Induced drag on the prop and 'prop-wing-lets'

falstro
  • Induced drag on the prop and 'prop-wing-lets' falstro

    Turbine engines are covered, which of course is to contain the process (just like a super/turbocharged engine is as soon as the air enters the intake). But it got me thinking, doesn't this also reduce - or even completely remove - the induced drag around the tips of the fan blades?

    Would it be possible to have a similar setup for a standard prop, it doesn't even have to be stationary, it could be a ring connecting the tips of the prop, spinning with it. Like an infinite winglet. :) It has the added safety benefit that it'll be visible when the prop is spinning. And I figure if the ring is strong enough to maintain its circumference, the load on the prop should be marginal since it's spinning around its own center of mass.

    Is the induced drag on the prop not large enough to warrant any thought, or would such a prop-winglet-ring (I'm sure there's a real name for it, anyone know what I'm talking about?) cause other disruptions of the airflow? Or perhaps there are other reasons, like it would simply be too hard to solve for constant-speed props?

  • What you're talking about does exist, they are called Q-Tip Propellers.

    Remember that a propeller blade is just an airfoil - like a wing - and the basic aerodynamics are no different than a wing. But the rotation of the blade creates more phenomena than a wing, in particular the helicoidal vortex one sees behind the prop and causes all sorts of propeller effects.

    In theory, nothing would prevent us from having winglets at the prop tips : the advantages would be

    1. Making the prop more efficient by reducing the induced drag (same as a winglet on a wing )
    2. Reducing noise
    3. Keeping the propeller tip speed subsonic by decreasing its length

    The big problem is in aerodynamic stresses, and as far as I know there have been some quite spectacular failures during testing, so the solution is now to give a greater sweep to the tips (see that as equivalent to the 777 wing tip compared to the 787 for instance). Try and find articles on the Hartzell Q-tip.

    As naval propellers are wider and capable of deealing with bigger torque stresses, modern ones do have winglets. You could find some pictures on the web.

  • A ducted fan comes close to what you're describing, although the ring around the propeller is stationary instead of attached to and spinning with the propeller.

    The main advantage of a ducted fan is higher efficiency due to reduced propeller blade tip losses (essentially induced drag) but this efficiency advantage is lost at higher speeds and/or lower thrust demand.

    In "normal" aircraft, the drawbacks of a ducted fan outweigh the efficiency gains. Ducted fans are mainly used in airships and VTOL aircraft like the infamous Bell X-22. They are also used in most jet model airplanes.

  • For historical perspective do some research on the Culver Channel Wing. This twin engine plane has two ducts that do not completely encircle the prop.

  • Q-Tips and Ducted fans are the big ones to solve the problems you're thinking of.

    Your ring idea would be very difficult to implement for a number of reasons, weight being primary. A metal ring all the way around the propeller would add a significant amount of weight to the aircraft, which would likely cancel out any efficiency gains you get from stabilizing the airflow. In addition, the tips of a propeller are already experiencing several thousand G's at normal operating RPMs. This is acceptable because the prop gets continuously lighter as you approach the tips. But if you were to attach a ring of metal weighing a few dozen pounds, the forces would be astronomical, and your prop would very quickly fail.

    A second issue is that in order to have efficient propellers, we rotate the blades slightly to change the angle at which they bite into the air. These are called constant speed propellers, and they're already somewhat complicated. If you go and add a second pivoting point to the prop tips so that they can move inside the ring, you're just adding a bunch of bearings, grease, weight, and another failure point.

    Finally, balancing the ring would likely be a difficult task. First your ring would have to be manufactured to very precise tolerances which would be quite expensive. The slightest nick or dent in the ring (which happens frequently to propellers) will cause it to become unbalanced and would at minimum require work, and at most could cause the whole propeller to shake itself apart. This is already a minor concern for props, but when you put your heavy disk out on a long arm from it's fulcrum, then subject it to incredibly high G forces, you're just amplifying any faults it may have.

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