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Downward force from hovering toy greater than weight?

  1. Sep 7, 2014 #1
    A toy similar to this one


    weighs 30 grammes on a digital scale. It hovers and flies. Holding the toy over the weighing scale with the motor turned on, with sufficient thrust to fly, I was astonished to see the force of the air on the weighing scale register 35 - 40 grammes! Thrust greater than weight? What is hapenning here?
  2. jcsd
  3. Sep 7, 2014 #2


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    You might be inadvertently pushing down on the toy slightly as you hold it in place. If so, the scale will be resisting both the weight of the toy and that additional downforce.

    There might be a net upwards flow of air as the downwards moving flow hits the pan of the scale and rebounds.

    Your scale might not be accurate (have you checked?)

    Others reading this thread will surely think of other possibilities as well. Getting experiments right is trickier than it looks.
    Last edited: Sep 7, 2014
  4. Sep 7, 2014 #3


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    What nugatory said, plus: possibly you were getting some "ground effect" when holding it close to the scale, so the same motor power was generating more thrust over the scale than in free air.

    Try putting it on a wire frame (or something similar that doesn't obstruct the air flow) above the scale, so it can actually take off as you increase the thrust.
  5. Sep 8, 2014 #4
    No pushing dwon on the scale- will try the take- off experiment.
  6. Sep 8, 2014 #5


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    Of course it's greater. If thrust was equal to or less than weight force then Newtons second law tells us it wouldn't be able to fly:

    (I assume by fly you mean ascend vertically)
  7. Sep 8, 2014 #6


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    You can hover and ascend vertically at constant speed with thrust equal to weight force. The question is if in his experiment the thrust was just set for hover or much higher for vertical acceleration.
  8. Sep 8, 2014 #7
    If I aimed a water hose or a high pressure jet of air at the weighing scale won't it register a weight depending only on the force of the jet, and not on anything else?

    The flying machine is kept up by accelerating air downwards, not by the column of air underneath. It so happens that a mass of air is accelerated and impacts the plate causing an increased in force on the plate.

    Is this it?
  9. Sep 8, 2014 #8


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    You have to make sure that it produces just enough thrust to hover, not more to accelerate or push up against your hand.
  10. Sep 8, 2014 #9


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    That's correct.
    That is what causes the force on the plate, but you also need to consider what happens to the air after it "impacts" the plate. The air is still moving somewhere, to get out of the way of the air that is following it downwards.

    Your thrust measurement assumes the air leaves the plate traveling horizontally. That is unlikely to be exactly true. If the air was deflected back up again, you would get a greater force on the plate.

    You might be able to show that experimentally if you put a shallow dish or bowl on the plate, so the air is deflected upwards over the edges of the dish.

    As Nugatory said, getting experiments to give accurate results is hard!
  11. Sep 8, 2014 #10
    As a practical application, for example a helicopter or a BAe Harrier will create a downward force of air much greater than the weight of the aircraft.

    Physics is fascinating, thanks to all.
  12. Sep 8, 2014 #11


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    It's most likely that while hovering in ground effect, some of the air is being reflected back upwards a bit or turned upwards due to vortices. If the plate of the scale was large enough the effects of the reflected air would be offset by reduced pressure just outside the reflected air. It might be possible to contruct a cardboard box with an opening on the top a bit larger than the induced downwash of the hovering model to prevent most of the reflected flow of air.

    ... or put the model inside a sealed box (closed system), and then check the weight with the model at rest on the bottom, hovering, or pinned against the top of the box (assuming protected rotors). Since it's a closed system, the total weight of box, air, and model will remain the same (as long as there's no net vertical component of acceleration).

    If out of ground effect, then I'd expect several newton 3rd law pairs of forces, one of which would the the downforce exerted by the aircraft onto the air equals the upforce exerted by the air onto the aircraft. If in a hover, then the downforce exerted by the aircraft would be equal to its weight. Eventually the weight of the aircraft is transmitted through the air and at the surface of the earth, the downforce exerted by the air equals the weight of the air and all of the objects being supported by the air, including lighter than air balloons supported by buoyancy (assuming no net component of "vertical" acceleration of the mass of the system over some period of time).
  13. Sep 8, 2014 #12
    If you are close enough to measure the air pressure on the scale you are getting some ground effect; as mentioned above, you need to make a wire frame that sits on the scale but holds the toy much farther away.

    Not sure if it comes into play with the short time and small distance to ground effect with your experiment, but there is something that happens with helicopters that try to hover for extended periods... which results in them pushing down-force much harder than the weight of the craft.

    When the helicopter first begins to hover, the surrounding air is relatively static and the only down force needed roughly matches the weight of the craft. With extended hovering, the surrounding air forms a toroidal vortex like a horizontal donut with the blades in the hole.

    The air pushed down curves back up all around the craft and re-enters from the top again, and as this sets up the vertical column of air flowing down through the blades progressively increases speed.

    The pilot corrects for this to maintain hover with the cyclic (blade pitch control lever)... but has to be careful. Lots of helicopters have crashed when they could no longer get purchase on the air because it was already flowing downward too fast. With the blades only cutting but not pressing down any air, the pilot doesn't control the craft well so it is difficult to get out of the column of downward air flow...
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