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Homework Help: Propeller rotating in horizontal plane

  1. Apr 14, 2014 #1

    I have a project whereby I am designing a toy that spins a horizontally positioned propeller, then releases it.

    After releasing it, the propeller gains height, but moves somewhat away from the operator of the toy.

    Is the direction of airflow through the propeller vertically? If so, is it ''thrust'' that makes it climb and ''lift'' that makes it move forward? (lift is perpendicular to thrust).

  2. jcsd
  3. Apr 14, 2014 #2


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    The flow is mostly vertical with a rotational component. The ideal primary air flow diameter narrows as the speed of the air increases, but real world viscosity results in the surrounding air also being affected.

    Lift is perpendicular to the relative flow. For a propeller, "lift" would be perpendicular to the relative flow over a propeller blade, but the relative speed and direction of this flow is affected by the propeller, and the resulting induced flow speed and direction (rotation) changes somewhat depending on radial distance from the hub of the propeller.
    Last edited: Apr 14, 2014
  4. Apr 14, 2014 #3


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    Is that a description of what it does in real life, or a description of what you want it to do?

    Initially the thrust force is along the axis of the prop. If the prop is not perfectly vertical when it is released, it will have a component that starts it moving sideways.

    As soon as that happens, there is a relative "cross-wind" component of the incoming air, relative to the prop. That means the blades on one side of the prop have a higher speed relative to the air than the blades on the other side, so the aerodynamic forces are unbalanced and create a bigger horizontal force in some direction. The aero forces will also tend to rotate the prop to point in a different direction.

    In helicopters, the angle of the blades is changed as they rotate, to control the forces generated by the rotor. In fixed wing aircraft, the relatively large inertia of the plane itself, and the other flight control surfaces, mean this effect from the propeller is not usually significant. And of course helicopters use the "sideways" component of force from the rotor to move horizontally relative the the ground, but fixed wing planes do not.

    If the toy is made from flexible material, there is a further effect, in that the centripetal and aerodynamic forces on the blades will probably change their shape, and the change in shape will affect the aero forces. Understanding that in detail could make an interesting (and challenging) project!
  5. Apr 15, 2014 #4
    I want the propeller to rise vertically, with minimal movement in the horizontal plane.

    I'm required to draw free-body diagrams of the propeller in flight. I know that there will be a ''lift'' force that is defined to be perpendicular to the air flow.

    If the thrust here is vertically upwards (or near enough), then I wonder if the ''lift'', which is defined to be perpendicular to airflow, is causing the prop to move away from the operator in the horizontal plane?
  6. Apr 15, 2014 #5


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    lift is perpendicular to the air flow relative to the moving blades of a propeller, so it's almost in the same direction as thrust.
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