Rolls-Royce brings propeller engines back in vogue | Hacker News Reader

Rolls-Royce brings propeller engines back in vogue

guardian.co.uk

23 pointspg29 comments

This type of engine is quite dramatic looking:

http://pic60.picturetrail.com/VOL1689/10590113/19957526/3259...

I wonder how they designed it. This looks like something that a GA combined with a CFD solver would create.

As a concept, there is nothing revolutionary about it. This was explored quite a bit back in the 1980s. It is a natural consequence of the physics of how engines operate most efficiently and is well known. So it wouldn't suggest that a GA was necessary to "find" this as a configuration.

At this point in time, CFD and the types of tools used in engine design do not lend themselves very well to applications using GA. CFD and other computational tools are very slow (on the order of hours to days) to run and still require a lot of human oversight to produce decent results (so much for physics-based tools, eh?). In addition, it's not quite as simple as a CFD code. It will also have to include the thermodynamic cycle analysis, structural analysis, and noise analysis tools (and dozens of other tools in order to properly assess a possible design). In a very large design space, this would make a zero-order random search method very inefficient. GA are best when the evaluation of the fitness function is computationally inexpensive. That's not to say that there are no applications of GA in aerospace design, but right now they are much more limited in scope.

Engine design is incredibly complex. The big engine manufacturers spend several BILLION dollars designing the "core" of the engine alone.

I had an offer to develop an library to compute pareto-optimal sets of parameters for car-engines at Bosch. (Pareto-optimal means that you can not get more of a good A (e.g. power) without less of another good B (e.g. efficency) at a point in the set.)

(Would have been quite interesting. But I chose to optimize schedules for a railway company instead.)

Thanks for the info. Also, check the amount of research on wings and GAs: http://www.google.com/search?q=wing%20design%20genetic%20alg...

but I haven't looked personally into mixing CFD and GA.

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.

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

What's also interesting is how it will affect what airliners look like. These engines pose a very significant problem for integration onto an aircraft. Projected diameters for an engine of this type range from 12-17 feet. Compare this with a diameter of about 56-60 inches on a current high bypass engine as found on a 737.

Picture: http://seattletimes.nwsource.com/ABPub/2006/05/09/2002983476...

Article: http://seattletimes.nwsource.com/html/businesstechnology/200...

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.

flying is still safer than any other method of transportation. You are something like 1000 times more likely to die in a car accident.

Is that per mile or per hour?

If you drive you do have some control of the risk. By how you drive, which car you drive, how it's maintained and when you drive etc.

Flying is very save by distance. Trains are a bit saver per time, but not per distance.

The safest vehicles I know of are elevators. You get to travel a mean distance of here to the sun between accidents.

If you look at statistics of deaths per 100,000 per year (for example), yes, flying is safer than driving. But if you look at the likelihood of death per hour, driving and flying are equally as safe.

It's just that you don't fly as much.

Or that planes travel at 7-15x the speed of cars... The real stat is fatalities per 100m passenger miles and planes lead significantly (a factor of several hundred).

Deaths per passenger mile is the stat that air travel happens to achieve spectacular figures for, so of course it's the stat that the airlines have always touted as evidence for the safety of flying.

In some ways, though, it's a silly measurement. In many (most?) cases, flying and driving are not equivalent alternatives. You can't drive to Hawaii and you can't buy a ticket on Northwest to the grocery store. A comparison of the deaths-per-passenger-mile stats for those two trips only sounds meaningful.

You'll have to back that statement up. There aren't nearly as many serious plane accidents as car accidents, by a factor of at least 100. At least if you consider regular airline traffic.

But remember that when a plane has a serious accident _hundreds_ of people may die.

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

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

Tell me about it... I really like food.

Planes still fly fine without engines -- you just might be landing soon.

Structural issues are another thing entirely.

> 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.

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

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?

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.

So what was revolutionary about jet engines, just the ability to go faster than mach .8?

That was certainly a big part of it. Jet engines, although revolutionary, were developed only about 40 years after heavier-than-air flight began. It's hard to group all jet engines into a single category and discuss trends of the entire group. One grouping you could make is the distinction between turbojets (and very low bypass turbofans) and medium/high bypass turbofan engines. With turbojet engines (like the original designs starting from the 1940s), you had the ability to produce a lot of thrust at the expense of fuel consumption. These are the types of engines that you had in military jets and supersonic planes like the Concorde.

However, for modern airliners, fuel consumption is important. So a second class of jet engines was used that send a small portion of the air through the "core" of the engine, where it is burned and power is extracted, and the majority of the air is accelerated through the fan without being combusted. This is much more efficient (and much quieter, since most of the noise you hear is from the interaction of the high speed air coming out of the engine with the surroundings). The open-rotor concept described in this article is just a natural extension of the high-bypass engine. You get a much more efficient engine when you can increase the ratio between the amount of air that you accelerate through the fan compared to how much you send through the core and burn (this is called the bypass ratio). Since the weight and drag penalty increase exponentially as you increase the size of your engine, removing the duct around the fan becomes necessary once you get to very large bypass ratios.

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?

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.

Reply on news.ycombinator.com