How fast to 747s accelerate?
- Thread starter Tommy B
- Start date
Whenever I'm on a passenger plane, takeoff occurs always within 30 seconds of moving off, give or take one second. This is true whether on a smaller plane like EasyJet (737?) or the larger ones used by national carriers (747?)
I had the same question some years ago and had looked it up, finding that the take off speed was, IIRC, 160mph. Of course the exact speed would depend on many factors including the weight, but I had worked it out (with many assumptions) that the acceleration to 60mph was about 8 seconds and only past about 120mph would the acceleration be comparable to a super car.
Or perhaps my calculations and/or memory are failing so don't flame me
I had the same question some years ago and had looked it up, finding that the take off speed was, IIRC, 160mph. Of course the exact speed would depend on many factors including the weight, but I had worked it out (with many assumptions) that the acceleration to 60mph was about 8 seconds and only past about 120mph would the acceleration be comparable to a super car.
Or perhaps my calculations and/or memory are failing so don't flame me
Soldato
- Joined
- 19 Oct 2002
- Posts
- 18,096
- Location
- Shakespeare’s County
That sounds likely in how the acceleration would feel.
Probably engine heat has something to do with it aswell, ie when the EGT get to a certain temp you can really start to press on it. You dont want things like turbine blades heating up considerable faster than the casing.
Probably engine heat has something to do with it aswell, ie when the EGT get to a certain temp you can really start to press on it. You dont want things like turbine blades heating up considerable faster than the casing.
Soldato
- Joined
- 19 Oct 2002
- Posts
- 18,096
- Location
- Shakespeare’s County
SB118 said:
Only if Hamster powered....
Where is Rick Moranis?
- Joined
- 3 May 2004
- Posts
- 17,721
- Location
- Kapitalist Republik of Surrey
Not really, thrust doesn't work well from a standstill and you get all sorts of nasty turbulence in the front of the engine when the air in front is stationary. Once you're moving at some pace you get ram air effect in the engine and the thrust really starts to become effective.Jonnycoupe said:
Static thrust is always a lot lower than dynamic thrust reaching a maximum, if I'm not mistaken, at its top speed
Soldato
- Joined
- 19 Oct 2002
- Posts
- 18,096
- Location
- Shakespeare’s County
Its not only ram air but also you get the axial gas flow speed matching the angles of the compressors and stators so your compression is not only of a higher adiabatic efficiency but you also get more mass flow through the engine. Then the turbine stage has higher exit velocities to give you more thrust and more shaft power to compress the air and also drive the Fan. But sure the compression is assisted by ram air as such. The pressure at station 0 prior to the fan is greatest at the point the aircraft rotates to take off as fromt then on as it climbs your ambient density is dropping away.
Most the thrust on the Turbo fan comes from the 'cold' thrust so ram air doesn't effect that as compression and combustion isn't part of that thrust creation.
I'm yet to see any stationary air effected by turbulence...
Infact the more I think about this the engine is close to full power so the later surge down the runway cant be down to the time it takes the engine spool. Im not familar with the pilot bit but are they at max power right from roll off or do that wind in the power as the Aircraft speed increases?
Im not sure if thats the case. As engines will be rated only on a big test rig in static airflow and that rating has to be sufficiently verified for any aircraft to get certified for flight; they dont tend to oversize Aero engines for obvious reasons.
Most the thrust on the Turbo fan comes from the 'cold' thrust so ram air doesn't effect that as compression and combustion isn't part of that thrust creation.
I'm yet to see any stationary air effected by turbulence...
Infact the more I think about this the engine is close to full power so the later surge down the runway cant be down to the time it takes the engine spool. Im not familar with the pilot bit but are they at max power right from roll off or do that wind in the power as the Aircraft speed increases?
Jonny69 said:Static thrust is always a lot lower than dynamic thrust reaching a maximum, if I'm not mistaken, at its top speed
Im not sure if thats the case. As engines will be rated only on a big test rig in static airflow and that rating has to be sufficiently verified for any aircraft to get certified for flight; they dont tend to oversize Aero engines for obvious reasons.
Last edited:
As people have said there are a lot of variables. Weight, temp, pressure, runway length etc.
What is most common is a de-rated take off. This basically means that the engines are not used at full power so as to save engine wear and tear (as well as save fuel, reduce noise etc). It gets a little complicated how its calculated basically but the thrust is reduced so that the V1 speed matches up with the ASDA (accelerate stop distance available)
These days its pretty rare to have a full thrust take-off as most runways are long enough.
oh and generally speaking the ram rise that a turbo-jet experiences on take-off is pretty minimal if I recall correctly.
To answer the question above, the engines have to stabilse, before take-off power is set. This usually means standing up the throttles to vertical, once both engines have accelerated to around 30-40% N1 (or EPR if thats your kettle of fish) the PM then calls stable and the PF calls set thrust, at which point the TOGA buttons will be pressed and the auto-throttle will set whatever the take-off thrust calculated to be (be it de-rated or full thrust). The PM obviously watches and makes sure that the correct power is being used.
What is most common is a de-rated take off. This basically means that the engines are not used at full power so as to save engine wear and tear (as well as save fuel, reduce noise etc). It gets a little complicated how its calculated basically but the thrust is reduced so that the V1 speed matches up with the ASDA (accelerate stop distance available)
These days its pretty rare to have a full thrust take-off as most runways are long enough.
oh and generally speaking the ram rise that a turbo-jet experiences on take-off is pretty minimal if I recall correctly.
To answer the question above, the engines have to stabilse, before take-off power is set. This usually means standing up the throttles to vertical, once both engines have accelerated to around 30-40% N1 (or EPR if thats your kettle of fish) the PM then calls stable and the PF calls set thrust, at which point the TOGA buttons will be pressed and the auto-throttle will set whatever the take-off thrust calculated to be (be it de-rated or full thrust). The PM obviously watches and makes sure that the correct power is being used.
Last edited:
Soldato
- Joined
- 19 Oct 2002
- Posts
- 18,096
- Location
- Shakespeare’s County
De rated would explain things then With the length of most runways achieving a balanced field length by V1 is probably comfortable under those conditions.
The ram rise on a TurboFan is even less significant, theres no real ducting or stagnation chamber if you will.
Im not familiar with N1 or EPR Acronyms, could you explain them please.
With the length of most runways achieving a balanced field length by V1 is probably comfortable under those conditions.The ram rise on a TurboFan is even less significant, theres no real ducting or stagnation chamber if you will.
Im not familiar with N1 or EPR Acronyms, could you explain them please.
N1 is the rotation speed of the first stage of the compressor. EPR is the engine pressure ratio, I dont use this but I think thats the ratio of turbine exhaust p to compressor inlet p.
As said before, the engines typically take 6-8sec to spool up from idle to take off thrust, and we initially set 50%, then quickly move thrust levers to the take off position after they're steady at 50. By the time the full take off thrust is achieved we've only moved a matter of 10-20metres or so in the 6-8secs. Then the real acceleration kicks in. Im not sure of the exact amount of time to get to the 120-150mph or so we actually rotate at, Im gonna look exactly tomorrow, quite interested.
Full TOGA take-offs happen occasionally, and I had one in Rome yesterday. The reason there is due to runway length and more importantly a rather large hill quite close to the airport.
As said before, the engines typically take 6-8sec to spool up from idle to take off thrust, and we initially set 50%, then quickly move thrust levers to the take off position after they're steady at 50. By the time the full take off thrust is achieved we've only moved a matter of 10-20metres or so in the 6-8secs. Then the real acceleration kicks in. Im not sure of the exact amount of time to get to the 120-150mph or so we actually rotate at, Im gonna look exactly tomorrow, quite interested.
Full TOGA take-offs happen occasionally, and I had one in Rome yesterday. The reason there is due to runway length and more importantly a rather large hill quite close to the airport.
I was wondering about this a few weeks so counted the number of seconds from stationary to rotation. In the A340-600 i was travelling to HK in it was ~55secs. Not sure what the load was on the way out but coming back we were close to max weight.
The A340 climbs using the curvature of the earth and the moon's mavity to gain altitude, it's not exactly the epitome of a performance heavy
yes, planes FMC normaly works out best engine setup/thrust rating to get the aircraft weight in the air. With long runways, the FMC wont be setting the plane to take off at 100% thrust, probably 92-95%? However if the runway is short 100% would be used...
also got to remember that jet engines take a while to spool up, and generally are set to 40% to check pressures and EGT's are "ok" before going smoothly to take of thrust.
even my 1.25 fiesta will probably beat the jumbo for the first 100 yards....lol
The A340 climbs using the curvature of the earth and the moon's mavity to gain altitude, it's not exactly the epitome of a performance heavy
lol that plane is scary to be under as it takes off.... always wonderd how the A340-600 gets into the air really, it seems so underpowerd.
The 300 series is even worse. There's nothing worse to go through your sector than an A340-300 fully loaded going to China on a really warm day with pilots who's English language skills extend as far as the numbers 1-10.
Isn't there a minimum regulatory climb rate of 500ft/pm in the UK? I know 340's are very slow climbers and there's variables such as the usual weights and balances, temperature, humidity, and wind speed/direction which affect it but isn't curvature of the Earth OTT?The A340 climbs using the curvature of the earth and the moon's mavity to gain altitude, it's not exactly the epitome of a performance heavy
It's actually surprisingly difficult to find data on this as well. Airbus have done a fine job of hiding the fact, clearly!
The AIP states that a minimum rate of climb/descent of 500fpm shall be adhered to and if it cannot, ATC must be informed. A lot of the heavies struggle to do it, I've seen an A340-300 climbing at 350fpm below FL100 when I had told him to expedite.
500fpm isn't so much a problem at high level but in the TMA where we have loads of bizjets and A319's etc shooting up at 4-5000fpm, an A340 in the middle of the melee doing 500fpm can really mess things up, especially when you have 4 Easyjet A319's and two Gulfstream V's stuck underneath
The 'curvature of the earth' statement was an exaggeration of course although sometimes I really do wonder.
Out of interest and unrelated, do you fly airways much?
500fpm isn't so much a problem at high level but in the TMA where we have loads of bizjets and A319's etc shooting up at 4-5000fpm, an A340 in the middle of the melee doing 500fpm can really mess things up, especially when you have 4 Easyjet A319's and two Gulfstream V's stuck underneath
The 'curvature of the earth' statement was an exaggeration of course although sometimes I really do wonder.
Out of interest and unrelated, do you fly airways much?
Interesting stuff there.The AIP states that a minimum rate of climb/descent of 500fpm shall be adhered to and if it cannot, ATC must be informed. A lot of the heavies struggle to do it, I've seen an A340-300 climbing at 350fpm below FL100 when I had told him to expedite.
500fpm isn't so much a problem at high level but in the TMA where we have loads of bizjets and A319's etc shooting up at 4-5000fpm, an A340 in the middle of the melee doing 500fpm can really mess things up, especially when you have 4 Easyjet A319's and two Gulfstream V's stuck underneath
The 'curvature of the earth' statement was an exaggeration of course although sometimes I really do wonder.
I'm a low hours PPL student and a high hours holiday maker. Never been in an A340 but have been in plenty of other Airbus and Boeings.