Thrust From United States of America, joined Sep 2003, 2673 posts, RR: 11 Posted (4 years 10 months 2 days 6 hours ago) and read 5129 times:
What's up. Listen, I've got some questions about the takeoff roll lengths of certain airplanes. Why do certain airplanes take longer to get off the ground than others? Like surely the heavier airplanes must be able to compensate to complete a flight in the same amount of time as a much smaller airplane on the same route? Like does a 747 typically accelerate faster than a smaller plane like the 757 once it is in the air, or what? I guess this is a question I've been pondering for some time. I would think that engineers would realize possible disadvantages of a heavy airliner and would probably try and design the plane in such a way that these disadvantages could be compensated for. If my question needs further clarification, please don't hesitate to let me know.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 1, posted (4 years 10 months 2 days 4 hours ago) and read 5091 times:
Quoting Thrust (Thread starter): Why do certain airplanes take longer to get off the ground than others?
- Thrust to weight ratio factoring in any derate.
- Take off weight for that flight.
- Decision to delay rotation in order to achieve higher climb speed.
Quoting Thrust (Thread starter): Like surely the heavier airplanes must be able to compensate to complete a flight in the same amount of time as a much smaller airplane on the same route? Like does a 747 typically accelerate faster than a smaller plane like the 757 once it is in the air, or what?
Most of any flight except very short ones will be at cruise speeds. Acceleration is nowhere near as important as, say, a car, with starts and stops, slowdowns, etc... So the acceleration time to cruise speed is not a huge factor. Time to climb to cruise altitude is probably more critical.
Quoting Thrust (Thread starter): I would think that engineers would realize possible disadvantages of a heavy airliner and would probably try and design the plane in such a way that these disadvantages could be compensated for.
Well, heavier airliners will have proportionally more engine power so there's the compensation.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
Vikkyvik From United States of America, joined Jul 2003, 8210 posts, RR: 28 Reply 2, posted (4 years 10 months 2 days 3 hours ago) and read 5079 times:
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Quoting Starlionblue (Reply 1): Acceleration is nowhere near as important as, say, a car, with starts and stops, slowdowns, etc...
A good explanation I read paraphrased it thusly:
"Airlines aren't buying a sports car. They're buying a truck."
If I could only remember who wrote that.................
Quoting Thrust (Thread starter): If my question needs further clarification, please don't hesitate to let me know.
Could you clarify? I'm not sure I understand what exactly you're asking.
"Two and a Half Men" was filmed in front of a live ostrich.
CosmicCruiser From United States of America, joined Feb 2005, 2212 posts, RR: 16 Reply 3, posted (4 years 10 months 2 days 3 hours ago) and read 5079 times:
Quoting Starlionblue (Reply 1): Thrust to weight ratio factoring in any derate.
- Take off weight for that flight.
- Decision to delay rotation in order to achieve higher climb speed.
Except "decision to delay rotation to achieve a higher climb speed". That would NEVER happen with the exception of a WINDSHEAR alert on t/o roll then you have a predetermined added factor you use. A higher rotation speed has really nothing to do with climb speed. On rotation the FD command bars direct you to a pitch that is, with all engs running, V2+10. After flap/slat retraction you begin your acceleration to your CLIMB speed. That can vary from airline to airline or particular SID.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 4, posted (4 years 10 months 1 day 22 hours ago) and read 5037 times:
Quoting Thrust (Thread starter): Why do certain airplanes take longer to get off the ground than others?
Because TOW varies, wings vary, and the thrust/weight ratio is different for different airplanes (and for the same airplane on different takeoffs).
Quoting Thrust (Thread starter): Like surely the heavier airplanes must be able to compensate to complete a flight in the same amount of time as a much smaller airplane on the same route?
Cruise speed has no direct connection to weight. Cruise speed is primarily a function of aerodynamics with some adjustment for the value of fuel vs. the value of time.
Quoting Thrust (Thread starter): Like does a 747 typically accelerate faster than a smaller plane like the 757 once it is in the air, or what?
Typically, no. A 757 has considerably more excess proportional thrust than a 747.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 5, posted (4 years 10 months 1 day 22 hours ago) and read 5030 times:
Quoting Vikkyvik (Reply 2):
A good explanation I read paraphrased it thusly:
"Airlines aren't buying a sports car. They're buying a truck."
If I could only remember who wrote that.................
Quoting CosmicCruiser (Reply 3): Except "decision to delay rotation to achieve a higher climb speed". That would NEVER happen with the exception of a WINDSHEAR alert on t/o roll then you have a predetermined added factor you use. A higher rotation speed has really nothing to do with climb speed.
Perhaps I'm not phrasing it clearly but I am pretty sure I am not hallucinating about this.
With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.
I assume this is only with certain types and if concomitant with company procedures.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
Vikkyvik From United States of America, joined Jul 2003, 8210 posts, RR: 28 Reply 6, posted (4 years 10 months 1 day 21 hours ago) and read 5010 times:
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Quoting Starlionblue (Reply 5): With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.
I assume this is only with certain types and if concomitant with company procedures.
It sounds like what Cosmic is saying is that this isn't just a spur-of-the-moment decision. I assume it's planned for ahead of time. Correct me if I'm wrong.
"Two and a Half Men" was filmed in front of a live ostrich.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 7, posted (4 years 10 months 1 day 20 hours ago) and read 5003 times:
Quoting Vikkyvik (Reply 6):
It sounds like what Cosmic is saying is that this isn't just a spur-of-the-moment decision. I assume it's planned for ahead of time. Correct me if I'm wrong.
Yeah well I agree with that. Certainly not spur of the moment.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
Zeke From Hong Kong, joined Dec 2006, 7726 posts, RR: 73 Reply 8, posted (4 years 10 months 1 day 16 hours ago) and read 4962 times:
Quoting CosmicCruiser (Reply 3):
Except "decision to delay rotation to achieve a higher climb speed". That would NEVER happen with the exception of a WINDSHEAR alert on t/o roll then you have a predetermined added factor you use. A higher rotation speed has really nothing to do with climb speed. On rotation the FD command bars direct you to a pitch that is, with all engs running, V2+10. After flap/slat retraction you begin your acceleration to your CLIMB speed. That can vary from airline to airline or particular SID.
It is called V2 overspeed (or improved climb procedure), and is used by airlines the world over.
V1/VR/V2 are increased (but still withing runway and screen height limits), such that the new V2 is closer to the optimum climb speed giving a better takeoff gradient or the same gradient with a higher takeoff weight.
Requires a runway/stopway in excess of the minimum length, good surface and the aircraft in good condition.
We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
CosmicCruiser From United States of America, joined Feb 2005, 2212 posts, RR: 16 Reply 9, posted (4 years 10 months 1 day 2 hours ago) and read 4881 times:
Quoting Zeke (Reply 8): V1/VR/V2 are increased (but still withing runway and screen height limits), such that the new V2 is closer to the optimum climb speed giving a better takeoff gradient or the same gradient with a higher takeoff weight.
The more I looked into it it appears it was initially an Airbus thing but Boeing & MDD also published some data on it. Interestingly it looks like V2+20 or in some cases +30 would be the best climb gradient. Here we never change Vr or V2 but an all eng climb is always V2+10. Lose an eng and it's back to V2. Vr, as I said can change with a windshear alert for the arpt. Then it changes for brute energy at rotation.
Lowrider From United States of America, joined Jun 2004, 3220 posts, RR: 11 Reply 10, posted (4 years 10 months 1 day ago) and read 4861 times:
Quoting Starlionblue (Reply 5): With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.
I assume this is only with certain types and if concomitant with company procedures.
We use a performance program that does this for us. It looks at current conditions and the various structural and performance limitations. If it can improve our payload by enhancing the energy for the second segment, it will automatically do so. This can make for some startlingly late rotations, but its not like the airport gives us a discount for using less runway. If we can get a few more pounds of revenue on board, it is worth it.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 15, posted (4 years 10 months 22 hours ago) and read 4834 times:
Quoting Starlionblue (Reply 5):
With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.
It will get you to optimum climb rate faster, but it won't change your climb rate. The steady state climb rate is fixed by aircraft geometry, weight, attitude, and engine power.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 16, posted (4 years 10 months 20 hours ago) and read 4817 times:
Quoting Tdscanuck (Reply 15):
It will get you to optimum climb rate faster, but it won't change your climb rate. The steady state climb rate is fixed by aircraft geometry, weight, attitude, and engine power.
So much to learn! Thx!
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
Pihero From France, joined Jan 2005, 3496 posts, RR: 72 Reply 17, posted (4 years 10 months 16 hours ago) and read 4791 times:
Quoting Tdscanuck (Reply 15): It will get you to optimum climb rate faster, but it won't change your climb rate. The steady state climb rate is fixed by aircraft geometry, weight, attitude, and engine power.
We are here talking *Takeoff performance*, and thus we are talking about *one engine out* climb (in this case).
So, a V2, whether "raw" or *improved* is still a V2 for piloting aspects, and the *improved V2* gives a better second segment climb gradient and over-obstacle performance with all engines running.
The acceleration comes later.
DRAIGONAIR From Netherlands, joined Oct 2000, 706 posts, RR: 6 Reply 18, posted (4 years 9 months 4 weeks 1 day 17 hours ago) and read 4708 times:
When you are taking off and use a lower flap setting you will need to go faster to generate enough lift to lift off. However, due to the lower flap setting, your climb performance will be better (an increase in VY (climb rate) because you use less flaps and hence less drag). This could be used if there is a limited obstacle take off clearance.
However, using less flaps will also decrease the maximum takeoff weight. So, the heavier the plane, the more flaps are needed to generate enough lift permitting there is enough runway.
Derated take off is also used to safe engine life: less thrust is used and hence it will take longer to accelerate along the runway.
Also depending on the weather, the warmer it is outside the longer it will take for the a/c to accelerate, this also is the same for airports that are located at a large altitude. (warm, air is less dense hence need more airflow over the wing is needed to generate the needed lift, same for engines)
Pihero From France, joined Jan 2005, 3496 posts, RR: 72 Reply 19, posted (4 years 9 months 4 weeks 1 day 5 hours ago) and read 4651 times:
Draigonair,
Quoting DRAIGONAIR (Reply 18): due to the lower flap setting, your climb performance will be better (an increase in VY (climb rate) because you use less flaps and hence less drag).
We are not really interested in rate of climb, but in climb gradient
Quoting DRAIGONAIR (Reply 18): However, using less flaps will also decrease the maximum takeoff weight.
You'll have to qualify that as there are a good number of runways that are long enough so that they do not impact on the TOD or the Accelerate-stop distance.
Quoting DRAIGONAIR (Reply 18): So, the heavier the plane, the more flaps are needed to generate enough lift permitting there is enough runway.
See above
As a general rule, on an international runway, the second segment limitation comes before the TOD, ASD limits (See the example of a hot-and-high runway) so a lesser flap configuration is generally a good idea to consider.
Look up somewhere on the net a publication called "Getting to grips with aircraft Performance".
Worth a read.
AAH732UAL From , joined Dec 1969, posts, RR: Reply 20, posted (4 years 9 months 4 weeks 1 day 4 hours ago) and read 4644 times:
The standard rate of climb is 200 feet per NM. That starts 35 feet above the DER(Departure End Runway). The OCS(Obsatcle Clearence Surface) is the 40:1 slope..... or 152 feet per NM. The 200 feet per NM is the standard used which allows the ROC(Required Obstacle Clearence) to be meet. This is changed when something is in the OCS and then a note is made in the ODP(Obstacle Depature Procedure) or SID that plane has to meet in its climb gradient to fly it. They use these numbers all the way to the MEA, MORA, MOCA, filed cruise etc. as well as positive course guidence (PCG). New TERPS has an initial climb area (ICA) set to allow at least 400 feet above the DER as well as PCG.
There is also a ton of other stuff that goes into this but the above is the BASIC(rest is a ton of math crap & stuff ) stuff used to design the SID/ODP, rate of climb, etc. This is according to FAA TERPS and pilots really don't need to know all this stuff, just how to fly them
I go into this stuff, because the designers have already taken into account the climb performace and has designed the stuff off their numbers, in turn the planes must meet these set fourth to fly the procedures!
That is why there will be different ways that airlines have to help planes meet these. I doubt the 757 would have problems most of the time BUT a fully loaded 744 has problems a lot of times. That of course depending on the TORA, TODA, etc etc.....
Sorry if I got to tech but at least this provides some light into how they figure this stuff out.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 21, posted (4 years 9 months 4 weeks 1 day 4 hours ago) and read 4632 times:
Quoting Thrust (Thread starter): would think that engineers would realize possible disadvantages of a heavy airliner
You are right. The engineers would have us fly around empty. Fortunately the accountants who understand the advantages of heavy airplanes, prevail and we sell as many tickets and as much belly freight as we possibly can.
Quoting CosmicCruiser (Reply 3): "decision to delay rotation to achieve a higher climb speed". That would NEVER happen
Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor. Involves selecting a higher V1 and VR for a takeoff. V2 would not be changed and the selected V1 speed ultimately will be limited by accelerate-stop distance. In practical terms, however, a given hotrod two-holer might be able to accelerate to tire limit speed and still reject the takeoff on a very long runway. The reason for the V1 and VR overspeed is twofold. (1) The plane will accelerate faster from original charted V1 to the new VR faster with the weight on the wheels than wallowing through the air, and (2) the new liftoff speed is nearer L/DMAX and that improves the initial climb performance.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Yes it sorta does..... I have been part of a sim session when they put DEN at crazy hot and stuff. It was V1, Vr, V2, and then called "rotate"...... like 10-15 knots higher then V2...... We were really moving but still climbed like crap
Pihero From France, joined Jan 2005, 3496 posts, RR: 72 Reply 23, posted (4 years 9 months 4 weeks 1 day 3 hours ago) and read 4608 times:
Quoting SlamClick (Reply 21): Quoting CosmicCruiser (Reply 3):
"...decision to delay rotation to achieve a higher climb speed". That would NEVER happen
Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor. Involves selecting a higher V1 and VR for a takeoff. V2 would not be changed...
Actually, it could be changed. There is now the possibility of a *V2 range* where :
1.1 Vs < V2 <1.35 Vs (VS here is the Vs 1g and my signs read "at most equal to...)
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 24, posted (4 years 9 months 4 weeks 1 day 2 hours ago) and read 4595 times:
Happiness is not seeing another trite Ste. Maarten photo all week long.
25 Zeke: Increasing V1 does not directly improve climb gradient, improving the V1/VR will improve MTOW, but will not improve the gradient. The best climb grad
26 SlamClick: Isn't that the entire point? I mean what company cares about the climb gradient, so long as it is equal to, or greater than the legally required grad
27 Zeke: If you are limited by obstacles, changing V1 will not change the MTOW. Increasing V1 beyond optimum can actually decrease your MTOW as you are moving
28 Lowrider: Only to the extent that not meeting climb gradients can get expensive. Particularly if terrain is the determining factor.
29 SlamClick: I know all that, I was only discussing a case where initial, or second-segment climb was the limiting factor in maximum allowable takeoff gross weigh
30 PGNCS: This is not necessarily correct. Heavier aircraft very frequently require lower flap settings to deal with climb limit requirements. The corollary is
31 SlamClick: You copied my post then appear to think we disagree. Read mine: "not that uncommon" - means it is common - means it does happen
32 EssentialPowr: Baloney. Engineers design an a/c to haul payload. If an a/c cannot be profitable, then the design is poor or short lived... Weight control of a new a
34 411A: Clearly you haven't been around long enough. The overspeed (improved climb) procedure was used on the first swept wing civil jet transport...the B707
35 PGNCS: SlamClick: I was actually trying to support what you said as correct. If I copied it in a confusing manner, I apologize. I agree with you.
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