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The mechanics of gyroplane lift.

  1. Apr 3, 2012 #1
    I understand how an aeroplane works, with the wings deflecting air downward and thus creating a lift force.

    I understand how a helicopter works, with the rotor pushing air downward and thus creating a lift force.

    However, I do not understand how gyroplanes, with a backward-tilted unpowered rotor, generate lift. By my understanding, forcing air through the rotor from underneath should make it rotate in a sense that would draw more air into it from underneath, creating a lift force pointing downward.

    So how does a gyroplane function? Does it use cyclic pitch to finagle the rotor into going the other way, or is there something else I've missed?

    Thanks in advance.
     
  2. jcsd
  3. Apr 3, 2012 #2

    K^2

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    Picture a paper airplane. It flies with wings tilted up, and yet it keeps gliding forward, not backward. Why? Because it is descending as well, and lift, which is perpendicular to direction of motion, has a forward component.

    You can picture each blade of an autogyro as such a glider, gliding downwards and forward, pushing air down in the process. So clearly, the rotor needs to be descending through the air in order for the rotors not to stall. But what if you are holding a level altitude? Well, that's where the rotor's tilt comes in. A rotor tilted backwards and moving forward gets the same air flow as a perfectly horizontal rotor does during descent.

    Does that help?
     
  4. Apr 4, 2012 #3
    So basically, the rotor is just being used as a parachute? It has a negative angle of attack and if spun that way while stationary would pull the gyroplane down into the ground?
     
  5. Apr 4, 2012 #4

    K^2

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    No, it has positive angle of attack. Again, think about a glider. It has positive angle of attack, but it glides forwards, not backwards.

    If you want to get a bit more technical, the autorotating rotor has a driving and driven regions. But both have positive angle of attack and both generate lift.
     
  6. Apr 4, 2012 #5
    How does pushing air upwards through a rotor that's rotating to push air downwards speed it up? Pushing air upwards pushes on the underside of the blades and thus against the spin.
     
  7. Apr 4, 2012 #6

    K^2

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    Vectors. Take a look at this picture.

    attachment.php?attachmentid=45873&stc=1&d=1333531363.png

    Segment from A to B is your wing/blade chord with positive angle of attack. C is center of pressure. Vector DC is the momentum of incoming air. Vector CE is momentum of outgoing air. As you can see, air was pushed down. The difference of two vectors, DC-CE=CC' is the amount of momentum transferred to the wing/blade. Momentum transferred per unit time is the force. So the force on the wing/blade will be in direction of CC'. As you can see, it is mostly upward, but that vector is also slanted slightly forward, so it will be pushing the wing/blade forward, rather than slowing it down.

    I understand that it seems slightly counter-intuitive, but this is how airfoils work.

    P.S. The actual angle of attack here is the angle DCA, which is way too high for a real wing. I had to exaggerate things a little to be more clear.
     

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  8. Apr 4, 2012 #7

    rcgldr

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    From what I understand, it's the air pushing backwards (while the gyrocopter moves forwards) that generates the torque that drives the rotor (auto-gyro), which in turns generates lift. A gyrocopter can't hover, it always needs some amount of forward speed. If the engine goes out, a gyrocopter can glide, something like a 3:1 or 4:1 glide ratio (maybe higher, these are just the numbers I recall).
     
  9. Apr 4, 2012 #8
    So basically, you're saying that the angle the air comes off the wing is less than the angle it comes on, by more than the pitch angle? I suppose I can accept that (it's converting the kinetic energy of the air into its own KE, yes?)... but it depends on forward speed to generate the large DC, yes (and on rotor speed to make it come from the right direction)?
     
    Last edited: Apr 4, 2012
  10. Apr 4, 2012 #9

    jim hardy

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    From my understanding that's their Achilles heel. If rotor ever tips forward and airflow reverses, well , it's not flying any more.
    Happened to a friend who was hot-dogging in his homebuilt. Was in a shallow dive eying the girls on the beach. Fortunately he was over water deep enough to break his fall.

    Pay attention to angle of attack.
     
  11. Apr 4, 2012 #10

    K^2

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    No. Airfoil pushes the air downwards. That's all it really does. The rest is about angle of attack and relative wind. Because relative wind comes from bellow, there is forward component to generated lift. That's what spins the rotor.

    The 4:1 glide ratio for autorotation is the maximum. You can go down steeper. Reason you usually don't want to is because with a glide you get more clean air in your rotors, and if you stall the rotor in autorotation, you are kind of screwed. Of course, the rotor on autogyro is tilted, which makes proposition of vertical descent without power even trickier. But the rotor itself, without significant load, can autorotate vertically down. Horizontal speed is not necessary.

    Pretty much. It's a little more complicated with energy. The way I've drawn it, the energy of the flow is conserved, but yes, realistically, it decreases. Part of it can go into any work that needs to be done on the rotor. The rest of it goes into turbulence.
     
    Last edited: Apr 4, 2012
  12. Apr 4, 2012 #11

    rcgldr

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    update There's a link to a diagram and an article in my next post (#13) that corresponds to what K^2 has already posted.

    A better explanation of what I was getting at is that the rotor blades have a slightly negative angle of attack - correction, the rotor is driven by the wind flowing through is similar to the way gravity and the wind cause a glider to glide forwards as it descends. The glider overall has negative pitch, but not the wings.

    I was thinking of gyrocopters that can descend vertically, but apparently the ones that can descend vertically have adjustable overall pitch (collective in addition to cyclic).
     
    Last edited: Apr 5, 2012
  13. Apr 4, 2012 #12

    K^2

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    Again, it's not. The rotor blade has positive angle of attack. It is not driven like a turbine at all. The reason a glider with positive angle of attack moves forward is the same reason that the rotor can spin forward with positive angle of attack so long as relative wind is from bellow.

    The angle of attack of a disk has absolutely nothing to do with any of it.

    The turbine works completely different because turbine needs to provide high torque to a generator or compressor, while providing minimal resistance to the flow. Rotor does exactly the opposite. It provides maximum resistance to the flow, while delivering very small torque on the rotor. This is achieved when an airfoil glides "down" the flow, and this requires a positive angle of attack, for the same reason that you wouldn't build a glider with negative angle of attack.

    If the rotor generated "lift" via drag of a turbine, like you suggest, the glide ratio of autogyro would be closer to 1:4 than 4:1.
     
  14. Apr 5, 2012 #13

    rcgldr

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    Example diagram that shows this, the resultant aerodynamic force has a forwards component relative to the axis of rotation, which spins the rotor as K^2 stated:

    http://www.jefflewis.net/graphics/autorotation.jpg

    link to the full article:

    http://www.jefflewis.net/autogyros.html

    Some gyrocopters can descend vertically, is this only possible on those that have collective control, allowing for negative pitch on the main rotor?

    In the case of a model radio control helicopter, in order to auto-gyro (motor idle or off) in a vertical descent, negative pitch is required. Just before landing, a transition to positive pitch is used to hover momentarily and land before momentum of the main rotor is lost. Is the same method used for the gyrocopters that can land vertically?
     
    Last edited: Apr 5, 2012
  15. Apr 5, 2012 #14

    K^2

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    Note that negative pitch is not the same thing as negative angle of attack. AoA remains slightly positive, while the pitch might have to go negative. It depends on the rate of descent compared to rotor blade speed, which determines angle of relative wind. For RC models, the blade velocity isn't that high, so negative pitch is a must for autorotation.

    I'm not sure if ability to descend vertically depends on ability to control pitch. It does seem like pitch would have to be reduced, so it might be one of the requirements. On the other hand, for the actual landing, you need pitch control to perform the collective flare. This is necessary if you are performing a vertical landing in autorotation, same as with RC models. Otherwise, your vertical velocity is too high. That means that any autogyro designed to land vertically will have pitch control.

    An autogyro without pitch control will perform a normal flare during landing, which obviously requires forward airspeed.
     
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