Wing lift distribution presentation

A very interesting and thought provoking explanation of why the conventional wisdom of elliptical lift distribution needs rethinking.  Interesting for lovers of fixed-wing but also might be applicable for propellers and rotors...?

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  • Yes, interesting point Andy.

    Although your ellipsoidal lift distribution can suffer the same fate if it's generated through washout and not chord/planform.

  • Developer

    Interesting, though I dont see this as that new? It has been covered in detail in various books on aerodynamics  from 70's 80's AFAIK

    I built a wing using a bell shaped lift distribution and found it performs well.

    Tensor V2 article on DiyDrones

    BUT ....

    There is one important point that was glossed over though made by the questioners at the end.

    The efficiency of the bell shape lift distribution is optimised very much for one airspeed.
    At high airspeeds  the bell shaped wings start physically bending down due to the load on the tips and cause high drag.
    The drag increases very quickly and has to be counteracted by lift at the root, so you run into a brick wall if you try to go fast..

    With an elliptical lift distribution, though the peak efficiency is less, you have a much wider flight envelope
    So the bell shaped lift distribution is potentially good for airliners or endurance, however, if you need to cover a wide range of airspeeds, then an elliptical lift distribution is better.

    The info regarding proverse yaw is useful though especially so for stealth aircraft I guess

    But the narrow flight envelope is why you dont see it used on general purpose aircraft





  • Andrew, I think we are agreeing on most points.

    In regards to the poat, you, yourself, brought It to my attention that I essentially got off topic and went on to discuss matters in design that are completely unrelated, and did so in length!

    You are right in regards to flutter and pitch stability. And you are most likely aware, some of the most efficient foils today have extreme pitching moment. With prandtl's lift distribution theory applied, the wing needs a total revamp in structural design. If not, then yes it could lead to flutter. in regards to say a flying wing, prandtl's lift distribution can help to eliminate the need for a reflexed foil if enough wing sweep is applied. Again, more complications with structural design, in that it creates a larger tortional load. It is much easier to create a structure that has a positive load across the entire span than it is for there to be a positive and negative load. Of course, the larger the scale, the more difficult.
  • Yes, it's true that Dr Prandtl wrote his paper in the 30's and some of us have seen it before.  There is a lot of received wisdom floating about and Mr Bower's presentation did a sterling job of blowing some of it away.

    I don't get your (now deleted) point about flutter and pitch stability w.r.t. the Prandtl lift distribution proposal. Flutter is an aeroelastic phenomenon related more to wing torsional stiffness and aerofoil pitching moment and pitch stability is driven mostly by CoG relative to the NP.  I don't see how wing lift distribution affects it in a generic way for all cases although it might be influential in some specific (esoteric?) instance.  Care to explain?

  • I guess in a way it could be misconstrued as some kind of rant...

    In reality all I was getting at is that prandtl's lift distribution theory was accepted and proven over the years several times by several individuals, including Nasa. Mr Bower and colleuges proved that by utilizing the lift distribution they could get rid of drag inducing yaw devices on a highly swept wing.

    As mentioned, even though Prandtl's work has been accepted, it is not easily achievable on the majority of large scale airframes.

    That is where I went on a tangent in regards to how far behind several "new" aircraft truly are.....
  • You seem to be conflating a whole bunch of disparate concepts there Justin.  Give us a call back when you've got your thoughts together as clearly as Mr Bowers obviously has.

  • Brilliant! Thanks for sharing

  • 100KM

    Ha, finally some else understands that elliptical lift distribution does not work as well for a loaded aircraft. I used a bell curve lift distribution when designing the techpod, although not quite as radically as done on the Prandlt wing in this example. 

  • Do I understand it correctly you have negative AoA of the tips to reduce the induced tip vortex drag by reversing the lift; and it also solves the adverse yaw problem ???

    Can we apply the same principle to improve the induce drag of the law aspect ratio wings???

  • Food for thought alright.  Plenty of the crashtest hobby combat wings do not have vertical fins.

    What I would like is someway to relatively easily calculate the twist/sweepback/airfoil combination.  I guess they are using low or positive pitching moment airfoils.

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