A diesel APU sounds like an interesting trade study. While it would be heavier than a turbine APU, at least some and perhaps all of that weight penalty would be offset by the reduced fuel consumption. As a general rule, turbines are most fuel thirsty at low altitudes. A diesel APU powering a generator wouldn't need to operate over a wide RPM range so it could be tuned to the correct power setting for even greater efficiency.

Good Aviation Week article here about the 787's electrical systems, including specific design concerns for the "electric airliner."

Regarding the starter/generators: during engine start cycles, the high torque/low speed of the starter motors will overcurrent - by design. One serious concern is the longevity of the starter/generators under such strain.

Also, due to the high current draw of the 787 systems, the turbine APU drives two 225 kva generators; the engines each have a pair of 250 kva starter/generators. We're talking some major electron flows here. Optimal engine starts use both starter/generators on the engine. A single starter/generator can perform an engine start, but takes much longer and can seriously overtemp the starter/generator.

The article goes on to say that it would be impossible for the 787 to perform a "normal" ground-power start (without the APU); one standard 90 kva ground-power cart doesn't have the juice to turn the starter/generator. A pure ground-power start would require two carts connected to different power buses.

There's a lot to not like about this new design.

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I looked up the specs of Union Pacific's "big blow" turbines, and discovered that in practice, there isn't a large weight difference between turbine and Diesel locomotives of comparable horsepower. The earlier (built 1952-'54) General Electric 4500 GTEL (gas turbine electric locomotive) 8-axle model produced 4500 traction horsepower and weighed 534,000 pounds. The later (built '58-'61) GE 8500 GTEL produced 8500 HP shared between two permanently coupled 6-axle units, weighing 408,000 lbs and 440,000 lbs (the claim that these locos were upgraded to 10,000 HP appears to be a myth, as the generator and traction motors were incapable of handling that power).

For comparison, contemporary Diesel locomotives produced 1500 to 2500 HP and weighed about 250,000 lbs for a 4-axle unit and 350,000 lbs for a 6-axle unit (give or take a few tons).

Modern locomotives are reaching the performance levels of the smaller GTELs (in terms of horsepower; the modern locos will outperform the turbines at low speeds thanks to computerized wheel anti-slip controls and AC traction motors). The GE ES44AC produces 4400 HP and weighs 416,000 lbs, while the Electro-Motive Diesel SD70ACe produces 4300 HP and weighs 408,000 lbs. Both builders offered 6000 HP models weighing 425,000 lbs, although these have been withdrawn from the domestic catalogs.

Some of the lack of weight difference could be attributed to the small auxiliary Diesel engine that the turbine locos needed for hostling and starting the turbine (a locomotive APU!), although it is standard practice to ballast locomotives to the desired weight.

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Larry, by the time a pilot resorts to using an APU, he/she is in deep kimshee.

Prior to that point, the less the APU weighs, the less it costs over the long haul, and turbine APUs cost WAY less than diesel APUs over the lifetime when they're needed.

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Larry, by the time a pilot resorts to using an APU, he/she is in deep kimshee.

They use APUs while on the ground and when the engines aren't running. For example, they can use APUs to power air conditioning and ventilation systems, engine start, etc. The point of the study is that turbine engines are thirsty while on the ground. A diesel APU should burn considerably less fuel during those operations. Depending on how much less fuel and how long it runs, the weight savings from the lower fuel consumption might well offset the higher weight of the diesel engine.

I am a huge jets fan! J-E-T-S JETS JETS JETS!

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The H-4 Hercules was not overly efficient, simply because of contemporary engines. It's not a great example of scaling propellers to large aircraft.

This, however, is an efficient example of a large aircraft with propellers:



That aircraft is the Tu-95 'Bear-H', and is about the size of and has similar operating parameters to a B-52 Stratofortress.

That is a good example, but it is still a jet (ok, technically a turbine powered aircraft).

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The A380, the largest commercial aircraft ever out into production, is still dwarfed (in wingspan at least) by a prop plane.

The Antonov A225 might be a better comparison though (another amazing aircraft).

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Hi, Interesting to note that the Hercules would have handled much better with
a split tail configuration like many other large craft of the day. I think the Martin "Mars" had this. But it was not to be. The vision of the day was to abandon
water as a base and to promote airfields as we know them today.
There must have been some turbulent flow indeed behind those eight
propellers. And there remains the cannard configuration, putting your control surfaces in clean air up front and the rudders outboard of the main wing.
I'll bet Burt Rutan could do it.

Best regards, Dan

Well, the topic title says "Jets & propellors[sic]", not "Jets & piston."

True.

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I'm not even sure whether I was aware, at the time I started this thread, of the existence of propellers driven by jet turbines, or of the fact that a substantial fraction of the thrust from most jet engines comes from the fan in the front rather than directly from the explosion blowing out the back...

Explosion isn't exactly the right word. Explosions in engines (even piston engines) are usually very bad things. In a piston engine, if the fuel mixture is too low, you can get detonation. These are essentially explosions inside the cylinders instead of burning. Detonation can ruin an engine very quickly.

Most jet engines operate on the suck, squeeze, bank, blow concept. Air is sucked into the intake and compressed. Fuel is added and burns continuously, where the resulting highly expanded combustion products are expelled out of the nozzle.

There are exceptions. For example, the pulse jet engines used on the old V-1 buzz bombs didn't have continous combustion. The only moving parts were these vanes in the intake. They were simple and they worked but not very efficiently.

Turbojets are more complicated. If the turbine spins a propellor, it's called a turboprop. If it powers something else - such as the transmission of a helicopter - it's called a turboshaft. Most modern engines spin a fan section that's similar to a ducted propellor. The ratio of the air that flows outside the main part of the engine to that that flows inside the engine is called the bypass ratio. Jet fighters usually have low bypass ratio turbofan engines. This allows for greater efficiency than a straight turbojet but keeps the engine responsive to rapid throttle changes. Airliners use high bypass ratio engines so most of the thrust comes from the fan.

A ramjet engine often has no moving parts. The engine has to be moving quickly so that the ram air pressure is sufficient to keep the combustion gases flowing out of the rear of the engine. An afterburner is little more than a ramjet section stuck on the rear of a jet engine. This greatly increases both the thrust and the fuel consumption. Most jet fighters can only run on full afterburner for about 15 minutes or so before the tanks are dry.

Ordinary ramjets are only good to a maximum speed of Mach 4 or so. A scramjet is a supersonic combusion ramjet. These engines are being tested to allow for much higher speeds (Mach 8-11, perhaps faster).

The engines used on the Blackbird were very powerful but complicated. They were turbojets for takeoff and low speed flight (Mach 2 was low speed for a Blackbird) and then essentially became ramjets. The design top speed for a Blackbird was Mach 3.2 but I've read that it could go quite a bit faster in emergencies.

Welcome to aviation history.

If I am not mistaken, every large propeller driven plane manufactured since about 1958 (mostly Lockheed C-130s and P3s) is a turboprop. That was when the last DC-6B, DC-7 and Constellation were built. (There may be some non-American makes I don't know about.)

The larger an engine is, the more prone it is to failure from metal fatigue, which in turn is a vastly greater problem for piston engines than for turbines with their purely rotary motion. Thus, for large engines, the high initial cost of a turbine is offset by greater reliability and reduced frequency of maintenance. Small piston engines are less prone to that type of failure, and remain the engine of choice for the smallest light planes.

The old JT3 turbojet on the first 707s and DC-8s was a pure jet, meaning all of the thrust was directly from the exhaust out of the combustion chambers. It was very efficient in level flight in the stratosphere above speeds of about 550mph, but was excruciatingly feeble and noisy on the ground. The first jetliners used up about half their fuel just getting off the ground and climbing to cruising altitude, and could not make the north Atlantic crossings without stopping at Gander for fuel. The successor engine, the JT3D, had a fan added to the front end which generated thrust with cool air that bypassed the combustion chambers and rear turbine. It gave more thrust with lower fuel consumption and noise on takeoff and climb, more than offsetting the slight loss of efficency in level flight cruising. Modern engines have even more bypass, and are functionally more nearly like turboprops.

One might ask why the greater durability of large turbines has not led to more of them on railroads. The answer is that a railroad diesel is just as durable because of structural brute strength, at the cost of being way too heavy for use as an aircraft engine.

Really? I hadn't really thought of it like that. I mean, I know they're high bypass but I hadn't really thought that the bulk of the thrust came from the fan.

Wikipedia has a pretty good article on turbofan engines and their advantages. For airliners and transport aircraft, high-bypass turbofan engines are the way to go. The produce high thrust with less noise and much lower specific fuel consumption (SFC) value than pure turbojets. A hi-bypass turbofan engine typically has a SFC (units of fuel to produce a unit of thrust for an hour) in the range of .3 to .4 for takeoff and perhaps 0.6 for cruise. A pure turbojet's SFC is often over 1.0, meaning the turbofan will consume considerably less fuel than a turbojet to produce the same amount of thrust.

Here's a NASA chart that shows the improvement of SFC over time. I've read that SFC has decreased approximately 1% per year over the past decades. This chart shows that trend. In all cases, the SFC is for a plane cruising at Mach 0.8 at 35,000 feet. Around 1960, a pure turbojet at cruise would have an SFC of about 0.9. That means that to produce 10,000 pounds of thrust for an hour, you'll consume 9,000 pounds of fuel. By 1980, a low bypass turbofan might have an SFC value of about 0.72, so you'd need 7,200 pounds of fuel to produce 10,000 pounds of thrust. By comparison, a GE90 hi-bypass turbofan has an SFC of about 0.55 so it would only consume 5,500 pounds of fuel under those conditions. That's only 61% of the fuel that the 1960 turbojet required.

Diesel turbines have been attempted but were abandoned when they caused permanent damage to highway overpasses.

On another front -- think about how a diesel-electric locomotive works. They (along with diesel-electric submarines, which in the past often used the same engines) are truly the first hybrid engines. In this case, they are serial hybrids, which are arguably superior to the crappy road-going hybrids we see today.

I may be wrong, but I don't think many diesel-electric locomotives use any form of regenerative braking or battery power storage. I've read that many of them do use dynamic braking but the electricity produced is ran through resistors to produce heat. I'm sure someone is working to capture that energy but you'd really need some very powerful batteries (or ultracapacitors) to handle so much current.

If that's the case, then they really aren't hybrids, IMO. A hybrid gets part of the drive energy from stored electrical power. For a diesel electric locomotive, the diesel engine turns a generator that powers electric motors to drive the wheels. This has proven a more efficient arrangement than using a transmission like on a diesel truck. Back in the 1940s, there were a few pure diesel locomotive made but their narrow torque band meant the transmission had to shift gears frequently. They were quickly pushed aside for diesel-electric engines that were more efficient and needed less maintenance.

For those interested in different arrangements for hybrid vehicle designs, there's a pretty good article here. I've often wondered why serial-hybrid cars aren't available. Perhaps there are cost constraints or other issues such