Follow Slashdot stories on Twitter


Forgot your password?

Comment Re:Motors in wheels as part of the package ... hmm (Score 1) 150

but on the same not do the engines actually stop spinning? I would have thought the air naturally moving through the off engine would cause it to spin too.

Yes and no. Turbojets and turbofans as well as fixed-pitch props do free-spin in the air. However, they do so at a very low rpm, usually in single-digit percent of their rated speeds. If anything, this is more of a detriment to performance because it actually acts as a big air brake. All turboprops (as well as some of the higher performance piston props) I know are equipped with variable pitch full-feathering propellers, so they actually do come to nearly a complete stop - this helps reduce their drag and increases performance in engine-out conditions. Turbojets and turbofans do have an in-flight minimum restart rpm. This can be achieved either by flying very fast, by cross-bleeding compressed air from the compressor of the working engine, or by using an auxiliary power unit (a small turbine engine designed to start the aircraft without ground assistance and to provide power when the main engines are off or failed) to feed compressed air to the air turbine starter of the failed engine.
Regardless, irrespective if the engine's internal turbo machinery remains spinning at some small fraction of rated RPM, the hot section of the engine cools off pretty quickly, since the heat source is gone and you've got very cold air going through there (not at a very high rate, but still after a few minutes of -50C air flow, it's going to be pretty much chilled). As a further example, here you have a Boeing 747-400 APU (a >1000 shp beast) starting up and going from zero to 100% rated output power in about 30 seconds. The APU is fully automatically controlled, the crew literally just flips a knob in the cockpit and that's it (here it is, near the center of the picture).

Apparently it was the turbine of an old 737-300 with the turbofan removed so one of the mech engineers told me.

Possibly an industrial variant of the CFM56. Don't know what they're called in industrial versions, I'm only familiar with GE's and some of RR's products. Industrial conversions of aviation engines do occasionally happen.

the engine was attached to a large gearbox. Maybe that's where the warm-up requirement came from

Not sure either. Gearboxes don't really need warmup either, they just need lubrication. It's mainly large castings (as occur in piston engines) that are susceptible to heat stress. Turbine engine oil has very low viscosity (far lower than automotive engine oil), so I don't think viscosity of the oil is much of a factor either... I dunno, maybe the manufacturer just wanted you to really baby the engine.

Comment Re:Motors in wheels as part of the package ... hmm (Score 1) 150

That's the strange part, it was an aviation engine.

What was it? Just out of curiosity.
Anyway, all I can say is I've never seen a warmup requirement in the operating manual of any turbine engine-powered aircraft, but maybe it's because the operational procedures were designed such that it's averted. Warmup is definitely required in piston aircraft (e.g. DA-40; after startup 2 mins idle, then 1200 rpm until oil in green; no takeoff before that). However, in-flight restart procedures don't mention warmup either. You can shut down an engine in flight, leave it off for as long as you like, restart it again and immediately apply full power. One would think if component temps were an issue that the designers of the procedure would warn about it, but apparently they don't. I don't know about industrial applications. I've seen an engine overhaul tech once mention that when they test out industrial engines they do run them up slowly, but that it's not really required.

Still not a good idea to run up to full power before ensuring that every surface is lubricated.

Curiously enough, since in turbine engines there's no sliding of surfaces going on (everything is on ball and roller bearings), oil is primarily used as coolant, not as lubrication. That's why it's okay to let a turbine engine freewheel in the wind on the ground. You won't see that happening with turboprops or helicopter rotors because these guys are using a gearbox between the engine and the prop/rotor (and a pretty aggressive one at that, usually in about a 10:1 ratio). In fact, first thing after parking you'll often see ground crew running up to tie the props and rotor down.

Comment Re:Motors in wheels as part of the package ... hmm (Score 1) 150

Actually turbine engines tend to require next to no warmup. Unlike piston engines, turbine engines don't have large blocks.
At least, this is what I've been told by turbine engine technicians and it's been reinforced by never seeing an "engine warmup" requirement in any AFM or operational procedure. For piston engines, warmup is always built into the after start procedure (or equivalent).

Comment Re:Motors in wheels as part of the package ... hmm (Score 5, Informative) 150

Less sexy would be to develop a tug that could not only push the plane back, but also perform taxi duties.

This is already done. The pushback tugs are also used for repositioning aircraft between gates and/or hangars. There are many reasons why aircraft start their engines at the gate. This serves primarily as a checkout of the aircraft systems. If an engine behaves oddly, or has trouble starting, pulling back into a gate is simple. Doing it at the runway would be a lot more complicated, as it would require a full back-taxi, which on congested airports is already a major PITA. In addition, many of the internal systems such as flight control hydraulics are powered by the engines, so for example you won't have all flight controls fully functional (meaning, you can't perform a F/CTL check) and you can't fully extend flaps for takeoff unless you have at least one of the engine-driven pumps running. Secondly, the air conditioning packs inside the cabin are engine-powered and they take a lot of juice as well as compressed air (or you'd have to carry a sizable battery just to keep them running for the 20-30 minutes on the ground). On very long taxis to takeoff or after landing, many aircraft already do reduced-engine taxi. 747s routinely shut down 1 or 2 engines right after landing. Twins routinely do single-engine taxi. When there is a long queue for takeoff, similarly, engines get shut down. But doing the whole taxi completely shut down and only starting once close to lining up would probably result in tons of operational complications and possibly safety issues.

Comment Re:Real time? (Score 3, Informative) 26

Deicing doesn't deal with snow, or, well, not of the light fluffy kind anyway. It's mean to remove thick layers of solid ice that can form on surfaces and significantly affect aircraft performance. The reason for deicing when you see snow on the aircraft is because you can never be sure that there isn't at least part ice underneath it. That's why they deice, just to be sure. I'm sure you'll rather sit through an unnecessary deice 1000 times over than die once when it was really needed. Ice is no joke and people absolutely have died in aircraft because of it.
That having been said, the way it works is that they have types of deicing fluid, each of which is certified for a particular temperature and protection time. So something like up to 15 minutes of protection at -10C, 10 minutes of protection at -15C and 5 minutes of protection at -20C. The aircraft then has that allowable time window to line up and get airborne. In flight, it'll then either have to fully rely on its own anti-icing equipment, or exit the icing conditions (which usually happens fairly quickly).
The reason why we don't use an aircraft's own anti-icing equipment on the ground is because it isn't very extensive. It usually only protects critical components (typically wing leading edges, engine inlets/props and the main cockpit windshield panels plus some external sensors such as pitot-static tubes and AoA vanes) and may not be even be available for performance-critical phases of flight (such as takeoff), because it robs too much power. Adding more anti-icing equipment would add lots of weight and cost, not to mention power demand. *That's* why we thoroughly de-ice on the ground. Give the whole aircraft a good rinse, takeoff and quickly leave the icing conditions.

Comment Non-fossil sources (Score 1) 115

Don't know about biofuels, but there are proposals and some development work going on along these lines which actually do help the fleet. The reason for that is simple: you know which is the most valuable and important ship in the fleet? The aircraft carrier? The guided missile cruisers? The landing craft? Nope, it's the ugly, lowly oiler. Unless these ships are successful in their mission, the entire fancy multi-billion dollar fleet grinds to a halt within a week. You don't see spots on them being promoted in military advertising, but the oiler is really the centerpiece. When the fleet is out, all the oiler does is continuously hop around the carrier battle group while everybody sucks on its pipe like a total addict. And once the oiler is out, it's in a mad dash to the nearest middle eastern port for a refill and a mad dash back to the fleet. Without a continuous supply of fuel (and the fleet goes through *lots* of it), all your carrier escort ships stop and all flight activity stops. Essentially, at that point the fleet is useless, a victory to the enemy without a single shot fired.
So the new idea is at least partially solving this problem by synthesizing fuels directly in deployment. The carrier has plenty of nuclear power. At the very least, in theory, this can be used to synthesize jet fuel and keep the air superiority. This could help significantly lower the burden on the supply line to the fleet and thus increase the fleet's combat effectiveness. Current problems involve cost, buying fossil fuels is just too cheap. But it will not remain so forever.

Comment Re:Rsync could have done this too! (Score 1) 150

Not quite zfs needs to contact the destination zfs fs to compare with the last snapshot

Ehm, no, sorry. No communication with the destination machine is required while generating an incremental send stream. How can I claim this? Well besides being quite intimate with the ZFS source base (and I can point you to the relevant source files if you so desire), just a quick read through the zfs(1M) manpage will mention this example:

# zfs send pool/fs@a | ssh host zfs receive poolB/received/fs@a

As you are no doubt aware, pipes are by definition unidirectional. There is no way the zfs receive can talk to the zfs send at all. Another way to verify this is to check out ZFS backup systems such as Zetaback, which by default store the ZFS send streams as files on a central server (which may or may not actually support ZFS - it's not actually required). Now if an incremental send stream is stored as a file and then at some later point restored, this clearly tells you that there can't be any bidirectional exchange of information going on.

Comment Re:Rsync could have done this too! (Score 1) 150

ZFS has metadata that permits detecting these sort of files

Side note for your entertainment in case it interests you, the way ZFS actually handles the rename case has nothing to do with trying to follow file name changes. In fact, in order to handle a rename, we don't need to look at the file being renamed at all. The trick is in the fact that directories are files too (albeit special ones) with a defined hash-table structure. ZFS send simply picks up the changes to the respective directories as if they were regular files and transfers those. The changed blocks then contain the updated name-to-inode# mappings, which is what a rename really is. From ZFS send's point of view, a filesystem is just a flat collection of objects and all it does is transfer the changes to these objects that happened between two transaction groups.

Comment Re:Rsync could have done this too! (Score 2) 150

If you read on a bit in the article, you'll come across the example of daily syncing of VM images across to a backup node. While ZFS send is done in less than an hour, rsync would take north of 7 hours just to read in the local state of the VM image, much less figure out what has changed and send the diffs. This is based entirely on ZFS send's unidirectionality. The critical difference is that rsync needs to trawl the entire local dataset state completely and compare notes with the other box (which also needs to read it all in) in order to figure out what's changed. ZFS send doesn't need to do that.

Comment Re:Rsync could have done this too! (Score 5, Insightful) 150

The crucial difference is ZFS send is unidirectional and as such is not affected by link latency. rsync needs to go back-and-forth, comparing notes with the other end all the time. ZFS send is also a lot faster and more efficient, eliminating entire large portions of the filesystem tree structure that haven't changed without having to read them in. This is not to say that rsync's authors were any less competent coders. ZFS simply has more information available about the filesystem than rsync, so it can make smarter decisions.

Comment Re:So the plane tells ATC where it is... (Score 1) 96

Ok, fair enough.

My bad sir. I should have been more clear.

Even in a transport category aircraft, I'm sure the pilot can pull the right breakers if he wants to go invisible.

At present they can. We'll see about the evolution of the ATS. Maybe in the future as SSR is further reduced and self-reporting becomes more well tested, things such ADS-B might become mandatory always-on features and we'll see battery-powered kits installed into aircraft that cannot be switched off.

I'm not sure that ADS-B was really designed with anti-hacking in mind. It seems to be designed to work as long as everybody is playing nicely.

All of ATS is traditionally very much a gentlemen's club. There's nothing stopping you from hopping into your nearest non-transpondered non-radioed Supercub and generally behaving like an ass in the air. Same as on the roads, this kind of fun ends only when the guys with the big guns arrive.

Slashdot Top Deals

An engineer is someone who does list processing in FORTRAN.