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Comment by anovikov | Hacker News Reader

But really, it was just about four-engine planes becoming too expensive to run. Two-engine planes won. 777 burns 30% less fuel per passenger and has almost the same cabin width. And top level became a flop because it's too narrow for a first class cabin by today's standards and all other uses for them make no sense. Top floor existed at all because it was Boeing's entry for a heavy cargo plane competition in which C-5 Galaxy won: it was meant to be a cargo plane with a small - top floor - passenger cabin.

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I'd guess they'll continue in cargo service for many more years, just as the DC10 and MD11 did (despite the grounding after the Louisville crash, I expect they will fly again before finally being retired).

Fedex continues to fly the MD11; UPS retired their fleet.

Yes. There are recently built 747-8's that will in service for a couple more decades.

> Top floor existed at all because it was Boeing's entry for a heavy cargo plane competition

Yes, but it turns out the hump is great for area ruling (aerodynamic drag reduction at transonic speeds), as observed by the 747-300's extended hump giving lower drag (but higher weight, of course) than the short-hump versions.

Slightly pedantic, but this is not completely correct.

It had a lower Cd with the extended hump, but it still had a higher drag, due to the larger wetted area.

Could you expand upon that for us interested laymen?

Ah. It was based on a series of performance graphs from Boeing, dating back to my aircraft-design days where that very question was discussed and I was baffled enough by the contradiction to ask for detailed clarifications to the presenting professor.

Unfortunately, I don't have access to those old notes, and couldn't quickly find what I was looking for online.

So for now, due to lack of proper supporting arguments, I would say: scratch that.

The Cd is normally normalized by the wing area, and the wing planform was the same between the 747-200 and 747-300. So if the Cd was lower for the -300, the total drag should be assumed to be smaller.

I remember hearing that the -300 cruised at a slightly higher Mach number, I assumed that was enabled by the center of pressure being moved back a few metres. But I have no idea what kind of drag the hump transition imparted on the upper fuselage. I kind of assumed that the low pressure zone above the rest of the aircraft would make up for most of the drag penalty of the transition (in general, not -300 specific). I'd love to hear your thoughts on the matter.

Assuming that the boundary layer is laminar for the entire length of the extended part of the fuselage during cruise - which is probably a good assumption, correct me if I'm wrong - then I wouldn't expect that the increased wetted area to add any significant drag. There is no additional frontal area, and no additional low pressure transition (just one moved back, obviously). But I'm not an aerospace engineer, just an interested laymen, so I'm just guessing. For all I know the straight staircase reduced drag compared to the classic spiral!

At transonic speeds, wave drag dominates both induced and parasitic drag. The hump reducing drag is about shape (area rule).

Thank you. I'll read about the different drag types.

> [..] cruised at a slightly higher Mach number, I assumed that was enabled by the center of pressure being moved back a few metres.

In high-subsonic cases, it normally points to slightly better aerodynamics in the areas flirting with transonic regimes. The name of the game at those speeds is to try to delay the onset of drag-inducing shocks. These shocks will typically first appear on the wing, or in an area heavily influenced by the wing.

In the case of the -300, with the extended bubble coming down at about half the wing root chord, it is possible that the bubble downflow / low-pressure area positively influenced the flow over the wing to slightly delay the transonic onset and further effects.

Pulling the bubble further aft made things worse again.

> Assuming that the boundary layer is laminar for the entire length of the extended part of the fuselage during cruise

Mm. No, I would expect the flow to be turbulent well before that.

> and no additional low pressure transition (just one moved back, obviously).

Careful where you put your low pressure zones, the wing aerodynamics are critical and sensitive! :-) (see explanation above)

But to make things even more interesting, the 747- 300/400/8 cargo all use the short bubble. :-)

Thank you.

  > But to make things even more interesting, the 747- 300/400/8 cargo all use the short bubble. :-)

I only recently noticed this on 3 minutes of aviation. Considering the 4 7 was originally designed for the passenger jets to be easily converted to cargo, I found it quite peculiar that the two -400 variants left the factory with different fuselages.

I think the top floor is there because the crew cabin has to be high so the nose can swing up. The cables and wiring from the cabin can't be easily disconnected to allow such access. You will notice other large cargo variants of airliners load cargo only through the side of the fuselage.

Yes and no. The C5 has an upper level too. The whole setup solves a lot of problems at once. Opening nose makes for faster cargo operations which the military cares about for a bunch of reasons. There are usually people associated with military cargo so might as well seat them up there.

Any large cargo aircraft has primary loading inline with centerline, side doors just aren't efficient. It's either via front, via rear or both.

Me321/323 was I think first heavy cargo with nose clamshell doors, but after that everyone settled on nose rising up, clamshell rear. It also had the top deck.

I understand that for the 747, they initially just had a cockpit bulge atop the fuselage. However, this created too much drag, which they reduced by extending the bulge aft. They didn't need this space for flight operations, so it was naturally then used for additional passenger space.

This is correct.

For context, when the 747 was being developed, simultaneously Boeing was developing the SST, Britain and France were developing Concorde, and the Soviets had their own supersonic Tupolevs in development. Boeing was anticipating that supersonic aircraft would render the subsonic 747 obsolete for passengers overnight, so it was designed to be easily convertible to freight.

And interestingly, the Tu-144 flew passengers before the Concorde did!

> it was just about four-engine planes becoming too expensive to run

Four engines are also less safe than twin engines. I know it sounds counter-intuitive, but Boeing did the math and made the case.

Isn't that because the twins are designed to a higher standard, due to regulations such as ETOPS? It's not something inherent in two vs four engine designs?

I remember when the US Navy was rejecting single engine aircraft, and the F135 had be be proven literally bulletproof before they would even look at the F-35.

Engine designs get better and better over time, and tend to become new minimums in safety standards.

My understanding is that the 747 has the highest safety record of any commercial plane. Is that not true?

Hard to beat other planes that have not had fatal crashes or hull losses, especially when you consider the Tenerife crash.

The Tenerife crash was due to pilot error, not any problem with the 747.

While true, aircraft fatality statistics generally are not split that way.

The 747 is a safe aircraft, but there have been a lot of fatalities associated with it, due to pilot error, terrorism, improper repair/maintenance, etc.

Can you link to the source?

My time working on Flight Controls at Boeing.

Not sure if you ever make it over to HKG but there are quite a few cargo 747s that still seem to be making the rounds. CX and others have them on the GA / cargo side, and when on Lantau you often see (and hear!) them on final or takeoff.

I think CX would hate to retire them because from what I understand their load capacity is unparalleled. And you can load horses in them! There'll always be money from that for HAECO to keep them aloft.

It's even better when you're at the bus terminal at HKIA and watching them fly overhead...

My dad took me to Frankfurt AM to watch the first 747 to land there. Wow!

Can you at least explain the reasoning then?

there's a book on this, "How Boeing Defied the Airbus Challenge: An Insider's Account"

I don't know if it lays out the safety math but it goes very deep on twin engine safety and some of the fights over the a340 vs. 777 regulations

Engines became reliable enough for regulators to allow two engine planes to cross large bodies of water. (ETOPS) That's what really killed 4 engine planes.

Twin engines are actually safer than four.

Is this because of something analogous to an expected value calculation? Like the increased probability of failure of a single engine when carrying 4, combined with the lower excess power per engine, means that the safety margin of a 4 engine plane is worse? Or is it something to do with the doubling of the number of critical parts that could fail? I'm struggling to come up with a workable argument why 2 is safer, it seems counterintuitive. I don't have enough of the puzzle.

EDIT: Never mind, I see you wrote more down below about this. It's increased risk of a catastrophic engine failure bringing down the entire plane, not an engine simply dying and forcing a landing. Four engines = twice as many chances for that to happen.

There's an enormous amount of energy in a jet engine with that spinning turbine. Then there's all the heat, fuel, and fire in it. When that energy lets go, you don't want to be anywhere near it.

It's amazing that jet engines work at all! Yet the safety record of the engines is incredibly good.

But twins typically have even larger turbines, no?

High bypass fans are newer engine designs, nothing to do with being used in twins.

Though in general, a new engine design necessitates a new wing design, which means a new airplane.

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