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Some questions about the 777X

Posted: Sat Jan 14, 2017 2:56 pm
by VC10er
I was looking again at the Boeing website and at the 777X. I noticed this time that the engines don't have the "saw tooth" (my lay term) trailing edge on the engine nacelles, as on the 787, 747-8 and upcoming 737MAX. I was never 100% sure of what they do, (noise reduction I think.. so can someone a) explain what they do, and b) why are they not on the 777X? c) why is it just on Boeing aircraft: are the ending manufacturers restricted from adding them to engines they sell to Airbus? I personally LOVE they way they look, they are part of what screams new-tech to me, along with the new feather-like wings and in the case of the 787, the cockpit windows and nose.

Last, how big will the windows be, Boeing says they will be larger and higher than the current 777, will they be as big as the 787?

Thanks zillions for your expert responses. R

Re: Some questions about the 777X

Posted: Sat Jan 14, 2017 3:16 pm
by Stitch
VC10er wrote:
I was looking again at the Boeing website and at the 777X. I noticed this time that the engines don't have the "saw tooth" (my lay term) trailing edge on the engine nacelles, as on the 787, 747-8 and upcoming 737MAX. I was never 100% sure of what they do, (noise reduction I think.. so can someone a) explain what they do, and b) why are they not on the 777X? c) why is it just on Boeing aircraft: are the ending manufacturers restricted from adding them to engines they sell to Airbus? I personally LOVE they way they look, they are part of what screams new-tech to me, along with the new feather-like wings and in the case of the 787, the cockpit windows and nose.


They do indeed reduce engine noise, however Boeing has subsequently developed a new means of reducing said noise which allows them to dispense with the chevrons. This new means involves a new nozzle design that disrupts shock cells by mechanically mixing fan stream in the bypass duct with the ambient flow. The chevrons do much the same thing, but via forced mixing which adds drag. The new design has less drag and weighs less.

As to why Airbus aircraft do not have them, the design is a co-venture between NASA, Boeing and General Electric so I am guessing patent reasons restrict it to the Trent 1000 on the 787 and does not allow RR to use them on the Trent XWB or Trent 7000 on the A350 and A330neo.


VC10er wrote:
Last, how big will the windows be, Boeing says they will be larger and higher than the current 777, will they be as big as the 787?


I do not believe an actual dimension has been set (Boeing claim they will be 15% larger than "the competition", which I assume is the A350), but I would expect it to be similar in size to the 787's.

Re: Some questions about the 777X

Posted: Sat Jan 14, 2017 3:34 pm
by WIederling
Stitch wrote:
As to why Airbus aircraft do not have them, the design is a co-venture between NASA, Boeing and General Electric so I am guessing patent reasons restrict it to the Trent 1000 on the 787 and does not allow RR to use them on the Trent XWB or Trent 7000 on the A350 and A330neo.

Chevrons reduce (excess) noise. ... at cost ( more fuel )
If you don't have the noise issue you do not need them.

patents? chevrons ( aka mixing devices ) are rather old.

for the entertainment value:
https://nintil.com/2015/12/31/you-didnt ... s-edition/

Re: Some questions about the 777X

Posted: Sat Jan 14, 2017 11:10 pm
by AA737-823
The A321 with a CFM56 will sometimes feature them, albeit on the core exhaust nozzle, rather than the bypass.
Apparently the noise reduction is a meaningful amount, but so is the increase in fuel consumption.

Re: Some questions about the 777X

Posted: Sun Jan 15, 2017 4:17 pm
by benbeny
AA737-823 wrote:
The A321 with a CFM56 will sometimes feature them, albeit on the core exhaust nozzle, rather than the bypass.
Apparently the noise reduction is a meaningful amount, but so is the increase in fuel consumption.

I don't get it. How can it increase fuel consumption?

Re: Some questions about the 777X

Posted: Sun Jan 15, 2017 7:05 pm
by VC10er
Thank you to the Mr Spock's of aviation. However, I feel like I am once again 8 years old watching STAR TREK!

What is/are:

"shock cells?"

what part of an engine is the "bypass ducts" - I do understand both "bypass" and "ducts". I never truly understood where the main thrust comes out from, I assumed the center bare metal cone and the outer ring being perhaps bypass? Also why some engines have a solid cone in the center, some don't, some have a tube extending out. (?) Basically if I look at the rear of an engine and see 2/3 concentric circles, does the outer ring (which I can see the through to the blades in the front) provide thrust too, or just the centers?

Thank you: I seem to get that the chevrons "mix" now, and therefore reduce noise, but add weight. Given the newer tech with the "nozzle" will the chevrons go away on future 787's etc?

It is true that Boeing only seems to say "larger windows" for the 777-X, but no other details (e.g; same size as a 787, or 15% larger than current"...have they announced if they will have shades or the 787's dimmer?

GOSH, CANT THANK YOU ENOUGH!

Re: Some questions about the 777X

Posted: Sun Jan 15, 2017 7:19 pm
by VC10er
"FASCINATING" article. I guess nothing is ever entirely new!

In that article is a beautiful view of a 787 engine. So, every time I taxi away and I can see an engine from this kind of rear view, I am always wondering: How much thrust comes out from the nacelle cover with the chevrons and the gold colored metal, vs from around the blue cone, and from the hole in the cone (sometimes there is no hole)?

If I am not mistaken: the cone keeps the thrusting air to a tighter, stronger stream in high altitudes vs being dispersed inflow psi air pressure, am I somewhat correct?

Thanks again

Re: Some questions about the 777X

Posted: Sun Jan 15, 2017 10:28 pm
by WIederling
VC10er wrote:
"FASCINATING" article. I guess nothing is ever entirely new!

In that article is a beautiful view of a 787 engine.

looks rather good, "techy". But as you can see looking good can be misleading.

If I am not mistaken: the cone keeps the thrusting air to a tighter, stronger stream in high altitudes vs being dispersed inflow psi air pressure, am I somewhat correct?

IMU air from some faster stream mixing with a slower stream ( either core with bypass or bypass with free air flow ) you distribute and spread the mixing area ( essentially you elongate the border along which the mixing happens). you loose a bit of thrust from mixing.
fuel consumption stays constant :: efficiency goes down.

Re: Some questions about the 777X

Posted: Mon Jan 16, 2017 1:25 am
by VC10er
Another quick question and again thanks!

I have read many times the the 757 is "over powered", which is why it's unique for hot and high missions. I assume that means the 2 engines provide more thrust (perhaps wings too?)
Why do those engines have a simple round aperture with no cone or anything else? I realize it's no the only aircraft with engines with this design.

Re: Some questions about the 777X

Posted: Mon Jan 16, 2017 2:32 am
by Starlionblue
VC10er wrote:
what part of an engine is the "bypass ducts" - I do understand both "bypass" and "ducts". I never truly understood where the main thrust comes out from, I assumed the center bare metal cone and the outer ring being perhaps bypass?


The airflow around the core is bypass, since it bypasses the core. This air is driven by the fan. Blue air in the diagram.

Image

Thrust doesn't "come out". That is, thrust doesn't come from the exhaust pushing on the surrounding air. Thrust is generated by pressure differences. The pressure is lower in a rearward direction as you throw air out of the exhaust, while it is higher in a forward direction because more air is being compressed along..

In a modern turbofan, 80-90% of thrust comes from the pressure difference created by the fan, with the rest from the core.



VC10er wrote:
Another quick question and again thanks!

Why do those engines have a simple round aperture with no cone or anything else? I realize it's no the only aircraft with engines with this design.


The cone and core cowling are still there. They're just hidden inside the outer cowling on Rolls-Royce RB211 and Trent 700 engines. See here the rear end of a Trent 700.

Image

Extending the outer cowling past the core and tailcone aids in controlling bypass airflow. The downside is more weight.

Compare with a Pratt powered 757s, which has the more traditional "short" outer cowling and visible core cowling.
Image

VC10er wrote:
Another quick question and again thanks!

I have read many times the the 757 is "over powered", which is why it's unique for hot and high missions. I assume that means the 2 engines provide more thrust (perhaps wings too?).


"Overpowered" is a subjective definition. It has enough power to perform the required missions safely. It certainly has a solid thrust to weight ratio for an airliner, but contrary to legend, it is within 10% of the 767 and 321.