Tailless Aircraft - Reflex and other design issues

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Sockmonkey, Jan 10, 2019.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Feb 25, 2019 #581

    Aesquire

    Aesquire

    Aesquire

    Well-Known Member

    Joined:
    Jul 29, 2014
    Messages:
    2,295
    Likes Received:
    912
    Location:
    Rochester, NY, USA
    If span is not constrained, but stall speed is, then increased AR allows lower area.

    Correct me on that, please.

    Or.... Increased L/D allows lower area?

    Either assertion fits the facts if you look at wing area on modern hang gliders. http://www.moyesusa.com/products.html

    These are all roughly the same planform, a flying wing. The Malibu has a thinner airfoil and much more exposed structure than the other 3, more twist, so if you prefer to keep it as oranges to oranges, just consider the 3 models with the buried cross spar.

    None of these is as efficient as an ideal wing. Laminar flow is very limited, as the first seam in the upper surface trips the flow, if near imperceptible span wise ripples haven't already. The exposed pilot can be more than half the total parasitic drag. However, as a side by side comparison, those factors are about equal.
     
  2. Feb 26, 2019 #582
  3. Mar 9, 2019 #583

    Himat

    Himat

    Himat

    Well-Known Member

    Joined:
    May 5, 2011
    Messages:
    2,764
    Likes Received:
    636
    Location:
    Norway
    No, area and coefficient of lift do set the stall speed.

    In some cases, yes. An increased L/D, or rather Cl/Cd make it possible to operate at a higher Cl for the same Cd. If power constrained, the airplane can then have a smaller wing
     
  4. Mar 10, 2019 #584

    Himat

    Himat

    Himat

    Well-Known Member

    Joined:
    May 5, 2011
    Messages:
    2,764
    Likes Received:
    636
    Location:
    Norway
    Evaluated at constant span and equal speed the formula for induced drag can be rearranged to:

    Cdi = k*Cl^2*AR

    If the airplanes are built to equal standards the only drag part that reduces with aspect ratio is the skin friction. This will change with the area that follow:

    Area = constant / AR

    If the high aspect ratio airplane does achieve a higher Cl/Cd than a low aspect ratio airplane then depend on if the skin friction at some point get larger than the induced drag. This at least at speeds below where wave drag gets significant.

    My conclusion is that increasing the aspect ratio work by reducing span loading. In some ways the dimensionless aspect ratio number mask this.
     
  5. Mar 10, 2019 #585

    BJC

    BJC

    BJC

    Well-Known Member

    Joined:
    Oct 7, 2013
    Messages:
    9,334
    Likes Received:
    6,114
    Location:
    97FL, Florida, USA
    Not exactly. That is a formula for a non-dimensionalized induced drag coefficient, not the induced drag. If you work with actual induced drag, you will see that it depends on span, not aspect ratio.


    BJC

    edit. I’m a big proponent of always carrying / showing units with formulas and calculations. Doing so helps avoid confusing a coefficient with a physical quantity.
     
    Himat likes this.
  6. Mar 10, 2019 #586

    Himat

    Himat

    Himat

    Well-Known Member

    Joined:
    May 5, 2011
    Messages:
    2,764
    Likes Received:
    636
    Location:
    Norway
    Yes, imprecise of me, I should have said induced drag coefficient.

    Edit:
    Dimensionless numbers are fine when scaling designs, but in the case of the induced drag coefficient I am not sure. The trouble to me is it is not really scaling a design or comparing two similar designs. Used for design it alter the design and mask the change in physics between the designs. The difference in span loading.
     
    Last edited: Mar 10, 2019
  7. Mar 15, 2019 #587

    Aesquire

    Aesquire

    Aesquire

    Well-Known Member

    Joined:
    Jul 29, 2014
    Messages:
    2,295
    Likes Received:
    912
    Location:
    Rochester, NY, USA
    Digging through the intraweb, I found a glider I mentioned much earlier in this thread.

    a19780263000cp2.jpg

    valkyrieDRAWNG.jpg

    Short coupled little beast, and marginal in pitch stability. I wonder how I would have reacted to it if I'd had much more experience with short chord wings, as I later had. I might have kinder words to say. But performance wasn't all that impressive, no matter how much my adrenals were squirting at the time.
     
  8. Apr 10, 2019 #588

    henryk

    henryk

    henryk

    Well-Known Member

    Joined:
    Mar 8, 2010
    Messages:
    4,627
    Likes Received:
    437
    Location:
    krakow,poland

Share This Page

arrow_white