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Thread: Bad take-off computation.

  1. #61
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    You said it yourself: V1 cannot come before VMCG and V2 cannot come before V1. VMCG is only affected by two things: air density and thrust component, so weight doesn't move it (though it might affect the rate of the yawing moment - that's your department - uncontrollability is uncontrollability). But weight affects performance. So on a standard day using full thrust, when VMCG is coming at 133kts and your fast-accelerating empty 744 is reaching V1 at 103kts and VR of 116kts, it seems unwise. Derating to TO2 moves VMCG down to 122kts. Even then it seems unsafe to rotate before 127kts.
    If VMCG is 133kts, then V1 cannot be less that 133 kts. So even if your accelerate-stop vs accelerate-go balanced field calculated V1 gives you 103kts, you will need to scrap the balanced field V1 and the balanced filed length and have enough runway to accelerate to 133 kts then stop, which will be way more than what you need to reach V2 and 35 ft after an engine failing at 133 kts, since by then you'll likely be past V2 on the runway and with so much excess speed over the natural Vr the plane will zoom up the moment you start to rotate. In other words, you hard-fix V1 (and Vr and probably V2) at 133kts, so if an engine fails before that you abort just as you would do if you had a weight where the "natural" V1 was 133 kts.

    There must be something more to that. Maybe the ground handling becomes dangerous (beyond the statement "you will end up on the grass" which is acceptable and is what WILL happen if an engine fails at low sped in some airplanes and some scenarios). Maybe the problem here is that you will end up in the grass at say 120 kts and that was (understandably) deemed very dangerous. Maybe the ground handling on 3 points at 130 kts is very difficult, or maybe the plane will become airborne by itself even with zero pitch before Vr.

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  2. #62
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    Manure shoveling vs. aeroengineer moment: Weight on the nose wheel.

    Remember that MD-80 that laid into it a bit too hard as he turned onto the runway?
    For the sake of Vmcg only aerodynamic means can be taken into account. Any nose wheel authority is extra margin (very much like reverses in the RTO and landing calculations).

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  3. #63
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    Quote Originally Posted by Gabriel View Post
    For the sake of Vmcg only aerodynamic means can be taken into account. Any nose wheel authority is extra margin (very much like reverses in the RTO and landing calculations).

    In the 74, there is just not enough rudder, on the others, I have no idea. Even when you know in advance it is going to happen in the sim, it is very hard not to leave the pavement. Tiller and brakes included.

  4. #64
    Senior Member 3WE's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    In the 74, there is just not enough rudder...I have no idea...
    Unfortunately, you don't get it...

    You gotta imagine the inertia of a light weight versus a heavy weight being accelerated by engines out on a lever...

    More weight is gonna mean more drag against the pull of an engine and thus more yaw tendency.

    It defies logic (yes, simplistic logic) that lighter weight is where you have control problems...because you have 'the same rudder authority' whether you are light or heavy...you'd think the higher weight would make for more yaw, not the lighter weight.

    Gabe, the aeroengineer is trying to take the logic to the next level*...and to read between the lines, he is not finding an obvious answer.

    I too am trying to take the logic to the next level...and yes ATL 40 ft-wide planters and diagonal braces come to mind along with the fact that it's not simply a mass at the wing root...if it's spread out, you get a 'flywheel' that 'resists' yaw.

    Apparently the nosewheel friction forces do not figure in to the procedure, in spite of my attempt at cowboy improvisation.

    Complicated stuff.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  5. #65
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    Quote Originally Posted by 3WE View Post
    Unfortunately, you don't get it.
    I guess not. But I made it for 47 years with no accidents, and no major incidents. 11,000 hours in every model of the 747 ever built except the SP and the NASA 200 that carried the space shuttle. So I guess I will just live with my ignorance.

    Here is one for you, 2 identical 747's at FL 350, one is heavy, the other very light. You pull the thrust levers to flight idle and descend at 300 knots. Which one gets to a lower altitude first?

  6. #66
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    If VMCG is 133kts, then V1 cannot be less that 133 kts. So even if your accelerate-stop vs accelerate-go balanced field calculated V1 gives you 103kts, you will need to scrap the balanced field V1 and the balanced filed length and have enough runway to accelerate to 133 kts then stop, which will be way more than what you need to reach V2 and 35 ft after an engine failing at 133 kts, since by then you'll likely be past V2 on the runway and with so much excess speed over the natural Vr the plane will zoom up the moment you start to rotate. In other words, you hard-fix V1 (and Vr and probably V2) at 133kts, so if an engine fails before that you abort just as you would do if you had a weight where the "natural" V1 was 133 kts.

    There must be something more to that. Maybe the ground handling becomes dangerous (beyond the statement "you will end up on the grass" which is acceptable and is what WILL happen if an engine fails at low sped in some airplanes and some scenarios). Maybe the problem here is that you will end up in the grass at say 120 kts and that was (understandably) deemed very dangerous. Maybe the ground handling on 3 points at 130 kts is very difficult, or maybe the plane will become airborne by itself even with zero pitch before Vr.
    Here, muse over this...

    http://www.b747400.com/pdf/told.pdf

  7. #67
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    Quote Originally Posted by Evan View Post
    Here, muse over this...

    http://www.b747400.com/pdf/told.pdf
    This looks like a TOLD card made for a flight sim program. Not something that is used on the line.

  8. #68
    Senior Member 3WE's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Here is one for you, 2 identical 747's at FL 350, one is heavy, the other very light. You pull the thrust levers to flight idle and descend at 300 knots. Which one gets to a lower altitude first?
    1. I don't know- but would expect you to.

    2. For some of us, rote memorization is not quite as fun as knowing why.

    3. Do you have any idea why?- that I would like to hear.

    4. I will guess the light one descends faster. Why: Weight in the sky = potential energy and gliders will sometimes add ballast to glide further.

    5. Why not? More weight = more lift needed = more induced drag = faster descent. The aircraft pictured at 5:10 seems to descend much faster with added weight. https://www.youtube.com/watch?v=VklTs-Tid_I

    6. Wow, it could descend faster, it could descend slower. And dang...heavy planes tend to not be able to fly as high as lighter ones...since there isn't enough power to generate the lift and overcome the drag. I also guess that Gabe, the aeroengineer could give us a dissertation on when you descend faster and when you descend slower- I'm sure there's some complex, interacting factors 'pulling' both directions.

    7. I do know it is possible to land an 'old' 172 with 40-degrees of flaps and no added power...I used to worry when the instructor said, 'you might need to add a little power'...and I thought 'no I don't.' Remember that when we run the sim. I also think it's a bad idea to burn your last few hundred feet altitude for a few extra knots...even if you are a tad high.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  9. #69
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    Quote Originally Posted by 3WE View Post
    1. I don't know- but would expect you to. I do.

    4. I will guess the light one descends faster. Why: Weight = potential energy. Ding Ding Ding, we have a winner

    By the way, anyone having difficulty logging on with Google? It makes me logon over and over.

  10. #70
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Here is one for you, 2 identical 747's at FL 350, one is heavy, the other very light. You pull the thrust levers to flight idle and descend at 300 knots. Which one gets to a lower altitude first?
    The lighter one.

    Here is one for you, 2 identical 747's at FL 350, one is heavy, the other very light. You pull the thrust levers to flight idle and descend at best glide speed. Which one gets to a lower altitude first?
    The heavier one.

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  11. #71
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    More weight is gonna mean more drag against the pull of an engine and thus more yaw tendency.

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  12. #72
    Super Moderator brianw999's Avatar
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    I remember our old Canberra Bomber/Photo reconnaissance aircraft from the Cold War era when, if you lost an engine on takeoff all the operating engine was any good for was to take you to the crash site !
    If it 'ain't broken........ Don't try to mend it !


  13. #73
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    Quote Originally Posted by Gabriel View Post
    The lighter one.


    The heavier one.
    Funny guy you are!

  14. #74
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Funny guy you are!
    It was not funny. It is accurate. Take a closer look. The second quote is not the original one (hence I removed the "Originally Posted by BoeingBobby" part).

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  15. #75
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    Quote Originally Posted by Gabriel View Post
    It was not funny. It is accurate. Take a closer look. The second quote is not the original one (hence I removed the "Originally Posted by BoeingBobby" part).
    I now see that you removed the quote, and the airspeed, but the answer is still the same, given a constant speed decent. The lighter airplane will come down faster.

  16. #76
    Senior Member Evan's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    I now see that you removed the quote, and the airspeed, but the answer is still the same, given a constant speed decent. The lighter airplane will come down faster.
    uh oh...

  17. #77
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    uh oh...
    As stated earlier- added weight seems to increase the descent of the aircraft in the video below at about the 5:10 mark. I guess Bobby missed that. I suppose one might try to blame drag from the object in-tow, but the airspeeds are somewhat slow.

    I also forgot to point out the brill-yunt use of control surfaces to expedite the descent of the lighter aircraft shortly before 5:10. Flying is beautiful!

    EDIT: I forgot to note how, instead of pulling up relentlessly, the operator is using fairly critical attitude management, including pointing the nose downward to gain needed airspeed.

    https://www.youtube.com/watch?v=VklTs-Tid_I

    By the way Evan...please note the use of extreme cowboy instinct and low-silica FMS, passive 300-800 nm RADAR with virtually no FCOM nor checklists, TOPMS, GPWS, and some likely violations of weight and balance figures...hoping your head does not explode.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  18. #78
    Senior Member 3WE's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    I now see that you removed the quote, and the airspeed, but the answer is still the same, given a constant speed decent. The lighter airplane will come down faster.
    You have 40,000 hours and have indeed memorized a lot of stuff.

    What's a bit scary is sometimes the parlour talking ass hats here know some rather basic stuff. You'll dismiss it as unnecessary extraneous fluff, but I dunno...there's that TOD acronym that even 3BS knows.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  19. #79
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    Quote Originally Posted by 3WE View Post
    You have 40,000 hours and have indeed memorized a lot of stuff.

    What's a bit scary is sometimes the parlour talking ass hats here know some rather basic stuff. You'll dismiss it as unnecessary extraneous fluff, but I dunno...there's that TOD acronym that even 3BS knows.

    Look up weigh, I never said anything about the use of flaps or speed brakes, see post #37

  20. #80
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    I now see that you removed the quote, and the airspeed, but the answer is still the same, given a constant speed decent. The lighter airplane will come down faster.
    Hey BB, I acknowledge my own signature and all. But by now I expected that that you knew me well enough that if I say something, and then insist, you would consider that I may be right.

    So I'll insist a third time. It is accurate. You still didn't look close enough. I didn't just remove the speed. I replaced one speed by another speed. And that changes everything. Ane yes, you said "given a constant speed decent, the lighter airplane will come down faster" and, if by "constant" you mean "same speed for both planes", that's correct (within the flight envelope). But what I say is also correct (and also within the flight envelope).

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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