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Comment Re:The technical problems with this are immense. (Score 1) 345

A F119 is a low bypass turbofan and even a wimpy one at that (bypass ratio of 0.3:1), i.e. it's almost a pure turbojet, but not quite. When I was talking about a fan-driven engine, I meant an electrically propelled aircraft that has no engine core (well, at least not in the traditional sense), only an electrically driven fan - essentially infinite bypass ratio. In another sense, it could simply operate like a half turbojet. It would feature an electrically driven multi-stage axial compressor, but no combustor and no turbines. Instead, the compressor imparts potential (pressure) energy to the gas stream through direct mechanical action. This is then fed straight into the exhaust nozzle, accelerated and expanded to produce thrust, same as all jet engines currently do. The trick here would be to control adiabatic heating and heat loss in the gas, as that could represent lost expansion potential and thus lost power. Of course assuming all the electrical parts have been resolved (which is quite a big ask).

Comment Re:Why batteries? Hydrogen much denser. (Score 2) 345

Actually hydrogen has great specific energy (energy per unit mass), but lousy energy density (energy per unit volume). Even ignoring the massive weight of a vessel capable of holding hydrogen at >5000 PSI (needed for it to stay liquid at room temperature), liquid hydrogen has only on the order of 1/10 the density as compared to jet fuel (70.85 g/L vs ~800g/L).

Comment Re:The technical problems with this are immense. (Score 5, Informative) 345

So batteries don't necessarily need to directly compete with combustion engines because electric engines could (and I stress "could") have higher efficiencies than either piston or turbine engines (both of which actually lose a fair amount of the energy in the fuel to their engine cycle as exhaust heat). At 20MJ/kg a battery would compare very well to jet fuel. A good high-bypass turbofan can get maybe 40-45% efficiency. An electrically driven fan, might go as high as 80-90%. But AFAIK Musk wanted to make these aircraft supersonic. While a fan-driven engine *theoretically* can go supersonic, in practice it's so horribly inefficient that it's unlikely to be practical. That's where the Brayton cycle comes in and we're back to the 40% efficiency range (regardless if the reaction mass heating is provided by hydrocarbon fuels or an electrically-sourced heating mechanism) and batteries in that case are dead in the water.
The kicker though, as you correctly identified, is mass loss during flight. Aircraft get a lot of efficiency from this mechanism and also significant mission flexibility (for shorter missions you can take less fuel and more cargo). An electric aircraft would pretty much have to be factory-built for max range from the factor. I highly doubt it's ever going to be practical to reconfigure an electric aircraft on the flight line for shorter haul by taking some batteries out - keep in mind how tricky even comparatively tiny electrical systems are (see Boeing 787 Li-Ion battery fires). Plus the red tape on this is would boggle the mind.
Lastly, we needn't rely on fossil fuels. The public at large always thinks "smelly fuel = dirty". Not necessarily. We can synthesize a wide range of synthetic jet fuels already. Provided that the carbon source for that fuel is "renewable" (e.g. dissolved carbonic acid in ocean water), we could keep the venerable jet engine in place and simply source the fuel in a renewable manner. Then the fuel simply becomes a liquid chemical battery with fantastic power density and deployment flexibility.

Comment Re: It's what they say (Score 1) 112

The use of "y" at the end of a word and "i" in its place in the middle of a word was a convention by printers which made it easier to deal with the "y" descender in a stylish way.

I would not say this in many forums, but this is slashdot....

*I* find it convincing and interesting though a reference would be nice. And this is /. so it is appropriate to learn something new and slightly odd.

A pet hate of mine is faux archaic signs like "Ye olde cheese shoppe". The "ye" is just a misunderstanding AFAIK of "the" written with a slightly open, regional form of the letter thorn which vaguely looks like a "y" though it just means "th".

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.

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