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  • #16
    I am but an 18hr lapsed student pilot (and many years lapsed at that) but there's one factor in the above excellent arguments that I couldn't give a shit about and I'm speaking of noise.
    If staying airborne means that the NIMBY's below have to put up with a bunch more noise then so be it.

    It will still be quieter than the sound of me hitting their rooftops !!

    (for the uninitiated, NIMBY = Not In My Back Yarder.... A.K.A. the people who moved to live next door to an airport long after the airport was opened......and then complain about the noise !!)
    If it 'ain't broken........ Don't try to mend it !

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    • #17
      Originally posted by Gabriel View Post
      Clearly, "b" is a much more efficient climb: you are climbing more feet each minute, making more miles each hour, and burning less fuel. Yet, "a" has a much steeper climb gradient.
      ...I seem to recall that you pointed out that:

      When you are in a situation of negligible climb perforance, there is 'no difference' between angle of climb and rate of climb. (zero feet per minute = zero feet per mile, even though the units don't match)

      In other words, your flap condition is more draggy and the plane isn't going to climb at all but descend, but if you are clean, then you can maintain altitude/eek out your miniscule climb rate.
      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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      • #18
        Originally posted by brianw999 View Post
        ... there's one factor in the above excellent arguments that I couldn't give a shit about and I'm speaking of noise.
        If staying airborne means that the NIMBY's below have to put up with a bunch more noise then so be it.
        Of course. The noise abatement procedures don't apply in an emergency, and even if they did, in an emergency the captain has the legal authority to deviate from any regulation he/she deems necessary to cope with the situation.

        --- 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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        • #19
          Originally posted by 3WE View Post
          ...I seem to recall that you pointed out that:

          When you are in a situation of negligible climb perforance, there is 'no difference' between angle of climb and rate of climb. (zero feet per minute = zero feet per mile, even though the units don't match)
          That's correct. Vx and Vy converge to each other as you approach the ceiling.

          In other words, your flap condition is more draggy and the plane isn't going to climb at all but descend, but if you are clean, then you can maintain altitude/eek out your miniscule climb rate.
          That's correct (with the previously explained exception that if you are at a speed that retracting the flaps would put you very close to the stall, you might get more drag without flaps than with them).

          The typical take-off and climb sequence was for normal situations. Not emergencies.

          Remember, unlike GA flights, commercial flights have to be "dispatched", and that includes making all the take-off weight calculations to ensure, among other things, that if the most critical engine was to fail at V1, you'd still be able to take-off and achieve V2 and 35ft on the remaining runway, and then achieve a certain minimum positive climb gradient (with a margin, since the regulations mandate to degrade the demonstrated performance).

          That is why it's very unlikely that you see an incident like this one in a commercial jet flight.

          That said... I'm not sure I'd want to know the outcome if these planes had lost an engine shortly past V1:

          Ilyushin IL-76 IL76 Using It All Airliners usually use no more than 2/3rds of the runway to take off. Watch in amazement as this fully-loaded Russian car...

          Auf YouTube findest du die angesagtesten Videos und Tracks. Außerdem kannst du eigene Inhalte hochladen und mit Freunden oder gleich der ganzen Welt teilen.

          Auf YouTube findest du die angesagtesten Videos und Tracks. Außerdem kannst du eigene Inhalte hochladen und mit Freunden oder gleich der ganzen Welt teilen.


          (anyway, these planes had more problem with taking off within the runway than with climbing afterwards, so taking-off with less flaps was not an alternative and I don't think that raising them immediately after lift-off would have been a good idea either)

          --- 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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          • #20
            Originally posted by Gabriel View Post
            The typical take-off and climb sequence was for normal situations. Not emergencies.
            Still...you are at Las Vegas, in a valley, with a full load of Gamblers and their luggage and a good load of fuel, going to New Yark.

            It's 110 degrees, but fortunately LAS is only ~2000 MSL.

            The book says takeoff works well.

            The book says you will climb and eventually clear the mountains.

            But I have rode planes that did 270 degrees of circling on climb out- I assume to get over mountains...

            So, you would want to retract the flaps 'as soon as possible' for the best climb performance (I did catch the Vx vs Vy comment- there could be exceptions).

            However, one final disclaimer/wiggle word special- The "normal" climb out procedure probably does not delay flap retraction to any huge level of practical significance- climb out costs $$ and flaps are probably ALREADY retracted 'as soon as possible' (ASAP = as soon as REASONABLY possible and not as soon as aerodynamically possible)
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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            • #21
              Fixed:

              Originally posted by 3WE View Post
              Still...you are at Denver with Mountans to the west and a full load of tourists and their luggage and a good load of fuel, going to Japan.

              It's 100 degrees, and you are ~5000 MSL.

              The book says takeoff works well.

              The book says you will climb and eventually clear the mountains.

              So, you would want to retract the flaps 'as soon as possible' for the best climb performance (I did catch the Vx vs Vy comment- there could be exceptions).

              However, one final disclaimer/wiggle word special- The "normal" climb out procedure probably does not delay flap retraction to any huge level of practical significance- climb out costs $$ and flaps are probably ALREADY retracted 'as soon as possible' (ASAP = as soon as REASONABLY possible and not as soon as aerodynamically possible)
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

              Comment


              • #22
                Originally posted by 3WE View Post
                It's 110 degrees, but fortunately LAS is only ~2000 MSL.
                That's at least 5500 ft of density altitude!!!
                And it can be up to 7000ft if it's a low pressue and very moist day!!! (unlikely in LAS)

                --- 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 ---

                Comment


                • #23
                  Originally posted by Gabriel View Post
                  In general, but not always, any amount of flaps or slats in any condition adds drag. Parasitic drag is the obvious factor here, but typically also induced drag is increased. It's a bit tough to explain, but the most efficient way to produce lift is to push down the air with the same downwash all along the wingspan. Pushing down "half of the air" with less downwash and the other half with more downwash might produce the same lift (than all the air with the same downwash), but will necessarily take more energy to do, and hence more induced drag.

                  The exception is below some threshold at very slow speeds. Because of the flow separation, the parasitic drag skyrockets as the plane nears, reach and exceeds the stall AoA. Thus, if at a given slow speed you are somehow far from stall with a given flaps/slats setting but would be close to stall or stalled without flaps/slats, then probably the "clean" configuration will be more draggy and inefficient (in fact, "stalled" is not only inefficient but unsustainable).

                  In planes like the Tomahawk, where the stall speed is nearly the same with or without flaps, the above is not an issue.

                  In transport jets, with full span slats and maybe 70% span multi-slotted Fowler flaps, the difference in stall speed can be quite substantial (especially between the clean config and a take-off-ish config, once the slats are extended and the flaps are at say 15°, further extending the flaps add a lot of drag and little lift).

                  Hence, the issue here to retract the flaps is not as much achieving a positive rate of climb and some altitude, but attaining some speed where you will still be with a margin above the stall speed once the flaps are retracted to the next notch.

                  The usual take-off / climb schedule is:
                  Rotation
                  Lift off happens at a speed with a margin above stall
                  Positive climb - gear up
                  The plane keeps accelerating and pitching up until stabilizing at V2 +10 / +20 and a pitch of some 15 / 20°
                  The plane could keep climbing like this almost forever (okay, you typically have a 5 minutes limit for TOGA thrust), but it's not efficient and it makes a lot of noise (both the engines at TOGA and aerodynamic noise of the "dirty" plane), so "as soon as possible" (*) the idea is to level off (or substantially lower the nose and reduce the climb rate), reduce thrust to "climb" setting, accelerate, and start retracting the flaps and slats in increments as the increasing speed allows (this is called flaps retraction schedule) and keep accelerating to the optimum climb speed or 250kts (whichever is lower) and then pull up again to keep all the excess thrust (that so far was being used to increase airspeed) in climb.

                  (*) As soon as possible:
                  - Typically the reduction in pitch and thrust and the flaps retraction schedule is not done below 1000 or 1500 ft AGL. There are several reasons for this: First, while in the end cleaning up the plane will mean a more efficient climb, the initial reaction when you lower the nose to accelerate is to worsen the climb (in favour of speed). You want to gain a "healthy altitude" quickly before any level-off, so as to have margin to manage a situation like an engine failure, a windshear, etc. Also, to avoid workload saturation, it is good to give a little of time to the pilots to stabilize in the initial climb and check that all is Ok before proceeding with the next sequence of tasks. Remember, it will take just 20 to 30 seconds to reach 1000 to 1500 ft.
                  - Compare this two situations: a- You are in take-off config climbing 3000 fpm with TOGA at 170 kts. b- You are clean, climbing 3500 fpm with climb thrust at 250 kts. Clearly, "b" is a much more efficient climb: you are climbing more feet each minute, making more miles each hour, and burning less fuel. Yet, "a" has a much steeper climb gradient (you are climbing more feet per mile): 3000/170 is quite more than 3500/250. Also,a s said before, before climbing "better" (after cleaning) you first have to climb "worse" (level off for the flaps retraction schedule). Because of these two factors, terrain and obstacle clearance my require to delay the level-off and flaps retraction.
                  - Sound abatement: While climbing clean and at climb thrust is much more quiet than doing so at TOGA and "dirty", depending on some factors sound abatement procedures might call for the level-off, thrust reduction and flaps retraction to be done LATER (say at 3000ft). Imagine the normal profile: The plane climbs at TOGA to 1000ft, levels off, accelerates and retract flaps and slats, keeps accelerating to 250 kts. Pause. Let's call this point above the ground "point X". After point X the plane will resume climbing with climb thrust and 250 kts. Now imagine the "sound abatement profile". The plane climbs at TOGA, up to 1000ft both profiles are equal, but in this case the plane keeps climbing to 3000ft, and only then levels off, reduces thrust and starts the flaps retraction schedule. When the plane overflies the "point X" of the normal profile, it will be at 3000ft instead of 1000. Maybe making more noise (if the flaps retraction schedule is not yet completed), but the noise received on ground is proportional the the amount of noise generated and inversely proportional to the square of the distance, so being 3 times higher the noise on the ground will be 1/9 (if the intensity of the noise generated was the same, but in any event the intensity of the source it will not be 9 times louder).

                  If performance was the only concern you would level off and retract flaps / slats ASAP (or you would take-off clean if you could). But it is not.
                  I haven’t read your post all the way to the end, but I think you may have the parasitic and induced drags reversed. It is parasitic drag that is directly proportional to the speed. Induced drag tends to be greater at lower speed, higher angels of attack and all those fancy high lift devices deployed. The price you pay for high lift is induced drag. Parasitic drag is basically a skin friction.

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                  • #24
                    Originally posted by Peter_K View Post
                    It is parasitic drag that is directly proportional to the speed. Induced drag tends to be greater at lower speed, higher angels of attack and all those fancy high lift devices deployed. The price you pay for high lift is induced drag. Parasitic drag is basically a skin friction.
                    That is correct, but...

                    I think you may have the parasitic and induced drags reversed
                    Where?

                    --- 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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                    • #25
                      Originally posted by Gabriel View Post
                      Where?
                      Because of the flow separation, the parasitic drag skyrockets as the plane nears, reach and exceeds the stall AoA.
                      IMHO it would be the induced drag.

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                      • #26
                        Originally posted by Peter_K View Post
                        IMHO it would be the induced drag.
                        I see,

                        Well, there is a simplification often done that the parasitic drag coefficient is constant and that only the induced drag changes with angle of attack.

                        This is a good simplification for LOW angles of attack, where the flow remains attached.

                        EDIT: If you haven't read this psot yet, plase skip to the next post first and then return here if you still want.

                        Have you ever seen a Cl vs AoA, Cd vs AoA, and Cl vs Cd (polar) graphs for an airfoil?

                        Well, airfoils, as tested or as computer-simulated, DON'T HAVE induced drag.
                        When tested in a wind tunnel, they are placed all the way between the tunnel walls, and that's equivalent to an infinite span (the characteristics of the flow are the same in all cross-sections, and there is no spanwise flow or wingtip vortex) and hence an infinite aspect ratio and hence zero induced drag. When computer-simulated, they are 2D and hence also equivalent to to infinite span for the same reasons. The induced drag is a characteristic of the finite-span wings.

                        So in the airfoil performance graphs, Cd is all parasitic. Why would anyone bother to plot a graph that is just a straight line of constant value? (the same Cd for any AoA and for any Cl). Nobody, so they don't. The Cd in those graphs is pretty low and constant away from the stall. As you near the stall, reach it and go beyond, you can see the Cd curve suddenly climbing. And, again, this is 100% parasitic drag.

                        Think of this: you are flying at one given speed, say that it's somehow close to the "official" stall speed. But you can have the same CL (and hence same lift because the speed is fixed) a bit before the critical AoA and a bit after it. Do you think you'll have the same total drag? Well, you wont, you'll have much more drag beyond the stall. And is this increase due to induced drag?

                        Well, you've said it. Induced drag is what you have to pay for lift, and lift is the same in both cases. While technically the induced drag will not be exactly the same (because the spanwise lift distribution will likely not be the same in both cases), the bulk of the increase of drag will be parasitic drag.

                        Again, you've said it: Induced drag is what you pay for lift. What you pay for making chaotic vortexes and turbulence atop of the wing (separated airflow) is parasitic drag. Think spoiler.

                        The flow also separate from the upper surface of the flaps past maybe 15 degrees of deflection. That's why after that the flaps add quite a bit of drag but not so much more lift. And slats and Fowler flaps also accelerate the air between the wings and the device or between segments of the device (multi-slotted flaps), thus adding more "wet surface" and zones where the air circulate at high speed. All that is parasitic drag too.

                        The only reasons why flaps increase the induced drag are:
                        1) If you use the additional lifting capacity to fly slower (something you usually do), because you need higher CL to get the same lift at a slower speed.
                        2) Because it changes the lift distribution. Ideally, for minimum for optimum induced drag, each inch of span of the wing will produce the same lift per unit of chord (ok, that's a complicated way to say that each cross-section should be working at the same Cl). Unless the flaps are full-span (and they aren't), when you extend them you are clearly placing more load (higher Cl) on the flapped sections and less load (lower Cl) on the non-flapped sections, thus moving away from the ideal "constant Cl" rule. It has nothing to do with flow separation and the like. If you had full span flaps and could configure them to keep the same spanwise Cl distribution, the induced drag (at the same speed) would not change at all.

                        --- 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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                        • #27
                          Ok, I thought of a simple, clear, straightforward, and undisputable way to show that the parasitic drag is NOT constant with AoA:

                          Imagine a wing. A very simple wing. Rectangular planform (i.e. constant chord). No sweep. No geometric twist (i.e. no washout). All made of the same airfoil. This airfoil:



                          Now, imagine such a wing flying at 100 kts in ISA+0 sea level conditions.

                          Please tell mehow much do you think the lift, induced drag, and parasitic drag will be in the folowing two cases:

                          a. AoA = 0°: Lift = ______ lb. Induced drag = ______ lb. Parasitic drag = ______ lb.

                          b. AoA = 90°: Lift = ______ lb. Induced drag = ______ lb. Parasitic drag = ______ lb.

                          It doesn't matter if the number is right or not. I just want to see the relative sizes of the numbers you put.

                          --- 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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