stability of helicopters and related phenomena

Discussion in 'Architecture & Engineering' started by DRZion, Apr 27, 2010.

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  1. DRZion Theoretical Experimentalist Valued Senior Member

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    What is the easiest way to stabilize a single thruster (ie helicopter blade, rocket or something)?

    For instance, what prevents rockets from just tipping over as they start? I'm sure its got to do with at least a bit of design and engineering ..
     
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  3. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Because the thrust line goes directly through the centre of gravity. Why would it tip over, it's balanced. Hence the requirement for good weather conditions (i.e. low wind speed).
    Once it's moving then control surfaces/ thrusters can take over.

    Helicopters are a different sort of problem, but they still require the thrust line (mostly) through the CoG.
     
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  5. DRZion Theoretical Experimentalist Valued Senior Member

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    Hmm. I'm fairly sure a helicopter does tip slightly. Otherwise, how would it produce thrust anywhere but the vertical? I'm talking about helis that use blades only, cuz I know there are some that also have rocket thrusters. But they also have those secondary blades for ?turning? .
     
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  7. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    That's why I wrote "mostly".

    Huh?
    Which?

    I know of helos with tip jets (in the blades), but rocket thruster helicopter?
    If I know of any they've slipped my mind for the moment.
    Remind me and I'll see what I can remember.
     
  8. DRZion Theoretical Experimentalist Valued Senior Member

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    I can't be sure, but I think there are.. It would make sense though, with the thrusters (be it rocket or otherwise) providing horizontal thrust while the blades would provide vertical.

    As for the rocket, it must be damn well balanced if it doesn't tip in its 100 km trajectory away from earth's gravity. I have a feeling there is more to it, but rocket science is beyond me

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    Correct me if I'm wrong, but it seems that helicopters can alter the tilt of the main blade, right? This would be enough to keep it from falling unless of course the wind got strong.

    The reason I'm asking is that I want to put my recoilless thruster to work, but I don't want to end up falling hundreds of feet to my death after takeoff... and I sure as hell won't go into rocket science or imitate heli engineering to make it work!
     
  9. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Rockets seem a pretty intensive way of getting forward thrust - they're generally not throttleable: i.e. they're on or off.

    Pfft you asked about when they start. Once they're moving there's control surfaces and verniers. Or even in some cases the main rocket nozzles themselves can be steered slightly to make sure the thrust line goes through the CoG.

    Quite a bit. The blades don't even always operate in the same plane.

    Not sure what you mean about "falling". How strong a wind do you mean? The world speed record for helos is ~250 mph/ 400 km/ hr (unless you count hybrids like the so-far-not-achieved-design-promise ABC Sikorsky X-2)- that's a pretty strong wind.

    Recoilless thruster?

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    I think you just made NASA's day, or even year, Marc Millis is the guy to get in touch with.
     
  10. DRZion Theoretical Experimentalist Valued Senior Member

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    True.. but I suppose it could be made to have a tunable forward and rear exhaust.. not sure how well that would work inside a burning stream of rocket fuel.

    The what and the what?

    Oh, I see. You meant that rockets don't take off unless the winds are calm. It would be silly to think that helis don't work in high winds since they effectively experience high winds going 400 km/h .

    Well, its just a theory for now.. or maybe its already been done. If its not recoilless then its propellant-less, something like a radiation thruster but more efficient since it produces something more massive than photons... unless I'm completely fudging some math (quite likely). I should probably look at the link to see if anything sounds familiar.

    However this is not the point of the thread. The point is - how can I show off my hypothetical thruster without killing myself in the process. Given one source of thrust, how difficult would it to build a safe flying vehicle? It took Sikorski many years to build a working heli, I want something simpler.
     
  11. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Plus there's the problems of storing the fuel and liquid oxygen.

    Control vanes? The little wingy-things that stick out.
    Verniers? Little rocket nozzles that stick out the sides - they can be fired to correct any off-course movement.

    Well a rocket isn't as controllable as a helo (for low speeds anyway), but helicopters don't fly in really high winds because the pilot would have to spend most of his time correcting for gusts etc. Plus there's the drain on power if the thing's trying to fly against a head wind.

    AFAIK most (proposed and built) back-pack type units used the pilot's body for control - swing your legs about to change CoG line.
     
  12. DRZion Theoretical Experimentalist Valued Senior Member

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    I would think these are compartmentalized (you don't want to store the oxygen with the explosive fuel), and I agree that opposing exhausts would probably interfere since the path from the burning fuel to the exhaust is straightforward for obvious reasons.

    Hmm, this is a good point. A back-pack type device may be an easy way to go, and i could probably test my hypothetical machine in a room with a tall ceiling, or even outside with my legs tied to a rope. Simplicity is key.



    So, I have come up with this contraption to that should maintain stability. Imagine you have a long pipe. At one end of this pipe you stick a propeller so that its blades are perpendicular to the length of the pipe (like a helicopter). At the other end you tie a counterweight. Now, pretend that there is a high powered motor in the pipe and so the thing flies.

    I would think that it wouldn't tip over and here is why - if you remove the propeller and you hold the pipe, its basically like holding a well balanced hammer. You can do this at home.

    The easiest way to hold this thing is with the counterweight at the bottom. If you try to rotate (ie like tipping the heli) it to place the counterweight above your arm, it will be difficult since each extra degree of rotation will increase the sine and increase the required force to tilt it further. Hence, the most energetically stable geometry would be with the counterweight at the bottom and the propeller exerting force upward.

    This would be fun to play with using some kind of conventional motor and propeller - it should be fairly easy to balance a flying object! However, I do not know how much thrust conventional motors could generate and if this would be enough to lift the motor itself. If it is however, feel free to experiment and use the above mentioned balancing - ballast , keeping in mind that the trade off here is weight vs thrust. The ballast removes the need for sophisticated steering gadgetry at the expense of being heavy. Your call.

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  13. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    They are, but it's twice as much storage required (or half as much total fuel).

    Yep. But not round your neck.

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    It wouldn't tip over (unless serious control problems got it horizontal): for the simple reason that the centre of gravity is below the centre of thrust (effectively that being the support point).

    Agreed.

    Ack! Now you're getting into calculations.
    It's going to depend (among other things) on number of blades, size of blades, length of blades (i.e. total disc area), and rpm of the rotor. The larger the rotor diameter the slower the required rpm. But the larger the blade length the more chance of running into compression problems when moving forwards. (Although that'll usually only happen at 200 mph+ speeds which may be out the picture to start with).
    Here's a good start.
    And I can (probably) recommend the book Military Helicopter Design Technology, (I say probably because I've only had chance so far to skim my copy - I hit Amazon pretty hard over last month or so and getting through £1,000 worth of books does take time

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    ).

    Meh, I'd suspect that there'd be little need for ballast as such, the powerplant (and "fuselage") itself may well be heavy enough. That'd be more calculations for you...
     
  14. sifreak21 Valued Senior Member

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  15. DRZion Theoretical Experimentalist Valued Senior Member

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    So, do you see what I mean? Its like when you tie a string at the end of the hammer and put it on a nail in the wall. That is effectively the support point. The weight of the hammer keeps it vertical with the hammer head at the lowest gravitational potential. I think the propeller would act just as the nail does in supporting the mass; hence the thing would require some serious forces to tip.
     
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