Balanced V1 lower than Vmcg?
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And I have explained , TOM will change V1. To keep it within limits you will be able to adjust V1, in this case you want V1 to increase, without going outside your criteria of a balanced V1.
What you explained would have given you an unbalanced V1. I do have a graph on this, but not going to start uploading this now, because havent got time for that just now.
What you explained would have given you an unbalanced V1. I do have a graph on this, but not going to start uploading this now, because havent got time for that just now.
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Your graph didnt show up...... but thanks for trying....
I said that V1 is already LOWER than VMCG, therefore reducing takeoff MASS will have no affect. As I see it, your Balanced V1 is less than VMCG, so you have to increase the V1, you have said that you can increase the V1 by decreasing the Takeoff Weight, I really would like to see you explain this, because even in the world of computerized performance software, I have never seen it happen
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What do you mean by the best chance either way? If this was true, why dont airbus use balanced field as default? Or even Boeing? Are you saying that their methodologies are less safe than Balanced Field?
For us, reduced thrust is called Flex and it has no impact on the VMCG
Mutt
I said that V1 is already LOWER than VMCG, therefore reducing takeoff MASS will have no affect. As I see it, your Balanced V1 is less than VMCG, so you have to increase the V1, you have said that you can increase the V1 by decreasing the Takeoff Weight, I really would like to see you explain this, because even in the world of computerized performance software, I have never seen it happen
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the whole point of a balanced field is to give you the best chance either way,
However to use reduced thrust,
Mutt
Last edited by mutt; 5th Jun 2012 at 17:38.
Keith
One smallish correction, using a reduced thrust would not help in the case as Vmcg is based on rated thrust. A derate thrust setting would work, however.
GF
One smallish correction, using a reduced thrust would not help in the case as Vmcg is based on rated thrust. A derate thrust setting would work, however.
GF
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We use Optimized, or Improved Climb, or Balanced, or Minimum, or Maximum V1 policies, and we also have to option to adjust the V1/VR ratios. Lots of ways to do the same thing.
If you stick with Balanced V1 when you are not field length limited, you might find that you are climb or obstacle limited, so if you opted to unbalance, you can obtain better second segment gradients than balanced, this in turn leads to better climb limit weights or greater thrust reductions.
And as GF has pointed out, reduced thrust only helps you if you use a fixed derated reduction, not an assumed or flexible temperature reduction.
Mutt
If you stick with Balanced V1 when you are not field length limited, you might find that you are climb or obstacle limited, so if you opted to unbalance, you can obtain better second segment gradients than balanced, this in turn leads to better climb limit weights or greater thrust reductions.
And as GF has pointed out, reduced thrust only helps you if you use a fixed derated reduction, not an assumed or flexible temperature reduction.
Mutt
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Sorry about that, seems I might have uploaded the wrong image, will try again. Even got the text that explains the theory behind this.
If you look at previous post I have now changed the graph there, hope they visible, if not use the links here, and you should also see them.
[img=http://img266.imageshack.us/img266/7171/v1vmcgnew2.th.png]
[img=http://img715.imageshack.us/img715/3533/v1vmcgnew.th.png]
[img=http://img688.imageshack.us/img688/6361/v1vmcgnew3.th.png]
If you look at previous post I have now changed the graph there, hope they visible, if not use the links here, and you should also see them.
[img=http://img266.imageshack.us/img266/7171/v1vmcgnew2.th.png]
[img=http://img715.imageshack.us/img715/3533/v1vmcgnew.th.png]
[img=http://img688.imageshack.us/img688/6361/v1vmcgnew3.th.png]
Last edited by truckflyer; 6th Jun 2012 at 09:40.
One smallish correction, using a reduced thrust would not help in the case as Vmcg is based on rated thrust. A derate thrust setting would work, however.
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OK, if you are reducing Mass, then using the Maximum V1 associated with that Mass, you have unbalanced the V1, if you are willing to use an unbalanced V1, then there is nothing stopping you from unbalancing the V1 based on the original Mass, as i said we do this all the time with contaminated runway calculations.
It sounds like you have just become an advocate of optimized V1
BTW, that chart uses the term NORMAL V1, and shows it associated with a field length limit, this is somewhat misleading as it indicates that NORMAL V1 will always be balanced and field length limited, which is incorrect. It also shows that the Field Length Limit is longer with the lower weight, and this is once again incorrect, with a lower weight you will have a lower Field Length requirement.
If you plot the balanced V1's associated with a particular weight and field length, you will discover that they are linear and show that a higher weight gives a higher balanced V1, and inversely a lower weight gives a lower balanced V1.
Mutt
It sounds like you have just become an advocate of optimized V1
BTW, that chart uses the term NORMAL V1, and shows it associated with a field length limit, this is somewhat misleading as it indicates that NORMAL V1 will always be balanced and field length limited, which is incorrect. It also shows that the Field Length Limit is longer with the lower weight, and this is once again incorrect, with a lower weight you will have a lower Field Length requirement.
If you plot the balanced V1's associated with a particular weight and field length, you will discover that they are linear and show that a higher weight gives a higher balanced V1, and inversely a lower weight gives a lower balanced V1.
Mutt
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I am still trying to get my head around the graphs, to be honest. But I have been reading about something like this before too, where they stated that if you you was not FLL, than V1 would increase with mass, as we both agree on.
Same place I read that if FLL, than by reducing mass you would also be able to increase your V1 - I can't find this "book" or article where I read about this now, where I can find all the details about this.
But would this than be an unbalanced V1?
My logic behind the reducing the mass theory, was that you would be able to increase V1 - (with sufficient mass reduced), so that you would reach this higher V1 earlier. (as F = m x a which would be: a = F/m)
In my mind, I might be wrong, but this was my own theory behind it, you accelerate faster to a higher V1, because you have less mass, this means even though you have increased your V1, you will reach it at an earlier stage on the runway, because you are faster now.
You will also need a longer ASDR, but because you are lighter, there will be less inertia, to make you stop faster, although the balance between higher speed and less mass, will have be done trough performance operations/calculations.
Are these assumptions and logic above completely wrong? Or is there some major issue that I am failing to see with this?
The main reason I see that V1 increases with mass, when you are not field limited, is that you must have sufficient speed to be able to continue your take off, VR will increase with mass.
That means if you are very heavy, the gap between V1 and VR will most likely increase. So increasing V1, is to ensure that you can reach VR with one engine inop before end of runway. Which if the gap is to big, you might not reach, so you can than reduce VR by reducing mass!
If V1 is less than Vmcg, that means you reach V1, you will not be able to take off, but the only way I see from logic, is to make V1 equal to Vmcg. But obviously those calculations will be have been based on your current performance calculations.
Please I apologise if for what I have said, if I have completely missed the target here on this, but for me it did seemed logical way to explain, how that reducing mass would be able to increase your V1! And I would have thought there would be a mass that would still keep you within the balanced field requirement.
Your V1 must be high enough to stop and go, if you want to increase your V1 to a higher speed, you must find a way to reach that speed within a shorter distance than your older "to low V1", because as your V1 will be higher, you will need more distance to stop too.
So for me the logical conclusion that you can reach a faster V1 at a shorter distance, is to increase your acceleration, and to do this you must reduce mass, as Thrust / Mass = Acceleration
Same place I read that if FLL, than by reducing mass you would also be able to increase your V1 - I can't find this "book" or article where I read about this now, where I can find all the details about this.
But would this than be an unbalanced V1?
My logic behind the reducing the mass theory, was that you would be able to increase V1 - (with sufficient mass reduced), so that you would reach this higher V1 earlier. (as F = m x a which would be: a = F/m)
In my mind, I might be wrong, but this was my own theory behind it, you accelerate faster to a higher V1, because you have less mass, this means even though you have increased your V1, you will reach it at an earlier stage on the runway, because you are faster now.
You will also need a longer ASDR, but because you are lighter, there will be less inertia, to make you stop faster, although the balance between higher speed and less mass, will have be done trough performance operations/calculations.
Are these assumptions and logic above completely wrong? Or is there some major issue that I am failing to see with this?
The main reason I see that V1 increases with mass, when you are not field limited, is that you must have sufficient speed to be able to continue your take off, VR will increase with mass.
That means if you are very heavy, the gap between V1 and VR will most likely increase. So increasing V1, is to ensure that you can reach VR with one engine inop before end of runway. Which if the gap is to big, you might not reach, so you can than reduce VR by reducing mass!
If V1 is less than Vmcg, that means you reach V1, you will not be able to take off, but the only way I see from logic, is to make V1 equal to Vmcg. But obviously those calculations will be have been based on your current performance calculations.
Please I apologise if for what I have said, if I have completely missed the target here on this, but for me it did seemed logical way to explain, how that reducing mass would be able to increase your V1! And I would have thought there would be a mass that would still keep you within the balanced field requirement.
Your V1 must be high enough to stop and go, if you want to increase your V1 to a higher speed, you must find a way to reach that speed within a shorter distance than your older "to low V1", because as your V1 will be higher, you will need more distance to stop too.
So for me the logical conclusion that you can reach a faster V1 at a shorter distance, is to increase your acceleration, and to do this you must reduce mass, as Thrust / Mass = Acceleration
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Firstly, I agree with the other posters that the original question is poorly worded and ambiguous.
The debate that has led on from it has shown some confusion, both about take-off optimisation and about the effect of V1 on take-off performance.
Effect of V1
The term "field length limited" covers 5 basic performance scenarios:
1) You are limited by take-off run, all engines operating.
2) You are limited by take-off run, one engine inoperative.
3) You are limited by take-off distance, all engines operating.
4) You are limited by take-off distance, one engine inoperative.
5) You are limited by accelerate-stop distance.
So, if we change V1 but keep all other factors the same (this includes weight), we can have a variety of effects:
a) By definition, V1 has no effect on the all engines operating cases (1&3).
b) If V1 is increased, the accelerate-stop distance required will increase. This is intuitive, as we must assume that the aircraft will stop from a higher speed than before.
c) The take-off run and distance required, with one engine inoperative will decrease. This is not as clear as point b, but remember that we said that all other factors will stay the same. So, we will still have to reach the same Vr for rotation. The higher V1 means we can assume that the point of engine failure (Vef) happens later and the aircraft spends more off the take-off run with both engines operating, thereby accelerating faster and using less distance.
So, the effect of a change in V1 on a field length limited take-off depends upon which of the factors is actually limiting!
The debate that has led on from it has shown some confusion, both about take-off optimisation and about the effect of V1 on take-off performance.
Effect of V1
The term "field length limited" covers 5 basic performance scenarios:
1) You are limited by take-off run, all engines operating.
2) You are limited by take-off run, one engine inoperative.
3) You are limited by take-off distance, all engines operating.
4) You are limited by take-off distance, one engine inoperative.
5) You are limited by accelerate-stop distance.
So, if we change V1 but keep all other factors the same (this includes weight), we can have a variety of effects:
a) By definition, V1 has no effect on the all engines operating cases (1&3).
b) If V1 is increased, the accelerate-stop distance required will increase. This is intuitive, as we must assume that the aircraft will stop from a higher speed than before.
c) The take-off run and distance required, with one engine inoperative will decrease. This is not as clear as point b, but remember that we said that all other factors will stay the same. So, we will still have to reach the same Vr for rotation. The higher V1 means we can assume that the point of engine failure (Vef) happens later and the aircraft spends more off the take-off run with both engines operating, thereby accelerating faster and using less distance.
So, the effect of a change in V1 on a field length limited take-off depends upon which of the factors is actually limiting!
