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Newton? or Bernoulli?

Discussion in 'AviatorChat.com' started by airfoil, Feb 9, 2008.

  1. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    It may be time to start that old discussion (sic) again. When I was twelve years old, my friends' father, Captain Hank Evans (Pan American), tried to teach me the fundamentals of something he called "lift". I've always been the rebellious sort, so I listened attentively before I asked him some hard questions.

    Is it the job of the Wing to split the airstream into Above the Wing and Below the Wing components?

    When equal "amounts" of air divide, will the Wing Lift, anyway?

    Why on your Boeing is the Front (Leading) edge so fat and so thin at the Back(Trailing Edge)?

    Wasn't Bernoulli's field Hydraulics? Didn't he describe flow of a liquid in a pipe?
    What has that got to do with a Wing supporting an aircraft on a dynamically viscous "cushion" of air?


    I'm now a LOT older, but still sceptical as to why anyone would want to invoke the name of a Swiss Plumber in a discussion of the basics of Lift.

    This is my personal explanation of a Lifting Wing. Work, (Thrust) propels an airplane to a speed where the air beneath its wing has fewer and fewer options as to where it can flow, and cannot quickly enough "escape" the under portion of the moving wing. In a temporarily viscous state, it will support the aircraft until the wing moves further along and insults newer regions of Air. Now comes the Bernoulli guy, insulted at my audacity. "Wait, you forget the "vacuum" on top of the Wing!" He will argue at great length that it actually is NOT the "temporarily viscous" air under the Wing that is supporting the aircraft, but the "temporarily" thin Air atop our aluminum plank that is Lifting it.
    Gotcha! And that thin, weak Air is "lifting up" the Wing to "create Lift"?
    What is it holding on to? Do the Air Molecules have sticky mittens?

    There is a far longer way to frame the discussion, but in the interests of all,
    why don't we leave mathematics out? That debate is a draw, and causes average people to get "Glazeneye" .

    Bernoulli: Swiss Hydraulics guy
    Newton: Push back Third Law guy.

    If this Bait doesn't cause at least a mild uproar, this website is on life support.
     
  2. planeflyr

    planeflyr Karting

    May 27, 2006
    174
    This is a very common question about some principles which are not fully understood or appreciated.

    The actual answer, however as to what keeps an aircraft in the air is....

    MONEY!

    But there are some physical forces working on a wing which allow said money to do its job.

    The principle of fluid mechanics (and after all air is, in fact, a fluid) which Bernoulli described states that if you split a volume of a fluid and one part of that volume has to travel a further distance than the other volume before they recombine, then it must move faster to do so. As it moves faster, it's pressure decreases. This is what a wing does as it's cross-sectional shape has a bump or "camber" at the forward part. This camber provides a greater distance for the fluid air to travel to make it to the trailing edge and it therefore has to accelerate to do so. As such it's pressure decreases. With the lower pressure on top of tha wing and a higher pressure below it the wing is now being either pushed from the bottom or sucked from the top depending on how you wish to think about it.

    If you don't believe that this works, try this little experiment right now. Take a piece of long-ish lightweight paper and bring the edge to just below your mouth. BLOW HARD across the top of the paper making sure you don't blow across the bottom. The paper will lift up and be horizontal. Your fast moving air across the top of the paper lowers the pressure relative to the bottom of the paper and generates lift.

    Wings have many different shapes and develop lift in different ways, including angle of attack and angle of incidence.

    More to come.

    Planeflyr.
     
  3. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
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    Russ Turner
    There are also many misunderstood physical characteristics, as the laws of physics seem to vary depending on who you fly for.. A friend of mine, a superb f-15 pilot in the New Orleans Guard, never flew outside the hours of 8 AM to 4 PM, and only in good weather.
    When I asked this sage fighter pilot about his choices, he calmly responded "We in the Air Guard recognize that dark air has no lift"
     
  4. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    I would say that the Air above the Wing accelerates, and therefore loses pressure relative to the airstream beneath it, but just because that is true, don't assume that the Wing cares even a little bit. In a Wind Tunnel, Air can be pumped back into the superior air stream, to equal the value of pressure beneath it, and guess what? The Wing still lifts! It is also accurate to say that the "Fat" leading edge creates a functional angle of attack, relative to the lower surface. But back to equal pressure "Lift". First you must promise not to abandon Monsieur Bernoulli. Beneath the sea's surface, a submarine wishes to change its angle of rise, and pitches up its bowplanes. Liquid water does not compress, (not for our purposes here), and the Bowplane has a symmetrical chord and Foil. Water flowing over the top of the surface travels as far as that underneath, and the huge ship angles up. Pressure had nothing to do with what happened, not the kind Bernoulli Lifters utilize, anyway. What happened? Ask Newton. The Plane is "Inclined" forcing water
    down, and itself up. This is exactly what happens when a flying Aircraft, above ground-effect and speed above Stall stays airborne. Converting thrust to Drag to Lift. It is my position that the Low Pressure area above an airplane Wing is a byproduct of its angle of attack, a RESULT of the Mechanics involved in its Flight, not the genesis of it.
     
  5. Bob Parks

    Bob Parks F1 Veteran
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    Nov 29, 2003
    6,684
    Shoreline,Washington
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    Robert Parks
    DON'T ever ask an aerodynamicist how a wing lifts. You will be plowed under by a blizzard of equations and gibberish that makes no sense to the normal individual. The air flowing over the curved upper surface succumbs to the rule that an increase in velocity produces a decrease in pressure ( the particles of air are spread thinner). The flow under a wing with a curved upper surface is not spread so thin so it has higher pressure and pushes upward towards the lower pressure. Another element is the down flow aft of the wing that produces a reaction in the opposite direction...UP. A symetrically curved wing flies because it flies with a slight angle of attack and there is still a differential of flow over the top vs. the bottom. A single cambered wing flies the same way when inverted, with an angle of attack.
    If you can be fortunate to see a big tin bird taking off on a wet cold day you may be able to see the pronounced down flow of condensation aft of the wing. Even when the flaps are retracted there is strong down flow. Newton would be proud.
    Switches
     
  6. planeflyr

    planeflyr Karting

    May 27, 2006
    174
    Ah, yes. There is certainly more to aerodynamics than the brief and well known explanation of the airflow principles of a common cambered wing.

    There are symmetrical wings, wings with different chord arrangements, long/skinny wings, and short/fat wings. And there are wings which tend to defy explanation.

    Wing design is mostly a fit to the aircraft's intended operating environment and the speed at which it will fly.

    Long/skinny wings are suitable for high altitude or low power operations, creating a lot of lift in thin air or under slow airspeeds due to their airfoil shape and surface area. Examples of these types of wings are used on the venerable U-2 spyplane and on sailplanes.

    Short/Fat wings you see on most general aviation aircraft as a compromise in the airspeed/altitude vs. lift category. From "hershey bars" to semi-tapered, they are all quite similar. Think Piper Cub, Beech Bonanza, Cessna 182.

    And then there are those which defy explanation to all but Kelly Johnson.

    The F-104 [man-in-a-missile] Starfighter was one such aircraft. The wings are exceedingly short and thin and, I believe, were symmetrical in airfoil shape as well. (someone correct me here if I'm off on this one) One wonders how it stayed in the air. It often didn't! At that speed, a small angle of incidence will provide lift via pushing on the underside, just like when you stick your hand out a car window while driving and rotate it. The F-104 was KJ’s successful attempt to prove that the oft stated cliché that with enough power you can fly a barn door was physically obtainable. A very small barn door at that too!

    Submarine diving planes are frequently built in an airfoil shape as are the keels of racing class yachts, the latter having designs which are some of the most coveted secrets in the sport. The reason is identical of that for aircraft, reduction of parasitic drag and improvement in the aerodynamic physics of traveling through the fluid medium of seawater.

    In summary, it is not about Bernoulli vs. Newton. It is all Newtonian physics which govern the aerodynamics of flight. Bernoulli’s discoveries are merely one of many applications of Newtonian physics.

    Planeflyr
     
  7. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    The stubby wings of the F-104 are notable in at least two ways. They are attached to the fuselage Anhedrally (drooped down) to purposely destabilize the aircraft in Roll Axis. Second, they are "virtually" symmetrical in airfoil shape. Very thin, Bernoulli (Had he ever actually seen an airplane, or imagined one), would scratch his head. They offer no explanation or deference to the stubborn accolytes of the Swiss Plumber. Which brings me to why MOST wings have a fat front. The 104 launches at 200 KNOTS, and not with alot of confidence instilled in it's pilot. My Skylane eagerly breaks ground at 80, thanks to the induced angle of attack of its upper wing surface, and Msr. Bernoulli. At 120 Knots, I'd like to shed the fatty planks for skinny high aspect ones, but that's not possible, or practical. I fly two airplanes at the same time, as do most pilots. Compromises in low speed handling/safety and high speed cruise/economy, most GA platforms do a great job of low landing speed and cruise speed compromises.

    As an example, let's look at PlaneFlyr's example again. The F-104 is a Mach 2
    straight line interceptor, created to confront Ivan's Tupolevs. The U-2 is a subsonic high altitude look down spy with hours of loiter and a potty.
    Both were designed and built by Kelly Johnson at Lockheed. At virtually the same time. They are the Same Aircraft, literally. Except for powerplant and wings, they are identical. Goes to show the difference a wing makes.

    At low speed, Bernoulli trumps Newton; at high speed Newton wins. This isn't precise, but for most reasonable people, it suffices to explain Lift, and most flight regimes.
     
  8. boffin218

    boffin218 Formula Junior

    Oct 8, 2005
    817
    Philadelphia
    Full Name:
    Chris

    Both the Bernoulli and Newtonian explanations are right, but neither offers a complete explanation of how wings lift.

    And here we should note that the description above of Newtonian flight theories is a bit awkward. More typically it is explained in terms of 'turning' (or changing the direction of) a flow. Which isn't so much 'forcing' air down as redirecting its flow. (If it were a matter of forcing, the flapping of flat panels would be an effective means of flying)

    The 'Newtonian' part of this comes with his third law (often summarized as: for every action there is an equal and opposite reaction). In order to change the direction of a flow, force needs to be applied. In the case of our submarine, deflecting the flow downward produces an upward force: lift.

    Now, here's the rub. Nice though it is, this explanation is concerned with momentum - with questions of mass and velocity - and its conservation. It neglects two of the three principles that govern gas flow. When we discuss the movement of gases our equations must conserve three things: mass, energy, and momentum.

    Here's where Bernoulli comes in. His equations were designed to go beyond Newton and provide a solution that conserves energy, and explain (in part) the forces that act on a wing.

    Add in conserving mass - which yields bizzarro findings like the fact that changing the velocity of a gas in one direction also yields a change in the gas' velocity perpendicular to the original change - and we come to a more complete explanation.

    Which is embodied in the Euler equation (which neglects viscosity) and the Navier-Stokes equations (which don't)).
     
  9. Bob Parks

    Bob Parks F1 Veteran
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    Nov 29, 2003
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    Robert Parks
    In the late 60's the RCAF demonstrated the F-104 every year at the Abottsford air show. Usually had four airplanes and a back up. I talked to the pilots quite often and could hardly believe the figures they were quoting. The most interesting, and scary, was the dead stick routine. After a flameout the pilot had to maintain speeds of over 245 knots or the airplane became a streamlined brick. As one guy put it, " It would respond to the controls but it would not assume a change in trajectory." In other words you could do anything and everything with the controls but you were just a passenger riding it down.
    The each wing panel was seven and one half feet in span and composed of machined aluminum planks, a total of six or seven planks in each panel bolted together to form a diamond shaped symmetrical airfoil. The leading edge radius was equivalent to a dull kitchen knife and when on the ground a protective sheath was put on them for protection....you and the airplane.
    One of the most exciting things that they did was a series of touch and go's in front of the crowd, You could gain an appreciation for the speed of the approach, the thud of contact with the runway, and the blast of after burner when they left. Take a look at www.parkzart.com for my impression of an F-104.
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  10. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    Boffin, I am unable to connect your paddle visual w/ Newton, or any.

    Some other questions?

    1. Conserving momentum? Don't you mean conserving energy?
    2. Conserving mass? Only applies in nuclear physics, not Newtonian
    3. Bernoulli was born in 1700, and safe to say never saw other than a bird's wing, his equations apply to airflow how? (according to him?)
    4. Bernoulli applies to Inviscid media; believers claim air is not functionally
    compressible below velocities of 200 knots. (Nothing to "pack" against: this is patently false)

    To be consistent, I'm hoping to keep the discussion within the "perceivables" range of the majority of participants, Euler? Navier/Stokes? Why not Coanda, Blown Wing/ vectored thrust, etc.
    I am sticking to my theory (mine) that Isaac trumps Daniel in all aspects of Aero"dynamic" Flight. If you are one who insists on parsing Diverting vs. Turning Airflow, we might not be able to debate coherently.
     
  11. drjohngober

    drjohngober Formula 3

    Jul 23, 2006
    1,930
    Cville and Gbury Tex
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    Dr.John Gober
    Just did a ground school on Bernoulli and Newton's laws as it relates to helicopter blades ( which change angle of attack twice as making one revolution). Trying to figure out how to answer. Let me work on it, could be a masters thesis...
     
  12. boffin218

    boffin218 Formula Junior

    Oct 8, 2005
    817
    Philadelphia
    Full Name:
    Chris

    Such are the horrors of quickly written responses on complex subjects.


    The flapping paddle is a reference to your description of a submarine 'forcing water down' in a post above, which isn't quite right. There are forces involved, definitely, but the force is related to flow. I chose the 'air paddle' description because a) we've all seen it before in very early attempts to fly and b) it hopefully made readers think about 'forcing' air/water.

    1) If we look at a 'Newtonian' description by which a wing/airfoil alters the flow of a fluid (water/air) and, in return, is forced upwards, what we're typically looking at isn't actually an energy equation but one of momentum (mass * velocity), which can help us calculate lifting force.

    The Bernoulli equation most commonly referenced was derived from/ relies on conservation of Energy.

    In practice, the two aren't far removed, which is part of why the pressure differentials aren't a byproduct of lift - aren't a secondary phenomenon - but are part and parcel of it.

    2) No, I meant conservation of mass. In this case, it is mass flow rate (density * area * velocity) - and it is very much a part of Navier-Stokes' continuity equation:

    Continuity: dr/dt + d(r * u)/dx + d(r * v)/dy + d(r * w)/dz = 0
    (where d is a partial derivative, x/y/z are spatial coordinates, r is density, t is temperature, u , v, w are the x,y, and z components of the momentum vector)

    3) That Bernoulli's equations weren't used by him for airflow isn't really germane. Newton's laws weren't designed to study airflow either, and Newton certainly didn't try to explain aerodynamic lift in his lifetime.

    Bernoulli's work owes much to Newton, and in fact was a continuation of Newton's study of fluid flow.


    4) I am not trying to make a 'trumps' argument, nor I do believe it is productive to do so given the fact that both men describe fundamental elements of aerodynamic lift. (i.e. Bernoulli's observations are not a "result of the mechanics of flight").

    If you prefer it can be said this way: Bernoulli's equations relate pressure to a gas' local velocity, solving for pressure. Newtonians can examine changes in the velocity of a gas, and thereby derive a picture of the force that is acting on a airfoil. In practice, integrating changes in either velocity or in pressure will give us the aerodynamic force acting on a surface.

    In short, both views have something to offer and pictures of aerodynamics that rely on one tend to oversimplify. (most Bernoulli explanations underestimate the velocity of airflow over the top of an airfoil, most Newtonian explanations neglect the upper surface of the airfoil's contribution to lift)


    I don't share your conviction that 'turning' is something people can't 'perceive' (or visualize). I may be wrong.
     
  13. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    Now yer talkin aircraft. I have a Patent Pending on rotorcraft directional control. My thing is the Boom, and Tail rotor, but let me ask a silly question about the main rotor disc. Does it change AOA twice /revolution? I think just
    once, unless your cyclic (Swash) plate looks different than mine. Or did you mean that it's AOA changes Dynamically, once per rev.? What are you flying, Robby? Where? You lucky dog. My instructor had the same question for me after every preflight. Bill, you ready to cheat Death ?
     
  14. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    Well said. We're down to just a couple of small disagreements. Conservation
    of energy should include "mass", as I said, Physics, other than Nuclear, requires that all mass be accounted for at all times. "Turning" fluid, as opposed to "forcing it in another direction" ( which I think was obvious )
    really wastes time in understanding common ground. I'm sure you knew what I meant as did everyone else, and Maybe I overreacted when you compared "forcing" to "powering" or "paddling". Also, was it you who claimed that Submarine bowplanes have an "airfoil" ("hydrofoil")? That would be silly, since the plane has to have the same capabilities "up" OR "down". Same for aircraft elevators and ailerons, both of which articulate and would not work well if prejudiced in one or the other "direction". Lastly, as you know, D. Bernoulli's field was Hydraulics, he did no work, theoretical or actual relative to Aerodynamics. Newton did posit his Third Law in the gaseous realm, though predating Bernoulli. Do you actually own a Staggerwing? If so, you can be right about anything, for the cost of a ride? I think both gentleman have alot to offer any discussion; what a shame neither saw an aircraft fly. Can You Imagine?
     
  15. Bob Parks

    Bob Parks F1 Veteran
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    Nov 29, 2003
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    Robert Parks
    I thought that the retreating blade increased AOA and then as it enters the advancing quadrant, it decreases AOA. Ain't that twice ?
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  16. airfoil

    airfoil Karting

    Feb 1, 2008
    50
    Well, yeah. It was late, I'd just gotten back from the Hospital, and I was in low gear. Focused on my Tail, and knowing that the Disc "cycles" once per revolution in cyclic, my synapses went into the Big Wonderland. May be that's why I quit flying the thing. Seriously, helos are a fascination, my Lawyer flew Blackhawks, now a Beech Diamond, don't show him my post. Is it time to switch to Thrust instead of Lift and Einstein instead of Bernoulli?
    Switches maybe you know, does the "advancing" blade change position on the Disc depending on the direction you wish to fly? I saw your Artwork on your Mac site... breathtaking.
     
  17. Bob Parks

    Bob Parks F1 Veteran
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    Nov 29, 2003
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    Robert Parks
    My knowledge of the whirlygigs is limited and I should not pontificate on their operation. I think that to move in a direction , the disc is tilted thus changing the vector and to stay aloft the blades maintain their cyclic changes to keep even lift. I just finished my first morning cup of coffee and I'm not the sharpest senile in the ward yet.
    Thanks for your comments on the art. I'm working a lot of new stuff so I'd better get to it. BTW, I'll bet that Boffin can answer your question re disc changes of AOA.
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