1. Keith Beachy

    Keith Beachy Active Member

    Gravity is not going to pull the nose down, the plane is flying based on a center of mass, the CG (center of gravity is very important in flying). The only time gravity pulled "my nose" down, is when I was going straight up (T-38) and hit zero knots, then the plane stopped going up, and the nose fell straight down, and we dropped until we could fly again in air. If the CG is off, the aircraft can crash, see the cargo jet with the load shift, it crashed, it failed to fly after a massive CG shift to the rear. We load aircraft to maintain a safe CG. Flying is a balancing act, we don't care if the earth is round, we are flying in air on the earth. I never trimmed the KC-135 to adjust for the curved earth on 9 to 10 hour flights from Okinawa to Diego Garcia.

    The plane is flying in air, there is no need to pitch for the curvature of the earth. We trim the plane for level flight and make adjustments as we go to compensate for weight change, CG change, and when we are not free to fly a set AOA (like max Range AOA), we must constantly make small changes to hold an exact altitude per standards. If we have an autopilot the altitude is automatically maintained. The Autopilot was designed by assuming the earth is flat and not spinning, because the math is more complicated for a spherical spinning earth. For an aircraft flight dynamic the effects of a spherical spinning earth are negligible (but not for navigation, we must model spherical spinning earth).

    We can assume a flat earth for flying. Designing a plane in a perfect flat earth wind tunnel to fly level, it would fly level. The autopilot which flies commercial aircraft already assume a flat earth, and it works. The only pitch changes are made to maintain a pressure altitude, changes in CG, changes in weight. If I fly by hand and have the plane perfectly trimmed, I add thrust, we go up, reduce throttle, we go down, it is that simple. For subsonic flight and maybe up to and past MACH 3, we can assume a flat earth, there are no changes made for the curvature of the earth, or coriolis. If we do the math based on a curved spinning earth we find the terms related to curve and spinning are negligible, we then make the assumption of flat earth, no spin for the flight dynamics, the flying part.

    If it helps, think of a particular pressure altitude, FL350, as an ocean of air, and we are at that exact altitude, which does automatically curve with the earth, no need to pitch down each mile eight inches, we are already there as we fly. I prefer to fly max range AoA, or use 99 percent max range on the high side, which I usually used .81 MACH in the KC-135 on the MACH indicator after adjusting for errors in the system.

    The aircraft has no clue the earth is round, and flies steady at an altitude when trimmed properly. As we burn fuel the lighter plane will rise or go faster, based on holding an altitude or free to roam in altitude.

    When developing flight systems assumptions are made, and we can assume flat earth without a problem. Irony, the plane you designed to fly on a flat earth perfectly trimmed is how we do it, and gravity acts on the entire plane, not the nose, and at each instant we are level there is no need to pitch for a curve, it does not make sense for flying.
    This makes it easier to do the math, and it worked, because this is how our aircraft and their systems for flight are designed. Plus, there is no doubt the earth is a spherical spinning mass.

    Let's take an aircraft trimmed for climb, throttles set for climb, what happens. Payne Stewart, his flight crew failed to pressurize the aircraft properly and the crew and passengers passed out because they failed to recognize their hypoxia signs. The aircraft was not being flown by anyone, and continued to climb set trim, no changes, and did not stall, but reached a service ceiling for the specific throttle setting. The aircraft ran out of fuel and descended and crashed. The plane automatically followed the curve earth.

    Accident report for Payne Stewart's aircraft, trimmed for climb attained max altitude with a set pitch/trim the entire flight. An altitude based on a set pitch/trim, set throttle, no adjusting for the curve of the earth. An aircraft has to maintain a pressure altitude usually, the trim changes to account for weight changes, CG changes, etc. With set throttles to maintain an altitude, as the weight is reduced/fuel burned, the plane goes faster, the pitch is changed to maintain an altitude. Usually an altitude is set, speed control by reducing throttles as we burn fuel. No pitch change for the round earth. If we gain lift flying east, the pitch would still be set, not changing due to the curve, and if flying west, it would be set. The effect of round earth would be related to lift, no need to pitch down to follow a curve. It makes no sense.
    Payne Stewart's accident report.
    https://www.ntsb.gov/investigations/AccidentReports/Pages/AAB0001.aspx
    https://www.ntsb.gov/investigations/AccidentReports/Reports/AAB0001.pdf
    The accident reports for Payne Stewart's aircraft, after cleared to FL390 the crew passed out and left the plane in a set trim, as you suggested, no change in trim/pitch. It automatically follow the earth's curve. I can trim an aircraft for level flight at a throttle setting, if I increase the throttle, we climb, if I reduce power, we descend. If I level off and hold altitude and increase the throttle we go faster at that altitude. etc

    There is no pitch down, anything related to the round earth would be a lift component, it is negilagable for most aircraft below MACH 3. , climbing, and descending flight.

    Aircraft don't pitch down, aircraft follow the earth at a pressure altitude, there is no pitch down, it would change the speed of the aircraft to "pitch" down, and you would not longer be at the right pressure altitude. A flat earth would not have anything like we experience. As you walk away from the center of the flat earth you would have to lean, and flying would not work the thickest atmosphere were be stuck in the center of the flat earth, with almost no air at the big wall of ice, and the water would all be at the center of the flat earth, or in silly reservoirs on hills, where the water would not be level. Thus flat earth has no gravity, but some crazy claim of density which has people and things accelerating toward the flat earth, no gravity. What is acceleration on a flat earth. Without gravity there is no air on earth, no flying, no joy. The OP level experiment, the person doing the experiment denies gravity exists.

    If we pitch down, we would go faster and faster, by the time we get to 6 degree pitch down, the aircraft would exceed the max speed and crash at high speed. At each instant we are flying at a set pressure altitude, we don't change the pitch or trim for a round earth as we fly, we remain in level flight.

    To understand pitch and power, take a look at this.
    http://www.boldmethod.com/learn-to-fly/navigation/pitch-and-power-on-a-glideslope/
    Pitch for airspeed, power for glideslope. Lower pitch go faster, higher pitch go slower, power to go up, reduce power to go down. It makes no sense to dip the nose to follow the curve of the earth. Dip the nose, pitch down, you go faster. Any lift associated with a curved earth is part of flying, and you would trim your plane for level flight, and in a perfect world, no weight change, no CG change, no pressure changes, no need to change anything once level with set power. Payne Stewart's plane was at a set trim/pitch, and it crashed when it ran out of fuel, and climbed higher as it lost weight, and most like.y oscillated in altitude, because things change. The level experiment is not evidence for a flat earth. A flat earth would be evidence... and gravity does exist.
     
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  2. EricL

    EricL Member

    Keith provided quite a long answer to this, but I'll try a more generalized approach. Remember that a plane in steady-speed, level flight has many forces acting on it, all of which are in equilibrium. One of those forces is gravity. In which direction does gravity act? The answer is, 'straight toward the center of the earth'. If you fly forward by a distance of two feet or two-hundred miles or two-thousand miles, gravity is still acting in a direction that is straight toward the center of the earth, and at all times, the orientation with respect to gravity of a plane in level flight remains the same. Since the plane in level flight is in equilibrium with gravity and all other forces at all times, "down is always down" as the plane travels, no matter how far it goes, and there is no need to adjust for that.

    What you are envisioning is level flight through the air as being a straight line through a medium, but it's actually a path controlled by balanced forces aligned in various directions, and a level flight path remains constant relative to the curvature of the earth since one of those forces that remains in perfect balance is gravity.

    You envisioned gravity affecting all parts of the plane, but what you did not consider is that the plane is balanced on its flight surfaces with respect to gravity. Therefore, any change in the orientation of gravity affects the orientation of the plane. IF your frame of reference is independent of the earth, then the orientation of the plane keeps changing as the miles go by. If your frame of reference is the earth, the plane always remains aligned with the surface that's below. Either way, gravity is the controlling factor.

    To put it another way, flying level means flying a course which is exactly perpendicular to the force of gravity. Think of a plane in level flight which is always balanced on its flight surfaces against the force of gravity, just like a teeter-totter that always remains balanced on its pivot point. If you put a teeter-totter, which was perfectly balanced and level, lengthwise on a flatbed truck and drove it across the country (on roads which magically were level at all times, or if you just made sure to make your observations of the teeter-totter when the road you were on was level), would you need to re-adjust its position every 50 or 100 miles just to keep it level? Of course not. Would gravity "pull the nose down" on the teeter-totter to keep it level as you traveled, as you said is the case in the unquoted part of your post regarding a moving car? No. If you can understand why the teeter-totter carried long distances on a truck will always remain level no matter how far it is transported, then you can understand why a plane in level flight does not need to pitch down to remain at a constant altitude or to remain aligned with the surface of the earth below. The teeter-totter remains balanced on its fulcrum with respect to the orientation of gravity, and a plane in level flight remains balanced on its flight surfaces with respect to the orientation of gravity.
    (Edited a few times for better clarity).
     
    Last edited: Jun 13, 2017
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  3. mcdouble

    mcdouble New Member

    Thank you for the detailed replies, but I believe I may have not put across what I was trying to say well. I'm aware of and agree with almost all that was said, I'm just not really sure about the idea quoted in particular.

    A teeter-totter will remain balanced as it moves across the Earth because it is at rest, and levered against a fulcrum. An aircraft may be balanced to sit level around its CG, but it is also acted on by many other forces, notably aerodynamic lift from its wings. It seems to me there could not be a perfect balancing of the aircraft in this way, although it could be "good enough" for us to never notice anything.

    Take my thought experiment again, I design a plane with locked control surfaces that will fly perfectly level in a wind tunnel. I then take it into Earth's atmosphere, point it 30 degrees up and fly it at the appropriate speed. Does it immediately pitch down and fly level relative to the ground? Or would it fly upwards, at least for a time? There must be some resistance to the plane being balanced around the CG caused by aerodynamics.

    Just to be clear again, I am in no way saying that there is any need to account for curvature in the real world, or that planes designed to fly across a flat Earth would not function completely fine on the actual Earth. I am just wondering about this from a purely theoretical standpoint. If I'm wrong and an aircraft will in fact perfectly balance itself around its CG regardless of external forces, then I retract anything I've said.
     
  4. Z.W. Wolf

    Z.W. Wolf Senior Member

    Edit: I'm just seeing that I'm restating some things that have already been said.


    I'm not sure this is valid, but aircraft have a center of gravity (which must be carefully calculated when loading).

    Thought experiment:

    Walk around a perfectly spherical planet holding a plumb bob. The plumb bob will always point to the center of the planet - which on this perfect planet is the center of mass. You don't have to adjust anything.

    The same thing would happen on a spherical magnet. If you hold a small magnet on a brass chain just over the surface of the spherical magnet, that small magnet will always point to the center even as you move it across the entire equator. No adjustment on your part is necessary.

    Now use a plumb bob that is shaped like a little airplane. The chain holding the model airplane will have to be fixed to the center of gravity of the model for it to hang in the correct attitude - parallel to the ground, as a plane in flight. Now take that model airplane plumb bob all around the earth. No adjustment will be needed to keep the model horizontal on the chain. It hangs from the center of gravity all the way.

    You could do the same with a balance scale. Even a balance scale with different reference masses on each pan - comme ça - will maintain the same attitude all around the planet without any adjustment.
    [​IMG]
     
    Last edited: Jun 13, 2017
  5. Z.W. Wolf

    Z.W. Wolf Senior Member

    For one thing it wouldn't fly upward very long without enough thrust. It would run out of energy and stall. Then it would pick up speed in the dive and level out.

    How much experience have you had with model airplanes? Balsa wood gliders were a big thing when I was a kid in the '60's. I played with a million of them. A kid quickly gets an intuitive feeling for how the center of gravity - and the way the wings and tail are adjusted - affects the flight of these things.

    I suggest getting some simple balsa wood gliders and see how they fly.
     
    Last edited: Jun 13, 2017
  6. Keith Beachy

    Keith Beachy Active Member

    30 degrees is steep, unless you are flying an F-15.

    Payne Stewart's plane was trimmed for climb, on autopilot (to keep the wings level and hold a heading). The plane flew at one trim setting, and after climbing through 30,000 feet to FL 390, the crew did not respond. The plane continued to fly with one set trim, with the autopilot holding the wings level. The plane had the throttles set at a climb thrust, and the plane climbed above 40,000 feet. When first intercepted the plane was flying at 46,400 feet, and would climb higher as fuel burned off. The aircraft reached a maximum altitude of 48,900 feet. When the fuel ran out, one engine flamed out first, and this asymmetrical thrust caused autopilot to disengage, and turn and stall, and ended up spiraling into the ground.

    Payne Stewart's plane is your test plane, it was set, no one touched it, it flew to a max altitude ceiling because it was set for climb, with climb thrust, but reached equilibrium of the max altitude for lift and power and the speed it was trimmed for. https://www.ntsb.gov/investigations/AccidentReports/Reports/AAB0001.pdf

    The Germans made your set plane during WWII. https://en.wikipedia.org/wiki/V-1_flying_bomb
    But it had a autopilot setup to maintain altitude.

    I don't need to make up a new aircraft with set controls, each time I level off, I would trim my aircraft for the speed and altitude, and it remained.

    Your test aircraft would, if stable, attain the airspeed you trimmed it for, and an altitude based on the power setting you pick, like the V-1. Your aircraft if it speeds up due to upset, or something, it would climb, slow down to the trim speed, enter a decent, speed up, climb, etc, hopefully seeking a stable speed and altitude. As you burn fuel, the plane will climb. When you run out of fuel, you would land at the trim speed.

    Not the curve of the earth. In the KC-135 we had a yaw damper to keep the dutch roll moment under control. What resistance do you have in mind.


    Source: https://www.youtube.com/watch?v=t9vpgxYJ9HE


    The aircraft is balanced because of the lift from the wings, there is a "perfect" balance, lift of the wings, gravity, drag, thrust. There is perfect balancing, pilots have thousand of hours seeking and maintaining this perfect balance.

    http://howthingsfly.si.edu/forces-flight/four-forces
     
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  7. EricL

    EricL Member

    This is a critical point, but I think, not in the way you expect. All of the forces which are not generated by the plane itself are external forces. So the plane is not balanced 'regardless' of external forces. It is balanced because of them, and you can't consider that form of balance if you ignore any of them. Though all the forces interact and will likely have some affect on what the other forces do, to look at it in basic terms, all the forces come in opposing pairs, and those opposing pairs are in perfect balance if the plane is flying at a constant speed, and level. Propulsive thrust is balanced by frictional resistance, and lift is balanced by gravity. And yes, if the plane is in this steady-state condition, the plane is balanced on the flight surfaces that carry it, and changing the orientation of gravity will therefore change the orientation of the whole plane, just like those examples in the post by Z.W. Wolf just above, and just like my teeter-totter example above.

    I'm repeating myself now, but keeping all these forces in exact balance is the thing that causes steady-speed level flight. Steady-speed level flight is not caused simply by propelling the plane through a medium with the control surfaces properly set. That method would only work in a system where gravity does not play a role in the process, such as the travel path of fish-shaped object in a fluid medium (perhaps a submarine), and only in a case where the fish-shaped object has exactly the same density as the fluid.

    I'm just guessing regarding your thought process now, but could it be that you are assuming that the plane can't be so perfectly balanced that it stays in level flight indefinitely? That's actually not relevant to the point, for the same reason that you can't make a car go a straight line forever simply by locking the steering system in the proper position. Minor corrections are needed and expected, but not because the model that explains what is happening is not accurate (we can talk about a car going a straight line if the steering is locked and understand why this is so, even if no matter how careful we are in setting the steering position, the car will eventually, gradually, veer off course. We understand the system isn't perfect in that way).
     
    Last edited: Jun 13, 2017
  8. mcdouble

    mcdouble New Member

    I'm not sure how a Learjet's autopilot works, but surely it doesn't just set a particular trim and then lock the controls? It would make constant adjustments to maintain a certain rate of climb or airspeed etc, depending on how it was set. And once it reached the ceiling altitude that was set, it would again make adjustments to maintain that altitude. The throttle being locked at climb thrust would have interfered with that process, but it's not the same thing as the control surfaces being locked in place.

    Likewise, I don't see how the V-1 demonstrates the same thing either, as it had sensors to detect pitch and adjust its course accordingly.

    I agree, I just think that one of the factors at play when finding that balance is that the Earth is every so slightly curving away. Even if that factor is so microscopic to be absolutely unnoticeable.

    I guess I'm having trouble with this idea. Such a plumb bob is levered against the ground, so it balances according to gravity. If it was orbiting the Earth it would not balance in such a way, just as an aircraft orbiting the Earth does not change its alignment relative to the Earth's gravity.

    Now, an aircraft in flight is obviously not in orbit, it is passing through the atmosphere. But clearly aerodynamic lift is not a force that is dependent on gravity, as we see when planes make sharp banking turns, or perform loops etc. So surely while gravity must be playing a part, there must also be a part being played by the movement of the airframe through the atmosphere, which does not change as the aircraft moves, while the effect of gravity does (as the Earth curves)?

    I feel that I am not explaining this very well, but I'm not sure how else to put it across.
     
  9. mcdouble

    mcdouble New Member

    Actually I was assuming the opposite for my thought experiment. My question basically boils down to: take a theoretical aircraft with control surfaces locked to make it hold level flight in a wind tunnel, or over an infinite flat plane. Now fly that aircraft over the Earth, with all other factors being equal. Will that craft perfectly pitch its nose to match the Earth's curvature and fly around the Earth indefinitely?

    My initial assumption was that it would not, but it would seem I may have been wrong.
     
  10. Hevach

    Hevach Senior Member

    Consider if it did not. At constant speed, air pressure is a major factor in lift, and you can measure changes in it over very small changes in height (a good electronic meter will register differences just within arm length).

    When the plane is even a tiny degree out of level and has risen even a tiny amount, air pressure has dropped and lift has decreased, causing the plane to nose down and lose altitude until it reaches equilibrium again.

    These effects are continuous in reality, the plane doesn't have to get out of equilibrium to return there like that. Once the plane is in equilibrium it will stay there unless something changes to put forces out of balance. This includes your thought that the plane would try to continue tangent to the ground - equilibrium would have to be broken for that to happen, either increased thrust or altered aerodynamics via control surfaces. And what would be happening would not truly be entering a tangent path, but simply one with increasing altitude. It would still be one conforming to curvature.


    Actually entering a tangent path is beyond this scenario. The velocity required to maintain that for even a short time leaves lift and aerodynamics secondary, you're even beyond the envelope for orbital insertion and in the realm of escape velocity.
     
    Last edited: Jun 14, 2017
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  11. mcdouble

    mcdouble New Member

    I guess I just don't understand why the plane would perfectly nose down when lift decreases, rather than it just... generating less lift. If an aircraft is trimmed to fly directly at the horizon, and I then pull back on the yoke and let go, it will not immediately snap back to flying at the horizon. It will keep going "straight", at the angle that I pulled up to, at least for a while. In the same way a plane trimmed to fly a perfectly straight line shouldn't follow the Earth's curvature perfectly.

    At this point I have to assume I'm just missing some fundamental thing though, so I will defer to what others have said.
     
  12. Trailspotter

    Trailspotter Senior Member

    Let's look at this thought experiment from a 'higher ground'. The Moon orbits the Earth always facing it by the same side. Why? Because it rotates around its own axis with the same angular velocity as it orbits the Earth. The conservation of angular momentum keeps the Moon's orientation relative the Earth fixed. The same is true for satellites on the geostationary orbit and any other near-round orbit around the Earth. They do not need to constantly spend fuel to keep their orientation relative the Earth. Once established, the exact amount of angular momentum will do the job perfectly.

    The track of a hypothetical plane, flying with the constant speed, 'straight' and level relative the round Earth, is also an 'orbit'. For the plane staying on this orbit, there has to be a tiny imbalance between the gravity and the aerodynamic lift in favour of the former. The exact difference depends on the plane speed. There also can be a preset tiny angular momentum that will keep the plane's orientation relative the ground fixed without any pilots' interference, provided this orientation does not create an external torque.
     
    Last edited: Jun 15, 2017
  13. Hevach

    Hevach Senior Member

    The aerodynamic surfaces stop it from just dropping, but the reduced lift puts lift out of balance with weight (i.e. gravity), so the net acceleration is down.
     
  14. Mick West

    Mick West Administrator Staff Member

    Only for a very short while - seconds in my experience of flying small planes. If the plane is in equilibrium then moving the controls and then releasing them will just return to the previously state of equilibrium. This is a property that is designed into most aircraft, with the notably exceptions of highly maneuverable planes like fighter jet and aerobatic planes.

    http://www.boldmethod.com/learn-to-...-of-static-and-dynamic-stability-in-aircraft/
     
  15. mcdouble

    mcdouble New Member

    Yes, but the moon acquired that rotation due to tidal locking, and satellites must be given it with an initial firing of their thrusters or use of gyroscopes. It will not just happen automatically.

    Right, this is something that probably applies more to very powerful aircraft. With a high level of thrust they are able to "overpower" the plane attempting to balance itself, and make steep climbs, banking turns etc. That is all I have been trying to establish really, that the combination of thrust + aerodynamic lift will determine the aircraft's orientation at least somewhat independently of gravity. That effect will be much less in a commercial airliner than a jet fighter, and much less again in a Cessna, but it still must be there to some degree.
     
  16. Trailspotter

    Trailspotter Senior Member

    Yes, but once the right amount of angular momentum is established, its conservation will keep the satellite facing the Earth by the same side without the need for continuous adjustment.

    The same applies to your thought experiment with a hypothetical plane flying in uniform air. There are several controlled surfaces that are used to set up the right amount of angular momentum after take off and ascent. At a level flight these surfaces are fixed in the positions, which do not create a torque of external forces, allowing the plane to 'adjust' its orientation relative the ground 'automatically' due to the conservation of angular momentum.
     
  17. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    "Gravity-gradient stabilization (a.k.a. "tidal stabilization") is a method of stabilizing artificial satellites or space tethers in a fixed orientation using only the orbited body's mass distribution and gravitational field. The main advantage over using active stabilization with propellants, gyroscopes or reaction wheels is the low use of power and resources.

    The idea is to use the Earth's gravitational field and tidal forces to keep the spacecraft aligned in the desired orientation. The gravity of the Earth decreases according to the inverse-square law, and by extending the long axis perpendicular to the orbit, the "lower" part of the orbiting structure will be more attracted to the Earth. The effect is that the satellite will tend to align its axis of maximum moment of inertia vertically."

    https://en.wikipedia.org/wiki/Gravity-gradient_stabilization
     
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  18. Fred Derf

    Fred Derf New Member

    Actually it's more like you're in a car driving along a road that has a banked curve and you have to hold the wheel straight in order to go around the curve. The pilot doesn't have to pitch down the plane does it automatically, a plane trimmed for level flight will stay level with the center of earth's mass with no inputs to its controls. The controls would only have to be adjusted to change away from level flight.
    Note the above assumes a perfectly consistent non-moving atmosphere, in reality the plane's controls must be continuously adjusted to account for variation in the atmosphere. But they're never adjusted for curvature.
     
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  19. Fred Derf

    Fred Derf New Member

    Pretty much. A plane is balanced on its lifting surfaces, it will stay balanced as it transits around the earth.

    Think of a model plane hanging from a single string balanced so the plane hangs level. Take the model (and string) and hang it in your car, then drive to another location (say from the New York to Los Angeles) the plane will swing around (of course) because of the vehicle accelerating in various directions but whenever you stop and allow the plane to settle down, you will never have to adjust it to get it level again. It will remain level. Just like a plane in flight.
     
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  20. Fred Derf

    Fred Derf New Member

    You'd have to explain how gravity works on a flat plane before I could make such a concession. I can concede that the results exactly match what we would expect to see on a spherical earth.
     
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  21. Fred Derf

    Fred Derf New Member

    Not possible.

    Gravity doesn't stop just because a vehicle isn't touching the ground anymore. A plane is just as affected by gravity while in the air as it is on the ground. A plane is balanced on its lifting surfaces and will maintain level relative to the earth as it flies.


    So, a plane.

    Yes, you got it in one.

    Again, the plane is balanced on its lifting surfaces. It will remain balanced as it transits around the earth.

    They don't compensate for the curve of the earth. It's automatic. Take a bottle, filled most of the way with a liquid. See that the top of the liquid is level with the pull of gravity in your location. Take the bottle somewhere else on the surface of the earth (say Australia, unless you're in Australia in which case say Germany), look at the bottle and see that the liquid is still level with the pull of gravity. You didn't have to compensate for it, did you?

    That might be an easy answer but it's wrong. A plane in flight is balanced perpendicular to the pull of gravity and will remain that way, no adjustment is necessary to account for the curvature of the earth.
     
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  22. Fred Derf

    Fred Derf New Member

    It won't immediately pitch down bit it will immediately start to pitch down. A plane is balanced on its lifting surfaces. Think of a model plane hanging balanced from a tread such that it's level. Tilt it up and let it go, what happens? It falls back to level. The whole point of trimming a plane to fly level is so that it will fly level (i.e. self correct).

    Now in reality there are a lot more forces operating on a plane and it wouldn't be this clean but we're ignoring them for this thought experiment.
     
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  23. Fred Derf

    Fred Derf New Member

    It adjusted to account for atmosphere, not for gravity or curvature. Gravity doesn't change enough to worry about and curvature is handled automatically by the plane being balanced.

    You may think that, but you're wrong.

    So why did you bring it up?

    Aerodynamic lift is a force used to counter the force of gravity. It's not dependent on it, it opposes it.

    It's balance. The plane is balanced. The plane will remain balanced as it transits around the earth. It really is that simple.
     
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  24. Fred Derf

    Fred Derf New Member

    You've just changed the scenario! We were discussing a theoretical plane with fixed controls flying in a theoretical perfect atmosphere and now suddenly you're talking about a real fighter jet flying in the real atmosphere with variable controls. It's no wonder you're getting confused.
     
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  25. Excelllinkus

    Excelllinkus New Member

    Imagine that Darryle Marble's world view was correct, i.e., that in the case of "if the world was round," the bubble in the spirit level would move as the aircraft followed the curvature of the earth. Say you flew from Houston (30° North) to Minneapolis (45° North), keeping the spirit level on your tray table and watching it move (in D. Marble's world) from the middle to the end of the bubble tube. According to D. Marble's model, when he landed in Minneapolis, all liquids in all containers would be tilted at 15° and would stay that way. If you had, say, an upright bottle of Coke, the liquid would be titled at 15° away from horizontal. You would have to compensate for the ground being tilted at 15°. Yet mysteriously, the water in Minneapolis would be level to the ground, or would it?
     
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  26. Mick West

    Mick West Administrator Staff Member

    Essentially yes, if you take his experiment to its logical conclusion. I'm not sure he's done that though.
     
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  27. Z.W. Wolf

    Z.W. Wolf Senior Member

    Yes, I think this is very much the way he pictures things. Down is an immutable direction. Level is immutable. A plane surface.



    If he took it this far, he might think of this as a kind of reductio ad absurdum.
     
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  28. mcdouble

    mcdouble New Member

    Not entirely sure why you decided to resurrect this thread as you are just repeating things that had already been said. I still have not seen any reason to believe that an aircraft's aerodynamic balance from air flow over its lifting surfaces should be exactly equivalent to its gravitational balance around its centre of mass.
     
  29. Z.W. Wolf

    Z.W. Wolf Senior Member

    That's not really the issue. My advice to you is to start with the basics and educate yourself to think in terms of kinetic energy and potential energy in general, and then kinetic energy and gravitational potential energy. Every object that gains altitude uses kinetic energy to do so, and gains gravitational potential energy.

    The missing ingredient in your thinking always seems to be energy. When the plane loses lift it is falling and converting gravitational potential energy to kinetic energy. It falls straight down. There are two velocities. One forward, and one down (due to gravity). It can lose altitude, fall, without going nose down. In WWII that was called "mushing through the air." The plane is level, but losing altitude. It doesn't have to fall nose down and gravity doesn't have to pull the nose down selectively. It's pulling the whole plane down, just like any object.
     
    Last edited: May 24, 2018
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  30. MikeC

    MikeC Closed Account

    Not quite - you can convert between types of energy - you can gain altitude by sacrificing speed - ie exchange kinetic energy for potential energy. Always assuming you have sufficient kinetic energy to do so and still maintain enough lift.....
     
  31. Z.W. Wolf

    Z.W. Wolf Senior Member

    This is the way I think of it. There are people here much more knowledgeable in physics than I am, so I welcome corrections.

    In the case of the plane traveling around the globe: If it is flying level, it's actually falling straight down at the same time... slowly. At exactly the rate the curved surface below it is falling. The plane flies level and stays level with the ground, though following the earth's curvature. Something like an orbit.

    Thus a spirit level on board will always be level.
     
    Last edited: May 24, 2018
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  32. Z.W. Wolf

    Z.W. Wolf Senior Member

    Yes.

    I deleted that part, because it was getting too complicated. I really should learn to delete the whole post when I'm doing complicated re-re-edits, then just do a new post; so that people don't catch me with my pants down like this.
     
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  33. Z.W. Wolf

    Z.W. Wolf Senior Member

    Example of a glider falling straight down... slowly. It doesn't have to fall nose down.



    I anticipate the objection that it isn't following the earth's surface - if it were following the earth's curvature it would have to nose down. But that's not true. On any part of the earth's surface, it would still be falling straight down. The local straight down. Gravity is always affecting the whole plane, not any part of it selectively.

    Another example: If you walked around the world with a plumb bob in your hand the plumb bob would always hang straight down. The local straight down. There would be no need for you to adjust it.

    Thinking in terms of energy: It was given kinetic energy (the throw), it gained gravitational potential energy (the climb), it converts gravitational potential energy to kinetic energy (the glide).
     
    Last edited: May 25, 2018
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  34. Nth

    Nth Member

    I suppose this conversation is getting a bit off-topic from the spirit level, but I'll just throw in my two cents Re: planes needing to nose down. Please refer to the follow image:

    Metabunk 2018-05-25 14-24-45.

    The lift force acting on the airplane is proportional to the medium density, the square of the velocity, and the wing area, all multiplied by a lift coefficient. The lift force directly opposes the gravitational force, or weight, of the airplane as given by Newton's third law, F=ma. In this case, we'll consider it as W=mg, or the weight of the airplane being equal to the mass of the airplane multiplied by local gravity. Gravity varies with altitude and latitude, but the variance is probably small enough that g can effectively be considered constant.

    Upon reflection, the plane would lose mass as it burns fuel, but for simplicity's sake we'll ignore that too. In this case, then, the airplane must maintain a certain velocity within a given medium density to stay aloft. Because air density decreases with altitude, and engines can only provide so much thrust (yielding a top speed), there is a point where the plane will be physically unable to ascend any further.

    So considering the atmosphere to be made up on concentric rings of decreasing medium density on a spherical earth, the plane would have no choice but to follow the curvature. And, as the direction of the gravitational vector changes, if I'm not mistaken, a slight torque would be applied to the airplane pulling it back to level. I'd need to go over the math on that more extensively, though.
     
    Last edited by a moderator: May 25, 2018
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  35. Kevin McMillen

    Kevin McMillen New Member

    "Fuzzy Math" that is what a FEer uses to "prove" their point about airplane flight.

    First we all know many forces go into play during a flight, but honestly we don't have to worry about any of them to debunk this.

    I'm using 600mph to simplify things.

    A FE would correctly calculate the drop in 600 miles as 240,000ft. Now they go to fuzzy math. Since the plane is going 600 miles in an hour they divide 240,000 by 60 and get 4,000 ft drop per minute. Divide 4000 by 60 seconds and you get a drop per second of 66.7 ft per second.

    Anyone would notice that right? a spirit level should show that. Right?

    Let's now use correct math. 600 mph is 10 miles per minute. That would be 1 mile in 6 seconds.

    Ok, now we have 1/6th of a mile per second. That 's .1667 x .1667 x 8 = tada .222 inches.

    Yep, that's less than 1/4inch per second. Anyone going to notice that?

    Ok, let's go further. Just how much would a plane have to nose down to fly around the world. Remember we are forgetting gravity cause after all "it doesn't exist".

    Remember that 1/6th of a mile drop is .222 inch. That 1/6th of a mile is 880ft. Most commercial planes are at or just over 200ft. That's four plane length per sixth of a mile.

    If we want we could just divide that .222 by 4 and get .0555. That's less than 1/16th of an inch in the length of the plane that it would have to "nose down" to travel around the whole world if it had enough fuel.

    Couple things though probably miniscule. The calculation is for earths circumference, diameter, and drop. Planes fly at about 5 miles, so we'd have to add ten miles to the diameter, the circumference that it flies would be longer, so wouldn't that make the drop per second even less?I

    Yes, I'm anal, here goes.

    A plane 220 ft long is actually 1/24th of a mile. That's .0416 so.......... .0416 x .0416 x 8 = .013 of an inch. 13 thousandths of an inch drop in the length of the plane. That's less than 1/64th of an inch that the plane would have to "nose down" consistently throughout the entire flight.

    I'm a bricklayer. Any bricklayer knows that if they can get a 50ft foundation within an eighth of an inch level they are doing dang good. No level will show that much less 1/64th in 220 ft.

    Kevin
     
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  36. DavidB66

    DavidB66 Member

    As there has been some mention here of balance scales, it may be worth saying a bit about how they maintain their balance. (There is a Wikipedia article on 'Weighing Scale', but it rather neglects the important issue of stability.) There are many discussions of this in old science texts, written when scales were more widely used than they are now.

    A balance scale usually has a straight and symmetrical beam pivoting on a central fulcrum. If the weights suspended on each side are equal, it will be in equilibrium when the beam is horizontal. However, this is not enough to secure stability. This depends on the position of the fulcrum. If this is under the centre of gravity (mass) of the beam, depressing one side of the beam brings more of the mass of the beam onto that side of the fulcrum, which depresses it further, in a positive feedback loop. It is therefore unstable. This is usually the case with a simple see-saw, which is essentially a beam on top of a roller. If the beam is slightly displaced, it will fall to one side or the other, with no tendency to return to its original position.

    If on the other hand the fulcrum is above the beam's centre of gravity, the equilibrium should be stable. Depressing one side of the beam then moves some of its mass to the other side of the fulcrum, which tends to pull the beam back to horizontal. Historically, various types of balance were designed to enhance stability. For example, mass might be added to the beam below the fulcrum, so that it would react more quickly to any displacement. Alternatively, or in addition, the beam might be bent or angled so that when one side is raised the vertical force of gravity acts with greater leverage to pull it down. The beam might also be adjustable to raise or lower the centre of gravity. There were many variations in design, depending on the purpose of the scales and the trade-off between different objectives.

    If a well designed balance scale in stable equilibrium is transported around the curved surface of the earth, it will adjust itself automatically to the changing direction of gravity. The effect of the changing direction of gravity is equivalent to a displacement of the beam from horizontal, and equilibrium is restored accordingly.

    Whether the same process would also apply to an aircraft is not so clear. As many comments have pointed out, the stability of an aircraft depends on a dynamic balance of forces. The shape and distribution of weight of the aircraft is far more complicated than that of a balance scale, and there is no single fulcrum. However, the distribution of weight in relation to the wings (the nearest equivalent to a fulcrum) must be one of the factors affecting an aircraft's stability. The design might conceivably be sufficient in itself to ensure that the aircraft remains in level flight as it moves round the curve of the earth, and even where it is not, it may be a contributory factor. This need not be something that the designers or users of the aircraft consciously aim for, but it would be a by-product of the quest for a stable design in general.
     
  37. Mick West

    Mick West Administrator Staff Member

    Beware of over-complication.

    All that's actually happening in the real world is the pilot (or the auto pilot) sets the controls so the plane maintains level flight. "Level" for flight navigation means flying in the same external air pressure, and not the same distance above the geoid. The shape of the earth makes very little difference to this control setting. The setting will need frequent small adjustments (up and down), but not because of the curve of the Earth.

    Theorizing about a no-adjustments flight on some idealized perfectly spherical world with no weather (meaning no sun) is fun, but not really helpful. Even on such an idealized and impossible planet the fluid motion of the atmosphere is probably too chaotic to actually maintain altitude for ever.
     
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  38. Kevin McMillen

    Kevin McMillen New Member

    Correct, not helpful to us. We accept all the forces, but a FEer doesn't, so if we are going to successfully debunk their tests, we must debunk as they understand flight.

    First, I know of no plane that flies perfectly level. Up, down, faster, slower, bumbs (turbulence) along the way, etc. Just watch that bubble in the video move.


    This is why I merely gave the drop for the length of the plane. 1/64th of an inch in 220ft. Less than 1/4 inch per second of flight instead of 67 ft as they claim.

    There's no point debunking anything if we can't make it easy to understand by those who don't believe in gravity.

    Kevin
     
    Last edited: May 27, 2018
  39. Kevin McMillen

    Kevin McMillen New Member

    Have been considering how a fairly straight airplane at about 220ft would fly level on a round/globe earth.

    Technically, level is not a straight line on a globe.

    Consider a straight line 220ft long. The left end we'll label point a, that will be the front of the plane, the exact center point b, the right end point c.


    If the center, point b is at 30,000 ft elevation and the line is perpendicular from that point, then point a's elevation would be 30,000ft plus .0035 of an inch. The drop in 110ft would be about .0035 inches. The elevation of point c would also be 30,000 ft plus .0035 inch.
    Moderation: deirdre
    see Math edit in post #80


    If both point a and point c were at 30,000ft then the elevation of point b would be .0035 inch lower than 30,000ft but the line is still straight.

    If point a represents the front of the plane and is at 30,000 ft, point b the exact center and we'll imagine point b also as the center of gravity also at 30,000 ft. Point a, the front of the planes drop from point b would be .0035 inch though at the same elevation.

    Point c would be .0035 inch higher in elevation than point b, and as I said in the first post the drop from point c to the front of the plane would be about 1/64 th of an inch.

    Now that's in a perfect world with the only force being gravity.

    In reality if you place a dinner plate on a vertical 1/16 inch diameter steel rod and balance the plate then you basically have an idea of a plane in flight. You'll have different forces pulling, pushing, etc. in every direction.

    Kevin





    .021
     
    Last edited by a moderator: May 29, 2018
  40. Kevin McMillen

    Kevin McMillen New Member

    Being new to the forum I'm unable to find the edit function. Is editing only possible for a short time after original posting?

    Anyway, in my post #79 I mistakenly said that if the front of the plane was at 30,000ft and the center was at 30,000ft then point c, or the rear of the plane would be .0035 of an inch higher in elevation. I'm pretty sure that should be .007 of an inch higher or 30,000ft plus .007 of an inch.