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How does a glider glide ?

  1. Jun 6, 2008 #1
    in an aircraft, the lift is due to the force that the air pushes vertically on the aircraft and also in a fast speed, the low pressure zone on top of the wing provides a very small amount of lift called the Bernoulli law.

    Now, if it is a glider, without the engine power, how can it keeps itself in the mid air? Is it because the Bernoulli law or is it something other than that?

    please forgive my poor description consider i not even a college student(grade 12 right now) and my first language is not English.
  2. jcsd
  3. Jun 6, 2008 #2


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    Assuming no vertical component of air flow, a glider doesn't remain in the air; it descends at a constant rate, using the component of force from gravity in the direction of travel to oppose the force of drag.

    Large (80+ foot wingspan) gliders are very efficient, with 60 to 1 glide ratios (same as lift to drag ratio for a glider). Moderate sized high end gliders (15 or 18 meter wingspan) are around 50 to 1.
    Last edited: Jun 6, 2008
  4. Jun 6, 2008 #3
    Lift is defined as perpendicular to the airflow. So in air that is not in motion, as a glider decends in steady flight, the direction of lift is upward and slightly forward.

    the lower relative pressure above the lifting surfaces and the greater relative pressure below the lifting surfaces provide all the lift. The top of the wing does most of the lifting.

  5. Jun 6, 2008 #4


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    Technically, "gliders" don't stay in the air, they glide until they reach the ground. Having a negative pitch angle makes them decend and provides forward speed to keep them from just plain falling.

    Sailplanes, on the other hand, are like gliders in that they keep their speed up by angling their nose down, but they are kept aloft (or propelled upwards) by vertical wind that is a greater velocity than their downward speed. Ie, thermals.

    I suppose a sailplane is a type of glider, but a glider isn't necessarily a sailplane.
  6. Jun 6, 2008 #5


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    Usually but not always. (Yet another oppurtunity to post a picture of the cool looking M2-F2 lifting body).

    M2-F2 Glider, flat top curved bottom:


    Powered version, M2-F3, (top speed reached was mach 1.6):

    A summary of posts I've made in previous threads:

    After visiting a large number of web sites, my conclusion is that lift is a combination of Coanda effect, "void effect" and simple deflection, all of which result in the "downwards" acceleration of air. Coanda effect explains how laminar flow follows a convex suface. "Void effect" explains how turbulent flow follows a convex surface. Concave surfaces simply deflect airflow. The curvature of air flow accelerates the air and generates lift. "Void effect" explains how drag is developed "behind" a wing, while direct forward deflection of air accounts for the drag in front of a wing, along with friction along the surface of a wing.

    Except for a special class of airfoils, most of the air flow over and under a wing is turbulent, with only a portion of the air flow being laminar near the leading edge. For most wings, the flow transitions from laminar to turbulent flow above and below a wing, detaching during the transition, but reattaching after the transition. This happens in the first 30% of the chord length or sooner on a "normal" airfoil, and between the first 30% to 75% of chord lengh for a "laminar" airfoil (by definition). In some cases, rough surfaces and/or turbulators are used to cause the transition to occur at a specific position on an air foil. In the case of gliders, an "oil flow test" is done to visualise this transition. A bead of oil is placed on the leading edge of the wings, the glider is flown for a while at a fixed speed, then landed and the oil pattern observed. It's common practice to do this in glider magazine reviews.

    http://www.standardcirrus.org/Turbulators.html [Broken]

    Oil flow testing is also done in wind tunnels:

    http://www.hisacproject.com/news.html [Broken]

    At this web site, pages 4 and 5 discuss how little air flow is laminar over many wings, and how "laminar" air foils increase laminar flow to 30% or more over the chord length of a wing. In the case of gliders, laminar "bubbles" result in either more drag or less lift so the laminar air flow is deliberately broken up sooner than it normally would via rougher surfaces or turbulators (this is mentioned in the article). The laminar section starts mid way down page 4:


    "All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30% and 75% of chord."


    The next web site does a descent job of explaining lift, but with a bit too much emphasis on Coanda effect, ignoring void effect and turbulent flow, but towards the end of this web page, there's a diagram of a wind blowing over a roof, and although the air downwind of the roof is turbulent, it's also at lower pressure, due to void effect. Since both laminar and tubulent air flows contribute to lift, both cases should have been covered better than it was at this web page:

    "The physical cause of low or high pressure is the forced normal (perpendicular) acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect.":

    http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html [Broken]


    Assuming this next video isn't GGI, it appears to be a series of pictures of a flame aimed at various angles over an glowing (from the heat) airfoil at a fixed angle of about 45 degrees. As the flame angle is made more horizontal, the effective angle of attack becomes higher. What I call "void" effect is more evident here, as the flame flow is detaches from the aft end of the airfoil at low effective angle of attack. At higher effective angle of attack, the flame flow detaches from the "upper" surface of the airfoil, but it's still accelerated (curved) "downwards", while below the airfoil there is significant direct deflection. About 28 seconds into this video (you can hold it at this position), the downwards curvature of the flame over the "top" of the wing is still evident, in spite of the large amount of apparent detachment.


    Next is a link to a small wind tunnel video, considered a "2d" airflow (equivalent to a 3d wing with infinite wingspan). Air speed is slow, chord length is small, so the Reynolds number is quite low, and the air flow is much more laminar and the angle of attack before stall is much higher than it would be if everything were scaled up to a faster speed and a larger size. The smallish wind tunnel also prevents any significant upwards or downwards flow of air, so the air flow is not the same as it would be in an open environment. Wind tunnels that are much larger than the wing or model being tested, such as the one linked to above showing oil flow testing are much closer to "real world" environment. The transition into the stalled condition is very abrupt. In the segment annotated as "stall", there's virtually no lift, but near the end of the video, that starts off "flow attached", then "stall", there's still significant lift although there is a stall.


    For this model, the stalling angle of attack is fairly small:

    Another wind tunnel, slow air speed, short chord, but not as much as the first video. Again the nature of the wing tunnel (proably drawing air inwards from the right), prevents the air flow from remaining deflected, and skews what would happen in an open environment:

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  7. Jun 6, 2008 #6
    Jeff, you've certainly been doing your homework.

    There's a classic text you might want to pick up if you dont have it. It's old but basic, and cheap. "Theory of Wing Sections". You can read through it without following the math and get something out of it. It's well illustrated with plenty of graphs and diagrapms.
    Last edited: Jun 6, 2008
  8. Jun 6, 2008 #7
    Unfortunately, saying that lift is a result of the Bernoulli effect is a non-explanation.

    A wing provides lift because it imparts a downward force on the surrounding air. This is straight from Newton's mechanics. If the air is to provide an upward force on the wing, the wing must provide a downward force on the airsteam.

    The force of the air on the wing distributes itself over the wing surace; top and bottom. So this is pressure. Normally, the bottom of the wing is pushed upward, so the pressure is increased from the atmospheric pressure. And the top of the wing is pulled upward, so the pressure on the top of the wing is less than atmospheric pressure.

    This is as far as I go. I keep hoping that Jeff, in his dilligence, will eventually supply the answer as to how pressure on the wing and impulse to the air are related in qualitative terms.

    You could say that the Burnolli effect causes lower pressure over the top of the wing because of its increased velocity. But unless you say how this increased velocity imparts a downward force to the airsteam, you haven't explaned lift using Burnolli.
    Last edited: Jun 6, 2008
  9. Jun 7, 2008 #8


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    Force = mass times acceleration. However the issue is that the cross sectional area of air affected by a wing passing through it is huge compared to the size of the wing and the amount of acceleration varies with the distance above a wing and the distance below the wing. There are localized interactions of the air (turbulence) that don't contribute to overall lift or drag. For calculation purposes, where to draw the line for what constitutes air affected by a wing, for example air that is only accelerated by 1 nm / sec2?

    Bernoullii principle doesn't apply well to wings, because the air flow is turbulent across a wing, and is only laminar for the leading 30% or less at the leading edge (except for "laminar" air foil which require extrememly smooth surfaces, which even a small amount of debris, like a squashed insect, or moisture will disrupt). The turbulent air flow invovles the mixing of nearby air flow into existing streamlines. Bernoulli only works well for undistrubed streamlines. The more I find out about the nature of wings in the real world, the more I wonder how Bernoulli ever became a popular theory for explaining lift.

    In the videos above, even in the case of small wind tunnels with restricted vertical movment of air, it's clear that above a cambered afoil, the smoke trails used to identify streamlines are closest to the wing at the leading edge of the "peak" of the cambered air foil, then flow away from the wing (but still diverted downwards). What's going on with the air between the "inside" streamline and the top surface of the wing? This is not stagnant air, and more likely to be flowing in a circular pattern, probably as small eddies, and certainly not a good candidate for applying Bernoulli principle.

    The bottom line is that there is no simple answer. Air foil programs are very complicated, and produce close to real world numbers (called "polars" for air foils), but wind tunnel (large relatively open ones) testing and then real world testing is still required to confirm that a particular air foil doesn't have some unexptected qualities. Radio control and full scale gliders get the most attention, because there is more empahsis on effeciency in spite of cost (unlike powered civilian aircraft), and in the case of radio control models, new models are created much more often than other high research type aircraft, such as large commercial aircraft or military fighters.
    Last edited: Jun 7, 2008
  10. Jun 7, 2008 #9
    I would be happy to see a qualitative description for a ideal nonviscous, incompressible (and adiabatic=no heat conducted away, or into) fluid over some general convex surface.

    Hmm. This sounds a bit off. From what little I remember from my cr*p fluids class, the fluid in a pipe might go from laminar to turbulent flow in a distance of a few diameters. (diameter is the characteristic length for Reynolds number in round pipes) The whole cross-section goes turbulent in short order. But I don't recall that Bernolli becomes unapplicable in the turbulent flow regime.

    If you were to cancel-out the average longitudinal flow velocity, I think you would see something very much like this, but the Bernolli pressure-velocity relationship would still hold, I think.

    Now my memory is getting jarred. Something about 'theory of thin sections'...
    Last edited: Jun 7, 2008
  11. Jun 7, 2008 #10


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    Bernoulli assumes a constant total (pressure and kinetic) energy. A wing peforms work on the air increasing the overall energy. Even if you adjust for the increase in total energy, another issue is that the total amount of air invovled changes as it flows across the wing, with turbulent flow causing air outside of the "streamline" to be "pulled" into the flow, so you take this into account also, but then how do you define velocity in the turbulent, moving eddie flow that occurs accross most of a wing, and then relate this to pressure?

    I thought of another example of what might be called "semi-detached" stream lines. If you roll down the windows in a car movnig at speed, you have to stick your hand out a bit beyond what was the outer boundary of the window before you actually feel the full relative speed of the air.
  12. Jun 7, 2008 #11
    Yes, of course, and you've expressed this before. I didn't, then, know how to answer. You've seen the usual demonstration of the Bernoulli effect with clear glass tubes. Some small tube runs off perpendicular to the fluid flow to measure the pressure. The pressure translates into the volume of the small tube (minus the effect of head pressure, if it runs vertically and is open at the top). If the small tube were closed with a pressure transducer in it, it would measure the same pressure as within the flow. The pressure in fairly uniform across the crossection of the pipe. The pressure also translates through the boundry layer in th same manner as into the tube.

    I cracked upon a book on section theory. I don't think Im going to get to 80 pages of integral and vector differential equations anytime soon.
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