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Downwash from the rotor of a helicopter creates drag on the helicopter body acting against the lift force of the rotor. Why are there no helicopters with the rotor under the main body of the helicopter to eliminate this force?

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    $\begingroup$ What does the helicopter land on? The rotor is still spinning when the helicopter is on the land, so the gear can't extend through the rotor. The hub is too small for the weight to be balanced on uneven ground. Having long arms for the gear that extend past the rotor might work, but would be ungainly $\endgroup$
    – CSM
    Oct 23, 2023 at 10:00
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    $\begingroup$ I think that's called a lawnmower. $\endgroup$
    – keshlam
    Oct 24, 2023 at 6:27
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    $\begingroup$ Mandatory gif: i.stack.imgur.com/IqEqj.gif $\endgroup$
    – Federico
    Oct 25, 2023 at 7:20

4 Answers 4

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If the rotor would be underneath the helicopter, then:

  1. The helicopter body would still generate vertical drag from being exposed to the vertical airflow, but now above the rotor,
  2. The rotor would work in a disturbed turbulent airflow, because of said body, increasing vibration and reducing efficiency,
  3. You still need some landing gear below the rotor to land on.
  4. The slightest roll control input during start or landing would cause a blade strike on the ground,
  5. Uneven terrain would cause a blade strike
  6. Loading and unloading passengers and goods would require the rotor to come to a halt first.
  7. Whilst the helicopter is parked, the low rotor could easily be accidentally damaged by inattentive ground operations.
  8. The body would need to be on a rigid mast above, instead of hinged below the rotor. From a stability point of view, this is challenging at least may be beneficial for stability.

In summary, putting the rotor below the helicopter has many downsides, but gives very few benefits.

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    $\begingroup$ Good answer, however your point 8 about stability is a counterintuitive misconception. The mounting point of your rotor in respect to your CoG actually affects the stability in a positive way. See this answer: aviation.stackexchange.com/questions/42513/… $\endgroup$
    – U_flow
    Oct 23, 2023 at 10:32
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    $\begingroup$ @U_flow thank you for the link. I have updated the answer accordingly. $\endgroup$
    – DeltaLima
    Oct 23, 2023 at 11:05
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    $\begingroup$ One additional consideration for point #8. Rotor masts are typically narrow metal cylinders, which are stronger in tension than compression. The rotor attachment to the transmission would likely need to be radically different. $\endgroup$ Oct 23, 2023 at 18:24
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    $\begingroup$ Another point - autorotate would be extra-difficult, with the pilot having to essentially "balance the broomstick" rather than hanging under it. Noone needs that extra work when its all gone badly already. $\endgroup$
    – Criggie
    Oct 23, 2023 at 19:16
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    $\begingroup$ Points 1 and 2 are if not entirely then at least mostly wrong. The air flow above a helicopter rotor is not supposed to be linearly downwards at all, on the contrary the whole working principle is that it directs air coming in horizontally (from the front during flight, from all sides during hover) downwards. It is true that the movement right above the rotor has a vertical component and would cause some vibrations, but nowhere near as much as that's the case for pusher props. $\endgroup$ Oct 23, 2023 at 19:50
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Actually, there are/were (nearly/sort-of)

Have a look at: Lackner HZ-1 Aerocycle on Wiki

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  • $\begingroup$ Interesting! So they invented the Breath of the Wild minecart flyer in 1955. "it was discovered that the Aerocycle's forwards speed was limited by an uncontrollable pitching motion," $\endgroup$ Oct 24, 2023 at 10:57
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    $\begingroup$ That thing looks terrifying. $\endgroup$
    – Brad
    Oct 24, 2023 at 18:53
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    $\begingroup$ That guy's not even wearing a chute, because, what's the point with that slicer underneath his feet. $\endgroup$ Oct 24, 2023 at 20:05
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    $\begingroup$ Another one: hiller.org/event/flying-platform $\endgroup$
    – Florian F
    Oct 24, 2023 at 21:09
  • $\begingroup$ @CptReynolds And apparently the XMP-2 parachute, designed for the HZ-1 and VZ-1, "proved to have insufficient reliability for use as a personnel parachute". $\endgroup$
    – screwtop
    Oct 25, 2023 at 22:24
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Some AAVs have the rotors mounted outboard and mostly below the cabin. Whether that's a 'helicopter' or not is arguable. Some definitions specify overhead rotor(s).

enter image description here

So it seems more practical when there are multiple (non-coaxial) rotors.

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    $\begingroup$ I would be so scared to get anywhere near one of these. Imaging accidentally falling or having one of those blades pull in a piece of clothing! they look cool though! $\endgroup$
    – coblr
    Oct 23, 2023 at 22:02
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    $\begingroup$ @coblr I imagine a practical air taxi version of that would not allow the blades to spin up unless the area around it was clear and doors closed. They're electric so they don't have to idle. It would have to have a lot of redundancy in the control systems to get approvals I would think. $\endgroup$ Oct 24, 2023 at 0:36
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    $\begingroup$ @coblr How is it different to turbo prop planes? $\endgroup$
    – Darren
    Oct 24, 2023 at 6:34
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    $\begingroup$ @Darren: It's different because the rotors are so low that if you trip and fall, you could fall into them instead of under them. And they're right next to the door because this thing looks tiny, unlike a typical turboprop. coblr isn't just talking about not wanting to get near the blades, which is something you should avoid on any aircraft. They're saying that there's no way to get near the whole aircraft without being near a blade. So yeah, you'd want to make sure the blades aren't moving before opening the door from inside, or before walking up to it from outside. $\endgroup$ Oct 24, 2023 at 10:20
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    $\begingroup$ @SpehroPefhany I'd be more worried about what can go wrong (externally, of course the doors can be interlocked) in the time it takes them to spin down again $\endgroup$
    – Chris H
    Oct 24, 2023 at 10:36
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@Spehro Pefhany mentioned "outboard rotors", but I would like to mention an exotic example - AeroVelo human-powered helicopter. Its rotors are placed pretty low :-)

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