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Hump on an aeroplane can be on the bottom

  1. Sep 25, 2010 #1
    "hump" on an aeroplane can be on the bottom

    https://www.physicsforums.com/showthread.php?t=133173


    On this question, someone stated that the "hump" on an aeroplane can be on the bottom. If the hump is on the bottom, surely the pressure difference would suggest higher pressure on the bottom (because of greater velocity). If so, how do planes fly.

    I know the above link also explains why fluids at greater velocity have a lower pressure but I found the information far to overwhelming.

    I was wondering if someone could explain it simply, possible giving an every day analogy (in terms of pressure specifically)
     
  2. jcsd
  3. Sep 25, 2010 #2
    Re: Aerodynamics

    Lift also depends on the angle of attack, not only camber.
    http://en.wikipedia.org/wiki/Camber_(aerodynamics)

    As for a simple analogy for the angle of attack, try running with a sheet of plywood (obviously no camber) held at different angles. In a certain range, you can actually feel some lift.
     
    Last edited: Sep 25, 2010
  4. Sep 25, 2010 #3

    rcgldr

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    Re: Aerodynamics

    A wing performs work on the air through mechanical interaction. Using the air as a frame of reference, before a wing arrives, the air is not moving, but after a wing passes through, the air ends up with a downwards (related to lift) and forwards (related to drag) velocity, corresponding to an overall increase in mechanical energy.

    Although pressure may be reduced above a wing and/or increased below a wing, eventually the disturbed air pressure returns to ambient, but there will still be a non-zero downwards and forwards velocity, corresponding to the total increase in mechanical energy due to interaction with a wing.

    When work is done and the total energy is increased, then both pressure and speed of the air can be increased. The effect is similar to a propellor but the pressures and energy increase are greater with propeller than with a wing. Explanation for propeller, explaining the energy increase of the air:

    http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

    The airfoils with the "hump" on the bottom are less efficient than conventional airfoils, there's more drag created for the same amount of lift (a lower lift to drag ratio). The mechanical interaction on the bottom of most wings simply pushes the air downwards (and somewhat forwards) similar to a propeller, resulting in higher pressure, higher velocity air (relative to the air).

    Pushing the air from below using higher pressure consumes more energy than pulling the air from above using lower pressure, so it's more efficient to get most of the lift from above a wing (reducing pressure) which is how most effcient air foils works.

    Increasing the amount of air affected by a wing by using a longer wing span increases efficiency the most, because the increase in velocity is smaller if a larger amount of air is involved. The aerodynamice forces involved are related to mass x velocity, while the energy consumed is related to 1/2 x mass x velocity2. If the amount of affected air is doubled, then the amount of mass is doubled, while the velocity is halved, and the energy consumed is 1/2 x (2 mass) x (1/2 velocity)2 = 1/4 m v2 = 1/2 the energy consumed.
     
  5. Sep 25, 2010 #4
    Re: Aerodynamics

    Without the wing pushing and pulling on the air around it the air around it would not be accelerated, so any motion of the air caused by the wing is caused by the wing pushing on the air or pulling on the air. Once in motion pressure may go down but there would be no motion on the top of a wing in flight without the top of the wing physically pulling on the air (measured in low pressure).

    So the top of the wing is having a tug of war with the air, who’s going to win? The winner is the wing as a direct result of its shape. Now the only thing better than shaping the top of the wing to use low pressure to divert relative airflow is to shape the bottom to do the same and in the same direction as with an under cambered airfoil. Despite the lake of length differentials of the top and bottom it generates the most lift at zero angle of attack than a flat bottom airfoil.

    So how much lift is caused by the wing pulling on the air to divert it as opposed to the amount of lift cause by the low-pressure air pulling the wing up? You can levitate a ball in an upward jet of air, while drag supports its weight lift keeps it in the flow. Mathematicians will tell you that the ball is being pulled into the jet of air by its low pressure or you can think of it as the higher atmospheric pressure pushing it into the area of low pressure. The real reason the ball tends to stay in the flow is because as the ball starts to drift out of the flow the airflow on the side of the ball still in the flow travels farther around the ball exiting in a direction away from the flow. The ball turns the flow as a result of its shape.
    It is easy to separate the lift caused by diverting the flow (Newtonian) and pressure differentials (Bernoulli). Take an air hose (or kitchen faucet) and while the air is coming out hold the curved back of a plastic spoon into the flow with the handle parallel to the flow, notice how much force it takes to pull the spoon out of the flow. Now take a plastic knife and do the same thing. Despite the fact that the pressure differentials are the same the spoon generates an easily noticed amount of lift and the knife generates none. The spoon generates a dramatic amount of lift by diverting the flow as a result of its unique shape and the knife generates lift by being pulled into the flow as a result of pressure differentials although you hardly can tell it.
     
  6. Sep 25, 2010 #5

    rcgldr

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    Re: Aerodynamics

    You have equal and opposing forces here, the "winner" is the object with more mass. If the wing was a very thin shell filled with helium, then the wing would be accelerated upwards more than the air downwards. To sustain this condition, the wing would have to follow a circular path, with the lift generating centripetal acceleration, otherwise the wing's upwards acceleration would result in upwards velocity, eliminating the lift force.

    If the presure below a wing is reduced below ambient, then lift is reduced since air would be accelerated towards the low pressure zone from all directions, except downwards, where the presence of the wing prevents downwards flow, resulting in a net upwards acceleration of air, reducing lift. The same thing happens above a wing, air accelerates towards the low pressure zone above a wing from all directions except upwards, because the presence of the wing prevents any flow through the wing, resulting in a net downwards acceleration of air, increasing lift.

    Diversion of air over a cambered surface (Coanda like effect) results in low-pressure that "pulls" the air the same as any low pressure zone. There's no magical lift force generated that doesn't involve the downwards acceleration of air (or pressure differential if in ground effect). The Newton third law pair of equal and opposing forces are the downwards force the wing exerts on the air, and the upwards force the air exerts on the wing.
     
  7. Sep 26, 2010 #6
    Re: Aerodynamics

    Just a quickie. Would there be a higher pressure underneath the wing when the hump is on the bottom?
     
  8. Sep 26, 2010 #7
    Re: Aerodynamics

    if the hump were on the bottom, with a horizontal flat top, then the air on the bottom of the airfoil would move more quickly than the air moving across the top (assuming that movement is horizontal). Therefore, the pressure below the wing would be less than the pressure above the wing, i.e. the wing would be heavier due to aerodynamic force. However, if you change the angle of attack so that the flat top is no longer horizontal you will change aero dynamic force. Changing the angle of attack can change the point where the air splits to cause the air flowing over the top to move further and therefore faster than the air on the bottom.
     
  9. Sep 26, 2010 #8
    Re: Aerodynamics

    Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object.
     
  10. Sep 26, 2010 #9

    rcgldr

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    Re: Aerodynamics

    Faster moving air doesn't mean lower pressure air, in the case when work is done, which violates Bernoulli. The air just aft of a propeller is high speed and high pressure:

    ... the propeller does work on the airflow. We can apply Bernoulli'sequation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine (by the propeller) violates an assumption used to derive the equation.

    http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

    A wedge like airfoil with the hump at the bottom and near the aft end of the airfoil (imagine a flat bottom air foil rotated 180 degrees along it's wing span axis, essentially an inverted flat bottom airfoil moving backwards), with the top horizontal, and the bottom angled downwards, will divert the air downwards (and forwards), increasing both the pressure and the speed of the air (relative to the surrounding air), because it peforms work on the air. It's not an efficient design (relatively high drag) but it will generate lift.

    All wings perform some amount of work on the air. Similar to the propeller article above, the results of interaction between wing and air is a non-zero "exit velocity" (the velocity of the affected air when the pressure of that affected air returns to ambient), because work is done, increasing the total mechanical energy of the air.

    For a simple example, imagine an efficient 1500lb glider with a 60:1 glide ratio (these exist), with 60 mph forward speed and 1 mph downward speed. The power input due to gravity = 1500lb x 1 mph / 375 (conversion factor) = 4 horsepower. All of that power is being used to increase the energy of the air by 2200 lb ft every second (4 x 550 lb ft / second), some of it an increase in temperature, but most of it an increase in mechanical energy.

    The downwards force translates into an downwards impulse (a packet of increased mechanical energy) carried through the air, expanding while it travels downwards, but eventually transmitting all of the downforce onto the surface of the earth. The atmoshpere and any objects flying or gliding in the atmosphere are a closed system, and the downforce on the earth's surface equals the sum of the weight of the atmoshpere and any objects in the atmosphere (as long as those objects don't have a vertical component of acceleration).
     
    Last edited: Sep 26, 2010
  11. Sep 26, 2010 #10
    Re: Aerodynamics

    If we were to reverse the question, why are there high speeds with low pressures, would the following answer do:


    If we take two objects with the same mass. One is at high pressure and
    the other is at low pressure then when force F is applied to both bodies,
    the body with the lesser pressure will move faster.
    (Pressure acts against the force applied)
     
  12. Sep 26, 2010 #11

    rcgldr

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    Re: Aerodynamics

    Acceleration = force / mass, internal pressure doesn't matter. Friction or drag due to environment matters, but you could have high pressure hydrogen versus low pressure water, and the water would have more drag.

    Bernoulli's equation relates to a situation where a gas or fluid accelerates from a higher pressure zone to a lower pressure zone with no external work involved during the transition (how the high and low pressure zones are created and maintained is ignored). During the acceleration from the higher pressure zone to the lower pressure zone, pressure decreases and speed increases. Bernoulli equation just states the relationship between pressure and speed during this transition. It is sometimes expanded to include a gravitaional potential component for fluid or gas movement if there is a vertical component (relative to gravity) involved.

    Bernoulli doesn't handle the case where work is involved and the total amount of mechanical energy is changed. To deal with real world flows, Navier-Stokes equations (somewhat simplified, as these are rarely determinable in the real world) are used.
     
  13. Sep 26, 2010 #12
    Re: Aerodynamics

     
  14. Sep 26, 2010 #13
    Re: Aerodynamics

    Yes the air behind a propeller is high pressure and high speed but if it splits to flow around an airfoil, then the air that travels further (to maintain laminar flow) will travel faster relative to the air on the other side. So there will there will be a pressure differential across the wing. The side with the faster moving air will have a lower pressure than the other side and will pull the wing toward the lower pressure.

    The way it was explained to me while doing my pilots licence was, "as the air molecules vibrate they bump into the wing, when the air is moving quickly then each molecule has less time to bump into the wing." I suspect that there is more going on than that but it made it easier to remember.
     
  15. Sep 28, 2010 #14
    Re: Aerodynamics

    Perhaps there is another way to explain it. Air has inertia and while the flow around an airfoil remains laminar, then the surrounding air will resist changing speed and direction. So air molecules that split at the leading edge of a wing will need to arrive at the trailing edge at the same time. If the same number of molecules are moving along the top and bottom surfaces of a wing but the top ones are moving further then there must be more space between each molecule (other molecules don’t rush in to fill the gap because of inertia). It’s similar to pulling the plunger back on a syringe with your finger over the end, the air in the syringe will be at a lower pressure than the air outside, and if you let the plunger go it will be pulled toward the lower pressure.
     
  16. Sep 28, 2010 #15

    rcgldr

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    Re: Aerodynamics

    There's no tendency for the molecules to arrive at the trailing edge at the same time. Relative to the wing, the molecules above move further back than the molecules below. Relative to the surrounding air, the molecules above a wing will move much less forward (in some cases they move backwards a bit), than the molecules below. This is demonstrated in the video linked to from the previous thread:



    A similar effect is shown in the diagram and animation on this web page:

    http://www.avweb.com/news/airman/183261-1.html

    Getting back to the original post, even with an unusual wedge like airfoil with the hump below and at the back of a wing, the air below is pushed forwards more than the air above, separating the flows in the same manner as a conventional wing.
     
    Last edited by a moderator: Sep 25, 2014
  17. Sep 28, 2010 #16
    Re: Aerodynamics


    Yeah your right, the air doesn't always arrive at the same time. The force created (i.e. work done) by the movement of the wing will be shared between lift and disturbance to the air. The effects you are refering to are mainly due to the angle of attack. The angle of attack mainly affects the air under the wing, slowing it down in a wind tunnel or pushing it forward in the atmosphere. The you-tube footage is of a symetrical wing that will not produce lift without angle of attack. The angle of attack will force air down, the inertia of the air resists the downward movement which increases the air pressure and adds to the overall lift produced.

    A different reason why the air below the wing may not reach the trailing edge at the same time is because some of the air leaks around the wing tips from the bottom to the top, forming wing tip vortices.

    If you look a little closer at the nasa stuff you'll discover that the lifting bodies are like Buzz Lightyear, they don't fly, they fall with style. They are a pretty awsome concept, launched from a mother ship at 45000 feet and rocket propelled to 60000 to 70000 feet. The rockets create the lift at those altitudes cause there's not enough air to for aerodynamics to be of much use for lift. The lifting bodies have just enough aerodynamics to control the fall back to earth and land safely. The proto types were tow launched.

    And if you look a little closer at Roger Longs page http://www.avweb.com/news/airman/183261-1.html you will find a small disclamer down the bottom. I have taken the liberty of taking a quote from it.
    "In order to save postage for a least a couple of letter-to-the-editor writers, I should discuss one special case of wing flow. If a Clark Y-style wing with a flat underside is operating at an angle of attack where the bottom is exactly aligned with the airflow, there will be very little disturbance of air below the wing. The increase in air pressure below the wing will only be that caused the intrusion of a thick object. The top of the wing, its aft portion being lower than the leading edge, will still be sweeping out a space into which air will rush creating an acceleration of air over the top and a modest amount of circulation. Although the air pressure below the wing will be very close to ambient, there will still be a pressure difference between top and bottom." by Roger Long
     
    Last edited by a moderator: Sep 25, 2014
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