Future materials usage in Rotorcraft.

[Graviman]

While studying the engineering for rotorcraft, i have come inescapably to the conclusion that the rigid rotor offers the best way forwards for rotorcraft performance. This includes:

1. Controlability (particularly if gyro augmented) for both training and application.
2. Efficiency since the blade may be actively twisted for optimum AOA.
3. Reliability which the latest range of FEA optimised elastomeric bushes offer.
4. Performance envelope from hover up to high mu.
(Carter Copter have not yet been able to demonstrate a repeatable high mu capability with a teetering head - 1.5 sec was only enough for a single rotor turn!)
5. Cost (speculative) since the inherent reduction of mast bump incidents will reduce insurance and maintenance costs.

The above points can only be addresses if a carefully thought out heli concept is the basis of a (final) production machine. My personal preference (and still a likely point of debate) is a feathered retreating blade, outboard advancing, intermesher. My reasoning is that the hover disk pressure distribution will be close to a single rotor, and this distribution may be maintained right up to Vne, as each blade is fully unloaded only at 270' position, and half unloaded at 0' and 180' positions. Since retreating rotor is feathered rotor mu may far exceed 1.0, which allows variable RRPM, as long as the total heli lift moment is balanced. Also listening to the Chinooks overhead during the G8 convinces me that the vortex slap noise, against wings or other rotors, is not inconsiderable!

This design concept will inevitably put very large bending moments on hubs and bearings, similar to fixed wing roots. Other design concepts such as teetering and reverse velocity utilisation, applied to either convertiplanes or interleavers, seek to reduce these moments but suffer various aerodynamic/mechanical complexities. To minimise the weight in such a design carbon composites become necessary, but are there any other material considerations such a concept should consider? I believe the work Dave Jackson is doing on rotor systems is important, and feel it is worth asking the question are there any other factors to consider?


This thread is going to be difficult without Lu's informed opinion keeping the theory on a sound footing... :uhoh:

Mart

[edit: for coffee break during another post that ended up too long... :rolleyes: ]

[slowrotor]

We have four blade materials now:
1) Wood ---- good for fatigue .... not so good for balance in a humid area. Getting hard to find clear wood but could be laminated from small pieces. A material called compreg is used for props, compreg is similar to paper sheets glued into a block then shaped.

2) Aluminum---- has lower fatigue life than steel but is often used.

3) Steel------ used in the past for autogyro and helo blades. I am considering steel for myself because of the weldability. I dont think steel could be used for a rigid rotor, most likely a teetering design will be used.

4) composite---- some materials absorb water and give balance problems. Materials are much more expensive and labor is more as well I think.

Anything else.... I think thats it for materials.

[Dave_Jackson]

Graviman,

You mentioned "variable RRPM" in your post. A new thread entitled [Variable Speed Rotors and Prop(s)] has been post, to place this idea into the public domain. You may wish to critique it.


Slowrotor,

IMHO, there is only one choice ~ composites. :ok: Composite construction is no longer the future, it is the present. Aerospace is into composite construction in a big, big way and the prices get lower and lower.

The composite that absorbs moisture is Kevlar and there is probably little reason to use it in a rotorblade. Man-made silk is a future composite under development. It is said to have the greatest strength to weight ratio of any known material.


Dave

[Graviman]

Man made silk composite - impressive stuff. I guess diamond fibre composite is the next step. Carbon fibres (Bucky tubes) may hold the secret to room temp superconductivity too...

I was looking at some Goodman diagrams of composite, and was suprised that the numbers didn't look that different to high tensile steels above fatigue limit stress (but density is obviously much lower). Do you have a preference for type of carbon composite? I gather that kevlar is intermixed with CRP in the EH-101 airframe, due to it's improved fatigue characteristics - moisture absorption would not be such a concern here. Using compressed resin to introduce fibre tensile loads will help to push up fatigue life too.

I am a great fan of wood, but was amazed to hear how little of (say) a spruce tree you can use. You can only use the trunk from above ground to below the first branch! I imagine that compreg is only one type of wood composite available to overcome this hurdle.

Interesting to learn how F1 uses photo stress to optimise layup too. The bonded platic layer for measuring strain (using polarised light) is marked up with isocline contours, to determine strain (or stress) gradients for particular load inputs. This helps perfect the fibre orientation after CAD/FEA first pass.


BTW Slowrotor, i would seriously steer you clear of welded fabrications for rotor blade assys. Although high tensile steel may be good for 200MPa fatigue max principle stress (metric - don't understand "English" units :confused: ), the weld is at best 100MPa - conservative estimates state 30MPa! Worse is that welding introduces preloads, and stress concs that result in local tearing. This is the main reason aero went off towards riveted ally, while auto developed spot welded steel...

Mart

[slowrotor]

Mart,
I think the problems with weld stress cracking is related to thick weldments (1/2 inch or thicker).
For thin sheet, the metal simply yields until all weld stresses are superceded by the normal service loads. Thin welded structures are used for the most highly stressed parts. Look at the center fuselage pylon on the Enstrom for instance, welded thin tubing.
The alternative is bonding..... joint strength maybe 3000-6000psi.
And a welded 4130 joint is ........ 70,000 to 90,000 psi.

I like to weld..... dont trust my skills at bonding.

[Graviman]

That makes sense, Slowrotor. I've been stressing up 12mm plate recently - without FEA optimisation we still see failures! Just make sure there are no initiators for weld failure, and that joints are prepared/dressed for maximum weld strength. Counldn't find Enstrom details, but i bet they still don't overstress the welds.

http://www.b-domke.de/AviationImages/Rotorhead.html

I just assumed that all heli steel bits were either forged, high pressure cast, or machined.

Mart

[delta3]

Hi,

I had the opportunity to visit Bruno Guimbal today in Aix right at the moment they where starting to fly/ certify the VH-diagrams.

The Gabri G2 certainly uses state of the art materials, F-1 alike use of carbon, including the crash seats.

Refering (in honest respect) to Lu, not theetering, but 3-blade with 87 mm excentricity, to be followed.

D3

[Graviman]

I'm not familiar with the Gabri G2 project, Delta 3. Any useful design pointers? I gather that composites absorb a lot of energy while fracturinging, so allow very good crash absorbing structures. This is in addition to the good strength/stiffness per unit mass.

Mart

[slowrotor]

My understanding is modern materials like carbon fiber do not absorb crash energy anything like metals. Carbon and fiberglass shatter like glass. For energy absorption the material will get warm when bent. Steel and aluminum absorb the energy and carbon alone does not. With foam maybe.
Carbon shatters into needles that can be inhaled by crash recovery crew (not to mention the unlucky pilot) so kevlar is often laminated in with carbon.
Most designs employ steel cockpit crash cages when the aircraft is made of carbon.
Race cars have steel tube crash protection. Steel yields (bends). Carbon shatters.

[Graviman]

"Steel yields (bends). Carbon shatters."

Are you saying that weight for weight a steel structure will absorb more energy than a carbon fibre structure? I thought that shattering, if design attention forces progressive failure, was a reasonably efficient mechanism. Agreed about the dust though.

I have found aluminium (or copper) very nice to work with, since it has a very high strain to tensile failure. It just keeps deforming at more or less yield stress. No strain rate dependance either, so you know it'll always behave the way you think it should.

Steel just buckles, unless reinforced or just plain thick. The only structures that work are basically rigid. The energy is absorbed by collapting one rigid structure after another. I can see why steel reinforced composites are popular in racing...

Mart

[slowrotor]

I dont think carbon can absorb energy progressively as needed in a crash. Carbon can absorb impact energy but it gives it back as a rebound and that is not good for the occupants. Or it breaks and thats not good either. Carbon has a better weight ratio in some respects so it can provide a cockpit that stays intact and thats good. But you want absorption and protection.
A perfect system will collapse with increased resistance to the stop point to keep g load below about 40g. Steel work hardens as it bends and that is why all cars use it I think.
I am not familiar with steel reinforced composite but it makes sense to me. How is it done?

Here is a thought picture:
Remember the 100 ton drop hammers that formed bathtubs from sheet steel?
The steel sheet absorbs the impact as it forms and gets harder and warm. Try that with a sheet of carbon fiber and the drop hammer will not survive. That is how I see it in my mind.

[Graviman]

"A perfect system will collapse with increased resistance to the stop point to keep g load below about 40g."

You really know your stuff! This almost perfectly describes design for the latest auto NCAP tests...

"Steel work hardens as it bends and that is why all cars use it I think."

Plus it's cheap. Not dirt cheap like it used to be though, which gives other technologies a good look in - expect composite skinned ally spaceframes to make more of an appearance.

"I am not familiar with steel reinforced composite but it makes sense to me. How is it done?"

I've not seen it, but i suggest bonding a carbon fiber skin to a tubular (high tensile) steel spaceframe. This would be ideal for helis, and i have even suggested it for DJs Unicopter.

"Try that (drop hammer) with a sheet of carbon fiber and the drop hammer will not survive."

F1 carbon nosecones seem to crush pretty well. I can see your point though: Carbon composite does not inherently want to absorb impact energy, and bounce back would be the worst thing possible. Any composites experts care to comment? :ok:

Mart

[delta3]

Graviman

Bruno really took the 'F1-crash-cockpit' as one of the major design options. It should take 35-G (hope I remembered this number well). I don't think scattering will be an issue because if it starts scattering G-forces will need to be so high that whats in it (the pliot....) will have no chance anyway (cfr F1)

Maybe too synical to mention, but flying the VH by other than the companies test pilot puts a higher probality on a real life crash test. Came close to it next week, but ended up with only slightly bended skids. Hope of course this does not happen for all involved.

D3