why helicopters are stable?

[jostpuur]

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?

[Cyrus]

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.

[HallsofIvy]

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.

[Cyrus]

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).

http://www.fas.org/man//dod-101/sys/ac/x-29-EC91-491-6.jpg

[FredGarvin]

I don't know of any forward swept wing aircraft in production.

[sophiecentaur]

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.

[jostpuur]

Could it still be that the gyroscope effect I mentioned brings some stability? That means that the helicopters like these


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

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


are more unstable than the traditional helicopter? And would require more computer assistance?

[Cyrus]

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:

http://www.amazon.com/gp/product/1575242095/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=0894649299&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0YXBS7H0AX7BEQ76XZCD

[Cyrus]

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.

[MotoH]

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.

[Cyrus]

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?

[MotoH]

mkay?

I saw forward swept wing and got excited. . .

[Cyrus]

I saw forward swept wing and got excited. . .

Haha, it's all good. :)

Now read, you.

http://www.nasa.gov/centers/dryden/news/FactSheets/FS-008-DFRC.html

[MotoH]

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.

[FredGarvin]

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.

[Brian_C]

Role Technology Demonstrator
Manufacturer Sukhoi
First flight 25 September 1997
Introduced January 2000
Status Experimental
Primary user Russian Air Force
Number built 1

:wink:

[Cyrus]

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.

[Brian_C]

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.

[MotoH]

Didn't know it needed to be production to be incredulously awesome.

[Danger]

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'.)

[jostpuur]

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?

[Phrak]

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....

[jostpuur]

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.

[Phrak]

No problem. It's still an open question.