How do helicopters remain stable, despite the potential for tipping over?

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Discussion Overview

The discussion centers on the stability of helicopters and the factors that contribute to their ability to avoid tipping over. Participants explore various explanations, including the role of angular momentum and gyroscopic effects, as well as the inherent instability in helicopter dynamics and control challenges faced by pilots.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that helicopters appear stable because their center of mass is below the rotor, but later corrects this by stating that unsupported objects behave as if hanging from their center of mass.
  • Another participant proposes that the gyroscopic effect of the rotor contributes to stability, comparing it to a spinning top, but questions the stability of helicopters with dual rotors that do not have significant angular momentum.
  • Some participants assert that helicopters are generally unstable and require careful stability analysis to understand their dynamics.
  • A participant mentions the complexity of flying helicopters, highlighting the interdependence of controls and the challenges of maintaining stability, even with electronic assistance.
  • There is a discussion about the evolution of aircraft design, particularly regarding wing configurations and their impact on stability and performance.
  • Several participants reference the book 'Chickenhawk' to illustrate the difficulties of flying helicopters and the inherent instability involved.
  • One participant raises the question of whether helicopters with dual rotors are more unstable and require more computer assistance compared to traditional designs.

Areas of Agreement / Disagreement

Participants express differing views on the stability of helicopters, with some arguing they are generally unstable while others propose explanations for their stability. The discussion remains unresolved, with multiple competing perspectives on the factors influencing helicopter dynamics.

Contextual Notes

Participants note that a comprehensive understanding of helicopter stability requires advanced knowledge of dynamics and control systems, indicating that the topic is complex and multifaceted.

jostpuur
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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:

http://en.wikipedia.org/wiki/File:Spanish_Army_Chinook.jpg

http://en.wikipedia.org/wiki/File:Kamov_Ka-50_MAKS_2005.jpg

Why are these helicopters stable too?
 
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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.
 
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.
 
HallsofIvy said:
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.

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).

x-29-EC91-491-6.jpg
 
I don't know of any forward swept wing aircraft in production.
 
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.
 
Again, you have to do a stability analysis looking at the poles on the root locus. Jostpuur, the question you are asking requires a graduate level understanding of helicopter dynamics. It's really not a simple answer.

If you really want to know, you need to get this book:

https://www.amazon.com/gp/product/1575242095/?tag=pfamazon01-20
 
Last edited by a moderator:
sophiecentaur said:
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.

That's right, there is significant coupling in the lateral and longitudinal dynamics.
 
  • #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.
 
  • #11
MotoH said:
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.

mkay?
 
  • #12
Cyrus said:
mkay?

I saw forward swept wing and got excited. . .
 
  • #14
Sexy.

The use of compressed nitrogen to increase control at high angles of attack is very interesting! I never knew about the VFC until today.
 
  • #15
MotoH said:
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.
Not a production aircraft.
 
  • #16
Role Technology Demonstrator
Manufacturer Sukhoi
First flight 25 September 1997
Introduced January 2000
Status Experimental
Primary user Russian Air Force
Number built 1

:wink:
 
  • #17
Brian_C said:
Role Technology Demonstrator
Manufacturer Sukhoi
First flight 25 September 1997
Introduced January 2000
Status Experimental
Primary user Russian Air Force
Number built 1

:wink:

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.
 
  • #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.
 
  • #19
Didn't know it needed to be production to be incredulously awesome.
 
  • #20
sophiecentaur said:
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.
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'.)
 
  • #21
https://www.youtube.com/watch?v=aVagmBvFLig

What a nice little toy!

What do that kind of toys actually contain inside them? Does it have a gyroscope and a small computer getting readings from it?
 
  • #22
Cyrus said:
Again, you have to do a stability analysis looking at the poles on the root locus. Jostpuur, the question you are asking requires a graduate level understanding of helicopter dynamics. It's really not a simple answer.

Can't some simple and rough deductions of stability be made without the detailed analysis?

A hinged rotor certainly looks like it contributes to stability in the same fashion dihedral does for aeroplane.

Edit: good grief, I didn't know this post was dated, but still...
 
  • #23
Phrak said:
Edit: good grief, I didn't know this post was dated, but still...

Must be my fault...

I was thinking that I have right to return to my own thread whenever I want :smile:

Originally I didn't respond to Cyrus' posts frankly because I didn't know what poles and root locus are. I've tried to look about them a little bit on Wikipedia now... but this isn't very serious. I'm just spending my time.
 
  • #24
No problem. It's still an open question.
 

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