comfortable flying as I was before I had that job.

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DenisM
It's good that you don't know how food is made. :)

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eru
Bismarck said, law is like sausages. It's not to known how they are made.

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DLWormwood
> Another reason for the higher efficiency of open-rotor

> engines is that, unlike traditional engines, they do

> not have a casing around the propeller.

"Traditional" engines? Isn't this nomenclature a little backwards, given that
propellers pre-date jet engines?

EDIT: I still haven't figured out how to quote correctly here... this site
seems to only use a tiny subset of Markdown.

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jcl
There is no correct way to quote. You can either use italics or quotes, but
there is no markup that will trigger the blockquote HTML tag.

There is a "help" link next to the edit text box (but not the reply text box,
for some reason): <http://news.ycombinator.com/formatdoc>

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tocomment
So propeller engines are more efficient than jet engines? Is this true
regardless of speed, plane size? I always thought jet engines were
revolutionary, were they really just quieter?

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designtofly
Yes, propeller based engines are much more efficient. They don't waste quite
as much energy "throwing" air out the back. It is generally true regardless of
size. However, using a propeller does limit the speed of the plane. In fact,
an airliner with an open-rotor engine (like the one described in the article)
will most likely fly just a bit slower (around Mach 0.75-0.8 instead of 0.85
like most airliners today). The propeller becomes very inefficient when shocks
start to form on it. Therefore, you have limitations in how fast you can spin
the blades and how fast you can fly. You eventually reach a speed where the
blades cannot generate any more thrust to keep accelerating the aircraft.

Jet engines were revolutionary, but not because they are quieter. Pure jet
engines are actually incredibly loud. The only way the noise was reduced in
modern airline engines was the advent of high bypass engines where most of the
air going through the engine is only slightly accelerated.

~~~
tocomment
Another thought. I heard that putting a duct around a prop gives you more
thrust ceteris paribus. Is this just not true, or maybe only true for slow
speeds?

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designtofly
Yes, it is generally true. There is not a big dependence on flight speed
because the inlet on jet engines slows down the flow before it enters the
engine. So even though an airliner might be flying at Mach 0.85, the face of
the engine only sees about Mach 0.3. The reason the duct improves the thrust
of the engine is because it reduces the pressure losses (improves efficiency).
A fan (or any stage in the engine compressor) increases the pressure of the
air. Since air moves from high pressure to lower pressure, the air that you
just pressurized wants to sneak back around to the front of the fan around the
tips. By putting a duct around the fan, you help keep air from sneaking back
and robbing you of the work that you just did on it. In fact, the efficiency
that you gain is highly dependent on the clearances between the duct and the
tip of the fan. A lot of work goes into reducing these clearances as much as
possible. You can imagine that that is a non-trivial task since the parts of
the engine expand and contract based on temperature, and also vibrate and have
dynamic structural behavior.

