How many engines on a plane?.. 2 or 3?
Well, why?
Would it not be more efficient to have say 10 - 15 Engines all 1/8th of the size and power output, and have them scattered across the wings and where ever else, this should eliminate
the problem of engine failure causing the crash (unless they're out of fuel) completely.
there would be 10 -15 times as many fuel pumps, stating systems, fire extinguishers etc.
I must admit I felt a bit nervous when they swapped from 4 engine 747s to 2 engine 777s, but it can still fly on the last engine running 
The VC10 from the 60s had 4 smaller engines, but they were so loud as evidently the tips of the rotor blades went through the sound barrier and made a distinctive crackle.
Boffin1:
there would be 10 -15 times as many fuel pumps, stating systems, fire extinguishers etc.I must admit I felt a bit nervous when they swapped from 4 engine 747s to 2 engine 777s, but it can still fly on the last engine running
The VC10 from the 60s had 4 smaller engines, but they were so loud as evidently the tips of the rotor blades went through the sound barrier and made a distinctive crackle.
Though I happened to watch a video on the airbus that was landed on the Hudson river a few years ago, and in that plane, they had two engines that were both disabled with a single bird strike.
10 - 15 engines would give you security in that it's almost 0 chance every single one of them will stop working, if they could get say just 4 of the 15 engines to simply fly at min speed until they could land, sounds a lot safer...
the fuel nozels would be a lot smaller and easier to pump around to each engine, easier to lift one out and to inspect, if an engine explodes at 30,000 feet, which you prefer? 1 small engine going or one mother of one potentially sending out sharded metal blades at you, having smaller engines would also produce less noise in mid flight, the engines could simply be switched off to conserve fuel or have them all come on even have a couple of engines dedicated for power..
I think we use big engines to do the job because we always have.....
Boffin1:
I must admit I felt a bit nervous when they swapped from 4 engine 747s to 2 engine 777s, but it can still fly on the last engine running
I believe there was a rule at one point that transoceanic flights had to use aircraft with four (or maybe three) engines. The change to allowing two I think predates the 777, maybe around the time of the 767 or maybe a bit earlier. This was predicated on improved engine reliability and AFAIK has worked out well.
There was a story about the tower asking a fighter to go around again to allow a B-52 to land first because it had an engine out. The fighter pilot replied, "Ah yes, the dreaded seven-engine landing."
"Ah yes, the dreaded seven-engine landing." LOL !
As per the amazing landing on the Hudson river, we actually have an ad on TV that shows that, and interviews the pilot.
He said they ran into a whole flock of birds, so I guess you could have got a bird in any number of engines ?
The very first jet liner, the Comet, had 4 powerful engines mounted in the " armpits " of the wings.
The RAF took over some of the surviving ones to use as air sea rescue, it could fly out to the search area quickly, and then shut down two engines while searching
Back in the 60s one of the apprentices that I worked with at BA ( it was BOAC then ) said that he volunteered as one of the " passengers" on a practice flight that flew up and down parts of England for a couple of hours in 707.
As they were flying around, they were asked over the radio to look out for a missing Cessna, and given the registration number on the wings.
They spotted a small plane on a small grass airfield and overflew it, and all peeled their eyes to see the number, but they were going too fast when it briefly came into view .
So the Pilot went around, and to reduce speed, lowered the landing gear and the flaps to fly over slowly. Evidently the landing headlights automatically come on then.
Everyone peered out of the window, and as they approached the hut, a guy came running out waving his arms in the air and and shaking his head as if to say " you can't land that thing here "
They trusted the comet?... I'm not sure i'd step inside one, it was a tin can waiting to explode....
Thats when it had square windows, a very expensive lesson that was, two lost off the same coast !
Well for one thing with 15 engines the chances of failure go up, surely? More engines to fail.
The more salient point is that a whole lot more things go wrong in aircraft than engine failures. The recent crash at San Francisco, for example, was nothing to do with engine failure. As are aircraft colliding, running out of fuel, landing in fog, bad decisions by the captain, flying into mountains, wind shear, lighting strikes, flying into the ground, etc.
Even a bird strike could easily go into enough engine inlets to make the plane unflyable. After all they (the engines) would need to be near to each other.
More engines would require more fuel, partly because of the extra weight. More fuel is more things to go wrong with fuel, plus more danger on a crash landing.
I think two engines is the practical lower limit because then you can hang one off each wing, away from the rest of the aircraft and the passengers, and have the fuel adjacent to it.
10 - 15 engines would give you security in that it's almost 0 chance every single one of them will stop working,
A manufacturing defect might conceivably affect all of them. As would lack of fuel or contaminated fuel. Plus all the other things I said above.
A good read is the book about the "Gimli Glider", a Boeing 767-233 jet which did run out of fuel mid-flight due to confusion about metric/imperial units.
Despite being unpowered the pilot glided it to land at a local airstrip without loss of life, or major damage to the aircraft. Of course it would be different over the sea.
Very interesting. Surely this was a major factor in the positive outcome:
Captain Pearson was an experienced glider pilot, which gave him familiarity with flying techniques almost never used by commercial pilots.
A point of concern as aircraft get more and more automated, is that while the pilots may be able to manage the automation just fine, the question remains whether, if they have to, they can actually fly. This guy obviously could!
Frozen fuel... http://www.singleprofessional40.com/othertopics/b52crash.html My grandfather helped with the investigation. According to my grandfather, the fault brought down several B-52s including one plane carrying a nuclear weapon that was never recovered (Atlantic Ocean). The problem was essentially impossible to find because everything worked perfectly except in extremely rare conditions. When the problem occurred, the plane was lost, completely destroyed, or the resulting fire would warm the parts and melt the ice. The cold South Dakota winter kept everything frozen for the investigation team.
the fault brought down several B-52s including one plane carrying a nuclear weapon that was never recovered
That's a bit frighting , not because it could detonate, but because someone ? could get hold of it, plus the pollution if the casing corrodes ?
I have no knowledge here.
As for brilliant landings, Captain McCormick managed to land a DC10 in 1972 with no operation of the rear control surfaces, after a cargo door blew out and wrecked the control lines which all ran under the floor ( which had been blown out )
He just used the throttles of the engines to control the aircraft.
I understand he never flew a plane again !
As an employee of the army...
Um sorry sir, i lost a nuke....
Boffin1:
That's a bit frighting , not because it could detonate, but because someone ? could get hold of it, plus the pollution if the casing corrodes ?
A single nuke at the bottom of the Atlantic doesn't bother me. Refining Uranium ore has to be significantly cheaper than trying to get that sunken treasure to the surface. (I grew up five miles from three missile silos and 40 miles from the #3 USSR strategic target so I may be a bit desensitized.)
However, this sort of thing... 2007 United States Air Force nuclear weapons incident - Wikipedia ...does bother me.
Boffin1:
He just used the throttles of the engines to control the aircraft.
That has to be unbelievably tedious.
Yes, just imagine if they were doing a training run in a valley near your town. Hopefully the missiles were not actually armed, as in, in a state that they would detonate when released.
That has to be unbelievably tedious.
"The crew was able to accomplish an emergency landing by using the ailerons, right elevator, some limited rudder trim and asymmetrical thrust of the wing engines." Wikepedia
I fly a SEP (single engine piston) light aircraft, as do many others on the Flyer forum
many discussions there about 2 engines vs. 1
it's a bit religious (Catholic/Protestant sort of discussion)
twin pilots say they carry a spare
SEP pilots say twice as much to go wrong!
YMMV
Isn't air travel already the safest mode of transportation? Personally, I might trade a bit of safety for a little more comfort and convenience, both in the aircraft and airports, better connections, less crowded planes, etc. etc. etc. 
I have mentioned it elsewhere but I currently am an intern at GE Aviation. I am not in the Jet Engine division, but I do get to listen in on their Lunch'nLearns. From these I have learned that as a general rule a bigger engine is always more efficient. They have even said facetiously they would love to be able to mount one huge engine to the bottom of the plane if only it would work into the designs. I probably couldn't explain every detail of why it is more efficient if they presented them for practical and legal reasons, but I think it can be explained by geometry.
In aviation everything is a trade off between thrust and weight. A larger engine adds burn space inside faster than it adds supporting material on the outside. It is a surface area vs. volume idea. Then there is a number of auxiliary systems that have to be in place for each engine.
While having redundant engines is nice, similar gains in safety can be had be making the existing engines more reliable. One of the products they make where I work links into sensors on the engines and monitors the data in real time sending data to teams on the ground. Airlines have already used it to preemptively service engine issues before they would necessitate grounding the plane and diagnose problems in flight. This seems to be the way the industry is headed. More engines just isn't practical on a number of levels. Here is an article* on the system I mentioned. http://www.gereports.com/doctor-on-board/