Benelli lift theory under question

Discussion in 'All Things Boats & Boating' started by Frosty, Dec 5, 2012.

  1. daiquiri
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    daiquiri Engineering and Design

    But even that situation can be reconducted the the starting vortex. Think about an abrupt and finite change in AoA of the foil. It will cause a formation of the starting vortex just like an abrupt start (seen previously) will cause it. Now reduce amplitude of the AoA change, and the vortex left behind will be proportionally reduced. At some sufficiently small, but still abrupt, change of AoA the wake wortex will be so small to become almost invisible. An infinitesimal approach which you know very well. A slow continuous increase in AoA of a real airfoil is reconducted to an infinite series of infinitesimal impulsive AoA changes. Consequently, an infinite series of infinitesimal vortices will be left behind it, becoming a part of the vortex sheet.

    Finite near-impulsive changes in AoA are well-illustrated in this video: http://youtu.be/WOabqXrjrMo , with consequent finite vortices well visible. :)

    After some time, the viscosity will do it's job by dissipating the trailing-edge vortices and transforming them into turbulence and heat, but that doesn't take anything from the validity of the concept, which is well demonstrated.
    Then, yes, we can discuss the real usefulness of such advanced concepts in teaching the basics of lift to a person with no background in math, or who is unfamiliar with words like "infinitesimal" etc... But that's another story. Sometimes a good old example of waiving one's hand in the air through a side-window of a moving car can be better than 100 CFD videos. :)

    Cheers
     
  2. DCockey
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    DCockey Senior Member

    I have never question the validity of the starting vortex concept. I am skeptical that bringing up the starting vortex and vortex theorems actually helps folks without an intermediate or advanced knowledge of theoretical physics understand lift generation by an airfoil.

    Why not just say that the flow approaching the wing divides so that the flow leaves the trailing edge smoothly. The details of the division of flow depends on the airfoil shape and the angle of attack. For an airfoil which generates lift upwards the portion of the flow over the top is going faster on average than the portion of the flow under the bottom and hence the pressure is lower on the top than on the bottom. Bernoulli's principle can be used to explain how faster speed means lower pressure. The net difference in pressure between top and bottom surfaces results in lift.

    Then if there is a desire to complicsate matters, or if more details are desired, the concept of circulation can be introduced as the average of the velocity around the surface of the airfoil, and how this corresponds to the average of the pressure differences and hence lift.

    Now those who already know about circulation and the Helmholtz theorems on vorticity, such as the theoretical physicists at Cambridge a hundred or so years ago, might want to know how circulation is possible without contradicting the Helmholtz theorems. For those folks the starting vortex is an essential element to understanding how an aerofoil can generate lift.
     
  3. petereng
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    petereng Senior Member

    Hi- I usually explain lift as follows: When you hold a water hose and use it as a jet you can feel the reaction in your hand. The more you angle the jet the more reaction you feel. If the hose is straight (no angle of attack) there is a small reaction (change in momentum stuff). Most people have experience with this type of force. So now take a wing - the more we bend the air the more lift the wing makes (ie increasing camber increases lift). This is the translational flow part of the situation. Then for the advanced people we can look at the rotational flow. This involves the boundary layer which I usually explain acts as a lubricant for the translational flow across the surface. The BL is rotating like small ball bearings and rolls across the surface and has to leave the TE forming a trailing vortex. Vortices are part of the drag side of the problem unless you have a crab claw or delta wing which uses the vortices to change the translational flow further. Things like aspect ratio, turbulent flow etc are a bit more advanced and need the right building blocks for understanding. You can't use Bernoullis strictly speaking as it is not a closed volume like flow in a pipe or venturi. But it can be used to calculate things along a streamline. By the way animals maximise the use of rotational flow as their wings or flippers are not static like our plane or boat foils. That is why if you use classic "theory" bees can't fly as they do not use translational flow to generate lift. Translational flow is usually visuallised by streamlines in wind tunnels using smoke etc. These streamlines have seduced us into thinking that lift is simple and its not. Cheers Peter S
     
  4. DCockey
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    DCockey Senior Member

    That is essentially the theory which was most commonly accepted until the early part of the 20th century. Unfortunately it is at best incomplete and is a in poor agreement with reality. For example it doesn't explain why a cambered airfoil at zero geometrical angle of attack (based on line from tip to tail) will have lift.

    Classic airfoil theory, which predicts lift very well for airfoils with attached flow, assumes irrotational flow except for the infinitely thin wake. Trailing vorticity is due to spanwise differences in flow and is relatively independent of the boundary layer thickness. The boundary layer of an two dimensional airfoil, ie an "infinite" span constant section wing, does not leave the trailing edge as a trailing vortex.

    Trailing vorticity from a crab claw or delta wing contributes to drag.

    Bernoulli applies along any streamline which doesn't have viscous losses. No need for a closed volume. Pipe flow and venturi flow usually have visocus losses in which case Bernoulli is not valid without adjustment, although in many instances it may be reasonable to neglect the loses.

     
  5. petereng
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    petereng Senior Member

    Hello David,
    If the foil has camber (asymmetric) it bends the flow therefore it has a momentum change therefore it has lift. Isn't it that simple? All vorticity is drag but in the crab claw and delta wings these vortices are used to change the translational flow which means the fluid changes direction further then just the geometry of the foil allows therefore the lift is greater. The more lift you create the more drag you create (you don't get a free lunch) Its fine to quote a theory but we are looking outside the assumptions here. The rotational flow you are talking about is tip vortices which is correct in that this is a spanwise problem. But I think the comment about no trailing edge vortice needs to be looked at. The BL is feed at the LE stagnation point and the BL leaves at the TE stagnation point and it therefore must be rotational as the BL is in rotational flow. I must say here I'm not a fluid dynamacist but I am a mechancial eng. I'm more up on materials then this stuff. But I have been involved with sailing rig design for over 20 years and theres lots of stuff we don't yet understand about lift. Sure the general theories work but then we are limited by the assumptions. To go fwd we must re assess the assumptions and learn more about the finer details. We are just getting into this area now as the BL can be modelled quite accurately now whereas its been shoved into the assumption of "irrotational" flow which removed its significance. If you want to study a bees wing you can't say "irrotational" flow therefore xyz. A bees wing is highly rotational and very small translational flow. When I say rotational I mean all the rotational flow just not the tip vortice. Unless my understanding of how the BL works is incorrect. ie its feed at the LE stag point then circulates around the foil and slowly works its way to the tip then sheds? I don;t think so. Only the tip flow sheds. The BL say at mid span must shed at the TE. The starting vortice is part of this rotational flow that allows fully attached and developed flow, then full lift etc etc too many words now Cheers Peter S
     
  6. tom kane
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    tom kane Senior Member

    Look at a helicopter rotor which spins without lift before take off as the angle of attack of the rotor blades have been set to zero. Use your collective pitch control to increase the angle of attack and away you go.
     
  7. DCockey
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    DCockey Senior Member

    Above was in response to ???
     
  8. petereng
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    petereng Senior Member

    Hello Tom - I think you will find there is still lift just not enough to lift the chopper. Cheers Peter S
     
  9. Frosty

    Frosty Previous Member

    Probably this one.

     
  10. PAR
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    PAR Yacht Designer/Builder

    Frosty, if you had a clue, you realize that the comment you've quoted is precisely correct (overly simplistic for obvious reasons and the length of this thread proves), but since you haven't the remotest idea of any of the principles, dynamics or physics involved, you couldn't possible make any tangible/relevant contribution to this here, except to show your absolute ignorance on the subject and of course their related subtexts.
     
  11. Frosty

    Frosty Previous Member


    Exellent contribution and relative to the thread thank you. your very kind. But yes I know you've said so once. Do you mind if post on my own thread. Your hatred of it is obvious this begs to ask why do you post ? You have given nothing in respect to the Question just insults and irrational anger.--- shooo,--- go away.

    Please fight the temptation to add further insults and display any further pointless anger that realy does nothing for any reputation you think you have.


    .
     
  12. watchkeeper

    watchkeeper Previous Member

    Par, Its amusing a troll finds that being smitten instead of being the smitter an unpleasant experience
     
  13. TANSL
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    TANSL Senior Member

    I think I should react to the contempt shown against PAR and declare my admiration and respect for what, through this forum, I know about him.
    Frosty, I try to respect everyone but when one takes hundreds of post acting like a fool, it is normal that others consider him a buffoon. And also greatly respect the office of jester, is much needed in these troubled times in which we live.
    I laugh a lot with your occurrences but, when you speak of scientific theorems and laws of nature, I'm dying of laughter.
    Best regards to all.
     
  14. watchkeeper

    watchkeeper Previous Member

    Well said
     

  15. Number4

    Number4 Previous Member

    A helicopter blade can have symetrical or asymetrical aerofoil the same as an aircraft wing.
    The blade's velocity is a function of the speed of the helicopter and the speed that it is rotating.
    The tip of the blades will be travelling much faster than the root.
    High speed aerofoils are necessary because the tips may exceed the speed of sound.
    A helicopter on the ground will not be at full power, although it may be at high rpm.
    Power and angle of incidence are controlled to provide lift and forward motion.
    RPM may not increase but torque does. RPM x Torque = power.

    An helicopter, with an asymetrical aerofoil, would take off, with enough rpm, at 0' angle of attack.
    A plane, taxiing along the runway is still creating lift, just not enough to become airborne.

    The amount of lift created by an helicopter can be very well calculated by applying..........Bernoulli's Principle!!!!!!

    P.S. I hope I never get in another one of those bloody things again.
     
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