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Takeoff Rolls And Climb Rate Questions  
User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Posted (6 years 1 month 1 week 3 days 10 hours ago) and read 6459 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.


Fly one thing; Fly it well
36 replies: All unread, showing first 25:
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17033 posts, RR: 67
Reply 1, posted (6 years 1 month 1 week 3 days 8 hours ago) and read 6421 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."
User currently offlineVikkyvik From United States of America, joined Jul 2003, 9953 posts, RR: 26
Reply 2, posted (6 years 1 month 1 week 3 days 7 hours ago) and read 6409 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.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 3, posted (6 years 1 month 1 week 3 days 7 hours ago) and read 6409 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.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (6 years 1 month 1 week 3 days 2 hours ago) and read 6367 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.

Tom.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17033 posts, RR: 67
Reply 5, posted (6 years 1 month 1 week 3 days 2 hours ago) and read 6360 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.................

 Smile

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."
User currently offlineVikkyvik From United States of America, joined Jul 2003, 9953 posts, RR: 26
Reply 6, posted (6 years 1 month 1 week 3 days 1 hour ago) and read 6340 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.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17033 posts, RR: 67
Reply 7, posted (6 years 1 month 1 week 3 days ago) and read 6333 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.  Wink Certainly not spur of the moment.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineZeke From Hong Kong, joined Dec 2006, 9041 posts, RR: 75
Reply 8, posted (6 years 1 month 1 week 2 days 21 hours ago) and read 6292 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
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 9, posted (6 years 1 month 1 week 2 days 6 hours ago) and read 6211 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.


User currently offlineLowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 10, posted (6 years 1 month 1 week 2 days 4 hours ago) and read 6191 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.



Proud OOTSK member
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 11, posted (6 years 1 month 1 week 2 days 3 hours ago) and read 6181 times:
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Quoting Lowrider (Reply 10):
its not like the airport gives us a discount for using less runway.

How interesting things would be if that were indeed the case...  scratchchin 

2H4



Intentionally Left Blank
User currently offlineLowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 12, posted (6 years 1 month 1 week 2 days 3 hours ago) and read 6178 times:



Quoting 2H4 (Reply 11):
How interesting things would be

It would make it harder to cater to photographers who like those end of runway overflight shots. That is, of course, the real reason we do it.



Proud OOTSK member
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 13, posted (6 years 1 month 1 week 2 days 3 hours ago) and read 6177 times:
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DATABASE EDITOR



Quoting Lowrider (Reply 12):
That is, of course, the real reason we do it.

And here I thought it was just to spite NIMBYs...

2H4



Intentionally Left Blank
User currently offlineLowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 14, posted (6 years 1 month 1 week 2 days 3 hours ago) and read 6173 times:



Quoting 2H4 (Reply 13):
And here I thought it was just to spite NIMBYs...

Just an added benefit. One of the many services we provide.



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User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (6 years 1 month 1 week 2 days 2 hours ago) and read 6164 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.

Tom.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17033 posts, RR: 67
Reply 16, posted (6 years 1 month 1 week 2 days ago) and read 6147 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."
User currently offlinePihero From France, joined Jan 2005, 4410 posts, RR: 76
Reply 17, posted (6 years 1 month 1 week 1 day 20 hours ago) and read 6121 times:
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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.



Contrail designer
User currently offlineDRAIGONAIR From Netherlands, joined Oct 2000, 708 posts, RR: 6
Reply 18, posted (6 years 1 month 1 week 21 hours ago) and read 6038 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)



cheers
User currently offlinePihero From France, joined Jan 2005, 4410 posts, RR: 76
Reply 19, posted (6 years 1 month 1 week 9 hours ago) and read 5981 times:
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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.



Contrail designer
User currently offlineAAH732UAL From , joined Dec 1969, posts, RR:
Reply 20, posted (6 years 1 month 1 week 9 hours ago) and read 5974 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  Smile ) 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.
     

[Edited 2008-07-27 15:29:12]

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 21, posted (6 years 1 month 1 week 8 hours ago) and read 5962 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 Starlionblue (Reply 1):
- Take off weight for that flight.

You meant to say weight AND configuration, right?

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.
User currently offlineAAH732UAL From , joined Dec 1969, posts, RR:
Reply 22, posted (6 years 1 month 1 week 8 hours ago) and read 5956 times:



Quoting CosmicCruiser (Reply 3):
That would NEVER happen

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  Smile


User currently offlinePihero From France, joined Jan 2005, 4410 posts, RR: 76
Reply 23, posted (6 years 1 month 1 week 7 hours ago) and read 5938 times:
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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...)



Contrail designer
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 24, posted (6 years 1 month 1 week 6 hours ago) and read 5925 times:



Quoting Pihero (Reply 23):

Yep. Fair enough.



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
33 David L : I think he was backing you up.
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.
36 SlamClick : Well I agree with us both, I guess.
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