I have my helicopter rating.
As others have said, helicopters are inherently unstable. I’ve said that hovering in a helicopter is very Zen. The reason is that if you think about the constant, minute control inputs you need to… well, input… you can’t do them fast enough or accurately enough. Instead, just let your arm move the control without thinking about it.
I’ve not flown an autopilot-equipped helicopter; just Robinson R-22s and Schweizer 300CBs. They will not hover themselves.
Agreed, although our conditions didn’t always permit that. We often were flying into cut LZs in a forest. Myself or one of my crew knew the rotor diameter, craft length and wind direction and would cut a suitable swath with a little extra thrown in, but not too much as the state was very stingy in what they’d let us cut. Compound that with the fact the ground was often unlevel and a pick-up or drop-off had to be performed with the skids not actually setting down, but doing more of a “power on toe in.” Watching pilots maneuver through such gave you a great deal of appreciation for the skill required to hover, or do anything approximating that.
Again, the mechanics required just makes me think the OP’s answer will be nearer to a “no.”
As David Simmons notes, some are, some aren’t. Certainly enough is known about fixed-wing design to make it easy to design for stability, but there may be good reasons not to do so.
By contrast, I’m not sure it’s possible for a hovering helicopter to be inherently stable (and I’m reasonably sure that no real-world design is). It should certainly be possible for an autopilot to do the job.
A helicopter tethered to a rope is different than a helicopter hovering. The amount of lift the helicopter needs to generate in the tethered case is enough to hover but not enough to break the rope. With a strong rope this could be a very large range, such that you could just set the throttle/ collective and be done with it. The other problem is the helicopter spinning this would need some kind of control or perhaps several ropes to keep the copter from spinning.
I can’t seem to remember or find the term, but isn’t there a danger in a low-altitude hover where the column of air below the helicopter loses pressure due to constant down-draft, resulting in the rotor blades no longer being able to sustain lift.
My chief concern would be breaking the helicopter. Even if you had a very strong rope and a very good attachment point, it might work if you could get the craft to pull straight up with the attachment point right beneath the center of gravity. A minute gust of wind and your helicopter is going sideways into the ground. Multiple ropes might work but I’m still doubtful.
Well, many helicopters routinely carry sizeable external loads slung beneath them, which looks to be much the same thing.
Balloonist here.
It’s perfectly realistic, given a calm enough day and an “autopilot” or remote control. You’d just need a system that would keep the envelope temperature constant.
I’ve climbed up a rope, but not quite in to a balloon (clearing a stuck release) because it was a nice calm day. A stronger wind and I’d have just cut the rope from the bottom (it ended up getting cut from the top)
Incidentally, one of the helicopters I rented was destroyed when the pilot entered a vortex ring state. The pilot and his passenger survived uninjured.
(Three other helicopters I used to rent also crashed. One pilot flew into the worst weather California had had in a long time and was forced into the trees. Bad case of ‘get-there-itis’. I heard he did the same thing in another helicopter a week later. One pilot attempted a ridge landing but came in too shallow. He was caught in a downdraft he couldn’t climb out of and hit the ridge. Rolled about 100m down the hill, destroying the helicopter. He suffered bruises and abrasions. Another rolled over on an autorotation-to-touchdown when it got a little sideways and got a skid caught on a rock. I don’t know if it was the instructor or the student was flying.)
The earth is a pretty sizable external load though. A helicopter providing lift and lifting something that dangles below and always pulls down with a pilot is completely different. Look again at this video that was posted above in the thread.
Actually, let me draw you a diagram:
Sorry for the bad drawing. My idea is that tethered hovering is inherently unstable. The load below (earth) cannot move, so a horizontal force on the helicopter will produce a slight tilt which introduces a horizontal component of thrust and exacerbates the tilt until the chopper is hugging the ground. Basically it attempts to revolve around the point of attachment but earth gets int he way. Am I wrong?
Argh! :smack: Not the collective, the CYCLIC, the stick which wibbles the rotor around. Apologies for the late bump but I couldn’t let that go, fighting ignorance ‘n’ that, don’t want to spread misinformation.
Actually, something more or less similar is frequently used to land helicopters on ships. It´s called a bear trap and basically the chopper reels down a cable that it´s attached to the ships deck, the cable is tensioned and the pilot slams full throttle, that way when the ship moves up and down in the waves the helicopter follows the motion so it doesn´t smash on the landing pad; the thing is then winched down to a landing.
Search on this page for “Bear trap”, near the bottom of the page you´ll see a link to a helicopter landing that way.
As for helicopters beign inherently unstable, that´s true, but the design can be changed to make them stable, you may have seen R/C toys like this the rotors are hinged so that they tilt to counteract any unintended lateral movement. Now that may not be a very practical solution for a full size thing (besides being extremely ugly), but the principle is the same.
No - I think you’re right. But for small displacements from vertical, this instability looks to be something that a pilot (or autopilot) could readily deal with.
Well there is this. If you are just hovering the lift force is only the weight of the helicopter. Any lateral components because of tipping will be of a certain size.
If you are pulling hard on a tether the lift force and therefore the latteral components from tipping are bigger and might be harder to accomodate.
I’d say it’s correct to conclude they would harder to accommodate. But I see nothing in the OP that requires the force in the tether to be anything enormous. Thus, reasonably small lateral displacements should be manageable.
It might be a function of the length of the tether. I notice that the heavy lifting jobs tend to have fairly long lines from helicopter to load.
Yes, the problem in the video was twofold:
They were attached to a ground based object that they were not going to lift off the ground. Thus, if there is a slight change in height of the helicopter (due to gust or something) there is a resulting change in the tension of the cable and the angle of the helicopter (possibly in any or all of the three axes).
The cable was attached away from the center of gravity, so that change in tension resulted in a large moment.
The change from hovering to flight is also one of the trickiest things to do in flying a helicopter, I’ve heard. You’re not generating any lift from lateral movement yet, and you’re not getting full lift from your rotors either. It’s sort of like the moment between one step and the next when you’re walking. If someone yanked on your shoelace while you were in the middle of a step, even if they let go right away, you might still fall on your face. In this case, the boat “tripped” the helicopter.