Can a helicopter stand on the ground upside down on its rotor head?

Question

From what I understand the rotor head needs to be strong enough to hold the craft weight hanging below it, and sounds pretty strong. The blades however look quite fragile (for a piece of metal).

In short, can a helicopter do something like this?

I found source of this GIF image: Escape to Witch Mountain (1975). This scene is around the 1h 19m mark. All hail @Peter Kämpf for identifying this copter as Hughes 500 which led me to list of copters in movies.

In the scene with humans and the Australian helicopter, a trained eye can spot the lack of a "green screen effect" (it's a 40-year-old movie!) so it seems this Hughes 500 was really put upside down! (Should we investigate it further to Movies.SE now?)

Followup on Movies.SE

Answer 1

Stand on its rotor head? yes, maybe. Spin? not for long.

The rotor hub and blades are quite strong and can take the helicopter weight at-least temporarily (leaving aside the balancing issue). The blades are designed to overcome stresses in flapping and can take the load. The rotor hub however, is not designed to take loads in this attitude (it is designed to lift the helicopter, and not push it) and its capability is an open question. You could put the helicopter upside down, but its going to be a brief affair with no guarantee that the parts will ever work again properly.

However, in case you want to spin the helicopter, you run into a number of issues. You could clamp the blades down and try to spin, but the rotor hub will struggle to take the load. The load path from the rotors to the fuselage is designed to take lifting loads and not this.

Helicopters are not designed for sustained inverted flight and as a result, the systems are not designed for anything like this. For example, the gearbox lubrication system isn't going to work for long in the upside down condition. The rotating control rods aren't going to take the gear box weight, let alone the helicopters'.

But first, the dynamic loads- helicopter systems below the rotor hub are not designed to take rotating loads, let alone the horribly unbalanced one like a fuselage. Even the lightweight helicopter blades are balanced to prevent and unsafe vibrations -in a case like this, the rotating fuselage would most likely tear apart the helicopter before anything else.

Answer 2

This looks like a model that was carefully set up to spin.

First the facts: FAR part 27 regulates the design criteria for rotorcraft. Specifically, FAR 27.337 states:

The rotorcraft must be designed for—

(a) A limit maneuvering load factor ranging from a positive limit of 3.5 to a negative limit of −1.0 [snip]

So yes, contrary to the unsubstantiated claims in the highest-rated answer, the inverted rotor shaft can support the weight of the (most likely empty) helicopter. The center of gravity is very close to the rotor mast, and gyroscopic forces might even stabilise the fuselage long enough to film one full revolution, but still the clip is impossible to film with a real helicopter without breaking things. Look at the drawing of the Hughes 500 C, the type used in the clip, below:

Hughes 500 three-side view (picture source)

Clearly, the tail extends above the rotor plane, so something must be broken first before the fuselage is free to spin around. Next, everything must be carefully set up and the fuselage must be spun around somehow. This film was not made after an inverted touchdown - this is impossible with regular helicopters. From the linked page:

To enable a commercial helicopter to fly upside down, manufacturers would need to make its rotor blades more rigid so as not to flex too close to the main body of the helicopter (otherwise they could rip off their own fuselage or other critical components). They would also need to redesign the joint that connects the rotor blades with the rest of the vehicle so it could bear the load of an upturned helicopter. Finally, they would need to develop new controls to allow the rotor blades to tilt downwards and reconfigure the engine so that fuel and lubricants could be distributed properly while the helicopter was inverted.

The page doesn't go into stability, but that would be another reason why this clip is not real. Flying upside down would be similar to balancing a broom on a fingertip.

Note that the FAR part 27 still demands a load that is equal to standing the helicopter on its head, regardless of the blades touching the body when loaded for -1g. Even though the helicopter cannot be flown inverted, the loads that would incur must still be tolerated, because they might happen in regular flight when a heavy gust hits the aircraft.

My explanation: Someone took his model helicopter, broke part of the tail off (note that the vertical part of it is missing in the clip!) and carefully balanced it on its rotor head. Mind you, he could even have fixed the rotor head to the ground with some tent stakes. There is enough structure inside of the hinges to facilitate this. Spinning it is easy, and no, there are no imbalances which would "tear the helicopter apart", especially not at such a leisurely rotation speed.

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EDIT: Thanks to the invaluable help of @PTwr it is now clear that this is a scene from the 1975 Disney movie "Escape to Witch Mountain" in which two kids are chased by an evil millionaire. His helicopter in one scene flies and lands upside down. From www.rotaryaction.com:

… The chopper winds up flying upside-down and landing in that position, still spinning, making the pilot and his passenger dizzy.

So it is a Hollywood special effect, and given the time of the movie (before CGI), this must have been shot with a scale model.

Answer 3

Nope, they cannot. There is a hinge in the rotor head that will make the helicopter want to topple over, it won't be spinning like in the video: it is not supported by the blades laying on the ground. Picture the blades not being there and you'll see how impossible it is to balance the whole construction on top of the rotor mast. And functionally (for this case) the blades are not there, they hinge up & down, see red circle in the picture underneath.

Some helicopters have a bearingless rotor head, dimensioned for making the fuselage follow the rotor disk while suspended in the air. Not for suspending the imbalance of a spinning fuselage when upside down: the CoG must be exactly in line with the rotor mast for this to occur, and real helicopters are designed with a useful CoG range.

When everything is up in the air, it might be conceivible for the helicopter to fly upside down if the collective could travel down as much as it can travel up, and if the fuselage could be supported like a broom on top of a hand, and would not spin. But not on the ground like in the video, no.

Update

Bearingless rotor heads have flexing beams instead of pure hinges, so the rotor head has some inherent stiffness to keep a fuselage upright when the helicopter is upside down. Some stiffness: when the rotor is spinning the centrifugal forces on the blades help in keeping them extended.

The vast majority of helicopters have flapping hinges or teetering hinges, both will make the helicopter flop over when in a situation like in the video. Yes the fuselage can stand on top of the rotor mast but the rotor mast won't stay upright, it'll hinge.

Actual rotor head of the Hughes 500. Check where the flapping hinge is.

Answer 4

According to the Web site, http://www.rotaryaction.com/e.html, the helicopter was a real Hughes 500 with extended landing skids.

I would assume that even if the frame was real, they would use a touch of movie magic to pull off the effect. To get the spin-rate right for a movie, I would assume they pulled the engine, used a rotary electric motor to control the exact rate of spin and balanced the helicopter to produce the effect.

Like others, my search for movie trivia came up blank. So I went to IMDB to see who the special effects people were and ask them the question. Unfortunately, Art Cruickshank passed away 1983, Danny Lee passed away in 2014. Hal Bigger is listed as an uncredited special effects guy, but I could not find out if he was alive or a contact address.

If anyone wishes to contact the crew and ask them how they pulled off the effect, that's one way to get an answer on how it was done. I'll defer to others on whether or not you could do this with a real helicopter. My assumption is that under perfect conditions, you could power up and take off. But not without a modified Hughes 500.

IMDB Cast and Crew: http://www.imdb.com/title/tt0072951/fullcredits?ref_=tt_cl_sm#cast

Answer 5

No. You have neglected to account for how drive systems work.

When the engines engage with the transmission, the blades begin to turn. The sprag clutch/freewheeling unit is only oriented in one rotational direction and to catch. If the load comes from the other direction of rotation, it disengages. (See page 4-6 of the link).

In this particular case - beyond the problem of the fuselage being well out of balance - most helicopter's free wheeling units would not engage properly.

A more pressing difficulty would be that (if the engines would start) the engine would run until the problems with the fuel system arose.

Typical helicopter fuel systems rely on gravity feed into pumps in the bottom of the fuel cell, which then pressurize the fuel and get it to the engine. When turned upside down means that the fuel will go to the new bottom (the top) and the air will go to the top (bottom) at which point the fuel pumps will start sucking in air instead of fuel. If you get enough air into your fuel lines you can expect the engine to have trouble providing power to the rotor system.

(@JanHudec observed that the sprag clutch would likely engage, so part of the answer may be incorrect. I may edit later after another estimate. What is a freewheeling unit? It's what allows you to autorotate when you lose drive power from the engines).

Answer 6

Can it...? Yes. But... practically No.

The points below explain why the answer is, strictly speaking, yes. But then, they explain why it,s not actually helpful in practice. Note how each case fails so fast that the technical properties of the helicopter are mostly irrelevant.

Can it stand on the rotor head?

Yes. The helicopter is in a nice equilibrium.

It's just that it is in an unstable equilibrium.
But the stiffness of the central drive shaft should be enough to keep it in balance for a while. And note that the drive shaft is somewhat tilted, not exactly vertical. If the pilot picks up his manual, searching for the right checklist, the helicopter will certainly fall over.

So, practically No.

Can it rotate on the rotor head?

Yes. The imbalance is no problem, when it starts slowly to rotate.

It will turn, but not a whole revolution. Probably not more one fourth of a turn. There are some problems caused by the lacking rotational symmetry helicopters. When it just stands on the rotor, the orientation of the shaft is such that the center of gravity is above the rotor head. Depending on the amount of fuel, and the use of seat belts by the pilot, the center of gravity will be some centimeters away from the shaft axis. That means a rotation axis would need to be tilted by some degrees against the shaft.

So, practically No.

Can the rotor head handle it without just flying away?

Yes. The rotor head would be stabilized by being pressed into the ground.

The problem is with the other parts of the helicopter. All other parts. They would all fly away.
After rotating for some angle, the helicopter would move to the side of the center of gravity, rotating around the rotor head on an horizontal axis too. Before touching the ground, it will tilt the rotor head slightly, which would, because of the long lever, would move the rotor blades violently, if they would not break before. Soon later, the connection of the rotor head and the upper drive shaft would break, allowing the helicopter to crash.

Assuming we want more things to not fly away, like drive shaft, blades and pilot,
practically No.

The rotor head, pressed into the ground, just sits there, happily ever after.