Wooden Helicopter blades?

[Thruthefence]

Y'all want to know the "first line of defense" when A&P mechanics inspect composite structure in the field? This includes wings, rotor blades, and propellers. Not being signed off on Airbus structure, I cant vouch for it, but I would be extremely surprised to find this method not in use.

Sitting down?

You "tap" the area under inspection with a "post 1964" American Quarter. You are trained to recognize the "DEAD" sound of a separation in laminates, as opposed to a solid "ringing" tap where the structure is intact.

Here's what happens when you don't pay attention:

(happened in flight, by the way)



That area with the thin line encircling it, is the area that SHOULD have been inspected by sounding, or tap checking the bond between the skin & leading edge extrusion. I bet this guy picked up an awesome 1/per rev when this puppy shucked loose!

The paint erodes away, exposing the seam between the honeycomb panel & the leading edge, and at 400 mph or so, sooner or later it will unzip, often with fatal consequences.

[digger_doug]

I read an article in an old copy of "Machinery" magazine
which I can no longer find, nor online.

It showed the Bell factory, and the making of those wooden
blades.

They set the wood blank on a long cart, much like a sawmill
carriage, with a upside down angle iron track.

A very large sanding belt, a little wider than the blade, ran up into
the rafters, and back down, parallel to the floor,
over the cart.

The contact surface (backing up the belt) was
a large wheel maybe 5' diameter, with an ever changing profile.

The cart was fed under the belt (and the wheel), and the wheel
was chain driven to the cart, to slowly rotate as the wood blank
passed under. The large wheel making 1 revolution (or slightly less)
the full length of the blade, a cam of sorts.


BTW the famous "bubble" was pretty much freeblown, only forms used
on the flat sides for the door cut-outs

[Alistair Hosie]

Just think about people trying new and dangerous things for the forst time.
I kind of don't want to insult them by saying they were very brave in actuallity the engineering is so well worked out and tested.
I find the old engineers making bridges for example, or can you imagine testing a parachute for the first time of an ejector seat and loads and loads of other things.

Let's see what do you think that the first time it was tested was very very brave? Alistair

[A.K. Boomer]

Some carbon fiber frames can last a lifetime - and if not nicked or damaged can be just as strong as the day you bought it - the trouble is is one with the same brand and model can break on the test drive off the showroom floor ------ there really is no rhyme or reason to it,

they've gotten it dialed allot better - but it's still the most unpredictable frame material out there,,, by far,,, very rare to have cro-mo or aluminum fail like this unless someone missed a weld..

I think the main problem with CF is it's an engineers nightmare -

it may be simple enough to figure out with things like fishing poles and such --- but now throw in a bunch of variable size tubes that have to be joined together and an aluminum bottom bracket that has to be "woven" with it (bonding issues) and blended into the main frame and it's extremely tough to gauge where you need more and where you can get by with less...

throw that on top of having to get the "grain" structures correct and then not having too much epoxy between layers and not having too little and things can get sketchy in a hurry...

The thing is is in a "controlled design environment" carbon fiber is not that bad of a material and in fact its superior to others in many ways,,, but the real world isn't some flat panel 5 layer plyboard... that being said -- one of the wonders of the material is to be able to shape it into just about anything you want...

That pic is unreal by the way --- how would you like to see that outside the window of the wing seat

[lazlo]

There are allot of high performance bike frames both Mt. and road that are made out of carbon fiber.

I've been riding a Kestrel tri bike for a couple of years now, and haven't had any issues, other than it's an incredibly stiff (and light!) frame.

The concerns about carbon fiber rotor flex made me think about the carbon fiber wings on the Boeing 787 -- up to 26 feet of flex!

[A.K. Boomer]

There are allot of high performance bike frames both Mt. and road that are made out of carbon fiber - Like Evan says it's great while its working but they have more issues with them than any other materials - your lucky if you find just a crack,,, many have failed catastrophically...

Personally - I like Aluminum - it's one of the harshest rides but delivers the most power to the rear wheels - while not a huge factor in spin mode its very measurable in a sprint - some of this is attributed not to just the material but the tubing size they have to use --- carbon fiber is amongst the worst for power transmission but many people like it due to it being so forgiving over bumps and such.

[Evan]

If you can find a solution Airbus would probably like to speak with you.

This is from an A310. The rudder fell off in normal flight. The flight control system compensated so well that the pilots didn't know it happened until the system notified them of a problem. The rudder wasn't being used for any large inputs, just normal autopilot commanded flight conditions.This isn't the only one either. At least two others have failed in the same manner.

[wierdscience]

I was discussing this recently with an engineer that is studying the problem. The problem is at a level where ultrasound doesn't have sufficient resolution. It seems that it may involve randomly distributed cracking of individual filament strands throughout the material. This isn't like gradual delamination although that can also happen. How to see this microscopic damage is the main problem.

I've made calibration samples for scanning various materials.Simple items like heat exchangers are just samples with various degrees of "pitting" machined in.

Some of the Fiberglass samples were lay ups that were then stressed to varying degrees producing varying degrees of damage.

Those were used with a healthy sample as a comparative tool.Given that Carbon Fiber lay ups are much more precise than Fiberglass it might be possible.

It probably wouldn't be a tool used by a freelance service,but a manufacturer such as Airbus or Boeing might be able to use it if they produced calibration standards matching the areas of interest.

[Evan]

Wouldn't Ultrasonic inspection work? It works wonders on Fiberglass boat hulls.

I was discussing this recently with an engineer that is studying the problem. The problem is at a level where ultrasound doesn't have sufficient resolution. It seems that it may involve randomly distributed cracking of individual filament strands throughout the material. This isn't like gradual delamination although that can also happen. How to see this microscopic damage is the main problem.

[lazlo]

Wouldn't Ultrasonic inspection work? It works wonders on Fiberglass boat hulls.

There must be some way that mainstream commercial and military helicopter rotors, most of which are composite, are inspected. Ultrasound would be an obvious candidate.

Nice picture,,, that's indeed a piece of work...

+1. Machine art.

[A.K. Boomer]

Nice picture,,, that's indeed a piece of work...

[wierdscience]

If anybody can come up with a reasonable way to reliably inspect carbon composites on site and in situ they will make a billion dollars.

Wouldn't Ultrasonic inspection work? It works wonders on Fiberglass boat hulls.

[Thruthefence]

"Rigid" is really a misnomer.

Here's a "Hingeless" main rotor. No lead-lag or flapping hinges. The rotor head is one big chunk of Titanium, as are the blade grips. A damn fine machine. MBB (now Eurocopter) Bo-105. The blades are composite.




The Hind has a fully articulated main rotor system.

[The Artful Bodger]

Are those rigid, semi rigid or articulated rotor blades?

[A.K. Boomer]

A bit more here though:



Phil

still pictures usually aren't the best gauge as the craft can be taking off at a great rate or in a big decent therefore not loading the blades as much,

but that one speaks volumes - the smoke streamers and the bending of the aft blades way more than the fore ones ensures that that bird is moving forward at a good clip,

when this is happening the rear blades are going through double time - for there not only responsible for the majority of lift of the craft but also the propulsion --- all controlled by the swash plate and blade angles...

Multiple blades can help share the load and therefore the deflection making it less --- but this is a catch 22 as in many cases multiple blades are required to rotate at lower speeds - this of course means more deflection due to less centrifugal force... it's basically all up to the math of the materials, RPM's, loading and number of blades which will have the least...