The Top Secret Boeing 797

[101st Airborne]

New GE Turbine Engine for Airbus

Meanwhile the engine pods are getting bigger and bigger.

I think that the first thing I though of when I saw these photos was just how much must be the safety margin in the construction of the 747's wing.

Attached are four photos of a GE Test Engine up in Mojave, CA.
They use a 747 aircraft as the test bed and remove one of their normal engines and put the test one on instead.

This engine is slated for the A380 Airbus Behemoth Double Decker that is being built.
You will get some idea of the size difference between it and the normal 747 engines.

The normal 747 engine is somewhere in the 60K lbs/thrust where this one is about 125K lbs/thrust!

[glyn]

[QUOTE=101st Airborne;357160]Yes, but they moved to pods for safety reasons.

The point about hanging engines in pods beneath the wing was to reduce the wing bending moment due to lift generated forces, especially on a slim swept wing like the B-47. Aerodynamically it is better to cluster the engines close to the fuselage which is a form of blended wing (using the Comet and CF-100 as examples). In the event of an engine out condition the thrust line of the remaining engines is still close to the centreline. With pods arrayed the way they are on modern 4 engined airliners the assymetry problems are compounded. However, it makes sense to have the engines where they can be regularly checked, and more readily pulled (although these days the reliability is such that this is seldom necessary.)

[pdf27]

Oh come on, air has to enter and leave sometime. putting the engines above on a flying wing deflects a lot of the noise upwards instead of down. I would have thought you got that from the link you posted earlier.

To an extent. The problem is that a lot of it is generated by airflow that has already left the engine and is behind the aircraft. Putting the engines above the wings provides little or no shielding against this noise. Incidentally, one of my accoustics/flow induced sound & vibration lecturers at university (Prof Ann Dowling) is somewhere close to running the SAI - not sure exactly as they were setting it up while I was there.

I don't have time to bat your whole post around, but this is BS. The GE90's composite fans are the lightest and most fuel efficient in the world. Yes, they are quiet too, but to say it is all about noise is just plain not true. The fuel efficiency of large fan engines doesn't come from thrust generated by the diameter (propeller effect), it comes from the hot gases working against a dense, cool layer of air. The bigger that air column's diameter, the longer those hot gases are contained. That is where the fuel efficiency comes from.

Err... what? That makes no sense at all, so you're going to have to go into far more detail. I think you're confusing sources of accoustic noise and propulsive efficiency here.
The propulsive efficiency effect of larger fans comes from the fact that thrust is related to the net momentum flux of air leaving the engine, while burning fuel generates power. The table below (generated by me on the back of an envelope for a nominal perfectly efficient fan being powered by a 30MW electric motor at sea level) illustrates the point.

Mass flow   Jet Speed @ 250m/s  Static Thrust  Thrust @ 250 m/s (M 0.83)
 100 kg/s           775 m/s       77.5kN         37.6kN
 250 kg/s           550 m/s      122.5kN         28.1kN
 500 kg/s           427 m/s      173.2kN         -5.2kN
1000 kg/s           304 m/s      346.4kN        -87.5kN

NOTE - just spotted an error with my ram drag numbers - I used the Mach number from 250 m/s @10,000m while the rest of the values were for sea level. This means that the results for thrust @ 250 m/s are unduly pessimistic, and will not fall off quite as fast as I have shown them doing. The general trend is correct however

There's more to quiet than burying the engines in the fuse- reducing landing gear drag, having 2 engines rather than 4, etc.

Very much so. This is another of the major advantages of a BWB - the large body area allows you to do away with high lift devices while maintaining efficient cruise L/Ds.

The sonic cruiser had the same fuel efficiency as the current crop of M 0.85 class airliners. The airlines asked Boeing for more, so the higher speed is what was sacrificed.

So Boeing claimed. I've not seen their internal numbers, but from my own experience with marketing departments and knowing how early a stage in development the project was binned at I remain deeply sceptical. There are very good reasons people avoid flying at exactly the speeds the Sonic Cruiser was designed to run at.

[pdf27]

The point about hanging engines in pods beneath the wing was to reduce the wing bending moment due to lift generated forces, especially on a slim swept wing like the B-47. Aerodynamically it is better to cluster the engines close to the fuselage which is a form of blended wing (using the Comet and CF-100 as examples).

Ummm... sort of. It does help reduce the bending moment at the root, but you also set up a point load at mid-span which has some problems all of it's own. If you do blent them in you end up needing a very long wing chord at the root, ending up with something that looks like the HP Victor. This kicks up your wetted area, so it isn't exactly a free lunch.

[glyn]

Ummm... sort of. It does help reduce the bending moment at the root, but you also set up a point load at mid-span which has some problems all of it's own. If you do blent them in you end up needing a very long wing chord at the root, ending up with something that looks like the HP Victor. This kicks up your wetted area, so it isn't exactly a free lunch.

Aviation is little different from the rest of the real world. Very few free lunches to be had.

[highsea]

...I think you're confusing sources of accoustic noise and propulsive efficiency here.

I didn't say that very well, lol. It was 3 a.m. Look, it's pretty simple- by decreasing the % of mass as core flow, you increase the bypass ratio. The percentage of duct flow to total flow is higher. e.g. you have more slow moving, cool air as a percentage of total flow.

...The propulsive efficiency effect of larger fans comes from the fact that thrust is related to the net momentum flux of air leaving the engine, while burning fuel generates power.

It's a function of the total air mass, not it's momentum. More air mass moving slower is more efficient than less air moving faster. This is why the fans are getting bigger. The GE-90-115B's fan is even larger than the original series, and the engine is rated at 115,300 lbs. If the large fan was a penalty, why would GE increase the diameter? It's because they need more mass flow to get the additional thrust, and a bigger fan does that more efficiently than running the same fan at a higher RPM.

Right now, the limiting factor is the ability to build larger fans, and the temperatures and pressure ratios needed in the core to power them. The ability to build supercooled turbine blades is what allowed these high temperatures and pressure ratios- The GE 90 has a 10 stage high pressure compressor plus an intermediate compressor to give it the power needed to drive the fan. The pressure ratio is the highest in the industry, which allows for a smaller core and a higher bypass ratio.

Even if a high bypass turbofan was no quieter than a turbojet, we would still be using them, because they are much more fuel efficient.

Back to work now, lol.

[pdf27]

It's a function of the total air mass, not it's momentum. More air mass moving slower is more efficient than less air moving faster. This is why the fans are getting bigger.

Impulse = force x time = change in momentum. Hence,
engine thrust = (mass flow) x (change in flow velocity across the engine). As power = 1/2 x (mass flow) x ((exit flow velocity)^2-(inlet flow velocity)^2) then it is fairly obvious that slower exit flow velocities give higher thrust and hence higher propulsive efficiency.
The bit you're missing, however, is Ram Drag. This is caused by the fact that an engine is effectively a great big blunt circle being pushed through the air. The bigger the frontal area, the higher the Ram Drag. For a given design airspeed there is an optimal bypass ratio - which is why turboprops are used at low speed with their huge bypass ratio, and turbojets with no bypass at all are used at high speed.

The GE-90-115B's fan is even larger than the original series, and the engine is rated at 115,300 lbs. If the large fan was a penalty, why would GE increase the diameter? It's because they need more mass flow to get the additional thrust, and a bigger fan does that more efficiently than running the same fan at a higher RPM.

Noise, pure and simple. An aircraft engine is in it's most heavily loaded condition at top of climb where it will probably be doing Mach 0.5+. By the time it gets to this level, ram drag is very significant and so increasing the bypass ratio will actually give you reduced levels of thrust. Larger fans (beyond a certain point we reached a few years ago) are purely and simply about reducing the velocity of the jetwash and hence reducing noise generation. The GE-90 is an example of doing this.

Right now, the limiting factor is the ability to build larger fans, and the temperatures and pressure ratios needed in the core to power them. The ability to build supercooled turbine blades is what allowed these high temperatures and pressure ratios- The GE 90 has a 10 stage high pressure compressor plus an intermediate compressor to give it the power needed to drive the fan. The pressure ratio is the highest in the industry, which allows for a smaller core and a higher bypass ratio.

Pressure ratio is good from a Carnot efficiency point of view, but it actually reduces the specific power of the engine rather than increasing it. Smaller core is also good, but you run into problems with the stability limitations of a two-shaft design such as used by GE and Pratt. Rolls run off a 3 shaft design which IIRC is somewhat heavier but allows a slightly slimmer core and give you a lot more design freedom to avoid unstable operating points.

Even if a high bypass turbofan was no quieter than a turbojet, we would still be using them, because they are much more fuel efficient.

Depends on the operating regime. Concorde used a pure turbojet, despite being built long after the invention of the turbofan. Turboprops have inherently higher bypass ratios and propulsive efficiency than Turbofans, yet are not in common use, largely because they are less fuel efficient than Turbofans.

[101st Airborne]

Hahahhahhahhahahha!

[highsea]

...Larger fans (beyond a certain point we reached a few years ago) are purely and simply about reducing the velocity of the jetwash and hence reducing noise generation. The GE-90 is an example of doing this.

Look, that is just not correct, but I am tired of arguing in circles with you.

When Rolls Royce designed their version of the Trent series for the 777, they knew they would have to increase the fan diameter of the 700 from 97" to 110" to meet the 80,000 lbs. thrust requirement. That's why the 800 series was created. The same situation applied with the A330- the 600 was underpowered, and they needed a bigger engine, so they created the 700 series.

In every modern commercial jetliner, when an engine manufacturer has needed more thrust, they go to larger fans. GE, Pratt, RR, it doesn't matter- they all do the same thing.

[dundonrl]

Meanwhile the engine pods are getting bigger and bigger.

I think that the first thing I though of when I saw these photos was just how much must be the safety margin in the construction of the 747's wing.

Attached are four photos of a GE Test Engine up in Mojave, CA.
They use a 747 aircraft as the test bed and remove one of their normal engines and put the test one on instead.

This engine is slated for the A380 Airbus Behemoth Double Decker that is being built.
You will get some idea of the size difference between it and the normal 747 engines.

The normal 747 engine is somewhere in the 60K lbs/thrust where this one is about 125K lbs/thrust!

those (atleast the bottom one) is a GE90 115B.. (you can see it writted on the engine) that engine is what's used on the 777, and the 115B is the one used on the ultra long range 777-200ER, and makes 115 thousand lbs of thrust..

GE - Aviation: GE90

[Predator25]

Unheard of. OK whatever you say.
I had a big group of aviation engine fire pics I have been look through my files for. I'm sorry but I seem to have deleted them.

Are u crazy? Engine fires virtually never happen because there's no reason for them to happen. This is mostly because jet fuel is harder to ignite than you think. The last time an engine fire happened was on British Airways Flight 009 where 40 feet flames blasted out of the engines. This happened because the engines backfired due to volcanic ash. As i said this was the most recent time it happened, and this happened in 1982.

[Predator25]

If thick wings were aerodynamically favourable (as with BWBs) then everyone would bury their engines in the wing, simply to reduce wetted area and (to an extent at least) ram drag.

There's no reason to bury engines in the wing. Besides, the newer planes made by Boeing have found a better use for the wings. They're now being used as fuel tanks. The 747-800 is using it, the 737-800, and the 787-9 are too. That's one the things that Boeing did on the 787-9 to achieve longer range.

[omon]

i,d hate to be drinking beer from a cup, when sitting in that flying wing away from the center, when it banks. or worst hot coffe.

[pdf27]

There's no reason to bury engines in the wing. Besides, the newer planes made by Boeing have found a better use for the wings. They're now being used as fuel tanks. The 747-800 is using it, the 737-800, and the 787-9 are too. That's one the things that Boeing did on the 787-9 to achieve longer range.

1) There are plenty of reasons to bury engines inside a wing, but they all depend on having a wing with an extremely long chord which can therefore be made very thick. This only works with very specific wing designs and in certain operating regimes. The HP Victor is a good example of this.

2) "Better use for the wings" is utter tripe. Aircraft have been burying fuel tanks in the wings since at least the Ford Trimotor (at least one Lufthansa airliner from the 1930s had a passenger cabin in the wings). Think about exactly what the internal volume of the wing is if it's a thin wing with podded engines, and then if it's thick enough to bury engines in it.

[Kernow]

New Boeing 797 Giant "Blended Wing" Passenger Airliner-Fiction!

Summary of the eRumor:
The story says that the Boeing Company is developing a radical new passenger jetliner that will carry 1,000 passengers. It's designated the 797 and is a "blended wing" design looking a lot like the old flying wing experiments of the 50's.

The Truth:
TruthOrFiction.com went straight to the source, the Boeing Company. A spokesperson said that it is not true that Boeing is developing a commercial blended wing aircraft. He asked that we help stop the perpetuation of the story.

What is true, according to Boeing, is that Boeing Phantom Works, the company's advanced research and development organization, is doing research on the blended wing body design as a potential military aircraft. Boeing has built a scale model to test its low-speed flying characteristics in a wind tunnel. There are also plans (as of 7/13/07) to flight test a scale model.