Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

747-8 engine choice

Tue Sep 21, 2021 7:56 pm

The 747-8 and the 747-400 have an almost identical thrust/weight ratio.

The 747-8 uses a variant of the GE-nx with a 6.5" smaller fan that produces less thrust than the other GE-nx. For less development cost the 747-8 could have used a bleed-air version of the normal GE-nx, with more thrust (74,100 vs 65,000). This would have added 1308 lbs of extra engine weight, plus some pylon reinforcement.

Would the 6.5" bigger fan have fit under wing?
Would this have allowed the 747-8 to fly higher, and therefore save fuel?
Does keeping the same fan size save lots of $$$ in development cost?
In actual operation, does the 747-8 have runway constraints? Because bigger engine means shorter runways.
 
trex8
Posts: 5700
Joined: Sat Nov 02, 2002 9:04 am

Re: 747-8 engine choice

Tue Sep 21, 2021 11:28 pm

I think a major issue was that as a quad you dont need as much take off thrust as a twin might with the same weight. You are paying weight wise and for extra thrust you dont really need. As for fan size, there must be some degree of a drag penalty but how much I will leave to the experts.
 
User avatar
747classic
Posts: 3977
Joined: Sat Aug 15, 2009 9:13 am

Re: 747-8 engine choice

Sat Sep 25, 2021 2:25 pm

kitplane01 wrote:
The 747-8 and the 747-400 have an almost identical thrust/weight ratio.

The 747-8 uses a variant of the GE-nx with a 6.5" smaller fan that produces less thrust than the other GE-nx. For less development cost the 747-8 could have used a bleed-air version of the normal GE-nx, with more thrust (74,100 vs 65,000). This would have added 1308 lbs of extra engine weight, plus some pylon reinforcement.

Would the 6.5" bigger fan have fit under wing?



No, because the main landing gear could not be made taller (not enough space in the main landing gear wheel wells !) , the distance between the engine cowlings and the runway would become critical, when installing a GEnx-1B variant, as installed at the 787 series.
At the GEnx -2B67, the fan diameter is optimal sized to fit under the 747 wing, creating enough distance between the engine cowlings and the runway, also the increased thrust of the GEnx-1B was/is not neccessary.

Note : The GEnx-2B67/P, did not receive a fan booster upgrade to increase the T/O thrust level, like the later certified GEnx-1B/P2 engines at the 787-9 and -10 did.
If more T/O thust would have been required for the 747-8 series, the GEnx-2B would have received an identical fan booster upgrade, still with the smaller fan.
 
r6russian
Posts: 64
Joined: Mon Feb 10, 2020 4:12 am

Re: 747-8 engine choice

Sat Sep 25, 2021 7:43 pm

748 already has a service ceiling of FL430, any higher and it wouldnt be able to descend to 10000 feet in case of depressuruzation in time. Im sure it can fly higher, but its illegal to go higher in commercial service
 
744SPX
Posts: 577
Joined: Mon Jan 27, 2020 6:20 pm

Re: 747-8 engine choice

Sun Sep 26, 2021 6:45 am

747classic wrote:
Note : The GEnx-2B67/P, did not receive a fan booster upgrade to increase the T/O thrust level, like the later certified GEnx-1B/P2 engines at the 787-9 and -10 did.
If more T/O thust would have been required for the 747-8 series, the GEnx-2B would have received an identical fan booster upgrade, still with the smaller fan.



GE did offer a 4% thrust bump version (69,500 lbs Boeing equivalent thrust) of the GEnx-2B around the time the PIP was introduced (2010-2012) specifically for the -8F but there didn't seem to be any takers. There also a 2009 "special conditions" document with the GEnx-2B69 listed along with the -67 (see below)

https://www.federalregister.gov/documen ... an-engines

The -2B demonstrated 71,250 lbs thrust in testing
 
a320fan
Posts: 960
Joined: Fri May 01, 2009 5:04 am

Re: 747-8 engine choice

Thu Sep 30, 2021 8:36 am

It’s already pretty easy to strike the outboard engines on runway if the crosswind technique isn’t applied correctly. https://m.youtube.com/watch?v=oRScivHIH10

Any larger diameter and the roll you can carry in the flare would quickly diminish to zero.
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Thu Sep 30, 2021 7:32 pm

kitplane01 wrote:
For less development cost the 747-8 could have used a bleed-air version of the normal GE-nx, with more thrust (74,100 vs 65,000). This would have added 1308 lbs of extra engine weight, plus some pylon reinforcement.

[...]

Would this have allowed the 747-8 to fly higher, and therefore save fuel?


Not at all. If anything, it would be lower. On climb and cruise, airplane is usually not thrust limited, but wing load limited, considering optimum cruise altitude. Same wing, (a bit) heavier frame means lower.
With modern engines, high altitude climb thrust still allows quite reasonable climb rate, and I don't think any modern heavy metal is limited by climb rate.

kitplane01 wrote:
In actual operation, does the 747-8 have runway constraints? Because bigger engine means shorter runways.


There are always conditions where you become runway/climb gradient limited, but not on 95% of airports. On most airlines, you'd have only a handful of locations where high, hot, or both limits aircraft.
However, higher thrust versions are more expensive to buy/service. Will it be offset by higher payload/benefit? I'd say it would not be a popular option, not even close, for reasons above.

Cheers,
Adam
 
User avatar
DocLightning
Posts: 22259
Joined: Wed Nov 16, 2005 8:51 am

Re: 747-8 engine choice

Thu Sep 30, 2021 8:01 pm

kitplane01 wrote:
Would the 6.5" bigger fan have fit under wing?


That was the issue.

Larger engines can be fit under the wing for engine testing purposes, but for regular commercial use, the clearance wasn't adequate. The larger engine size also meant that vortex control devices were installed on the engine cowlings for the first time in the history of the 747 family.
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Thu Sep 30, 2021 8:49 pm

gloom wrote:
kitplane01 wrote:
For less development cost the 747-8 could have used a bleed-air version of the normal GE-nx, with more thrust (74,100 vs 65,000). This would have added 1308 lbs of extra engine weight, plus some pylon reinforcement.

[...]

Would this have allowed the 747-8 to fly higher, and therefore save fuel?


Not at all. If anything, it would be lower. On climb and cruise, airplane is usually not thrust limited, but wing load limited, considering optimum cruise altitude. Same wing, (a bit) heavier frame means lower.
With modern engines, high altitude climb thrust still allows quite reasonable climb rate, and I don't think any modern heavy metal is limited by climb rate.


I don't understand this at all. I don't know what "wing load limited" means.

I'm a pilot. In my Cherokee at max altitude, I'm barely cruising along. If I were to pull back on the stick, I would lose speed and stall. In a 747, if I'm at max altitude and I pull back on the stick, is it not the same?

(I'm aware that other things can limit max legal altitude, like ability to descend or cabin pressure)
 
SteelChair
Posts: 1720
Joined: Fri Aug 25, 2017 11:37 am

Re: 747-8 engine choice

Thu Sep 30, 2021 9:20 pm

As far as the "flying higher" part, I've never seen a 747-400 or -8 above FL370 with anything like a normal payload and 340-350 is more normal. Even when lightly loaded, 38-390 seems to be about it...going higher costs you more. When very heavily loaded it is routine to see a 400 at FL300-310 for the first 2-3 hours. They're pigs. By comparison, on a standard day, an A330 will start out at 340/350 at MTOW and. 767-300 will always be able to make FL330.

I'm not an aerodynamicst. But it must be something to do with the number of engines and the clean wing on a twin versus a quad. When they are light, I see twins go up several flight levels versus the exact same city pair with a max MTOW.
 
User avatar
Starlionblue
Posts: 20814
Joined: Fri Feb 27, 2004 9:54 pm

Re: 747-8 engine choice

Thu Sep 30, 2021 11:57 pm

kitplane01 wrote:
gloom wrote:
kitplane01 wrote:
For less development cost the 747-8 could have used a bleed-air version of the normal GE-nx, with more thrust (74,100 vs 65,000). This would have added 1308 lbs of extra engine weight, plus some pylon reinforcement.

[...]

Would this have allowed the 747-8 to fly higher, and therefore save fuel?


Not at all. If anything, it would be lower. On climb and cruise, airplane is usually not thrust limited, but wing load limited, considering optimum cruise altitude. Same wing, (a bit) heavier frame means lower.
With modern engines, high altitude climb thrust still allows quite reasonable climb rate, and I don't think any modern heavy metal is limited by climb rate.


I don't understand this at all. I don't know what "wing load limited" means.

I'm a pilot. In my Cherokee at max altitude, I'm barely cruising along. If I were to pull back on the stick, I would lose speed and stall. In a 747, if I'm at max altitude and I pull back on the stick, is it not the same?

(I'm aware that other things can limit max legal altitude, like ability to descend or cabin pressure)


I'll try to explain and hope I don't make a has of it.

Wing load limited in the cruise means you don't need all the thrust but the wing can only produce so much lift. The 777-300ER is a good example. On longer flights, it tends to cruise significantly lower than the A350 because it is limited by its comparatively small wing. This despite having gobs of thrust.

In the Cherokee, you're probably thrust limited at max altitude. The wing would support cruise at a higher altitude, but you would not have enough power to maintain cruise speed.

If you pull back at max altitude, you would indeed lose speed, in both Cherokee and 747. However how you have reached that limit differs. Wing max vs thrust max.
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Fri Oct 01, 2021 4:53 am

Starlionblue wrote:
kitplane01 wrote:
gloom wrote:

Not at all. If anything, it would be lower. On climb and cruise, airplane is usually not thrust limited, but wing load limited, considering optimum cruise altitude. Same wing, (a bit) heavier frame means lower.
With modern engines, high altitude climb thrust still allows quite reasonable climb rate, and I don't think any modern heavy metal is limited by climb rate.


I don't understand this at all. I don't know what "wing load limited" means.

I'm a pilot. In my Cherokee at max altitude, I'm barely cruising along. If I were to pull back on the stick, I would lose speed and stall. In a 747, if I'm at max altitude and I pull back on the stick, is it not the same?

(I'm aware that other things can limit max legal altitude, like ability to descend or cabin pressure)


I'll try to explain and hope I don't make a has of it.

Wing load limited in the cruise means you don't need all the thrust but the wing can only produce so much lift. The 777-300ER is a good example. On longer flights, it tends to cruise significantly lower than the A350 because it is limited by its comparatively small wing. This despite having gobs of thrust.

In the Cherokee, you're probably thrust limited at max altitude. The wing would support cruise at a higher altitude, but you would not have enough power to maintain cruise speed.

If you pull back at max altitude, you would indeed lose speed, in both Cherokee and 747. However how you have reached that limit differs. Wing max vs thrust max.


I understand but disagree.

I've never read wing-load-limited in any aerodynamic textbook, and I don't think they do the math in those terms.

I believe the standard textbook answer is something like this ....

At max altitude the indicated air speed adjusted for air density is quite low. It's low because the engines cannot make much thrust due to low air density.
You're flying at a non-trivial angle of attack because of the low air density. If you had more thrust you could fly faster, which would cause the wings to generate more lift, which would move you higher.

Maximum altitude is exactly where the engines can produce only as much thrust as the weight of airplane divided by lift to drag ratio. Engines with more power can produce this amount of thrust higher.

One complication to what I wrote is mach effects. A plane like the 747 cannot exceed the speed of sound, even if the air speed adjusted for density is quite low.
 
User avatar
747classic
Posts: 3977
Joined: Sat Aug 15, 2009 9:13 am

Re: 747-8 engine choice

Fri Oct 01, 2021 5:32 am

On the 747-8 series the max cruising altitude (43100 ft) is limited by the time (4 minutes) to reach FL 140 in case of a rapid decompression, not by engine thrust.
This is caused by a better L/D ratio of the new 747-8 wing, compared to the wing at all previous 747 series.
Initialy the max certified altitude was limited to 42100 ft (747-8F), later, during certification of the 747-8I, the deflection and sequence of the spoilers was modified and the max certified altitude was increased to FL431 (747-8I and later built 747-8F aircraft)
Earlier built 747-8F aircraft can be modified.
See this thread for more background info. : viewtopic.php?t=1428971
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Fri Oct 01, 2021 6:27 am

kitplane01 wrote:
I've never read wing-load-limited in any aerodynamic textbook, and I don't think they do the math in those terms.


They don't. It's the construct to give a hint .

But since you want some...

Two important moments in flight that affect cruise level:
1. Capability to do final climb is (or could be) both wing load affected, and thrust affected, but mostly by thrust. The more thrust you have, it's better. The more wing you have, it's better (but less so, as wing would give you higher cruise as well). Good example by Starlionblue - 777 usually still has quite a ROC when approaching cruise.
2. Capability to maintain cruise. Two different reasoning here:
a) for a weight, wing area and aero, there's optimum altitude at which you could cruise. It's absolute minimum of drag in a given configuration (weight, speed, altitude, even MAC). The lower you go, the drag is increased due to induced drag raising faster than parasitic drag lowering. Opposite when you go higher than optimum. Assuming cruise based on TAS/Mach speed, of course,
b) for a given altitude, you need a certain thrust to maintain speed; still, that would not be enough, as the air is not still, you need more thrust to be able to compensate for wind change. General rule, most of the modern planes have 20-30% more thrust than required to maintain cruise (I did computational job for 734; it needs around 2*4700lbs thrust at optimum cruise in a still air, and is able to deliver around 6000lbs for each CFM56; numbers are from my memory, but more or less adequate for a given version and mass I did the computation).

So, the plane is always dependent on wing area to determine optimum cruise. Higher thrust allows to increase ROC of climb high, and in a very limited cases it would be useful (ATCs would hardly ever accept 50fpm climb; some heavies really struggled for climb approaching cruise). However, no jet plane I'm aware of is thrust limited at cruise, as the thrust they have is more than required to maintain cruise. Optimum FL is not dependant on thrust, but on wing/weight. The resulting drag then needs to be overcome by engine thrust. And planes do have enough and more than that nowadays.

Cheers,
Adam
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Fri Oct 01, 2021 5:43 pm

gloom wrote:
kitplane01 wrote:
I've never read wing-load-limited in any aerodynamic textbook, and I don't think they do the math in those terms.


They don't. It's the construct to give a hint .

But since you want some...

Two important moments in flight that affect cruise level:
1. Capability to do final climb is (or could be) both wing load affected, and thrust affected, but mostly by thrust. The more thrust you have, it's better. The more wing you have, it's better (but less so, as wing would give you higher cruise as well). Good example by Starlionblue - 777 usually still has quite a ROC when approaching cruise.
2. Capability to maintain cruise. Two different reasoning here:
a) for a weight, wing area and aero, there's optimum altitude at which you could cruise. It's absolute minimum of drag in a given configuration (weight, speed, altitude, even MAC). The lower you go, the drag is increased due to induced drag raising faster than parasitic drag lowering. Opposite when you go higher than optimum. Assuming cruise based on TAS/Mach speed, of course,
b) for a given altitude, you need a certain thrust to maintain speed; still, that would not be enough, as the air is not still, you need more thrust to be able to compensate for wind change. General rule, most of the modern planes have 20-30% more thrust than required to maintain cruise (I did computational job for 734; it needs around 2*4700lbs thrust at optimum cruise in a still air, and is able to deliver around 6000lbs for each CFM56; numbers are from my memory, but more or less adequate for a given version and mass I did the computation).

So, the plane is always dependent on wing area to determine optimum cruise. Higher thrust allows to increase ROC of climb high, and in a very limited cases it would be useful (ATCs would hardly ever accept 50fpm climb; some heavies really struggled for climb approaching cruise). However, no jet plane I'm aware of is thrust limited at cruise, as the thrust they have is more than required to maintain cruise. Optimum FL is not dependant on thrust, but on wing/weight. The resulting drag then needs to be overcome by engine thrust. And planes do have enough and more than that nowadays.

Cheers,
Adam


Of course the thrust needed to maintain an altitude is dependent on the airplane. Different airplanes have different lift/drag ratios in different conditions. If you're saying a 747 with a bigger wing would likely have a higher maximum altitude (and lower top speed) I'll agree. But it is still the case that a 747 with more thrust could climb higher.


Wind does NOT affect maximum altitude. Head into or away from a 100kt wind .. makes zero difference.
 
744SPX
Posts: 577
Joined: Mon Jan 27, 2020 6:20 pm

Re: 747-8 engine choice

Fri Oct 01, 2021 7:34 pm

Well, the 747SP on a pre-service demo flight cruised at FL460 with 200 passengers from NY to Tokyo and landed with enough fuel onboard for another 1000 nmi of flying. In testing it demonstrated 600 ft/min climb rate also at FL460

A combo of much lower wing loading and higher thrust to weight ratio.
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Sat Oct 02, 2021 6:53 am

kitplane01 wrote:
If you're saying a 747 with a bigger wing would likely have a higher maximum altitude (and lower top speed) I'll agree. But it is still the case that a 747 with more thrust could climb higher.


Both right (assuming by speed you mean lower IAS, but Ma/TAS staying more or less same). But once you have your aero and weights determined, it doesn't really matter how much thrust you have - assuming you have enough thrust to maintain cruise at cruise level and maintain required rate of climb until cruise. Which 747-8 already has (actually, it has much more than required). Optimum cruise level would not change with more powerful engines. Time to climb - sure, but is that a factor for long range airplane? Don't think so.

The max alt at a certain weight is a different thing, as higher thrust could compensate increased drag. However, it's not something airlines consider often (if any) - the only reason I could think of is avoiding traffic in crowdy areas, trading higher fuel burn for shortcuts.

Cheers,
Adam
 
Max Q
Posts: 9071
Joined: Wed May 09, 2001 12:40 pm

Re: 747-8 engine choice

Sat Oct 02, 2021 10:35 am

SteelChair wrote:
As far as the "flying higher" part, I've never seen a 747-400 or -8 above FL370 with anything like a normal payload and 340-350 is more normal. Even when lightly loaded, 38-390 seems to be about it...going higher costs you more. When very heavily loaded it is routine to see a 400 at FL300-310 for the first 2-3 hours. They're pigs. By comparison, on a standard day, an A330 will start out at 340/350 at MTOW and. 767-300 will always be able to make FL330.

I'm not an aerodynamicst. But it must be something to do with the number of engines and the clean wing on a twin versus a quad. When they are light, I see twins go up several flight levels versus the exact same city pair with a max MTOW.



It doesn’t really make sense to call a 747 a ‘pig’ because it can’t go the same altitudes an A330 can when they’re both at maximum weight. Its kind of ridiculous actually



What sort of maximum cruise level do you think an A330 could reach if it took off at 987000 pounds ?!
 
SteelChair
Posts: 1720
Joined: Fri Aug 25, 2017 11:37 am

Re: 747-8 engine choice

Sun Oct 03, 2021 1:50 am

Max Q wrote:
SteelChair wrote:
As far as the "flying higher" part, I've never seen a 747-400 or -8 above FL370 with anything like a normal payload and 340-350 is more normal. Even when lightly loaded, 38-390 seems to be about it...going higher costs you more. When very heavily loaded it is routine to see a 400 at FL300-310 for the first 2-3 hours. They're pigs. By comparison, on a standard day, an A330 will start out at 340/350 at MTOW and. 767-300 will always be able to make FL330.

I'm not an aerodynamicst. But it must be something to do with the number of engines and the clean wing on a twin versus a quad. When they are light, I see twins go up several flight levels versus the exact same city pair with a max MTOW.



It doesn’t really make sense to call a 747 a ‘pig’ because it can’t go the same altitudes an A330 can when they’re both at maximum weight. Its kind of ridiculous actually



What sort of maximum cruise level do you think an A330 could reach if it took off at 987000 pounds ?!


While I did specifically mention the A330, all the newer twins are race horses compared to piggy (I persist) quads. As I said, I'm not an aerodynamicst and haven't calculated the wingloading but it appears to me that quads just don't go up as well. The 787 generally cruises quite high like the 330.
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Sun Oct 03, 2021 3:09 am

gloom wrote:
kitplane01 wrote:
If you're saying a 747 with a bigger wing would likely have a higher maximum altitude (and lower top speed) I'll agree. But it is still the case that a 747 with more thrust could climb higher.


Both right (assuming by speed you mean lower IAS, but Ma/TAS staying more or less same). But once you have your aero and weights determined, it doesn't really matter how much thrust you have - assuming you have enough thrust to maintain cruise at cruise level and maintain required rate of climb until cruise. Which 747-8 already has (actually, it has much more than required). Optimum cruise level would not change with more powerful engines. Time to climb - sure, but is that a factor for long range airplane? Don't think so.

The max alt at a certain weight is a different thing, as higher thrust could compensate increased drag. However, it's not something airlines consider often (if any) - the only reason I could think of is avoiding traffic in crowdy areas, trading higher fuel burn for shortcuts.

Cheers,
Adam


Ignoring mach effects (which might be a mistake) max altitude is set by the ability of the engines to produce enough thrust to maintain level flight. Bigger engines can maintain this thrust to a higher altitude. Therefore with bigger engines you can climb higher. I'm quite sure this is basic aerodynamics (still ignoring mach effects, which might matter).

The reason you might want to climb higher is that in thinner air you can go faster (true air speed, not indicated air speed) for the same thrust, and therefore save fuel.

You wrote "optimum altitude" and not "maximum altitude" but for recent technology jetliners flying to maximum range, that's just about the same thing because the thinner air lets you go faster for the same thrust.
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Sun Oct 03, 2021 6:26 am

kitplane01 wrote:
You wrote "optimum altitude" and not "maximum altitude" but for recent technology jetliners flying to maximum range, that's just about the same thing because the thinner air lets you go faster for the same thrust.


No, not really.

Optimum altitude is an altitude where a given frame reaches minimum drag (actually, it's not a numerical minimum, as it's interpreted as maximum range configuration, but since we're talking drag difference, I'll use that simplification). It is one specific altitude for a set of a given parameters in flight - weight, center of gravity position, airframe aero config etc.
Maximum altitude is more or less what you defined - maximum altitude where a given frame can cruise at level (remember, air is not still)
Optimum altitude is nowhere close to maximum altitude.
If you go above optimum altitude, thinner air will not help the drag, as due to thinner air plane needs to increase AoA to maintain lift on wings, thus increasing drag over what you gained. Lower density also affects - increases - friction drag (by decrease of Reynolds number). All numbers together, above optimum the drag will be higher.

I've had a couple of flight plans where there were alt fuel burn numbers for above optimum. I could not find these yesterday, but believe me going above optimum is increasing fuel burn, not lowering. And yes, planes are capable of going quite a bit above optimum, sometimes as high as 4000ft above.

Cheers,
Adam
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Sun Oct 03, 2021 6:52 am

gloom wrote:
kitplane01 wrote:
You wrote "optimum altitude" and not "maximum altitude" but for recent technology jetliners flying to maximum range, that's just about the same thing because the thinner air lets you go faster for the same thrust.


No, not really.

Optimum altitude is an altitude where a given frame reaches minimum drag (actually, it's not a numerical minimum, as it's interpreted as maximum range configuration, but since we're talking drag difference, I'll use that simplification). It is one specific altitude for a set of a given parameters in flight - weight, center of gravity position, airframe aero config etc.
Maximum altitude is more or less what you defined - maximum altitude where a given frame can cruise at level (remember, air is not still)
Optimum altitude is nowhere close to maximum altitude.
If you go above optimum altitude, thinner air will not help the drag, as due to thinner air plane needs to increase AoA to maintain lift on wings, thus increasing drag over what you gained. Lower density also affects - increases - friction drag (by decrease of Reynolds number). All numbers together, above optimum the drag will be higher.

I've had a couple of flight plans where there were alt fuel burn numbers for above optimum. I could not find these yesterday, but believe me going above optimum is increasing fuel burn, not lowering. And yes, planes are capable of going quite a bit above optimum, sometimes as high as 4000ft above.

Cheers,
Adam


Actually I believe almost everything you wrote.

There is a best AoA.

If you go too high, you will not have enough engine thrust to maintain the optimum indicated air speed and so you have to increase your AoA above the optimum value. But with larger engines you could maintain this optimum air speed higher up. With larger engines you can maintain the optimum IAS and AoA at a higher altitude. In this case your true air speed will be higher even while your IAS will stay at the optimum, which increase true cruise speed at a given thrust level. (again, ignoring mach effects)
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Sun Oct 03, 2021 7:54 am

kitplane01 wrote:
If you go too high, you will not have enough engine thrust to maintain the optimum indicated air speed and so you have to increase your AoA above the optimum value. But with larger engines you could maintain this optimum air speed higher up. With larger engines you can maintain the optimum IAS and AoA at a higher altitude. In this case your true air speed will be higher even while your IAS will stay at the optimum, which increase true cruise speed at a given thrust level. (again, ignoring mach effects)


You need to realize one thing here:
Jet plane is Mach speed limited, not IAS limited at altitudes we're talking about. Cruise speed is based on Ma speed, not IAS. TAS is slowly going down with altitude (Mach speed is slower as pressure goes down). The reason plane speed is Mach-based is due to various parts of airframe nearing 1 Mach speed, thus increasing the drag rapidly.
So, the higher you go, you first climb on constant IAS, then you switch to constant Mach. This is usually somewhere around FL300, take or give a few thousand. This is of course true only for airliners/civil planes. Check the Vmo/Mmo concepts and climb strategies on that.

Also, see the Figure 3 in atrtached article:
https://www.boeing.com/commercial/aeromagazine/articles/qtr_2_07/article_05_2.html

Cost Index is probably beyond your current knowledge, but it shows nicely how MAX is not OPT, plus a few more concepts around typical speed in level flight.

Cheers,
Adam
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Sun Oct 03, 2021 6:19 pm

gloom wrote:
kitplane01 wrote:
If you go too high, you will not have enough engine thrust to maintain the optimum indicated air speed and so you have to increase your AoA above the optimum value. But with larger engines you could maintain this optimum air speed higher up. With larger engines you can maintain the optimum IAS and AoA at a higher altitude. In this case your true air speed will be higher even while your IAS will stay at the optimum, which increase true cruise speed at a given thrust level. (again, ignoring mach effects)


You need to realize one thing here:
Jet plane is Mach speed limited, not IAS limited at altitudes we're talking about. Cruise speed is based on Ma speed, not IAS. TAS is slowly going down with altitude (Mach speed is slower as pressure goes down). The reason plane speed is Mach-based is due to various parts of airframe nearing 1 Mach speed, thus increasing the drag rapidly.
So, the higher you go, you first climb on constant IAS, then you switch to constant Mach. This is usually somewhere around FL300, take or give a few thousand. This is of course true only for airliners/civil planes. Check the Vmo/Mmo concepts and climb strategies on that.

Also, see the Figure 3 in atrtached article:
https://www.boeing.com/commercial/aeromagazine/articles/qtr_2_07/article_05_2.html

Cost Index is probably beyond your current knowledge, but it shows nicely how MAX is not OPT, plus a few more concepts around typical speed in level flight.

Cheers,
Adam


Actually, I know that B747s have speeds scheduled by Mach number. And that there is an Mmo limit. If B747s slower, what I wrote is exactly right. And if B747s are at their Mmo limit, what I wrote is wrong. This is why I wrote over and over things like "not considering Mach effects". What I don't know is if B747s cruise at or very near their Mach limit. How much lower than Mmo is a B747s cruise speed?

And yes, I know about Cost Index.
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Sun Oct 03, 2021 8:52 pm

kitplane01 wrote:
What I don't know is if B747s cruise at or very near their Mach limit. How much lower than Mmo is a B747s cruise speed?


Typical cruise: 0.85 (some airlines will fly a bit faster, accepting increased a tad fuel burn, up to 0.86).
Mmo: 0.9

Cheers,
Adam
 
LH707330
Posts: 2520
Joined: Fri Jun 15, 2012 11:27 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 5:04 am

There are two things at play:
1. The 747 has less T/W in most conditions than modern twins because of the OEI consideration. This means that it usually climbs at a more leisurely rate, as do most quads vs their twin counterparts.
2. Initial cruise for many planes is to alarge degree a function of wing loading, span loading, and wing design. The 747 happens to be a heavier design with a lower initial cruise, like the 777-300ER. Some quads like the A340 (200 and 300) and the A380 often climb right up to FL330/340 or thereabouts.
 
bigb
Posts: 1455
Joined: Fri Nov 07, 2003 4:30 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 12:46 pm

SteelChair wrote:
As far as the "flying higher" part, I've never seen a 747-400 or -8 above FL370 with anything like a normal payload and 340-350 is more normal. Even when lightly loaded, 38-390 seems to be about it...going higher costs you more. When very heavily loaded it is routine to see a 400 at FL300-310 for the first 2-3 hours. They're pigs. By comparison, on a standard day, an A330 will start out at 340/350 at MTOW and. 767-300 will always be able to make FL330.

I'm not an aerodynamicst. But it must be something to do with the number of engines and the clean wing on a twin versus a quad. When they are light, I see twins go up several flight levels versus the exact same city pair with a max MTOW.


I find it hard that a A330 at Max Gross will make FL330 initially I could be wrong though. Usually FL310 is what I normally get on a max gross 747 doing a transpac with stepping up to FL330 after a few hours in flight.
 
bigb
Posts: 1455
Joined: Fri Nov 07, 2003 4:30 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 12:48 pm

A slight bigger fan will cause engine clearance issues from the ground as the current engines are already pretty damn close. The -8 is already a very efficient bird for a quad to be honest. It tends to make gas in-flight and had cause us to have to put fear out and flaps out fly around some to burn excess gas get under our MLW.
 
LH707330
Posts: 2520
Joined: Fri Jun 15, 2012 11:27 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 4:10 pm

One related question I've got on the 747-8 engines: did the thrust line get moved up relative to the wing versus the 400? The 400 pylons certainly look a lot deeper than on the -8, where the engines look much more cramped underneath the wing. Did they keep the bottom of the nacelle in about the same place and thus move the centerline of the engines up due to the bigger engine diameter?
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 6:52 pm

bigb wrote:
I find it hard that a A330 at Max Gross will make FL330 initially I could be wrong though. Usually FL310 is what I normally get on a max gross 747 doing a transpac with stepping up to FL330 after a few hours in flight.


Depends on weight config - we know this concept of flex MTOW. However, there is a LH FCOM published as pdf.

Check:
https://client.blackboxsimulation.com/forum/uploads/editor/a9/jx2i1yrinxpc.pdf
Section 2.05.20, pages from 230 onwards.

I guess LH only uses A333, so probably this table is A333. A332 should be able to climb higher at similar weights, since it's shorter (less skin drag).

General rule? Yes, it is capable of 330 at MTOW, even configured on 242t MTOW. Not always as optimal, but capable.

Cheers,
Adam
 
bigb
Posts: 1455
Joined: Fri Nov 07, 2003 4:30 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 7:51 pm

gloom wrote:
bigb wrote:
I find it hard that a A330 at Max Gross will make FL330 initially I could be wrong though. Usually FL310 is what I normally get on a max gross 747 doing a transpac with stepping up to FL330 after a few hours in flight.


Depends on weight config - we know this concept of flex MTOW. However, there is a LH FCOM published as pdf.

Check:
https://client.blackboxsimulation.com/forum/uploads/editor/a9/jx2i1yrinxpc.pdf
Section 2.05.20, pages from 230 onwards.

I guess LH only uses A333, so probably this table is A333. A332 should be able to climb higher at similar weights, since it's shorter (less skin drag).

General rule? Yes, it is capable of 330 at MTOW, even configured on 242t MTOW. Not always as optimal, but capable.

Cheers,
Adam


I guess I was looking for optimal cruise not max being tucked in the coffin corner.
 
gloom
Posts: 573
Joined: Thu Jun 30, 2016 4:24 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 8:18 pm

bigb wrote:
I guess I was looking for optimal cruise not max being tucked in the coffin corner.


Increasing cruise also increases max weight a bit (check .80 vs .82 cruise differences).

Still, it's only 242t bird that cannot reach 330 as optimum. 233t birds are off to 330 with no problem at all.

Cheers,
Adam
 
User avatar
kitplane01
Topic Author
Posts: 2014
Joined: Thu Jun 16, 2016 5:58 am

Re: 747-8 engine choice

Mon Oct 04, 2021 10:00 pm

bigb wrote:
gloom wrote:
bigb wrote:
I find it hard that a A330 at Max Gross will make FL330 initially I could be wrong though. Usually FL310 is what I normally get on a max gross 747 doing a transpac with stepping up to FL330 after a few hours in flight.


Depends on weight config - we know this concept of flex MTOW. However, there is a LH FCOM published as pdf.

Check:
https://client.blackboxsimulation.com/forum/uploads/editor/a9/jx2i1yrinxpc.pdf
Section 2.05.20, pages from 230 onwards.

I guess LH only uses A333, so probably this table is A333. A332 should be able to climb higher at similar weights, since it's shorter (less skin drag).

General rule? Yes, it is capable of 330 at MTOW, even configured on 242t MTOW. Not always as optimal, but capable.

Cheers,
Adam


I guess I was looking for optimal cruise not max being tucked in the coffin corner.


Suppose the most cost efficient cruise was rather up in the coffin corner. Would Boeing use those numbers in it's sales literature? Would airline management insist the plane be flown that way? Heck, are not planes tested to 10% beyond their max speeds just to cope with excursions?

Serious question: How large are uncommanded speed variations very in normal, non-turbulent flight? If I set the auto pilot for Mach=0.90, does it ever get to Mach = 0.91? In a Cherokee, we might see 5mph variations during up/down drafts, but I'm thinking Cherokee experience does not apply :-)
 
bigb
Posts: 1455
Joined: Fri Nov 07, 2003 4:30 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 10:15 pm

kitplane01 wrote:
bigb wrote:
gloom wrote:

Depends on weight config - we know this concept of flex MTOW. However, there is a LH FCOM published as pdf.

Check:
https://client.blackboxsimulation.com/forum/uploads/editor/a9/jx2i1yrinxpc.pdf
Section 2.05.20, pages from 230 onwards.

I guess LH only uses A333, so probably this table is A333. A332 should be able to climb higher at similar weights, since it's shorter (less skin drag).

General rule? Yes, it is capable of 330 at MTOW, even configured on 242t MTOW. Not always as optimal, but capable.

Cheers,
Adam


I guess I was looking for optimal cruise not max being tucked in the coffin corner.


Suppose the most cost efficient cruise was rather up in the coffin corner. Would Boeing use those numbers in it's sales literature? Would airline management insist the plane be flown that way? Heck, are not planes tested to 10% beyond their max speeds just to cope with excursions?

Serious question: How large are uncommanded speed variations very in normal, non-turbulent flight? If I set the auto pilot for Mach=0.90, does it ever get to Mach = 0.91? In a Cherokee, we might see 5mph variations during up/down drafts, but I'm thinking Cherokee experience does not apply :-)


Not very large, but the turbulence updrafts can hit anytime up in the flight levels with any sudden shift of wind and/or atmosphere condition sudden changes. So so it’s ideal to give yourself +/- 1000 to 1500 feet from the Max Altitude that the FMS is spitting out base on the current of the aircraft at the particular time. You don’t want to end up in a situation where a good bump will stall the aircraft because it literally will take a solid decent (serval thousands of feet in the flight levels) to recover. You also don’t want to end up on the top end of the coffin corner as well. But a change of .01 doesn’t take much.
 
Max Q
Posts: 9071
Joined: Wed May 09, 2001 12:40 pm

Re: 747-8 engine choice

Mon Oct 04, 2021 11:20 pm

SteelChair wrote:
Max Q wrote:
SteelChair wrote:
As far as the "flying higher" part, I've never seen a 747-400 or -8 above FL370 with anything like a normal payload and 340-350 is more normal. Even when lightly loaded, 38-390 seems to be about it...going higher costs you more. When very heavily loaded it is routine to see a 400 at FL300-310 for the first 2-3 hours. They're pigs. By comparison, on a standard day, an A330 will start out at 340/350 at MTOW and. 767-300 will always be able to make FL330.

I'm not an aerodynamicst. But it must be something to do with the number of engines and the clean wing on a twin versus a quad. When they are light, I see twins go up several flight levels versus the exact same city pair with a max MTOW.



It doesn’t really make sense to call a 747 a ‘pig’ because it can’t go the same altitudes an A330 can when they’re both at maximum weight. Its kind of ridiculous actually



What sort of maximum cruise level do you think an A330 could reach if it took off at 987000 pounds ?!


While I did specifically mention the A330, all the newer twins are race horses compared to piggy (I persist) quads. As I said, I'm not an aerodynamicst and haven't calculated the wingloading but it appears to me that quads just don't go up as well. The 787 generally cruises quite high like the 330.



Well twins have to meet a minimum climb gradient with the loss of 50% of their thrust so of course they are overpowered with two operating compared to a four engine aircraft


That doesn’t make the latter a ‘pig’ and that is a ridiculous statement


The 747-8 can carry an enormous payload compared to an A330, the Airbus would be even more of a ‘pig’ if you load the same weight on it as the Boeing !
 
SteelChair
Posts: 1720
Joined: Fri Aug 25, 2017 11:37 am

Re: 747-8 engine choice

Tue Oct 05, 2021 1:05 am

bigb wrote:
SteelChair wrote:
As far as the "flying higher" part, I've never seen a 747-400 or -8 above FL370 with anything like a normal payload and 340-350 is more normal. Even when lightly loaded, 38-390 seems to be about it...going higher costs you more. When very heavily loaded it is routine to see a 400 at FL300-310 for the first 2-3 hours. They're pigs. By comparison, on a standard day, an A330 will start out at 340/350 at MTOW and. 767-300 will always be able to make FL330.

I'm not an aerodynamicst. But it must be something to do with the number of engines and the clean wing on a twin versus a quad. When they are light, I see twins go up several flight levels versus the exact same city pair with a max MTOW.


I find it hard that a A330 at Max Gross will make FL330 initially I could be wrong though. Usually FL310 is what I normally get on a max gross 747 doing a transpac with stepping up to FL330 after a few hours in flight.


I don't have access to the charts, but an A330-300 PW at the 513.7 MTOW will make FL340 on a standard day, and routinely does so. Watch them on Flightradar coming feet dry off the tracks returning from Europe and you routinely see them do the crossing at FL350 and climbing to 380-400 when hitting the radar environment. 763s have to be almost empty to do that.
 
SteelChair
Posts: 1720
Joined: Fri Aug 25, 2017 11:37 am

Re: 747-8 engine choice

Tue Oct 05, 2021 1:10 am

Max Q wrote:
SteelChair wrote:
Max Q wrote:


It doesn’t really make sense to call a 747 a ‘pig’ because it can’t go the same altitudes an A330 can when they’re both at maximum weight. Its kind of ridiculous actually



What sort of maximum cruise level do you think an A330 could reach if it took off at 987000 pounds ?!


While I did specifically mention the A330, all the newer twins are race horses compared to piggy (I persist) quads. As I said, I'm not an aerodynamicst and haven't calculated the wingloading but it appears to me that quads just don't go up as well. The 787 generally cruises quite high like the 330.



Well twins have to meet a minimum climb gradient with the loss of 50% of their thrust so of course they are overpowered with two operating compared to a four engine aircraft


That doesn’t make the latter a ‘pig’ and that is a ridiculous statement


The 747-8 can carry an enormous payload compared to an A330, the Airbus would be even more of a ‘pig’ if you load the same weight on it as the Boeing !


Well yeah, they can carry an enormous load. Most of the time airliners aren't full, so flying at, oh say, both 70% of MTOW, in my experience the twin will climb higher than the quad. The twin also, in my experience see a greater benefit from planned redispatch and using closer alternates. You take weight off a modern twin (A330/787) it goes up and up. The quads not as much. Mho based upon my experience.
 
bigb
Posts: 1455
Joined: Fri Nov 07, 2003 4:30 pm

Re: 747-8 engine choice

Tue Oct 05, 2021 1:17 am

SteelChair wrote:
Max Q wrote:
SteelChair wrote:

While I did specifically mention the A330, all the newer twins are race horses compared to piggy (I persist) quads. As I said, I'm not an aerodynamicst and haven't calculated the wingloading but it appears to me that quads just don't go up as well. The 787 generally cruises quite high like the 330.



Well twins have to meet a minimum climb gradient with the loss of 50% of their thrust so of course they are overpowered with two operating compared to a four engine aircraft


That doesn’t make the latter a ‘pig’ and that is a ridiculous statement


The 747-8 can carry an enormous payload compared to an A330, the Airbus would be even more of a ‘pig’ if you load the same weight on it as the Boeing !


Well yeah, they can carry an enormous load. Most of the time airliners aren't full, so flying at, oh say, both 70% of MTOW, in my experience the twin will climb higher than the quad. The twin also, in my experience see a greater benefit from planned redispatch and using closer alternates. You take weight off a modern twin (A330/787) it goes up and up. The quads not as much. Mho based upon my experience.


A lot of 747 Freighters are loaded to damn near 98 percent MTOW….. At least most of my flights I’ve flown have been.

If you are are looking at Flight Radar at the A330, how do you know where those non A330 freight birds are loaded to unless you have access to weight and balance data from previous dispatched flights. You probably do, if not, how do you know? Most pax birds have quiet a bit of performance room with their rated MTOW. I’ll be curious to know what a fully loaded A330 freight bird will do?
 
B777LRF
Posts: 2885
Joined: Sun Nov 02, 2008 4:23 am

Re: 747-8 engine choice

Wed Oct 06, 2021 10:15 am

Max Q wrote:
It doesn’t really make sense to call a 747 a ‘pig’ because it can’t go the same altitudes an A330 can when they’re both at maximum weight. Its kind of ridiculous actually

What sort of maximum cruise level do you think an A330 could reach if it took off at 987000 pounds ?!


bigb wrote:
A lot of 747 Freighters are loaded to damn near 98 percent MTOW….. At least most of my flights I’ve flown have been.

If you are are looking at Flight Radar at the A330, how do you know where those non A330 freight birds are loaded to unless you have access to weight and balance data from previous dispatched flights. You probably do, if not, how do you know? Most pax birds have quiet a bit of performance room with their rated MTOW. I’ll be curious to know what a fully loaded A330 freight bird will do?


1) The comparison is quite valid, albeit the criteria you list are of course absurd. Climbing direct to a fairly high FL is a function of wing loading; the 747 and 777 have very high wing loadings and are therefore not able to initially go much higher than FL300-330 at or near max weights. A 787 or A350 can climb much higher initially, as can indeed an A380, and that's all because they have much lower wing loading. It's pretty obvious when you look at it - the 747 has a wingspan of around 70 meters which has to support a MTOW up to around 400 tons. An A330 has 60 meter wings, but "only" has to support up to 242 tons. (I know span is not everything, it's the wing area and shape which counts, but span works for a simple illustration of the issue).

2) A previous poster has offered information that a 233T A330 can climb directly to FL330/340 at MTOW. Doesn't matter if the aircraft is full of boxes or SLF; a Pax A330 will perform exactly the same as a Cargo A330.
 
SteelChair
Posts: 1720
Joined: Fri Aug 25, 2017 11:37 am

Re: 747-8 engine choice

Wed Oct 06, 2021 3:37 pm

B777LRF wrote:
Max Q wrote:
It doesn’t really make sense to call a 747 a ‘pig’ because it can’t go the same altitudes an A330 can when they’re both at maximum weight. Its kind of ridiculous actually

What sort of maximum cruise level do you think an A330 could reach if it took off at 987000 pounds ?!


bigb wrote:
A lot of 747 Freighters are loaded to damn near 98 percent MTOW….. At least most of my flights I’ve flown have been.

If you are are looking at Flight Radar at the A330, how do you know where those non A330 freight birds are loaded to unless you have access to weight and balance data from previous dispatched flights. You probably do, if not, how do you know? Most pax birds have quiet a bit of performance room with their rated MTOW. I’ll be curious to know what a fully loaded A330 freight bird will do?


1) The comparison is quite valid, albeit the criteria you list are of course absurd. Climbing direct to a fairly high FL is a function of wing loading; the 747 and 777 have very high wing loadings and are therefore not able to initially go much higher than FL300-330 at or near max weights. A 787 or A350 can climb much higher initially, as can indeed an A380, and that's all because they have much lower wing loading. It's pretty obvious when you look at it - the 747 has a wingspan of around 70 meters which has to support a MTOW up to around 400 tons. An A330 has 60 meter wings, but "only" has to support up to 242 tons. (I know span is not everything, it's the wing area and shape which counts, but span works for a simple illustration of the issue).

2) A previous poster has offered information that a 233T A330 can climb directly to FL330/340 at MTOW. Doesn't matter if the aircraft is full of boxes or SLF; a Pax A330 will perform exactly the same as a Cargo A330.


Great post.

In my experience, the 233T A333 can go to FL340/350 100% of the time at MTOW on a standard day. A guy told me once, "it can always make 340," and I found that he was correct.

Frankly, I always thought that the C5M engine choice was interesting. The PW2043 would have replaced the TF39 thrust wise but cut fuel burn by a substantial amount (50%?) and was available much earlier. But it seems it's always wise to go with more power and derate.
 
GalaxyFlyer
Posts: 8389
Joined: Fri Jan 01, 2016 4:44 am

Re: 747-8 engine choice

Wed Oct 06, 2021 4:18 pm

The C-5 needed more thrust, not replacement at the same thrust. Almost all of the fuel savings was from faster climb to a higher initial level. It has plenty of wing, just lacked thrust. It also needed the thrust for field performance, putting 2043 would have been a disaster. The HT90 hot section mod on the TF-39s were a decent improvement in climb and fuel burn.

With the TF-39, max gross weight at ETAF was about 807k: with the new engines it operates at all locations at 840k even in the summer, easily. A OEI return at ETAF at 807 was a tricky deal, easy with the CF-6.
 
744SPX
Posts: 577
Joined: Mon Jan 27, 2020 6:20 pm

Re: 747-8 engine choice

Wed Oct 06, 2021 5:55 pm

    Frankly, I was surprised that they de-rated the CF6 for the C-5M. Maybe the wing engine mounts are thrust limited? The 747-300 and AF1 had a lower MTOW at 833k but had significantly more thrust than the C-5M's derated CF6's.

    Before the specs were announced, I figured that they were going to go with the 60k or 62k versions of the CF6
     
    GalaxyFlyer
    Posts: 8389
    Joined: Fri Jan 01, 2016 4:44 am

    Re: 747-8 engine choice

    Wed Oct 06, 2021 8:40 pm

    Lockheed said it didn’t need that kind of thrust to meet the RFP. There are new pylons, but I suspect wing structure was a limiting factor. It did require beefing up of the crown and rudder, so that might have been a factor, too. Over the years, the torque which underlies the vin and rudder had various cracks and repairs. Some from original design weakness and some from air refueling, most of which was training, not operational.


    Another factor was maintaining the original design performance “red lines” esp Vmcg and Vmca. The thrust in the M is such that Lockheed offered 3 choices; a, test and increase those numbers; reduce the outboard engines thrust by 2% to maintain Vmcg or, and most expensive in money and delay, design an asymmetric thrust rudder compensation. In the end, I think they designed a sympathetic thrust reduction by 2% on the opposite outboard engine under thrust loss. I was only tangentially involved by going to a few manuals review conferences when the prototypes were in work.

    In any case, it has all the thrust needed basically because it has so much wing.
     
    744SPX
    Posts: 577
    Joined: Mon Jan 27, 2020 6:20 pm

    Re: 747-8 engine choice

    Wed Oct 06, 2021 8:45 pm

    Interesting! Yeah, I forget the wing is significantly larger than the 747's.
     
    GalaxyFlyer
    Posts: 8389
    Joined: Fri Jan 01, 2016 4:44 am

    Re: 747-8 engine choice

    Wed Oct 06, 2021 10:36 pm

    744SPX wrote:
    Interesting! Yeah, I forget the wing is significantly larger than the 747's.


    I was taxiing into the NASA ramp at Edward’s once. Edward’s is huge, I mean so big buildings look tiny until to approach them and realize the building is a full size that was speck a few minutes before. Except for the NASA ramp which does tightly accommodate their 747. We had a Devil of time parking there. The marshalling team couldn’t figure out why it was tight. “Well, we have 30’ more wing span, 16’ longer and 17’ of tail growth”. The t-tail was a big problem on tight ramps. 700 square feet more wing area and less span loading, too.
     
    SteelChair
    Posts: 1720
    Joined: Fri Aug 25, 2017 11:37 am

    Re: 747-8 engine choice

    Wed Oct 06, 2021 11:11 pm

    GalaxyFlyer wrote:
    The C-5 needed more thrust, not replacement at the same thrust. Almost all of the fuel savings was from faster climb to a higher initial level. It has plenty of wing, just lacked thrust. It also needed the thrust for field performance, putting 2043 would have been a disaster. The HT90 hot section mod on the TF-39s were a decent improvement in climb and fuel burn.

    With the TF-39, max gross weight at ETAF was about 807k: with the new engines it operates at all locations at 840k even in the summer, easily. A OEI return at ETAF at 807 was a tricky deal, easy with the CF-6.


    Thanks for that.

    Are you saying that there wasn't a fuel reduction in cruise as part of the M mod? I've seen those engines on 767, MD-11, and 744 and it seems like about 5,500 lb/hr is a nice rule of thumb. I've always wondered what kind of flows were seen on the C5. Doesn't it normally cruise lower than most airliners, and slower (.70-.74?)?
     
    GalaxyFlyer
    Posts: 8389
    Joined: Fri Jan 01, 2016 4:44 am

    Re: 747-8 engine choice

    Thu Oct 07, 2021 12:14 am

    I don’t fly the M, so can’t say. The old ones at mid-cruise burned about 25,000 lb/hr total. Cruise was .77; but the M, as I hear it, cruises at .79. Yeah, being thrust limited and at ISA+ temps, it started at F270 or F290 on a TATL leg, climb on coast in to 330-350. Lots of TPAC legs begin at 240 or 250, ended at 370. Now, the M can easily start at 340, climbing to 380 or 390. The gain was mostly in faster climb to a higher cruise level, only 3% or so due to better TSFC. That was from memory in discussion with Lockheed and GE during development.

    I came out of Qatar once, very high temps with an HT90 plane and was cruising 3,000’ higher than the chart. Engineer said we couldn’t climb and I was looking at 4% N1 below MCT. Of course, we could go up and did at 400-500 fpm.

    Who is online

    Users browsing this forum: No registered users and 14 guests

    Popular Searches On Airliners.net

    Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

    Military Aircraft Every type from fighters to helicopters from air forces around the globe

    Classic Airliners Props and jets from the good old days

    Flight Decks Views from inside the cockpit

    Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

    Cargo Aircraft Pictures of great freighter aircraft

    Government Aircraft Aircraft flying government officials

    Helicopters Our large helicopter section. Both military and civil versions

    Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

    Night Photos Beautiful shots taken while the sun is below the horizon

    Accidents Accident, incident and crash related photos

    Air to Air Photos taken by airborne photographers of airborne aircraft

    Special Paint Schemes Aircraft painted in beautiful and original liveries

    Airport Overviews Airport overviews from the air or ground

    Tails and Winglets Tail and Winglet closeups with beautiful airline logos