Blade manufacturing story

[tcasblue]

Worth a read....

https://www.jetwhine.com/2018/06/enstrom-helicopter-blade-maker/

[Fareastdriver]

Wooden blades. On the old Sycamore one would check the integrity of the wooden blades by grasping hold of the end, the blades drooped that far, and whipping the blade. As long as the wave went up to the root and back again smoothly the blade was OK>

[sycamore]

Thanks,FED, I do that with my arms and legs every day.....

[MLH]

Maybe the Enstom folks could school the Robinson's on how to build MRB's.

[timprice]

Compared to most, exceedingly good value, look after them and they will last a very long time.
I remember years ago we use to pop rivet the trailing edge if it debonded, Enstrom were not selling any blades so stopped that.:ok:

[SansAnhedral]

While the aluminum was bent and cracked, the bonding adhesive was unbroken. A composite blade would shatter and shred itself to an ineffective stump.

To believe that a given metal blade design is inherently more robust than a properly designed composite version is patently laughable to any rotors engineer who has any real experience from the last quarter century.

[Hughes500]

Sans,
Carbon fibre is not the utopia you think it is, when it let's go it really let's go and more worryingly with no warning. Have seen it on a very expensive carbon mountain bike. Yes I know it is not a blade but wow
!!!!!!!!

[Cyclic Hotline]

The Kmax has a Sitka Spruce spar in their rotor blades, although Kaman has recently initiated a program to replace them with a Composite design. https://www.kaman.com/news/kaman-launches-new-k-max-composite-blade-program

[SansAnhedral]

Sans,
Carbon fibre is not the utopia you think it is, when it let's go it really let's go and more worryingly with no warning. Have seen it on a very expensive carbon mountain bike. Yes I know it is not a blade but wow
!!!!!!!!

Never implied it was utopioan, but after 15 years of designing rotor blades, today I would never use metallic primary structure unless it was a design purely for cost (and not carrying humans).

[SansAnhedral]

The Kmax has a Sitka Spruce spar in their rotor blades, although Kaman has recently initiated a program to replace them with a Composite design. https://www.kaman.com/news/kaman-launches-new-k-max-composite-blade-program

Wood was the original composite material!

[dagenham]

interesting subject - My dad, lets call him Barking,was a racing yacht builder when they built large free-standing carbon fibre masts they found that if all the fibres where aligned and strong the thing made a certain ring when tapped and if not all correct the tone changed. Kelly the designer apparently sold the technique to the yeovil mob - perhaps one of you learned types could confirm or deny this

[Fark'n'ell]

Never implied it was utopioan, but after 15 years of designing rotor blades, today I would never use metallic primary structure unless it was a design purely for cost (and not carrying humans).

Could you elaborate please.

[[email protected]]

Dagenham - I think the 'tap-test' is more about identifying any voids in the structure - areas where the fibres and resin are not homogenous and weakness could be present.

The Lynx Composite Main Rotor Blades (CMRBs) were checked regularly in the hangar using a 2 pence piece which was tapped along the blade whilst listening for any change in tone.

[Evil Twin]

Didn't Robinson mandate a similar tap test to 44 blades some (12 ish) years ago? I remember watching a bloke do this so religiously that I expect those blades would probably look like golf balls now. Though they're robbo blades so would be fekked by now anyway. ;-D

[PassTheMarmalade]

Dagenham - I think the 'tap-test' is more about identifying any voids in the structure - areas where the fibres and resin are not homogenous and weakness could be present.

The Lynx Composite Main Rotor Blades (CMRBs) were checked regularly in the hangar using a 2 pence piece which was tapped along the blade whilst listening for any change in tone.

I remember hearing that the a tap-test procedure for Sea King blades uses a halfcrown. Whilst in-service non-destructive testing for current platforms includes far more advanced techniques (such as ultrasonics), Westland inspectors will still carry a halfcrown in their kit for the Sea King...

[Hughes500]

Sans

Wish you would go talk to HTC about 500 blades then. They can't even produce them weighing the same, ha da recent set where the tip weights were 60 grams different !

[SansAnhedral]

Could you elaborate please.

1. Largely isotropic materials like classic metallics (even forged parts) are inherently less weight efficient for a given design than composites that take advantage of the orthotropic structural aspect of fibers within a matrix on a blade.
2. Metal blades, for an equivalent fatigue life, will always outweigh a properly designed composite part (see #1)
3. Metal fatigue failure is usually sudden and typically less predictable than a properly designed composite equivalent.

[Paul Cantrell]

1. Largely isotropic materials like classic metallics (even forged parts) are inherently less weight efficient for a given design than composites that take advantage of the orthotropic structural aspect of fibers within a matrix on a blade.
2. Metal blades, for an equivalent fatigue life, will always outweigh a properly designed composite part (see #1)
3. Metal fatigue failure is usually sudden and typically less predictable than a properly designed composite equivalent.

I have a question about composites, specifically I wonder about the state of the art in NDT? I remember the middle east 139 that lost the empennage ( on the ground ) due to a failed repair in the carbon fiber... Given the large amount of composites being used in modern airliners, I'm guessing testing composites must have advanced in recent years. What kind of testing exists today to detect internal manufacturing defects / operational damage in modern composite structures? How does state of the art compare to eddy current testing, x-ray, etc used in metallic structures?

I must admit to trusting older composites like fiberglass, but still regard carbon fiber composites with some distrust as a "new" material, despite the fact that they're not that new anymore, and are widely used.

[Jhieminga]

First carbon-fibre compressor blades tested in service: 1968 (unsuccesfully).
First carbon-fibre floor panel installed in an airliner: 1970, flew in service for eight years with this panel.
First carbon-fibre brake pack installed on an airliner: 1972, test version for the Concorde brakes.
First carbon-fibre flight control certified for use: 1979, B727 elevator (five sets).

Not that new if you ask me.... ;)

[ShyTorque]

In the late 1970s/early 1980s the RAF began replacing the old metal spar blades of the Puma HC1 with brand new, composite items. We were shown photos of these chopping down telegraph poles etc which really impressed us.

A couple of years later it was discovered that a faulty batch of carbon fibre had been used on some blades and guess who had bought them?

An NDT rig was sent out to Germany. It consisted of a jig with a hydraulic ram to deflect the centre of the blade. Sonic transducers were attached to the blade under test. The idea was that how much it "creaked" determined how good (or bad) the blade was. We were walking across the hangar to morning brief as the first "non destructive" test was being carried out. There was a loud "BANG" and the blade snapped!

We weren't impressed after that. :eek:

[SansAnhedral]

Resin technology has progressed a great deal since the early days of composites in the 70s. Much of the experience that the older crowd has with carbon composites is with older fibers and first generation resin systems - combined made for some pretty spectacular failures due to the stiffer and lower strain capability of those materials.

Nowadays, most carbon composites leverage elastomer toughened resin systems that makes the matrix more compliant in concert with the high and ultra high modulus fibers - yielding (no pun intended) composite structures are are very strong and lightweight without being as "brittle" and flaw sensitive as the old days.

I wonder about the state of the art in NDT

NDI technology is also rather impressive today, ranging from handheld trandcucer based pulse echo to full water tables, x-ray, computer tomography, laser shearography to name a few. That said, mapping voids in thin laminates or skin to core is still most easily and directly done with a tap hammer and a well trained inspector.

[blakmax]

The major issue with bonding in blades is with the methodology that establishes the tolerable defect limits for bonded joints. Typically a damage tolerance analysis is undertaken and/or testing is undertaken to demonstrate that the structure can sustain the required loads in the presence of a defect of a certain size, and then NDT is used to demonstrate that all defects are smaller than that size. This valid for PRODUCTION defects, but may not be valid for defects that occur in service.

Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.

Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.

The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.

The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.

[Droop Snoot]

I have a question about composites, specifically I wonder about the state of the art in NDT?

Film radiography (X-ray) is be used to verify fiber orientation in a completed carbon fiber part, provided there is a representative radiopaque tracer present, such as fiberglass.

Radiography, either film type or real time fluoroscopy, has been known to find bond voids in a bonded structure, provided the adhesive is of sufficient radiopacity. That one was quite a surprise!

Automated vision systems are used to optically verify the fiber orientation of individual plies during layup.

Rap tests, or some variation thereof, which determine the natural frequency of a structure, will likely be used at some point in production and repair, if not being done now. The information could be used to verify the correct assembly was performed, the correct stiffness and mass distribution was achieved (for balance purposes), and to assess possible degradation in blades being repaired.

DS

[Droop Snoot]

Wooden blades. On the old Sycamore one would check the integrity of the wooden blades by grasping hold of the end, the blades drooped that far, and whipping the blade. As long as the wave went up to the root and back again smoothly the blade was OK>

Some version of that idea has been applied to metal and composite blades as well! :ugh:

[SansAnhedral]

The major issue with bonding in blades is with the methodology that establishes the tolerable defect limits for bonded joints. Typically a damage tolerance analysis is undertaken and/or testing is undertaken to demonstrate that the structure can sustain the required loads in the presence of a defect of a certain size, and then NDT is used to demonstrate that all defects are smaller than that size. This valid for PRODUCTION defects, but may not be valid for defects that occur in service.

Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.

Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.

The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.

The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.

Your dissertation comes off as somewhat alarmist and seems to overlook a few very pertinent points that are common in large OEM blade design:

1. Blade designs always include bond test coupon areas, typically within the molded assembly but outside the final machined EOP (as well as prior in the constituent detail parts), that contain all representative adhesive interfaces and laminate structures. Short beam shear, lap shear, block tension, and myriad other destructive tests are performed on these coupons to verify the bonding prep, process, and other specifications were met. It is at this point that defects like porosity and slick bonds (which you mention are undetectable by NDI, which is mostly true) are detected as a requirement prior to acceptance.

2. With respect to moisture susceptibility and "hydration" affecting bond lines, and the inability to reliably detect this, you comments on design allowables omits the fact that responsible manufacturers of composite bonded blades typically use a set of more stringent hot/wet allowables that are derived from destructive testing for precisely this sort of concern.

[Droop Snoot]

2. With respect to moisture susceptibility and "hydration" affecting bond lines, and the inability to reliably detect this, you comments on design allowables omits the fact that responsible manufacturers of composite bonded blades typically use a set of more stringent hot/wet allowables that are derived from destructive testing for precisely this sort of concern.

Sans…

The failure mode that blakmax describes is not addressed by the hot/wet environmental qualification testing you reference. The hydration mode is an adhesive failure mode at the metal/ adhesive primer interface, initiating at an edge and propagating along a line, creating a disbond. It is driven by exposure to moisture, calendar time, and to a lesser extent, loads.

As you know, the environmental qual tests that establish hot/wet design allowables for structural adhesives strive for a cohesive failure mode, to develop the full strength of the adhesive, with the presumption that the interfaces and adherends will always be stronger than the adhesive. If an adhesive mode failure mode is encountered in such tests, the data would be suspect and the failure surfaces carefully scrutinized. Of course, a failure could also indicate a fundamental problem with the adhesive/primer system, at least in a particular application.

Wedge tests can sometimes reveal the hydration mode, but the exposure time for the test, say up to a month, is orders of magnitude shorter than the years required to create a hydration mode in typical field usage .

To address your concern, the degradation does not occur in a bulk or volumetric sense. It requires an exposed edge that allows moisture ingress, so it is more of a localized and linear initiation. The resulting disbond is detectable by traditional NDI methods, presuming of course it is accessible and is of sufficient size. Propagation is slow, so it can be managed by periodic maintenance.

DS

[cavuman1]

I hate to admit that the last time I opened a Physics textbook was in college - one half-century ago! I have about five hours right seat time in a Bell 206 LR and was always fascinated at the exquisite design as well as inestimable strength of helicopter blades. Too lazy to look up the necessary equations to calculate load distributions, I found the following on Copters.com: "The rotating blades of a helicopter produce very high centrifugal loads on the rotor head and blade attachment assemblies. As a matter of interest, centrifugal loads may be from 6 to 12 tons at the blade root of two to four passenger helicopters. Larger helicopters may develop up to 40 tons of centrifugal load on each blade root. In rotary-wing aircraft, centrifugal force is the dominant force affecting the rotor system. All other forces act to modify this force."

No wonder these magnificent machines are known as "frantic palm trees"! I am astounded that they hold together as well as their fixed-wing cousins, which, as we all know, are simply "groups of rivets flying in close formation"!

- Ed

For the Geeks among us:
If you know the velocity of the object, simply use the following formula:F = mv²/rwhere:


F is the force (expressed in newtons) (https://www.omnicalculator.com/conversion/force-conversion);
m is the mass of the object;
v is the velocity; and
r is the radius.

If you know only the angular velocity ω, you can recalculate it to normal velocity by simply multiplying it by the circumference (https://www.omnicalculator.com/math/circumference) of the circular path. Use the following equation:
v = ω2πrin case your ω is in Hz (1/s).

Or the formula:
v = ωrfor ω in rad/s

Time to grab a beer....

[MLH]

For the math challenged:

https://www.thecalculator.co/others/Centrifugal-Force-Calculator-660.html

[Ascend Charlie]

It is interesting that centrifugal force is what gives the blades the strength to lift the aircraft. If you went to a stationary chopper and tried to lift it via the blades, they would bend right outa shape, but spinning around gives them strength. The vectors sort themselves out in beautiful style - the lift vector acting at 90 degrees from the blade, making it cone up, and the centrifugal force, depending on RRPM, acting straight out from the blade, trying to flatten the cone - the resultant is right along the blade.

A bit like a piece of nylon fishing line cutting through grass on a whipper snipper. Magic.