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B777 Takeoff Acceleration  
User currently offlineSpeedbird744 From United Kingdom, joined May 2001, 35 posts, RR: 0
Posted (13 years 2 months 6 days 17 hours ago) and read 5046 times:

Whats the takeoff acceleration like on the 777 compared with the 747-400? Does having the most powerful engines increase the acceleration on takeoff and pushes you into the seat?
Thanks.
John

10 replies: All unread, jump to last
 
User currently offlineFlight152 From United States of America, joined Nov 2000, 3389 posts, RR: 6
Reply 1, posted (13 years 2 months 6 days 13 hours ago) and read 4892 times:

The two engines on the 777 are more powerful but the 744 has 4 engines. Depends on weight.

User currently offlineFrequentflyer5 From United States of America, joined May 2001, 236 posts, RR: 0
Reply 2, posted (13 years 2 months 6 days 13 hours ago) and read 4887 times:

777=149KTS is decision speed, 153-155KTS is most likely your rotation for lift off. But this depends on load and weather. This is very comparible to the 747-400.

User currently offlineAmbasaid From , joined Dec 1969, posts, RR:
Reply 3, posted (13 years 2 months 6 days 12 hours ago) and read 4878 times:

The 777 have a better thrust to weight ratio than the 744, but i would be surprised if you could tell the difference at MTOW.

Remember that the 777 has to continue the takeoff with one engine following an engine failure at Vef, whereas the 744 is left with 3 engines.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6389 posts, RR: 54
Reply 4, posted (13 years 2 months 6 days 9 hours ago) and read 4871 times:

Accelleration is a question about thrust to weight ratio. All twins must have a quite favourable thrust to MTOW ratio (between 1 : 3.0 and 1 : 3.5) to overcome an engine out after V1 speed. The 777 is in this respect similar to other twins.

Quads need less power, typically around 1 : 4.0.

What makes the 777 different is that it is a very long range twin, which is frequently used for routes which are much shorter than max range, for instance US transcon routes. Then a 777 will always be very lightly loaded and therefore accellerate like a sportscar. With fifty or a hundred tonnes fuel "missing" the VR speed is also somewhat lower.

Here in Europe we hardly see a 777 take off for a non-intercontinental flight. They are loaded with fuel for long distance and consequently they roll along on the runway as any other twin.

My air force got its first load of N/A F-100 Super Sabre fighters when I was a kid. At an air show I saw these fighters climb almost vertically off the runway like a rocket.

A few years later, when in the AF, and a spotter, then those F-100s had been supplied with two large auxiliary fuel tanks and practise bombs for their daily training flights. I often wondered how they managed to clear the tree tops one mile off the two miles long runway. I couldn't see the planes out there, but I could see the smoke from the afterburner hang on those tree tops five minutes after a take-off.

It's all a question about power and weight.

Best regards, Preben Norholm



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 5, posted (13 years 2 months 6 days 9 hours ago) and read 4838 times:

Thrust to Weight ratios at MTOW:

B747-400: 0.259-0.310
B777-200: 0.266-0.328 (this includes -ER models)

So, there is a large range, but they're pretty similar overall (with the 777 being slightly more powerful).

I've yet to find evidence that twins have a great deal more power than tris/quads due to the engine out on takeoff scenario.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6389 posts, RR: 54
Reply 6, posted (13 years 2 months 6 days 7 hours ago) and read 4831 times:

Dear Flyf15, the 744 is probably the "worst" example when you want to find a quad which takes advantage of the its four engines having an overall lower thrust to MTOW ratio.

The reason for that is that the 744 has a relatively small and short wing compared to MTOW. The 744 has grown considerably in weight over earlier versions, but the wing hasn't grown that much. It is compensated by disproportionally more power compared to earlier 747s.

The "small" 744 wing is also the reason why it is the plane which has most problems with cleaning up and still obey to the 250 kts. speed limit below FL100. "Gimmy 280" a heavy 744 skipper will often pray.

Have a look at various versions of 707, DC-8 and A340. And also 741/742. Then you will see a clear trend.

Some early 747s even had to use water injection to give them the extra boost to make them airborne at MTOW, at least at less favourable conditions.

The A340 (present -200/-300 versions) is probably the clearest example. You will also notice, that even if it is a much lighter and lower capacity plane, then its wing span is actually slightly greater than a 741/742/743.

Take the very weakest 340-300 with 31,200 lbs CFM56-5C2 engines, the number is 0.209 (or 4.78 my way around). Which is directly comparable to a 330-300 with 0.254 (3.96). The more powerful versions are 10-15% better. They are directly comparable since they share the same wing. And both planes are designed to take advantage of long runways to optimize economy - they are certainly not "runway performers".

On the weakest versions available the twin has 22% more power to pull each pound. If you compare the most powerful versions of 333 and 343, then the difference is slightly bigger, around 24-25%.

Best regards, Preben Norholm



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 7, posted (13 years 2 months 6 days 4 hours ago) and read 4812 times:

I've just never seen proof that twins have the enormus extra amounts of power compared to quads to compensate an engine out on takeoff. This isn't to say that twins are lacking power, they obviously have enough, its just that quads have their share also.

True, while there are some very low powered twis/quads, there are also some highly powered ones:

BAe146-100: 0.333
L1011-200: 0.309
MD-11: 0.309

And, going back to twins of the days of the 707 and DC-8, we see twins of equally low amounts of power:

BAC 1-11-500: 0.239
DC-9-30: 0.240
Caravelle MkVI: 0.229


Now, to show you how I see a lack of evidence, lets compare four modern large airliners. I'm not going to use the A340 in this comparison as it is on the extreme low end of power compared to other airliners.

For a totally unbiased comparison, I'll use the highest power engine choice and highest MTOW of each of the current 300-seat sized aircraft for each calculation.

A330-300: 0.291 (one engine out: 0.146)
B777-200: 0.280 (one engine out: 0.140)
MD-11: 0.295 (one engine out: 0.197)
Il-96: 0.296 (one engine out: 0.222)

As we can see, each airliner has approximately the same about of power (0.280-0.300), be it a twin, tri, or quad. With the one engine out parameters, we see that the twins are to a definate disadvantage compared with the tri and quad aircraft. If they were to have surplus power over a quad during normal operation, the normal trust/weight ratio would be much larger, and the one engine out ratios for all would be quite similar. Yet, this is not the case.

Just for comparison, I'll add in the A340-300:
A340-300: 0.227 (one engine out: 0.171)

It has much less power than all the other aircraft, yet it still has more power than the twins with an engine out. You would think that at least the A340-300 would be surpased by the "excess power of twins for engine out takeoffs," yet this is not the case either.


User currently offlineSpeedbird744 From United Kingdom, joined May 2001, 35 posts, RR: 0
Reply 8, posted (13 years 2 months 5 days 18 hours ago) and read 4767 times:

Ok, no matter how thrilling the 777 is, can anyone say the same about the 747-400?, because I heard the reverse thrust is very quiet on the 777.
Also, despite how heavy the 747-400 is, does it not really push you into your seat and struggle to get into the air?


User currently offlineBruce From United States of America, joined May 1999, 5047 posts, RR: 15
Reply 9, posted (13 years 2 months 5 days 12 hours ago) and read 4759 times:

"pushing you into your seat" relates to the G-force. It would also depend on YOUR weight.

I felt "pushed" back into my seat on a 737.

space Shuttle astronauts are also "pushed' back into their seats hard, on takeoff.



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineBoeing747-400 From , joined Dec 1969, posts, RR:
Reply 10, posted (13 years 2 months 5 days 9 hours ago) and read 4733 times:

It depends on the weight and power. The 777 is lighter, but less powerful. The 747 is heavier, but it is also considerably more powerful. If both planes are filled with pax and fuel, the 777 has a little better power to weight ratio, so the 777 would be a little faster on take-off, but you couldn't tell a big difference.

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