Mass-Spring-Damper for Rotor/Fuselage

[davidwm68]

Hi All,
This is my first post and I am quite impressed with the range of topics and the knowledge held by all you guys.

I am currently working on a simulation in Matalb (Simulink) to determine to contribution to the 'vertical bounce' phenomena (PIO) that can occur with underslung loads.

I am basing the simulation on a simple dual mass system representing the helicopter and the load, with a spring between the two to representing the srping rate of the sling. The entire system is suspended below a fixed tip-path plane and the fuselage is joined to the TPP by a damper and a second spring.

The model is based on a CH53D and I have all of the parameters mostly figured out, except for the rotor-fuselage spring term. For the damping term I am using the Zw stability derivative (heave damping), but I'm at a loss as to how to work out the spring term.

I have extensive theoretical data, but no flight data. I've read about the centre-spring equivalent K(beta) but it is rotational based on integrated lift force over the blades starting at the flapping hinge, but I'm essentially after linear representation that would mimic the coning 'spring' action under load...

Any ideas? Any advice would help...

Cheers,

David

[Chippydriver]

David

Do a search on the NASA database for a paper by Chen & Hindson from the mid-'80's. Various models for hover (albeit CH47B) that encapsulate the phenomenon you're looking for. Or paper by Bob Feik of ARL Melbourne - same period - he used a RAN SeaKing and derived models from flight data.

End result is the same - coupled coning/heave/inflow 3 DOF model.

Good luck

[Udi]

Welcome to our forum, Dr. Houston. We would love to have your participation in many of the more challenging discussions. Your research has been mentioned more than once on these forums.

Regards,

Udi

[gyromike]

Dr. Houston! Welcome aboard.

Hope to see you become a regular here.:)

[davidwm68]

Chippydriver (Dr Houston??),

Firstly, I apologise for all the typo's in the first post that I made, but I hadn't reviewed the message before submitting...

Thanks for your advice! I am actually in Australia, but completing an Msc through Cranfield university having finished a rotary wing flight test engineers course at ETPS last year. My thesis sponsor is Dr Alastair Cooke (no doubt you would have heard of him), but as this is the festive season, he is not available to help at the moment. I am however, trying to progress my thesis on my own holiday, and looked to the forums for assistance..

My thesis is to look at the elasticity of load slings contribution to the vertical bounce phenomona with underslung load operations. The emphasis of the project is 'simple' modelling, and so far I have the equations of motion and simulink description completed, but populating the values (in fact the final spring term) is the sticking point...

I was able to retrieve a copy of Chen's paper through the Cranfield on-line library, but Feik's paper isn't available in an electronic format. I am separately involved with DSTO on this topic, but once again...no-one is available for the next few weeks. I will be able to get this paper when they get back (or through Cranfield library and post if that works out faster...)

Anyway, I briefly read Chen's paper and wasn't really sure whether that might help. I had a bit of trouble interpreting his work and putting it onto context for my own, and I also dont have that level of information to be able to model inflow etc from first principles on the CH53D.

I dont have any flight data, but was hoping for a 'silver bullet' for the spring term like I have for the damping term (heave damping). Having read the Feik abstract though, his work may help.

Opinion: In Chen's work, he talks about coning with the aircraft sitting on a hardstand. My thought was, if the helicopter is grounded (and obviouosly IGE) and power is applied resulting in coning prior to lift off, could one assume a linear rise in the tip path plane from zero to full thrust (AUW) and use the distance that the TPP rises for AUW as a spring constant? i.e. if the TPP rises 1 metre through an application of approx 15,000 kg thrust (150,000 N), would a spring constant of 150,0000 N/m be a reasonable approximation in this type of simulation? There is an issue with that data being IGE, when I am assuming OGE with a fixed TPP..another day...

Although I dont have the flight data available to determine this, I may be able to get it, or DSTO have other simulation packages available where I might be able to derive it...

Any further advice you may help would be greatly appreciated.

Cheers,

DM

[Matthew Parsons]

David,

I think the Zw term is giving damping of the rotor system (with aircraft weight attached) and not damping of the rotor to fuselage spring. I think you should be looking at a blade flapping system to work out the spring-damper constants for your model.

I can think of ways to get reasonable values through flight test, but I don't see how you can derive them from aerodynamic derivatives. If you have a trace of a damped oscillatory normal acceleration over time following a sudden drop in weight (load jettison) then the engineering parameters should be close to what you're seeking.

[davidwm68]

Thanks Matthew. I note you are a rotary TP, are you still at Rotes?

I hear what you are saying about the heave damping term. I'm using that as an approximation, as in aero-derivative terms that is indicating the entire rotor-fuselage system damping to vertical motion. To separate the load from the helicopter, I am not using the weight of the load (mg) as a force acting on the helicopter mass, just on the sling (spring)...

I am looking at rotor flapping dynamics, but because of the way I have set out to achive this, I am almost 'reverse engineering' this problem by using flapping dynamics as a spring-damper between the fixed TPP and the helicopter mass. It is virtual, and not really a physical spring between the rotor and the fuselage. (I hope that makes sense) I am fixing the TPP, and the linear 'motion' that would happen there is being 'referred' to the 'rotor-fuselage' spring.

i.e.

------- TPP (fixed)
||spring and damper (Zw)
--------
|| helicopter mass
---------
|
| sling spring (data from testing in the US)
|
---------
|| load mass
----------

Unfortunately, I wont be able to conduct any flight test, and getting that sort of data may prove troublesome...

Thanks,

DM

[Chippydriver]

Udi, gyromike, thanks for the welcome!

David, persevere with the Chen material - you may be reading a different paper from the one I'm thinking of. It does sound like you're trying to model via an 'equivalent systems' approach, if so you'll need flight data to 'match' engineering parameters.

I'll find out what you've done in some detail once you've submitted your dissertation - I'm the Cranfield external examiner with responsibility for the ETPS course!

[davidwm68]

Dr Houston,
Well, nothing like getting tips from an examiner huh?

I think what you have said in your last post is exactly what I'm trying to achieve, but I don't think I have been able to articulate that.

I am in fact trying to model based on an 'equivalent systems' approach, but trying to do this without flight data.

Unforutnatley, and as I explained earlier, the window I have availalbe to progress the work does not align with all those that have input regards oversight and direction...

Once Dr Cooke gets back to his desk I will be able to discuss this with him and seek guidance on the direciton of the paper. If I'm barking up the worng tree, then it may need a rethink...

Thanks again for your guidance, and I look forward to your feedback in due course.

Cheers,

DM