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Why helicopters are stable?

  1. Jan 31, 2010 #1
    I just realized couple of days ago that I had never known why helicopters were stable, and avoided tipping over. When you look at a helicopter, it creates an intuitive impression that it is stable, because the center of mass is below the rotor. It seems as if it the helicopter's body is hanging from the rotor, so that if the helicopter was going to tip over, then it would do it about an axis which intersects the rotor, and because the center of mass is below the rotor, therefore the helicopter remains stable. This thinking is wrong, however. Objects which are in the air unsupported, always behave as if they were hanging from the center of mass. If they tip over, they will tip over about an axis that intersects the center of mass!

    So if the rotor is not perfectly directed away from the center of mass, and a physical rotor will never be perfectly directed away from any point, the rotor's thrust will create a torque that tends to force the helicopter to tip over. So why are the helicopters so stable then?

    I succeeded in coming up with one explanation, and it is that helicopters don't tip over easily for the same reason why spinning tops don't tip over. The rotor has lot of angular momentum in it, and it keeps the helicopter stable. If the helicopter would tip over, it would do it very slowly, like spinning tops fall very slowly too, and hence pilots have plenty of time to adjust the direction of the rotor.

    I'm almost convinced that this gyroscope explanation is the right one, but there are some things which still bother me. There are also helicopters, whose rotors don't contain large angular momentum, because they have two rotors rotating in opposite directions. Like these ones:



    Why are these helicopters stable too?
  2. jcsd
  3. Jan 31, 2010 #2
    Helicotpers are (generally) unstable, you need to look at a stability analysis of the poles of the linearized dynamics on a root locus diagram.

    Aircraft are also unstable in the lateral dynamics (sometimes also longitudinal), but the degree of instability makes it acceptable for piloting.
  4. Feb 1, 2010 #3


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    Do you remember when jets had wings swept back? That was done for better stability but at a cost in speed. Now, with military jets flown mainly by computers, some instability is acceptable for better performance and wings are swept forward.
  5. Feb 1, 2010 #4
    Well, forward wingsweep is still very much at a NASA technology demonstrator stage. A lot of the X-29 data is still classified. But you are correct about stability augmentation systems (SAS).

  6. Feb 1, 2010 #5


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    I don't know of any forward swept wing aircraft in production.
  7. Feb 5, 2010 #6


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    Look in the first chapter or so of 'Chickenhawk' - by Robert Mason. It's about a pilot learning to fly helicopters for the Vietnam War. It describes, in a very graphic way, just how difficult it is to fly a manual helicopter. There are at least three controls, each of which affects the other two. It's more unstable than you could ever imagine; you change the tilt and that changes the lift which changes the heading so you correct for the lift and then you're turning in a direction you didn't want to go and find you're rising or falling and then . . . And I haven't even mentioned the precession problem.
    Even helicopters with electronic control are not easy.
  8. Feb 5, 2010 #7
  9. Feb 6, 2010 #8
    Last edited by a moderator: May 4, 2017
  10. Feb 6, 2010 #9
    That's right, there is significant coupling in the lateral and longitudinal dynamics.
  11. Feb 8, 2010 #10
    Look at the Sukhoi Su-47 Berkut. It is a forward swept wing design and is the gnarliest looking airplane Sukhoi has come up with.
  12. Feb 8, 2010 #11
  13. Feb 8, 2010 #12
    I saw forward swept wing and got excited. . .
  14. Feb 8, 2010 #13
  15. Feb 8, 2010 #14

    The use of compressed nitrogen to increase control at high angles of attack is very interesting! I never knew about the VFC until today.
  16. Feb 9, 2010 #15


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    Not a production aircraft.
  17. Feb 9, 2010 #16
    Role Technology Demonstrator
    Manufacturer Sukhoi
    First flight 25 September 1997
    Introduced January 2000
    Status Experimental
    Primary user Russian Air Force
    Number built 1

  18. Feb 9, 2010 #17
    It reminds me of the Ferrari F250GTO. Le mans said they had to build at least 500 to be a production car and be allowed to race. Only 36 ever got built (clearly a violation of the rules), but are now worth millions.
  19. Feb 9, 2010 #18
    If you look up military helicopter videos on YouTube, you will find a bunch of Russians bragging about how the Kamov Ka-50 helicopter is "superior" to the AH-64 Apache. It sounds very impressive until you realize that less than two dozen of them were ever built. The US has produced over 1100 Apaches.
  20. Feb 9, 2010 #19
    Didn't know it needed to be production to be incredulously awesome.
  21. Feb 11, 2010 #20


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    Thank you for mentioning that book. I'll look for a copy. I flew fixed-wing about 35 years ago, but have become interested in choppers within the last few years (owing in large measure to Fred Garvin's input and my ex buying me a flight at a local airshow a couple of years ago). I'm pretty sure that I could get a simple one off of the ground and back down again, but I wouldn't want to try anything fancy like leaving my back yard.
    By the bye, I applied to the US Air Force to go to Nam as a fighter pilot. Being a 17-year-old Canadian citizen who needed glasses, I was rejected. I'm disappointed that I never got my mitts on a Phantom, but more glad to still be alive. (And no, I did not understand the politics of the situation at the time. Once that became obvious, I was dead-set against the 'war'.)
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