qwertytam
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Why lower thrust for 777-9 vs. 777-300ER?

Thu Jan 04, 2018 8:01 pm

Apologies if this has been discussed before, but I'm curious why the 777-300ER comes with a 115klb thrust GE-90, where as the GE9x for the 777-9 has 10klb lower thrust rating, yet the planes have the same MTOW, and the larger diameter of the GE9x would cause more drag in an engine out scenario. How have Boeing/GE compensated for the lower thrust?
 
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zeke
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Thu Jan 04, 2018 8:25 pm

The “new” wing has less drag at low speed
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gloom
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Thu Jan 04, 2018 8:44 pm

Plus probably way more lift, even at low AoA, so rotation speeds should be finally normal, nowhere near tire limit speeds.

Cheers,
Adam
 
heavymetal
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Fri Jan 05, 2018 1:20 am

zeke wrote:
The “new” wing has less drag at low speed


gloom wrote:
Plus probably way more lift, even at low AoA, so rotation speeds should be finally normal, nowhere near tire limit speeds.

Cheers,
Adam


Both of these are the correct reasons for the lower thrust engines.

As Zeke said, the 10% increase in wingspan helps to reduce induced drag, which is the predominant form of drag at low speeds (takeoff and landing). A 10% increase in wingspan results in a 20% reduction in induced drag, since induced drag is inversely proportional to the span squared. Since thrust opposes drag, the decreased drag means you need less thrust.

Also, as Adam noted, the wing area of the 777X grew by ~7% compared to the 777-300ER. This lowered the aircraft's wingloading at MTOW, which means that the wing is helping to carry some of lift generation burden (remember, lift is dependent on both wing area and velocity squared). By having a larger wing, the 777X does not need to accelerate to the same speed as the 777-300ER to generate the same amount of lift, and thus the engines do not need to be as powerful. It should be noted here: the larger wing area does increase skin friction drag, since it increases the aircraft's wetted area, so some of the overall drag reduction benefit discussed above is given back. But, also as noted above, skin friction drag is not the predominant form of drag at low speeds when the engines need to run at at max thrust. This increased wetted area will come into play later, in cruise, when skin friction drag accounts for 50%+ of the aircraft's total drag.
 
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Faro
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Fri Jan 05, 2018 9:28 am

Lower thrust yes...but the GE9X nonetheless ended up heavier than the more powerful -115B...

http://www.airliners.net/forum/viewtopic.php?t=1379051

The reason seems to be heavier fan blade-off containment requirements...the GE9X has only 16 blades compared to the -115B's 22...

All the same, it seems that the GE9X failed its blade-off test...see post # 11 in the above-referenced thread...


Faro
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benbeny
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Fri Jan 05, 2018 1:24 pm

So, is it correct that basically 77L and 77W have wings that are not optimized for their size?
 
gloom
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Fri Jan 05, 2018 2:16 pm

benbeny wrote:
So, is it correct that basically 77L and 77W have wings that are not optimized for their size?


Yes - and no.

Yes: Higher thrust and other frame reserves allowed to push the TO weight from original 545klbs (772) to 766klbs (77L), so that's around 40% more. 77W was not as much dramatic improvement, stil also 766klbs maxed out. So, I'd say the plane has been pushed a bit over the limit, both in terms of tire limits and wing limits (and following consequences: lower altitudes, worse high and hot, etc.). It is the highest-loading ever, both on tires and wings.

No: On the other hand, the plane is still perfectly safe to fly, and had no counterpart on A side, being the most economical airplane in the class. Some minor drawbacks, but no plane is perfect after all. Basically, a class of its own.

However, since new Airbus is coming, 777X will definitely need better wing - especially that much of the overweight of 777 will be carried over (Airbus estimates 779 to be 35T heavier than 351). This will help compensate for lower thrust on takeoffs, but main target is to reduce drag on cruise - as always.

Cheers,
Adam
 
heavymetal
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Fri Jan 05, 2018 3:13 pm

benbeny wrote:
So, is it correct that basically 77L and 77W have wings that are not optimized for their size?


I would agree with Adam above - the wings are pushed to their maximum potential from a takeoff performance and initial cruise altitude perspective. However, for carriers/airports that don’t need superior takeoff performance, the economics of the 77W are/were unmatched since it offers so much more payload capability on essentially the same structure (save for stretching the fuselage). The 77L took the same wing and used it for incremental range, rather than incremental payload, and you can see the effect - airlines overwhelmingly chose the payload (77W) over the range. Extra payload helps pay the bills - extra range does not.

The principles above are a general rule of thumb for smaller family variants vs larger family variants: many families share the same wing, so the smaller variants generally have superior takeoff performance and range capabilities, while the larger variants trade that for payload. This equates to higher CASM on the smaller variants, and better CASM on the larger variants.
 
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lightsaber
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sat Jan 06, 2018 12:43 am

heavymetal wrote:
zeke wrote:
The “new” wing has less drag at low speed


gloom wrote:
Plus probably way more lift, even at low AoA, so rotation speeds should be finally normal, nowhere near tire limit speeds.

Cheers,
Adam


Both of these are the correct reasons for the lower thrust engines.

As Zeke said, the 10% increase in wingspan helps to reduce induced drag, which is the predominant form of drag at low speeds (takeoff and landing). A 10% increase in wingspan results in a 20% reduction in induced drag, since induced drag is inversely proportional to the span squared. Since thrust opposes drag, the decreased drag means you need less thrust.

Also, as Adam noted, the wing area of the 777X grew by ~7% compared to the 777-300ER. This lowered the aircraft's wingloading at MTOW, which means that the wing is helping to carry some of lift generation burden (remember, lift is dependent on both wing area and velocity squared). By having a larger wing, the 777X does not need to accelerate to the same speed as the 777-300ER to generate the same amount of lift, and thus the engines do not need to be as powerful. It should be noted here: the larger wing area does increase skin friction drag, since it increases the aircraft's wetted area, so some of the overall drag reduction benefit discussed above is given back. But, also as noted above, skin friction drag is not the predominant form of drag at low speeds when the engines need to run at at max thrust. This increased wetted area will come into play later, in cruise, when skin friction drag accounts for 50%+ of the aircraft's total drag.


Well said. One nitpick:

But if the plan can cruise higher, the added skin area is much less than the reduced drag. If cruise can be elevated in altitude. With CFRP wings, the optimal wing area is greater due to less weight per square area of wing.

Lightsaber
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benbeny
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sat Jan 06, 2018 2:05 pm

In the hindsight, 77L and 77W is so immensely popular, it might justify the cost of re-winging it when they first developed it.

But why didn't Boeing re-wing 77L and 77W in the first place?
 
gloom
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sat Jan 06, 2018 2:53 pm

benbeny wrote:
But why didn't Boeing re-wing 77L and 77W in the first place?


Why would it?

It costs a lot, doesn't add much value to producer (of course one may argue if it increases profits for airlines more than you paid for rewinging, then you can earn your extra, but probably there are better earnings than rewinging your almost brand-new planes), and locks your resources which could be used anywhere else (on 7E7 then, I think it started somewhere at that point when 77W was derived from original 773).

Cheers,
Adam
 
heavymetal
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sat Jan 06, 2018 7:29 pm

benbeny wrote:
In the hindsight, 77L and 77W is so immensely popular, it might justify the cost of re-winging it when they first developed it.

But why didn't Boeing re-wing 77L and 77W in the first place?


For starters, the 77L is/was not popular. Only 59 copies were sold worldwide, and of those, I would bet that the vast majority of them do not use the full range capability of the airplane, effectively making it a much less efficient 777-200ER. As I said above, payload pays the bills - range does not (oversimplification, but good rule of thumb).

The 77W, on the other hand, is very popular, and I don't think it necessarily needs to be re-winged. Airline customers are quite happy with it's existing payload and range capability, and seem to be content with it's takeoff performance, so why bother changing anything? If airlines were asking for more payload or more range, both of which would result in a need for a higher MTOW, then yes, you would likely need to give it a new wing or else airline customers would need to be comfortable only operating it on 3+ mile long runways. However, no one is asking for that, so no need to re-wing it: it's one of the most efficient airplanes from a CASM perspective as-is.
 
heavymetal
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sat Jan 06, 2018 9:59 pm

lightsaber wrote:
heavymetal wrote:
zeke wrote:
The “new” wing has less drag at low speed


gloom wrote:
Plus probably way more lift, even at low AoA, so rotation speeds should be finally normal, nowhere near tire limit speeds.

Cheers,
Adam


Both of these are the correct reasons for the lower thrust engines.

As Zeke said, the 10% increase in wingspan helps to reduce induced drag, which is the predominant form of drag at low speeds (takeoff and landing). A 10% increase in wingspan results in a 20% reduction in induced drag, since induced drag is inversely proportional to the span squared. Since thrust opposes drag, the decreased drag means you need less thrust.

Also, as Adam noted, the wing area of the 777X grew by ~7% compared to the 777-300ER. This lowered the aircraft's wingloading at MTOW, which means that the wing is helping to carry some of lift generation burden (remember, lift is dependent on both wing area and velocity squared). By having a larger wing, the 777X does not need to accelerate to the same speed as the 777-300ER to generate the same amount of lift, and thus the engines do not need to be as powerful. It should be noted here: the larger wing area does increase skin friction drag, since it increases the aircraft's wetted area, so some of the overall drag reduction benefit discussed above is given back. But, also as noted above, skin friction drag is not the predominant form of drag at low speeds when the engines need to run at at max thrust. This increased wetted area will come into play later, in cruise, when skin friction drag accounts for 50%+ of the aircraft's total drag.


Well said. One nitpick:

But if the plan can cruise higher, the added skin area is much less than the reduced drag. If cruise can be elevated in altitude. With CFRP wings, the optimal wing area is greater due to less weight per square area of wing.

Lightsaber


I agree on the first point - the larger wing area allows you to cruise as higher altitudes, provided the engines aren't limited at these altitudes (shouldn't be).

The second point I need to think more on - in my mind, the optimal wing area is still determined by the design payload and range combination (MTOW, altogether). All else being equal, the CFRP wing will drive a lower OEW (as you note), so if you are solving for a fixed payload/range combination, you could get away with a smaller wing than if you had an aluminum wing. Provided they have the same engines, the lower OEW would mean you get get the same range at a lower MTOW and hence, could design the wing to be smaller.
 
LH707330
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 12:56 am

heavymetal wrote:
For starters, the 77L is/was not popular. Only 59 copies were sold worldwide, and of those, I would bet that the vast majority of them do not use the full range capability of the airplane, effectively making it a much less efficient 777-200ER. As I said above, payload pays the bills - range does not (oversimplification, but good rule of thumb).


I've read that the 200LR is actually more efficient than the 200ER after about 2000-3000 miles due to the raked tips and the newer 115B engines. The problem was that those engines were more expensive, and by the time the 200LR hit the market in 2006, most of that segment was already saturated with 200ERs.
 
benbeny
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 5:18 am

And if I'm correct, 77L is a platform for 77F, right?
 
OldAeroGuy
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 6:59 am

benbeny wrote:
And if I'm correct, 77L is a platform for 77F, right?


Correct, so it wouldn't be wrong to combine the two sales figures.

59 + 165 = 224 (a bit more respectable)
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
OldAeroGuy
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 7:01 am

gloom wrote:
benbeny wrote:
So, is it correct that basically 77L and 77W have wings that are not optimized for their size?


Yes - and no.

Yes: Higher thrust and other frame reserves allowed to push the TO weight from original 545klbs (772) to 766klbs (77L), so that's around 40% more. 77W was not as much dramatic improvement, stil also 766klbs maxed out. So, I'd say the plane has been pushed a bit over the limit, both in terms of tire limits and wing limits (and following consequences: lower altitudes, worse high and hot, etc.). It is the highest-loading ever, both on tires and wings.

No: On the other hand, the plane is still perfectly safe to fly, and had no counterpart on A side, being the most economical airplane in the class. Some minor drawbacks, but no plane is perfect after all. Basically, a class of its own.

However, since new Airbus is coming, 777X will definitely need better wing - especially that much of the overweight of 777 will be carried over (Airbus estimates 779 to be 35T heavier than 351). This will help compensate for lower thrust on takeoffs, but main target is to reduce drag on cruise - as always.


Cheers,
Adam


Actually, the 77W MTOW rose to 775k lbs, the same as the 777-9
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
OldAeroGuy
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 7:16 am

heavymetal wrote:
zeke wrote:
The “new” wing has less drag at low speed


gloom wrote:
Plus probably way more lift, even at low AoA, so rotation speeds should be finally normal, nowhere near tire limit speeds.

Cheers,
Adam


Both of these are the correct reasons for the lower thrust engines.

As Zeke said, the 10% increase in wingspan helps to reduce induced drag, which is the predominant form of drag at low speeds (takeoff and landing). A 10% increase in wingspan results in a 20% reduction in induced drag, since induced drag is inversely proportional to the span squared. Since thrust opposes drag, the decreased drag means you need less thrust.

Also, as Adam noted, the wing area of the 777X grew by ~7% compared to the 777-300ER. This lowered the aircraft's wingloading at MTOW, which means that the wing is helping to carry some of lift generation burden (remember, lift is dependent on both wing area and velocity squared). By having a larger wing, the 777X does not need to accelerate to the same speed as the 777-300ER to generate the same amount of lift, and thus the engines do not need to be as powerful. It should be noted here: the larger wing area does increase skin friction drag, since it increases the aircraft's wetted area, so some of the overall drag reduction benefit discussed above is given back. But, also as noted above, skin friction drag is not the predominant form of drag at low speeds when the engines need to run at at max thrust. This increased wetted area will come into play later, in cruise, when skin friction drag accounts for 50%+ of the aircraft's total drag.


The 777-9 won't pay the full skin friction drag penalty due of its larger wing relative to the 77W. Remember that the 777-9 lower takeoff speed means the dynamic pressure is lower, reducing the skin friction drag is terms of force units.

This is similar to the cruise altitude effect mentioned by lightsaber in his post.

777-9 L/D will be superior to the 77W in all phases of flight (takeoff, cruise, & landing).
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
Aircellist
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 10:13 am

Sorry if it has already been asked, but what would be the order of difference in speeds, for take-off and landing, between the two generations of 777?
"When I find out I was wrong, I change my mind. What do you do?" -attributed to John Maynard Keynes
 
OldAeroGuy
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 2:52 pm

Faro wrote:
Lower thrust yes...but the GE9X nonetheless ended up heavier than the more powerful -115B...

http://www.airliners.net/forum/viewtopic.php?t=1379051

The reason seems to be heavier fan blade-off containment requirements...the GE9X has only 16 blades compared to the -115B's 22...

All the same, it seems that the GE9X failed its blade-off test...see post # 11 in the above-referenced thread...


Faro


Post #11 in the referenced thread says the GE90 failed a blade out test, not the GE9X.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
OldAeroGuy
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Sun Jan 07, 2018 3:17 pm

Aircellist wrote:
Sorry if it has already been asked, but what would be the order of difference in speeds, for take-off and landing, between the two generations of 777?


Heavymetal indicated that the 777-9 wing area is 7% higher than the 777W. Since the MTOW's are the same, takeoff speeds would be about 3.5% lower for the 779 if takeoff rotation angles were equal. As the 779 has a slightly lower takeoff rotation angle, it's probably more like 3%.

Of course, it gets to be a bit more complicated than that since the 779 better L/D allows use of increased flap angle for a given takeoff condition, compensating for the reduced rotation angle.

MLW will be higher for the 779 due to its higher OEW and higher payload capability. This will offset the benefits of the 779's greater wing area so its approach speed will probably equal the 77W's.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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Matt6461
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Mon Jan 08, 2018 8:25 am

OldAeroGuy wrote:
Of course, it gets to be a bit more complicated than that since the 779 better L/D allows use of increased flap angle for a given takeoff condition, compensating for the reduced rotation angle.


Whenever and if ever you find the time, I'd love to hear more about this complication.

Specifically, I'm wondering about the interplay of the V1/Vmu and V2 constraints on design of the high-lift system. Are you saying that the 777X's higher L/D would enable design/use of high lift configurations that are unavailable at takeoff for 77W because its lower L/D would fail the V2 condition with those configurations?

Related question - if the 77W has lift-generation capability that it can't use at takeoff, then why is that capability there? For approach/landing? I have read that the max Cl configuration is more dictated by approach/landing than by takeoff. Is that right for some planes? Would it be especially true where V2 is a bigger concern (e.g. twins) than when it is not (e.g. quads)?

Edited to change V1 to V1/Vmu
 
OldAeroGuy
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Wed Jan 10, 2018 5:45 pm

Takeoff is a very complex maneuver. The following sources are helpful in understanding all the factors and their interaction.

https://www.faa.gov/other_visit/aviatio ... safety.pdf

https://www.theairlinepilots.com/foruma ... rmance.pdf

- Takeoff field length (TOFL) is limited by V2, which in turn is limited by stall speed (Vs) or minimum unstick speed (Vmu)

- Vs and Vmu is a function of flap angle and takeoff rotation angle

- Flap angle selection is limited by 2nd segment climb gradient

Figure 21 from the first reference is helpful in understanding the situation we're discussing. It shows that for a given altitude, temperature and field length. from Figure 21:

Flaps 15, field length limit: 389.0k lb.
Flaps 15, climb limit: 393.6k lb
Flaps 20, field length limit: 393.6k lb
Flaps 20, climb limit: 383.0k lb.

For this case, the TOFL limited weight would be 389.0k lb.

The Flaps 15 climb limit weight is unavailable because of the field length limit.
The Flaps 20 field length limit weight is unavailable because of the climb limit.

This answers your first question. The lower wing loading of the 777X lowers Vs and Vmu, reducing TOFL for a given flap angle and weight. At the same time, the 777X increased span lowers induced drag, enabling use of a greater flap angle and lower TOFL. These improvements allow the 777X to have a lower T/W than the 77W with acceptable TOFL.

For your second question, the 77W does have the landing flaps available to reduce TOFL due to Vs and Vmu improvement. Operationally, use of these flaps for TOFL improvement is of low utility because of:

- 2nd segment climb weight limits
- Vmcg speed limits
- increased takeoff noise
- runway lengths available at 77W airports
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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Matt6461
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Re: Why lower thrust for 777-9 vs. 777-300ER?

Thu Jan 11, 2018 11:55 pm

Thanks as always, OldAeroGuy. Great resources and it's very helpful how the reference you cite breaks down very simply when and where the takeoff constraint comes in from lift or climb gradient. Looking forward to trying to puzzle through these docs as time allows.

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