Theoretically it’s not that hard - you only have to make blades stiff enough to withstand the weight of the vehicle…
They’d have to be that stiff anyway. The helicopter weighs the same upside down as right side up.
Good point, but still they would have to be stiffer - curved blades during upside down flight could possibly hit the helicopter.
I think the two things that would be the most trouble in trying this with an existing helicopter would be an engine that works upside down, and the stability. I don’t know if a typical helicopter engine could do that, but surely one be designed that can.
Stability would seem very hard though, roughly like balancing a helicopter with the blades removed on the shaft, versus hanging one by the shaft. I’d think you’d need it to be computer controlled, probably with a lot of development to get it to actually work.
Engine have nothing to do with it, you only have to change the angle of the blades to fly upside down.
Of course it has something to do with it. Even airplanes that fly upside down for an extended period need special engines or modifications. The main reason is that that standard fuel and oil pumps don’t work very well in an upside down engine and the engine can fail suddenly because of fuel starvation or even catastrophically when subjected to an upside down attitude.
I’m no engineer but it seems to me that by putting the center of mass above the blades, this might be akin to trying to balance a pencil or a broomstick on end. Technically possible, sure… but probably a difficult engineering feat for a helicopter. I may be way off the mark though.
Just as food for thought, radio controlled model helicopters are regularly flown upside down. Never done it myself, but have seen it done.
Balancing with the rotor below the helicopter is no more difficult than balancing with the rotor above the helicopter. Contrary to popular belief, having the rotor above the center of mass doesn’t lend any intrinsic stability to the helicopter. Helicopters already need active stabilization to stay level in flight, but this can be done with analog and/or mechanical systems rather than requiring a computer. Vehicles with the rotor below the center of mass have also been demonstrated, look up the Hiller Flying Platform for an example.
This may be a stupid question, but wouldn’t you also need to be able to reverse the direction the blades travel? I’d think that if you just flipped the helicopter, all the ‘lift’ generated would be straight down, no? So do helicopters even have a reverse gear? Or could you get enough lift from angling the blades to stay aloft?
What needs to be reversed on an upside down helicopter is each blade’s rotation along its own length (i.e., the collective pitch), not the direction in which the blades rotate.
I don’t have anything all that constructive to add (except for the above), but there’s a cool video online of a Red Bull helicopter doing aerobatics (including barrel rolls but excluding sustained upside down flight). Also, of course many radio-controlled helicopters can do this easily, but the power-to-weight ration in many radio-controlled aircraft is so ridiculous that they can basically be any shape and still fly aerobatically.
Finally, paging Johnny L.A., our resident helicopter pilot.
I don’t see how you can say the part in bold, and I don’t think it’s true at all. You’d be able hang a helicopter from the shaft just below the blades much more easily than you could balance it upside down on the shaft.
Also, the Wikipedia page on the Hiller Flying Platform certainly isn’t very convincing in terms of this showing that a helicopter can fly upside down:
An upside down helicopter won’t have the duct, and won’t have the ground cushion effect, so this doesn’t say anything about how a helicopter will behave.
For the model helicopter examples, I think the difference in scale is too large to be a good example. Could the shaft handle the torques needed to keep it upside down? I could easily imagine a (non-running) model helicopter being held by its blades sideways, with the shaft horizontal or close to it. Try that with a real helicopter, and I’d guess the shaft would snap or fold.
A helicopter doesn’t ‘hang’ from the rotor. Saying that the helicopter hangs from the rotor implies that the rotor will always produce an upward force, as if it were a balloon or something. A helicopter’s rotor doesn’t act like a balloon, always pulling upwards with the helicopter hanging below. It acts more like a rocket engine, producing a steerable force whose actual direction of thrust is relative to the body of the helicopter. Consider the pendulum rocket fallacy for why this doesn’t automatically stabilize the helicopter.
Now the thrust from the rotor is steerable by adjusting the angle of the rotor blades with the swashplate, and helicopters use that to stay stable by steering the rotor thrust. This can be done with a clever electronic system and servos or hydraulics, or with a surprisingly simple mechanical system as on cheap toy RC helicopters. Either way, it doesn’t actually depend on the rotor being above the center of mass as it’s an active stabilization system rather than the helicopter just passively hanging.
From your Wikipedia link:
In this context, “fixed” is the opposite of “steerable”. So this cite doesn’t support what you’re saying either.
The rotor on the right-side-up helicopter will produce a thrust that is directed roughly away from the center of mass of the helicopter body. To the extent that it isn’t directly aligned, the forces will tend to pull the mass closer to alignment. You’d have to apply a torque to keep it fixed at the same angle. Applying less torque than this will bring the center of mass closer to alignment, reducing the torque on the shaft.
For an upside-down helicopter, with the forces misaligned the same amount, that’s not the case. The forces will tend to push the mass farther out of alignment. The forces are working to help you stay balanced in the first case, and working against you in the second.