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Aerodynamics of gliding - simplified?

  1. Jul 20, 2014 #1
    For a glider to sustain flight at a constant speed, the aircraft must be placed in a dive so that the forward motion is maintained by the component of gravity acting along the glide slope ( as a ball rolling downhill ). This force must equal to the drag on the aircraft. The less drag, the lower the dive angle needed.

    For an aircraft wing there is an angle of attack at which the drag is the lowest value for the lift that is produced: ( The L/D) ratio. This is the most efficient angle of attack for the wing and corresponds to the angle of the dive (relative to the fuselage centre line) .

    My question is: how does one arrive at the correct weight necessary to maintain the forward force needed to maintain the required forward speed to obtain the best L/D ratio?

    How do you arrive at the correct weight for an Radio Controlled model glider for the lowest sink rate and the greatest distance?

    I am a little confused at the moment. This seems to address the issue:

    http://aviation.stackexchange.com/q...last-if-it-is-trying-to-stay-aloft-without-an
     
  2. jcsd
  3. Jul 20, 2014 #2

    A.T.

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    That is more a question of trimming than of how much weight you add. Distributing the weight and moving the COM allows to trim with minimal additional drag from the stabilizer.
     
  4. Jul 20, 2014 #3

    rcgldr

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    Lowest sink rate will occur with least weight (within reason), at a bit below the speed of best L/D ratio. On windy days, radio control gliders use weight (ballast) to increase air speed at speeds higher than best L/D ratio, needed to return upwind after chasing a thermal downwind. In addition, most contest type gliders can raise the entire trailing edge (both flaps and ailerons) slightly to reduce camber for lower drag at higher speeds (again well above the speed of best L/D ratio).

    The best L/D ratio remains about the same within a range of weight for a typical radio control or full scale glider, given the limitation of how much ballast the glider is designed to utilize. The speed of best L/D ratio is relative to the square root of the weight (note that lift and drag are related to speed squared), so adding ballast doesn't result in a big increase in speed.
     
    Last edited: Jul 20, 2014
  5. Jul 20, 2014 #4
    Suppose the glider is a model of a jet plane such as P-80 shooting star, is there such a thing as too light? To fly realistically it will have to fly rather fast, and be able to do loops and rolls and high speed passes. I know there are gliders capable of doing this, however with the fatter fuselages typical of jet fighters what can be done to reduce drag to the minimum and maintain long duration flights? On my model sim a light glider that is also high drag needs to have its nose pointed down quite a bit to obtain enough force to move forwards. Increasing the weight may help but then then wings may not sustain lift.
     
  6. Jul 20, 2014 #5

    rcgldr

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    Even if it doesn't fly, a very light feather will have a lower sink rate than most model aircraft, so for minimum sink rate, it's difficult to be too light. As an example of actual radio control gliders, the time duration contest ones used for discus / hand launch have a 1.5 meter wingspan (about 59 inches), thin wings, and weigh only 9 to 10 ounces.

    http://en.wikipedia.org/wiki/Discus_Launch_Glider
     
  7. Jul 21, 2014 #6

    CWatters

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    yes.

    Everything you say is correct. It's why modern competition gliders are very clean/low drag and have the ability to carry water ballast. You can't throw a light weight model biplane off a cliff into a gale and expect it to go forwards. You need enough weight to counter the drag at the required airspeed. If you want it to go forwards that airspeed must be faster than the wind. However you can't increase weight indefinitely. Not everything is capable of gliding.
     
  8. Jul 21, 2014 #7

    CWatters

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    PS Consider what happens if the terminal velocity of a plane in a dive is lower than the wind speed. There is no possibility of it gliding upwind of it's launch point.
     
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