Take-off questions
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Take-off questions
A couple of questions related to takeoff performance. Unfortunately, I have read some conflicting info and, while I have my biased answers, would love to hear anyone else.
1) Is takeoff weight a factor in Vmcg. In other words, with Vmcg change for a heavier aircraft vs. a lighter one.
2) How does a full power takeoff vs. reduced power takeoff affect total trip fuel burn. Which will result in a lesser fuel burn?
Thx.
1) Is takeoff weight a factor in Vmcg. In other words, with Vmcg change for a heavier aircraft vs. a lighter one.
2) How does a full power takeoff vs. reduced power takeoff affect total trip fuel burn. Which will result in a lesser fuel burn?
Thx.
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Kwik replies... may not be 100% accurate <G>
1. No. Pure aerody of 1 Eng Full thrust v Rudder Arm.
2. Negligible. Full Power may use less. Use of de-rated power is for engine wear / life reasons, not fuel.
Sure there will be other thoughts...
NoD
1. No. Pure aerody of 1 Eng Full thrust v Rudder Arm.
2. Negligible. Full Power may use less. Use of de-rated power is for engine wear / life reasons, not fuel.
Sure there will be other thoughts...
NoD
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The classic answer is that Vmcg is not considered to be a function of weight. But as with all things aeronautic that's a bit of a simplification.
For example, since deviation from track is the criteria for assessing Vmcg, one might expect that a significantly lighter aircraft might react so quickly to the disturbance that it exceeds the 30ft deviation, even if it were to come back towards the track just as promptly.
Also, the amount of weight carried on the gear will very much affect the tyre dynamics etc, and undercarraige forces can be an important factor.
The Vmcg reg actually calls for the "most unfavourable weight in the range of takeoff weights" so that does recognise the possibility of a weight effect, even though it's usually pretty much minor/negligible for the range one considers on a given aircraft.
For example, since deviation from track is the criteria for assessing Vmcg, one might expect that a significantly lighter aircraft might react so quickly to the disturbance that it exceeds the 30ft deviation, even if it were to come back towards the track just as promptly.
Also, the amount of weight carried on the gear will very much affect the tyre dynamics etc, and undercarraige forces can be an important factor.
The Vmcg reg actually calls for the "most unfavourable weight in the range of takeoff weights" so that does recognise the possibility of a weight effect, even though it's usually pretty much minor/negligible for the range one considers on a given aircraft.
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2) Older airplanes use a higher altitude at which takeoff thrust is reduced to climb thrust for noise abatement reasons. So theoretically a reduced takeoff thrust results in a lower trip fuel but because this time span is still very short it is almost negligible.
When reduced climb thrust is used the opposite is the case. As a longer time is needed to reach the optimum flight level trip fuel is increased.
When reduced climb thrust is used the opposite is the case. As a longer time is needed to reach the optimum flight level trip fuel is increased.
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Hi,
To the best of my knowledge Vmcg does not vary with weight. However RTOW(Regulated Take off Weight) is affected by Vmcg.
Vmcg is the absolute lower limit of V1 and when you have a V1 less than Vmcg, you cannot legally take off. The solution to this problem is to simply push V1 up........but if you are limited by your stopping distance, the only way you can push V1 up is by reducing the RTOW.
This generally occurs in practice when taking off well below structural limits off very short runways(ie off a balanced field).
Hope it helps
KTK
To the best of my knowledge Vmcg does not vary with weight. However RTOW(Regulated Take off Weight) is affected by Vmcg.
Vmcg is the absolute lower limit of V1 and when you have a V1 less than Vmcg, you cannot legally take off. The solution to this problem is to simply push V1 up........but if you are limited by your stopping distance, the only way you can push V1 up is by reducing the RTOW.
This generally occurs in practice when taking off well below structural limits off very short runways(ie off a balanced field).
Hope it helps
KTK
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Kennytheking......
You have totally confused me, surely if i reduce the RTOW i reduce the associated V1! Therefore i havent solved the problem of V1 less than VMCG.......
Guess Again...
Mutt.
You have totally confused me, surely if i reduce the RTOW i reduce the associated V1! Therefore i havent solved the problem of V1 less than VMCG.......
Guess Again...
Mutt.
Moderator
Mutt,
Methinks the scenario being suggested by KTK is ASD limited with V1<Vmcg.
Reduce RTOW sufficiently may permit an overspeed takeoff increase in V1 sufficient to force V1>Vmcg while remaining not worse than ASD-limited ?
Methinks the scenario being suggested by KTK is ASD limited with V1<Vmcg.
Reduce RTOW sufficiently may permit an overspeed takeoff increase in V1 sufficient to force V1>Vmcg while remaining not worse than ASD-limited ?
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Hi mutt,
Sorry about the confusion - let me try again.
Just a quick recap on V1. I am sure you know that it is the take off decision speed. It also has 2 requirements that it must meet to be valid.
1. It must be fast enough to permit the take off to continue on 1 engine(ie above Vmcg).
2. It must also be low enough to be able to stop before the end of the runway.
Let us consider where you are taking off on a relatively short runway, say 5000ft. Unfortunately the V1 turns out to be less that V1.
I am sure you will agree that if we take the same senario, but make the runway 10 000ft we could increase our V1 quite a bit.
Now just draw a parallel - we cannot increase runway length, however reducing the weight has the same effect(ie we have excess runway length).
Also remember that V1 is not a aerodynamic speed like Vr so it is in a way not affected by the aircraft weight. It is rather a function of runway length. Another example - if you have a V1 of 100kts and a weight of 30t and you are operating off minimum runway..........by reducing the weight, you could keep the V1 the same........or even increase it, and still stop before the end of the runway.
Ok I hope I haven't confused the issue even more.....
KTK
Sorry about the confusion - let me try again.
Just a quick recap on V1. I am sure you know that it is the take off decision speed. It also has 2 requirements that it must meet to be valid.
1. It must be fast enough to permit the take off to continue on 1 engine(ie above Vmcg).
2. It must also be low enough to be able to stop before the end of the runway.
Let us consider where you are taking off on a relatively short runway, say 5000ft. Unfortunately the V1 turns out to be less that V1.
I am sure you will agree that if we take the same senario, but make the runway 10 000ft we could increase our V1 quite a bit.
Now just draw a parallel - we cannot increase runway length, however reducing the weight has the same effect(ie we have excess runway length).
Also remember that V1 is not a aerodynamic speed like Vr so it is in a way not affected by the aircraft weight. It is rather a function of runway length. Another example - if you have a V1 of 100kts and a weight of 30t and you are operating off minimum runway..........by reducing the weight, you could keep the V1 the same........or even increase it, and still stop before the end of the runway.
Ok I hope I haven't confused the issue even more.....
KTK
Moderator
KTK,
(a) V1 is still restricted by VR which is restricted by V2 which is restricted by AFM scheduling limits
(b) if the takeoff is Vmcg-limited, reducing the weight generally will have no effect on Vmcg so the problem doesn't go away unless you increase V1
JimmyTAP,
While I don't have specific info re the machine, I suggest that
(a) Vmcg is independent of NWS so sideloads shouldn't come into it.
(b) the criterion is centreline divergence.
(c) at low weight, low fuel state the failure yaw rate may be such that pilot controlling inputs are inadequate to keep within the divergence limits .. hence the need for a higher Vmcg to improve lateral response to pilot control inputs. Flap setting may have some minor effect on yaw rate.
(a) V1 is still restricted by VR which is restricted by V2 which is restricted by AFM scheduling limits
(b) if the takeoff is Vmcg-limited, reducing the weight generally will have no effect on Vmcg so the problem doesn't go away unless you increase V1
JimmyTAP,
While I don't have specific info re the machine, I suggest that
(a) Vmcg is independent of NWS so sideloads shouldn't come into it.
(b) the criterion is centreline divergence.
(c) at low weight, low fuel state the failure yaw rate may be such that pilot controlling inputs are inadequate to keep within the divergence limits .. hence the need for a higher Vmcg to improve lateral response to pilot control inputs. Flap setting may have some minor effect on yaw rate.
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When the revised VMCG was measured there was a distinct effect at light weight. The VMCG value for the aircraft is only 81kts at heavy weights so is probably at the lower limit of what is possible anyway. At heavy weights it was considered that the tyre footprint was greater, providing more resistance to deviation than at light weight as Mad (flt) scientist mentioned.
Another factor in measuring these things is that at light weights the acceleration was so rapid that getting consistent measurements was difficult, especially if there was the slightest crosswind (and there almost always is).
The answer to Panama Jack's first question seems to be "yes but..."
JT
Another factor in measuring these things is that at light weights the acceleration was so rapid that getting consistent measurements was difficult, especially if there was the slightest crosswind (and there almost always is).
The answer to Panama Jack's first question seems to be "yes but..."
JT
Moderator
... and, of course, I should have emphasised that Vmcg ideally is independent of steering capability ... I can vouch for the observation that yaw rates become exciting in the last few knots as Vmcg is approached from above during the failure trials ... now you stay on the runway ... now you don't ....
John, Jimmy’s information re the 146 is correct. Whilst NWS is not an issue, the side force available from the nose wheels is. It would be possible to use the lower value of Vmcg at lower wts if the full effect of forward stick was allowed during the take off run. However, this was not possible as the certification requirements for control jam during take off limit the stick position to the mid range. 146/RJ operators note the AFM requirements for stick position during takeoff as this not only affects flight control in the event of a jam, but also the ability to maintain directional control on the runway, particularly when at light weight.
Another interesting effect was utilised by the HS125, in which the lateral / directional characteristics enabled the aircraft to be steered on the ground by the ailerons. I believe some credit was taken for this in the calculation of Vmcg.
Another interesting effect was utilised by the HS125, in which the lateral / directional characteristics enabled the aircraft to be steered on the ground by the ailerons. I believe some credit was taken for this in the calculation of Vmcg.
Moderator
I have no specific knowledge of the 146, having relinquished an assignment on the Type many years ago, so I must defer to the knowledge of those who do. However, I find it strange that a significant sideload is achieved during certification testing as the max aft cg consideration normally constrains what can be achieved. Learn something new each day, I guess ...
Aileron drag can be quite significant. A four-engined beastie of my past demonstrated a distinct preference for the grass unless appropriate full aileron was applied without delay ...
Re V1 and weight, minimum schedules will have V1 reducing with weight (until V1 becomes Vmcg-limited, at which stage the operationally permitted reduction ceases). If conditions and the AFM permit, however, one can then schedule an overspeed (or improved performance) takeoff with higher than minimum V1, VR, and V2.
Aileron drag can be quite significant. A four-engined beastie of my past demonstrated a distinct preference for the grass unless appropriate full aileron was applied without delay ...
Re V1 and weight, minimum schedules will have V1 reducing with weight (until V1 becomes Vmcg-limited, at which stage the operationally permitted reduction ceases). If conditions and the AFM permit, however, one can then schedule an overspeed (or improved performance) takeoff with higher than minimum V1, VR, and V2.
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Much of the discussion seemed to drift away from Panama Jack's original 2 questions, interesting reading in any case. May I have my 2 cents worth?
(1) Variations in Takeoff weight will have some, but negligible effect upon Vmcg. Variations in weight distribution will have a significant effect upon Vmcg, e.g. an aft loaded aircraft will have a higher Vmcg due to reduced rudder moment arm.
Vmcg is evaluated at the flight testing stage for the worst weight / weight distribution. In determining the 'official' Vmcg, nosewheel steering is disabled and is not a factor.
(2) Reduced thrust takeoff should result in a very slight reduction in sector fuel, but you have to be a bit of a nit picker to find it. Optimum Thrust Specific Fuel Consumption (Fuel used per unit of thrust) is generally found fairly close to engine speeds associated with Climb Thrust, anything higher or lower costs more fuel to produce each pound of thrust. TSFC at full takeoff thrust is worse than at Climb Thrust, and the lower end of takeoff thrust derates is usually at or about Climb Thrust.
What minimal fuel savings may occur would be insignificant compared to the massive savings in engine overhaul costs, not to mention the huge increase in safety commensurate with operations well below the engine stress limits.
(1) Variations in Takeoff weight will have some, but negligible effect upon Vmcg. Variations in weight distribution will have a significant effect upon Vmcg, e.g. an aft loaded aircraft will have a higher Vmcg due to reduced rudder moment arm.
Vmcg is evaluated at the flight testing stage for the worst weight / weight distribution. In determining the 'official' Vmcg, nosewheel steering is disabled and is not a factor.
(2) Reduced thrust takeoff should result in a very slight reduction in sector fuel, but you have to be a bit of a nit picker to find it. Optimum Thrust Specific Fuel Consumption (Fuel used per unit of thrust) is generally found fairly close to engine speeds associated with Climb Thrust, anything higher or lower costs more fuel to produce each pound of thrust. TSFC at full takeoff thrust is worse than at Climb Thrust, and the lower end of takeoff thrust derates is usually at or about Climb Thrust.
What minimal fuel savings may occur would be insignificant compared to the massive savings in engine overhaul costs, not to mention the huge increase in safety commensurate with operations well below the engine stress limits.
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Vmcg and the effect of weight
I'm jumping into this topic way after it seems to have ended but I still can not figure how weight (not the distribution thereof) affects Vmcg.
By definition, Vmcg is at the most 'unfavourable weight'. Therefore logic dictates that at other weights, Vmcg must be different. Have I missed something here?
How exactly does changing the weight affect the mechanics of balancing the moments involved?
TIA
Kanger
By definition, Vmcg is at the most 'unfavourable weight'. Therefore logic dictates that at other weights, Vmcg must be different. Have I missed something here?
How exactly does changing the weight affect the mechanics of balancing the moments involved?
TIA
Kanger
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Weight itself has a potential effect because Vmcg demonstration is a dynamic manoeuvre, not a static balance. So the actual inertia of the aircraft (both mass and moments of inertia) have some bearing on the behaviour of the aircraft.
Crudely, a heavier aircraft will respond less abruptly to the engine failure (which will tend to reduce the lateral displacement for a given case) but will also respond less to the pilot action to restore the aircraft to centreline (thus tending to make the displacement worse). Which is the more significant is a fine balance - to the extent that it may be not a practical concern.
There may also be design features that cause there to be a weight effect - thinking aloud, maybe tyre characteristics that cause the mainwheel moemnt generation to be different at heavier than light weights due to overloading the tyres, such that you get slipping/skidding at heavier weights?
Crudely, a heavier aircraft will respond less abruptly to the engine failure (which will tend to reduce the lateral displacement for a given case) but will also respond less to the pilot action to restore the aircraft to centreline (thus tending to make the displacement worse). Which is the more significant is a fine balance - to the extent that it may be not a practical concern.
There may also be design features that cause there to be a weight effect - thinking aloud, maybe tyre characteristics that cause the mainwheel moemnt generation to be different at heavier than light weights due to overloading the tyres, such that you get slipping/skidding at heavier weights?
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It's Easter, and now I truly believe in the resurrection of the dead. This thread has been dead for 9 months now, and suddenly back to life.
9 months, Hmmmmm......maybe it was only in gestation.
Gimme some of that old black magic Panama Jack.
Happy Easter,
Old Smokey
9 months, Hmmmmm......maybe it was only in gestation.
Gimme some of that old black magic Panama Jack.
Happy Easter,
Old Smokey