Characteristic Longitudinal Motion for Helicopter Question

[preet996]

Hi All,

I hope I've come to the right place. I'm a private pilot (fixed wing) and it turns out I'm writing a thesis on the static stability of helicopters. However, in my early stages of research, I'm not finding much information probably because I'm typing the wrong stuff in to search engines and looking for the wrong thing in libraries. Here's my question:

I understand that there are five characteristic modes of flight for fixed wing aircraft:

Short Period Mode
Phugoid Mode
Spiral Mode
Roll Mode
Dutch Roll Mode

There is plenty information on this and i've done research into these modes. However, my task is to find the characteristic modes for helicopters and research them deeply.

I know there is an equivalent for the dutch roll mode. What are the others? An explanation would be great, however if you can simply name them, at least I have somewhere to start with my research!

Just so my question is clear since I've written a massive post here when I only have one question:

What are the characteristic longitudinal modes of flight for a helicopter?

Thank you and look forward to hearing from you! :)

[SimFlightTest]

Dutch roll in a heli is usually called "LDO", and phugoid is called "long period".

[212man]

LDO being Lateral/Directional Oscillation.

I would highly recommend the book: Helicopter test and evaluation By Alastair K. Cooke, Eric W. H. Fitzpatrick, which I think would address most of what you need. There is a preview on google books here:

Helicopter test and evaluation - Alastair K. Cooke, Eric W. H. Fitzpatrick - Google Books (http://books.google.co.uk/books?id=B3-WN4wcf6YC&pg=PR8&lpg=PR8&dq=cook,+fitzpatrick,+helicopter&source=bl&ots=jqg1uyMpwD&sig=Jjo9RcwZXGrEM8CLmnJ8nUB-s3A&hl=en#v=onepage&q=cook%2C%20fitzpatrick%2C%20helicopter&f=false)

I suugest you also download copies of the FAA Advisory Circulars AC-29C and AC-27, which give some additional practical information.

[arismount]

Aerodynamics for Naval Aviators (PDF) | Ask a Flight Instructor (http://www.askacfi.com/3219/aerodynamics-for-naval-aviators-pdf.htm)

[oldbeefer]

I've yet to fly a helicopter which, without an AP, shows any sign of stability whatsoever!

[SimFlightTest]

Among all the helis I've flight tested (EC135, AS350, S92A, B407, B206B, B206L3, B429, EC225, S76C++) the only one that demonstrated a traditional phugoid/long-period mode without the AP engaged was the B206. All the other aircraft were divergent, particularly at the higher airspeeds.

For those machines with SAS or ATT mode engaged, the response was usually deadbeat when the augmentation was engaged.

Heli's are strange beasts with enough cross-coupling and interaction between modes that it will drive a fixed wing stability and controls engineer insane.

[preet996]

Wow!

Thank you everyone for your help over the day!

I am now a step closer to researching these modes of flight, whether they actually are possible to induce in a helicopter or not.

I now understand two flight modes discussed:

LDO
Long Period

Are there not any others?

Thank you,
Preet

[SimFlightTest]

Preet996,

LDO and long period aren't the only modes, if that's what you're asking. Like you mentioned in your initial post, you still have spiral stab, short period, and roll mode.

Helis are full of quirks like "digging in", pitching with sideslip, vortex ring, loss of tail rotor effectiveness, wake effect on aero surfaces, etc. It's those items that usually get most of the attention with regards to helis as opposed to the clasical dynamics modes.

When I develop a high-fidelity simulation model, the clasical modes have no chance of being correct without getting the quirks right first. But then I guess everybody has to start somewhere. :)

[preet996]

Dear SimFlightTest,

Thank you very much for your time and effort to explain!

I'm lucky that I got to do some flight training during the summer break between my degree, but man helicopters and their complexity just does not compare.

I mean you can type in "aircraft dynamic modes" into google and you get this:

Aircraft dynamic modes - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Aircraft_dynamic_modes)

And I was hoping to find something of the equivalent for helicopters which just states the main dynamic modes, I could research them and take them back to my tutor, but this topic is obviously a thesis because it's not as simple as that!

All the best everyone and I trust I'll be back on this forum asking more questions!

Preet :)

[Dave_Jackson]

Preet,

If you really, really want to look deeply into the subject try the following two books from AIAA ~ Educational Series.

Helicopter Flight Dynamics: The Theory and Application of Flying Qualities and Simulation Modeling ~ by Gareth D. Padfield
Rotary Wing Structural Dynamics and Aeroelasticity ~ R.L. Bielawa


They are so complicated that it takes about 5 minutes just to determine if the books have been opened upside-down or right-side-up. http://www.unicopter.com/RollEyes.gif


Dave

[SimFlightTest]

Hard to go wrong with a book by Prouty when trying to get a good qualitative feel for what goes on with a heli. Understand Prouty and then dig into the meatier text that DJ suggested.

The more you read, the more you'll realize that nobody knows 100% what's going on. Even recent papers on helicopter dynamics comparing predicted aircraft behavior against flight test data wouldn't past qualification criteria for a high-level flight simulator.

[AnFI]

Oldbeefer:

Did you try the H500 (D) or the R44?

Strangely - there seem to be two fundamental general shape classes for helicopters;
Aerodynamically 'drip-shaped' ones (like the R44 and H500) and ones like old fashioned cars with a bonnet (hood) and a steeply inclined windscreen - (Like the Lynx, Jet Ranger, A109 etc )

Is the first group pretty close to stable (almost fly by themselves?) and the second group less inherently stable? (they seem to make a bigger deal about having Stability Systems .... )

Is there any truth in this observation?

(Do Gazelles nearly fly by themselves without SAS?)

[preet996]

Thank you all for your replied.

I have Padfield's text which is probably going to be the most useful (although you're right - I still need to check once in a while if I'm holding the book upside down or not!) and I'm getting stuck in with the MATLAB simulations now!

All the best :)

[Matthew Parsons]

Oldbeefer:

Did you try the H500 (D) or the R44?

Strangely - there seem to be two fundamental general shape classes for helicopters;
Aerodynamically 'drip-shaped' ones (like the R44 and H500) and ones like old fashioned cars with a bonnet (hood) and a steeply inclined windscreen - (Like the Lynx, Jet Ranger, A109 etc )

Is the first group pretty close to stable (almost fly by themselves?) and the second group less inherently stable? (they seem to make a bigger deal about having Stability Systems .... )

Is there any truth in this observation?

(Do Gazelles nearly fly by themselves without SAS?)

No.

The external shape can have an effect on some stability modes, but the rotors, the fin, and the elevator/horizontal stab are the primary contributors. Streamlining of the external shape is more of a high speed thing, but not much of an effect. More important considerations are visibility, cargo dimensions, and marketing. I'm sure there are others, but little to do with stability.

I agree with oldbeefer that none are stable when you take a holistic view of stability, but many stability modes are naturally stable on proabably every helicopters, and others are naturally stable in some helicopters.

An example of a naturally stable mode is when in forward speed, if a gust speeds you up the rotor will flap back a little, causing you to slow down. Since the open loop (without input) dynamics removes the pertubation, this is considered stable. It is possible that this will start a dynamic oscillation (phugoid) but horizontal stabs are good at damping those, amongst other things. Also, if you consider that there is a pilot sitting there, then a dynamic oscillation that is slow and/or benign enough for the pilot to suppress is not a stability problem.