Problems with Rotorblade-cracks, MT03, MTOsport, Calidus

[Gyro_Kai]

The UK CAA recommendation states it with disarming honesty:
Cracks in the rotor blades may result in loss of a rotor blade and consequent loss of a gyroplane.

English Version:
http://www.caa.co.uk/docs/33/20100917MPD2010008.pdf

German Version:
http://www.dulv.de/_obj/FD06AB41-D43F-44D0-8568-ACA211470ADC/outline/AutoGyro_2010_04_sicherheitsmitteilung.pdf

Some pics from the German Version (which made me gulp)

Alledgedly this happened in the UK, when people mounted heavier, more vibrating engines on the frame, but that is just hearsay.

I heard some doubts about extruded aluminium not being a good choice for blades, maybe this is the proof.

Kai.

[RotoPlane]

I personally would not use any extruded blades unless it was mounted to the head in a way that would dampen the drag stresses on the advancing blades (2/rev). The SportCopter mounting system is one possible way to help alleviate these stresses. I wonder if these blades are 6063-T6 (requires less pressure, so easier to extrude) or 6061-T6.....

[animal]

Ouch, that dose not look good. who makes these Blades?

[Gyro_Kai]

Tim,

this concerns the Aircopter blades, used up until 2006 and the ones made by the manufacturer himself, used since then.

Kai.

[animal]

oh ok,I was not sure if they where made out side the company or not. I to wonder what Grade they are made from.

at any point, cracks in the blades are sure not good. have they had any break all the way yet? or just finding cracks?

hope they find the answer to the cause so they can get it worked out.

[Doug Riley]

Kai, we are not very familiar with these machines here in the U.S.

Is there any mechanism in the rotor head or mast (slider, pivoting mast) to relieve in-plane vibrations? I mean, anything besides the flexibility of the mast material itself?

[Oskar]

Ed might be onto something there.

When I first got my MTO Sport I would regularly fly at 100mph. Then I did some vibration measurements and found the following:
1 per rev vibration levels could be brought down to very low levels by rotor balancing, resulting in negligible stick and fuselage vibration.
2 per rev vibration levels, however, were not only high but also very speed dependant. Between 60mph and 110mph the 2 per rev vibration nearly doubles. Since 2 per rev vibration is built into the machine, the only way to manage it is to fly slower, and I now only exceed 90mph if in a real hurry.

[Gyro_Kai]

Is there any mechanism in the rotor head or mast (slider, pivoting mast) to relieve in-plane vibrations? I mean, anything besides the flexibility of the mast material itself?

Doug, I have not noticed any on the MT03 and MTOsport. The Rotorhead is milled out of a solid block of aircraft-grade aluminium. There is no side tolerance along the teeter bolt like in Ernie's slider head. The pitch axis has no noticeable tolerances and the mast is welded on the frame.

The photo is from the old Aircopter blade rotorhead (five screws as opposed to the new one with 8 screws per blade).

The Calidus, on the other hand, definitely has some dampening in the construction, but I don't know, how that is established.

Kai.

[hillberg]

looks like it requires a helicopter style root doublers to spred the loads from the bolt holes.-Ouch!

[helipaddy]

Are the blades anodised??

[C. Beaty]

The solution to the 2/rev problem of seesaw rotors lies not in isolation; avoiding resonance is the only way of solving the problem.

A seesaw rotor in conjunction with the mast or rotor pylon is a resonant system that receives periodic excitation from aerodynamic input and from mass above and below the teeter bolt.

To begin with, the rotor vibrates at 1/rev that we perceive as 2/rev because the rotor’s rotation doubles the frequency felt by the pilot.

Typical rotor rpm is 360 or 6 times per second. Rotor/mast resonance must be kept higher than 6Hz.

How?

The rotor must be kept as stiff as possible in-plane and mounted on the softest possible pylon. Mass or spring restraint at the rotor center always lowers the resonant frequency. Bolt an anvil to the rotorhead and the pilot might not feel rotor shake until the blades break.

The SkyWheels type hub that straddles the rotorhead is a good approach to stiffening the rotor in-plane. Various schemes of outboard drag struts are also helpful.

I explored this problem quite thoroughly some 20 years ago when I built a gyro with a well triangulated rotor pylon and the thing shook so bad it rattled my eyeballs. I was afraid to fly it higher than prepared to fall. Added drag hinges to no avail.

I didn’t understand the problem so built scale models from welding rod driven by magnetic actuators (Chrysler ignition pickups) fed by an audio oscillator. That gave me at least a ball park feel for what was happening.

From that evolved the “slider” mechanism.

Groen evidently experienced severe 2/rev shake on the SparrowHawk and precluded from copying the RAF magic bushing by a valid patent, used a slider even though they were using SportCopter drag hinged rotors.

The Bell solution to 2/rev vibration in B-47 precursors was certainly the antithesis of drag hinges.

[Oskar]

To begin with, the rotor vibrates at 1/rev that we perceive as 2/rev because the rotor’s rotation doubles the frequency felt by the pilot.

Just to clarify, I measured vibration levels at the top of the mast. The 1/rev component was at ~6Hz and the 2/rev component at ~12Hz. 1/rev levels after balancing were below 0.1ips, 2/rev levels were between 2ips and 4ips.

What the pilot perceives cannot be related directly to the numbers. Because 2/rev levels are more than 20 times as large as 1/rev one would assume that the vibration felt by the pilot would be dominated by the 2/rev, but this is not so.

A change in 1/rev vibration from 0.1ips to 0.2ips results in a very big change in perceived vibration, but when the 2/rev vibration changes from 2ips to 4ips there is no perceived change in vibration.

I haven't been able to explain this, maybe someone can enlighten me.

[C. Beaty]

Wherever you measure vibration on the airframe, Oscar, rotation produces an apparent frequency change.

Say that instead of the rotor, there is a large reciprocating saw mounted on the rotorhead. If you measure vibration at the mast with the rotor stationary, you’ll not be surprised; you’ll get vibration at the exact speed of the saw.

Now, imagine the rotorhead is rotating at the same speed that the saw is reciprocating. Possibly to your surprise, the vibration at the mast will be at double the reciprocation speed.
****
One/rev vibration, whether from out of track out of pattern or out of balance, produces a larger displacement for a given acceleration than does 2/rev. We sense displacement more so than acceleration.

[Master Roda]

Sooooo.... The blades are cracking from using the wrong alloy?

[C. Beaty]

Any aluminum alloy will eventually crack if the rotor is resonant at its rotational speed.

[gyromike]

Chuck,

You mentioned before that you excited a suspended rotor with a reciprocating saw to find the resonant frequency.

Do you recall what RRPM it would have been resonant at?
And was there a wide frequency/RRPM range where resonance occured, or was it narrow?

[WHY]

Have often wondered if extruded blades would be more prone to high frequency vibration traveling thru the blade more easily since there would be no interruption in the vibrations like you would have on a blade that was constructed of different sections of metal and had seams and joints.

Tony

[RotoPlane]

Chuck - To begin with, the rotor vibrates at 1/rev that we perceive as 2/rev because the rotor’s rotation doubles the frequency felt by the pilot.

I've not seen this statement before and it has me buffaloed. I know you know more than I Chuck...but I am having a real problem with this one. A gyro is traveling at 100 mph and its advancing blade hits this added wall of air. The blade will feel this sudden drag as a bump (as an aside; this puts an aft bending moment on the blades and hub).

The opposite blade doesn't feel this bump because its total airspeed has reduced 100 mph. When that opposite blade begins to advance we have another bump.

We are talking 1/rev/blade or 2/rev bumps total. So are you saying in this scenario that we will actually feel 4 bumps per rev....not 2/rev?

[Master Roda]

Any aluminum alloy will eventually crack if the rotor is resonant at its rotational speed.

That depends on the alloy though.

So what is the time to failure exactly for a given alloy?

One IS better than the other...no?

[Oskar]

One/rev vibration, whether from out of track out of pattern or out of balance, produces a larger displacement for a given acceleration than does 2/rev. We sense displacement more so than acceleration.

That’s correct, Chuck. When putting in numbers though we see that with a 2/rev velocity component (measured in ips) of more than 20 times the 1/rev component, the 2/rev displacement component is still more than 10 times the 1/rev displacement. Somehow the pilot does not perceive the large 2/rev displacement as vibration, but is very much aware of the much smaller 1/rev displacement.

The frequency doubling still has me puzzled. When balancing the rotors the 1/rev component went from 0.35ips down to 0.05ips (a factor of 7) but the 2/rev did not change. The only thing that seems to affect the 2/rev is the airspeed.

[C. Beaty]

Imagine, Ed, that you’re pushing a child on a swing; one of those playground swings with long chains so that the period of swing if fairly long.

You can give the kid one push per cycle or if you’re fast, can run around to the front side of the swing and provide 2 pushes per cycle. But you haven’t changed the period of oscillation of the swing in doing so. It still has a period equal to the square root of length divided by gravity x 2pi.

A rotor is the same thing except that the frame of reference has changed. Instead of you rotating, the swing (rotor) is rotating. The rotor gets 2 hits per cycle that excites its fundamental resonance, ½ the hit rate.

But because the rotor is rotating, the pilot gets 2 hits per revolution.
***
Mike, we suspended a set of blades by cords attached to the nodal points, ~¼ of diameter in from the tips with a saber saw clamped to the center of the hub and driven by a Variac for speed control. I measured speed with a Chrysler ignition pickup sensor sensing plunger motion and driving an electronic counter. The frequency as nearly as I can recall, was just above 6 Hz. I still have my notes around here somewhere.

A rotating rotor is resonant at a higher frequency because of centrifugal stiffening. Perhaps you’ve seen “spoke” diagrams in helicopter books that that plot resonance vs. rpm.

Clamping the hub center in a milling machine vise and measuring droop of the tips (blades standing on edge) gives a number from which the resonant frequency of a uniform beam can be calculated. Calculated and measured numbers were surprisingly close even though a rotor isn’t uniform.

The biggest reduction in vibration in a time before sliders was a ¼ aluminum plate sawn in a shape that generally mimicked a SkyWheels hub added to the blades.

But the moral of the story is to forget isolation; detune the rotor from resonance and it won’t break (or shake).

[C. Beaty]

The frequency doubling still has me puzzled. When balancing the rotors the 1/rev component went from 0.35ips down to 0.05ips (a factor of 7) but the 2/rev did not change. The only thing that seems to affect the 2/rev is the airspeed.Well Oskar, lets try this point by point.

Your reciprocating saw to complete one cycle starting with the plunger extended, goes from plunger all the way out, plunger all the way in (180º) to all the way out (360º). That’s a cycle.

Now mount your reciprocating saw on the rotorhead. With your accelerometer attached to the mast and without rotation of the rotorhead, your accelerometer measures the same frequency as described above.

Now with the rotorhead rotating at the speed of reciprocation, say the plunger is all the way out at 0º of the rotorhead. The accelerometer detects a forward bump at the mast.

By the time the rotorhead has rotated 180º, the saw’s plunger is all the way in but the saw is facing in the opposite direction and all the way in gives a forward bump at the mast.

Simple, eh?

A resonant rotor behaves just like the reciprocating saw.

Out of balance or out of pattern stirs the mast and stick in a 1/rev circle.

[RotoPlane]

Dang Chuck.....it is beyond me how you know all this stuff....at the tip of your fingers! I imagine by pictures, so by the time I finished running back and forth on the swing thing....I was out of breath. The good thing is that I now understand what you were saying and have saved it for future reference. I am also glad I really can feel 2/rev and it is not just in my head. I thank you for taking the time to set me straight.....

I have no idea how I can figure the resonance of my rotorhead and blades before they are built and assembled, so I will just hope I've got it right.....the number of logged gyro hopes are rising.....;).

[gyromike]

Chuck,

With your spreadsheet my rotor speed (22 ft. DW's) should be ~360 RRPM. I don't have a rotor tach on mine, but your spreadsheet has been accurate on other rotors and gyros I have tried.

My slider took out most of the stick shake, and the extended Bensen towers I machined, allowing me to use the top hole in the DW teeter block, took out even more. There is still a little left.

My slider uses flat springs and I have adjusted the tension several times to see what the effect would be, but not felt much difference. What would be the drawback to softening the spring tension even further(other than the gimbal block moving too far aft and contacting the pillow block)? Can it be too soft?

[Oskar]

Chuck, you’re a genius!

Correct me if I’m wrong, when you say “the rotor vibrates at 1/rev” you mean that the rotor is flexing at a frequency of ~6Hz. I understood it to mean that the inflexible rotor is causing a vibration at ~6Hz due to a mass unbalance.

This now finally answers a question that has had me puzzled for ages, namely what is the source of the large 2/rev vibrations I have been measuring. I had always thought that the source was aerodynamic, a flexing rotor had never entered my mind. There still has to be an aerodynamic component though because the measured 2/rev vibration is airspeed dependent.

Won’t be able to sleep tonight…

[C. Beaty]

Mike, you don’t want to run the risk of the U block jamming up against a stop with springs too soft.

You might try using soft springs with a curved backing plate that gives a progressive rate.

Your pivot bushings should be fabric lined DU style.

[C. Beaty]

Oskar, when Arthur Young started playing with electrically powered models barely out of his teens, he had no idea of the vibration problems he would face in a full scale helicopter.

His first man carrying prototype, the Bell model 30, shook so bad that it was unsafe to be flown faster than 20 mph because of 2/rev shake.

Arthur Young was a Ph.D. in mathematics and his assistant, Bartram Kelley had a Ph.D. in physics but both were stumped by 2/rev although they realized it was caused by resonance.

Their test pilot, Floyd Carlson came to the rescue and suggested the external bracing seen in the photo above. In his honor, they named it the Swedish yoke. Carlson was of Swedish ancestry.

[Mike G]

Chuck
I thought I read somewhere that the 2/rev vibration linked to increasing forward speed was due to the drag on the rotor each time it was at 90° to the direction of flight being greater than the drag when the rotor was parallel to direction of flight. Have I misunderstood something?

Mike G

[Gyro_Kai]

Are the blades anodised??

Hi Paddy,

no, just as they come out of the machine.

Kai.

[C. Beaty]

The driving or excitation force does indeed increases with speed, Mike. It’s a function of cyclic flapping angle as much as periodic drag variation.

Mass above and below the teeter bolt is forced to rotate in a 2/rev circle, which it resists.

Zero airspeed: zero flapping and zero aerodynamic drag variation.

But these forces are tolerable and can be isolated so long as they don’t occur at rotor inplane resonant frequency.

With the rotor resonant or nearly so at the excitation frequency, the inplane bending force can be dangerously high.

I’ve seen blades crack in just a few minutes with criss-crossed teetering rotors. The second set adds sufficient mass to tune the first set to resonance.

A normal seesaw rotor has a vibration pattern that resembles a glockenspiel or xylophone when mounted on a soft mast but with mass or stiffness added to the center, the vibration pattern most nearly resembles a tuning fork. The tuning fork mode is lower in frequency than the xylophone mode.
*********
Here would be the displacements, viewing the rotor from above of the 2 different modes of resonance.

In the LH view, imagine the mast to have the stiffness of a rope.

In the RH view, imagine the rotorhead to be bolted to a bulldozer.

The rotor blades don’t abruptly jump from one mode to the other; as the mast is stiffened or has added mass, the nodal points move toward center.

[Lee Scatt]

Chuck, Did you try hanging the rotor at its center and exciting it, tuning fork mode, as in the rigid pylon diagram?

[Master Roda]

Aside from harmonics,

Most anything that is manufactured that vibrates is isolated. A car engine for example isn't rigidly fixed to the frame. Maybe they should just "tune" the resonance and vibrations out of it?

Frames without isolation crack. Frames with isolation don't crack. Maybe I'm thinking way too simply. Is it more complicated than that? I don't think so.

[gyroplanes]

I have always had some concern over the double beam hub bar some models use. I don't see it as a model of flexibility. Perhaps it's forcing the everchanging coning angle to the bolt hole area?

[C. Beaty]

Chuck, Did you try hanging the rotor at its center and exciting it, tuning fork mode, as in the rigid pylon diagram?No but for a uniform beam, the calculation of natural resonant frequency is fairly simple.

The rotor turned edgewise and supported at its center is resonant in the tuning fork mode at:

F = 3.89/(droop)^0.5 cycles/min

In the xylophone mode the resonant frequency is:

F = 6.98/(droop)^0.5 cycles/min

It would have been more informative to have mounted the rotor on various forms of masts and then measured the resonant frequencies.
****
For those people having difficulty understanding this stuff, think in terms of a radio.

A radio doesn’t respond to frequencies to which it’s not tuned. The air is full of radio signals but the radio only responds to a single frequency at which the input circuits are resonant (or a narrow band of frequencies depending upon the sharpness of resonance).

The trick in a rotor is to make sure it is not resonant or tuned to frequencies generated by rotation and translation.

Arthur Young and his team at Bell Helicopter solved this problem 50+ years ago by stiffening the rotor inplane and mounting the rotor mast, gearbox and engine on very soft rubber bushings. The mast on a Bell-47 is downright floppy.

[Master Roda]

I have always had some concern over the double beam hub bar some models use. I don't see it as a model of flexibility. Perhaps it's forcing the everchanging coning angle to the bolt hole area?

Testing our own rotor systems, the stiffer hub bar we use (rigid/fixed) is less comfortable to fly than our "isolated" hub bar (pitch adjustable). That's why were phasing out the fixed/rigid hub bars (8").

[docrob]

Ron Herron's LW1 used, instead of a single piece mast as is common practice, a pyramidal structure which attached to the fuselage with 4 Lycoming engine mounts (Lord mounts,) one at the bottom of each leg of the pyramid. This left the entire "mast" free to move, somewhat independently of the fuselage. I've seen the structure, which he did not use on later models, and it is quite rigid. He says that it was very smooth. (He changed for ease of construction, cost, and ease of rigging.) I suspect this approximates the soft mounting of the power package/rotor system of the Bells. How would this enter into the vibration/resonance picture in a gyro?

Dr. Rob

[Doug Riley]

The only Bensen-style gyros that I've been completely happy with in the vibe department have been small, light ones with small, light blades, such as 23-foot DW's or 22-foot Bensens. Everything else I've flown or ridden in has vibrated to some degree. And, in all cases, the roughness got worse as you went faster.

An R-22 is silky by comparison. The R-22 has a mast-engine suspension something like a Bell.

So I think that Ron Herron was on the right track. Some gyro rotor vibes are in-plane and some are more or less vertical. Pivoting masts (like RAF) and sliders (like RFD) are steps in the right direction, but don't have enough degrees of freedom to isolate all the vibes.

[C. Beaty]

A reduction of pylon stiffness raises the inplane resonant frequency of a rotor
***********
And Tom, a rotor should be limber in a flapwise direction, the reason Bensen often stated for using segmented upper skins. Some Sikorskys also used segmented skins.

A helicopter does not benefit from the self correcting nature of autorotation so change of coning angle vs. load is considerable. The use of separate coning hinges in Robinsons is a good thing.

It’s not that the coning angle of a gyroplane rotor changes –it doesn’t under steady conditions- but the lift load along the blade span shifts more inboard to more outboard with rotation. This causes continuos flapwise flexing of a rotor blade.

That’s the real objection to extruded rotor blades.

[RotoPlane]

On the 2/rev vibration issue, to have a smooth rotor, we have been saying that it is best to have a stiff rotor (high frequency) and a soft mast where neither vibrates at the same mode. A small number of gyros appear to have very little stick shake but there are many others on videos and posts that do have it and it is noticeable. I'm beginning to think that it is difficult to achieve the above resonant condition.

It seems to me that the opposite would also be true…..a soft flexible rotor (low frequency) would need a stiff mast (high frequency). I'm beginning to think that this approach toward a smooth rotor would be much easier to accomplish. Please tell me if I'm wrong and why. This frequency stuff can boggle my mind.……

[RotoPlane]

Bump: I will sleep much better tonight if someone can answer post #39.....really. Yes, I am desperate....

[hillberg]

How about thinking more in the quality of design & Fabracation:suspicious:,You have a drilled hole and an attach area of a cambered airfoil and a flat plate bolted to each other :Cry:.And a concentration of clamping force and a reduction of area at the peak of the camber will cause a stress riser, :bored:If a doubler is added to releave the area and spred out the clamping force the part will last longer,or reduce the thickness of the plate at the outer ends so the part will flex with the blade and reduce the point area of bending loads:spy:,As for the Sikorsky blade pockets they are segmented for bending loads ( span & Chord wise)and for replacement in the field-You can fly with 14 missing pockets.:boom:

[C. Beaty]

Bump: I will sleep much better tonight if someone can answer post #39.....really. Yes, I am desperate....Beginning with Cierva, no one has succeeded in getting 2-blade rotors to work on gyros with rigid rotor pylons. Drag hinges, ‘isolated” rotors or whatever.

YouTube - Chuck Beaty and his unique gyro, 1994 (http://www.youtube.com/watch?v=qvdPL1pcG1A)

[RotoPlane]

But it seems so logical….apparently just having different resonating frequencies on the rotor and the mast is not the whole story….puke! I'm so far along with this design….perhaps my flexible-rigid head (oxymoron term) will act differently than a gimbaled teetering head….plus my mast rotates fore and aft and is hydraulically dampened….I could still make this work well….I'm rambling, huh….

Thank you Chuck….I'm sure you tire of people like me asking questions that for you seem so obvious. Imagine how much easier it would be if you had….you know, written a book.
Your drag hinges in the video did fly. I need sleep……

[C. Beaty]

Whoever said; “I wouldn’t get out of the electric chair to go for a ride in that thing” had it about right, Ed.

What’s really strange is that a human could be so dumb as to fly such a thing.
************
Having time to have thought things over, it is clear the MT blade cracks had nothing to do with 2/rev vibration. Cracks resulting from in-plane resonance always start at a trailing edge.

Cracks starting at root attachment holes and propagating chordwise almost certainly are the result of flapwise flexing.

A flexible, fiberglass hub or coning hinges are in order with extruded blades.

[Semler]

Is there even any pre-cone in that hub-bar arrangement?

If extruded blades were that dangerous i would of thought some of musters might have found some cracks by now, altho a good muster never stresses their rotors ;)

[C. Beaty]

I recall a rash of hub bar cracks in Oz, Semler. Do you know whether or not that was confined to any particular type of rotor blade?

[Master Roda]

Having time to have thought things over, it is clear the MT blade cracks had nothing to do with 2/rev vibration. Cracks resulting from in-plane resonance always start at a trailing edge.

Cracks starting at root attachment holes and propagating chordwise almost certainly are the result of flapwise flexing.

A flexible, fiberglass hub or coning hinges are in order with extruded blades.

I would think that about any blade. Extruded or not.

[Master Roda]

By the way, the Skywheel hubs crack. There's a pile of them under my bench. Repairable or not, I hate cracks. So maybe fiberglass isn't the best solution.

[RotoPlane]

If the design stressing and layup orientations are done correctly, those heads shouldn't crack. I haven't seen a Skywheel rotorhead but on something like this, the glass shape must "flow" to attach points to allow some flexing and those attach points should be reinforced with metal.

[Master Roda]

Ed,

I agree. We're working on a project right now that deals with those issues. Uni-directional fibers laid around ferrel bushings are a great way to connect attach points. You then feather them out like a leaf.

Jon

[SamL]

I recall a rash of hub bar cracks in Oz, Semler. Do you know whether or not that was confined to any particular type of rotor blade?

Hi CB
Link to ASRA AD's

http://www.asra.org.au/AD_AA.htm

Regards SamL...........

[C. Beaty]

Thanks, SamL. But not enough information in the Oz AD files to form any sort of conclusion.

The general rule is to provide flexibility in a flapwise direction to reduce stress and to make a seesaw rotor as stiff as possible in an inplane direction to avoid lead-lag resonance.

A filament wound fiberglass hub that straddled the rotorhead and offered a wider base for the blade attachment straps might be the way to go.

[Semler]

To Answer Your question Chuck, I believe no it wasnt just one brand of rotors, going off pure memory there was some in extruded and fabricated blades, i dont think there were any found in fibreglass blades (but with alu hub-bar).

Some time ago we had a defence force student do research on hub-bars and the stresses associated with them, his conclusion also was of some sort of fibreglass hubbar if i remember correctly, but also mentioned that for what we have and the hours we have imposed on our hub-bars that it was quite okay.

I'll see if i can find his report.

[Semler]

Here's the link

But buggered if i know... seem from his research it was due to alu surface defects.

Anyone know if these hub-bars have precone in them? Is the largest crack the last bolt hole?

seems to me the hub-bar might not be flexable enough transfering the flex to the blades.

[gyroplanes]

I can't see how a double beam hub bar can flex at all.

[Xavier AVERSO]

Porte-pales trop rigide pour des pales non renforçées au niveau du dernier trou de fixation.

too rigid hub bar for blades not reinforced on the level of the last clamp hole.

[Chopper Reid]

Is there even any pre-cone in that hub-bar arrangement?

If extruded blades were that dangerous i would of thought some of musters might have found some cracks by now, altho a good muster never stresses their rotors ;)

There have been a few "muster's" having trouble with cracking. To be fair though, most of them have had a lot more than 500 hours on them, some have gone over 2,000 hours and yes, both Patroney's and AK's have cracked, some out on the strap holes then some in the middle.

The 500 hour AD seems to have helped save lives but the "fix" hasnt yet happened.

I would love to see a set of Sporcopter rotors do some serious hours and see if their approach to rotor /hub bar building is the 'cure' to all hub bar cracking problems.

[Master Roda]

There have been a few "muster's" having trouble with cracking. To be fair though, most of them have had a lot more than 500 hours on them, some have gone over 2,000 hours and yes, both Patroney's and AK's have cracked, some out on the strap holes then some in the middle.

The 500 hour AD seems to have helped save lives but the "fix" hasnt yet happened.

I would love to see a set of Sporcopter rotors do some serious hours and see if their approach to rotor /hub bar building is the 'cure' to all hub bar cracking problems.

Is 1300 hours enough? Our trainer has seen the worst and our rotors and hub configuration seems to work very well. It has a rigid mast and shakes like crazy so all of the components are under alot of stress. No failure.

Here's a story. Two gentlemen take our trainer out for a checkride. Neither of them have the stick while in taxi. The blades start to flap and get cut in half at the root section from the prop. They fly it around the pattern. When they get back, they realise what had happened. They were thankful that our blades saved them from their stupidity. They didn't even notice a shake. I think the combination of the hub bar,torsional rigidity, and our spar strenghth is why they didn't really notice anything was wrong.

I'll post pics when I get back to the shop.

I also just noticed there aren't any good pics of our hubs either...I'll post those too.

Jon

[C. Beaty]

How could Bell Helicopter have gone so wrong?

74201

A rotor is the equivalent of a radio with respect to lead-lag (2/rev) vibration. It can be tuned to resonate with periodic aerodynamic and mass excitation or it can be tuned so that it does not.

Cracks like those of the MTO rotor appear to have their origin in excess flapwise stiffness. That’s easily relieved by coning hinges or a rotor hub that’s flexible in a vertical plane only.

[Master Roda]

Cracks like those of the MTO rotor appear to have their origin in excess flapwise stiffness. That’s easily relieved by coning hinges or a rotor hub that’s flexible in a vertical plane only.

Isn't that what we're talking about? Did I miss something?

[C. Beaty]

To me; “Automatic positioning for lead-lag” means drag hinges or rubber bushings that permit flexibility in a lead-lag direction. Lowers the inplane resonant into the range that can be excited by periodic aerodynamic and mass forces.

Not exactly what Bell was trying to accomplish with the external yoke.

[Master Roda]

I see what your thinking.

We haven't experienced any "exciting" of this nature. But, our flapwise dampening and flexablity are noticable. Like I said before, the difference between our fixed and adjustable bar is quite noticable.

When I post the pictures I think you will notice that the hub doesn't allow much movement for lead/lag.

[Master Roda]

I suppose "automatic positioning" isn't the right wording, but it means you don't have to set the lead/lag on the bench. It's built in.

The parts you can't see in the picture are a giant spherical bearing that the large NAS bolt goes through. There is also a large steel bushing between the second largest bolt and the urethane. The durometer of the urethane is such that it doesn't allow for alot of movement, but enough to keep things from cracking.

[StanFoster]

Jon- When I saw how nicely built my Sportcopter rotors and hub were built on my SparrowHawk, I was really impressed. That lead/lag joint really looks nice and beefy as heck.


Stan

[Master Roda]

Thanks Stan! That's nice to hear. I know it's a bit heavy, but the system just works very well.

[RotoPlane]

This is the first time I've seen this assembly up close and I am impressed. I see now how the rigidity of most rotors has been slightly uncoupled…..gee, it is somewhat similar to my head-arms to blade joint….maybe the reason I like it ;). Those plate (4130 ?) ears have spacers and washers….the nose side has a washer that is thicker; is that for setting the blade pitch? If so, I can see one reason for the spherical bearing....the other being easing of the coning and perhaps the flapping blade stresses. Thanks Jon....

[Master Roda]

Ed,

You are correct. We have a huge range of pitch settings we can use with this hub.
It really helps for our high altitude customers to get the best performance.

[C. Beaty]

Very pretty, Jon. But 2/rev vibration has nothing to do with prettiness or with isolation.

The way to get rid of 2/rev in a seesaw rotor is to increase inplane stiffness; decreasing inplane stiffness is going the wrong way.

The reason Stan’s Sparrow Hawk didn’t totally addle his brain from 2/rev shake with those soft inplane rotors is because it used an RFD style slider.

[Master Roda]

But that's not what the conversation is about Chuck.

I thought we were talking about flapwise strains and ways to correct or alleviate the problem.

I don't agree with your particular view on this 2/rev thing because the system proves it. The sliders don't work that well either. We have one, I don't think it does a damn thing. What about the RAF's and Xenon's that don't have those sliders (our conversions)? They all claim smoother sticks. Are my customers lying or trying to make me feel better about it? I don't think so.

So, back to topic.

[StanFoster]

Chuck- For your information......my brain has been "totaled" from vibes a long time ago.....nothing left up there to rattle! ha


Stan

[Master Roda]

We solved 2/rev in 2007. We tend to not release information, but I'm going to right now.

The mast in our SCII is allowed to orbit 3 dimentionally. A european gyro company has copied us since then. This isn't the end of the story though, Our gyro is a system. The combination of the mast and our hub/controls, etc eliminates the 2/rev.

Sliders only work in one direction....it helps, but does not solve the problem.

[C. Beaty]

An RAF has the magic rubber bushing that serves the same purpose as a slider, perhaps better.

The Xenons have a fairly limber mast that possibly keeps inplane resonance above 1/rev.

Rotor inplane resonance, involving both rotor and mast as the resonant structure is dangerous and must be avoided at all cost.

The more or less minor 2/rev shake that aggravates many people isn’t full blown resonance but occurs when excitation is just below resonance. The phase between force and displacement is always 90º at resonance but less than 90º for excitation applied below resonance. Stiffening the rotor inplane will often make it go away.

That’s the reason stick shake isn’t often purely lateral or fore and aft but on a bit of a diagonal.

The solution is a rigid inplane rotor mounted of a limber mast. Something that thanks to Bell Helicopter and Arthur Young has been known since ~1945.

[Master Roda]

Our system also allows movement up and down. Forgot to mention that.

[StanFoster]

Jon- You mentioned it....Post #71 .you said it orbits in 3 dimensions.


Stan

[Master Roda]

So I did.

Thanks Stan :)

[Master Roda]

An RAF has the magic rubber bushing that serves the same purpose as a slider, perhaps better.

The Xenons have a fairly limber mast that possibly keeps inplane resonance above 1/rev.

Rotor inplane resonance, involving both rotor and mast as the resonant structure is dangerous and must be avoided at all cost.

The more or less minor 2/rev shake that aggravates many people isn’t full blown resonance but occurs when excitation is just below resonance. The phase between force and displacement is always 90º at resonance but less than 90º for excitation applied below resonance. Stiffening the rotor inplane will often make it go away.

That’s the reason stick shake isn’t often purely lateral or fore and aft but on a bit of a diagonal.

The solution is a rigid inplane rotor mounted of a limber mast. Something that thanks to Bell Helicopter and Arthur Young has been known since ~1945.

Chuck,

As I said before, The RAF and Xenon conversions (to our blades and head) report a smoother stick. If what you are saying is true, wouldn't they experience MORE shake not LESS? Just trying to understand.

My thought is that the blades must be out of phase of each other (lead/lag).

How can you quantify the frequency of both when they are not in phase? Would they not cancel out?

I'm only bringing it up again, because we don't experience this 2/rev resonance with our adjustable bars, in fact the 2/rev is less.

Jon


P.S. We have rubber bushings in our new mast. They allow for more movement than the RAF. I don't think they are magic though :)

[C. Beaty]

When Arthur Young mounted the mast of the Bell-47 in rubber and stiffened the rotor in-plane 65 years ago, it wasn’t a by-guess and by-gosh procedure by tinkerers. It was a systematic approach to raising the in-plane resonant frequency of the rotor system by people who understood exactly what they were doing.

It was also the first successful of application of 2-blade rotors.

[Papa Smurf]

Why did the blades shown at the start of this thread crack?

[Master Roda]

@Chuck-That still doesn't answer my question Chuck. Why does it work for our gyro's? Our mast ARE limber...remember? It's also my opinion that helicopter systems operate differently than gyro systems.

@Papa Smurf- wrong alloy. Stiff hub bar (flapwise)

[C. Beaty]

It’s magic, Jon.

But perhaps not. If drag hinges are far enough outboard, in-plane resonance might be kept above 1/rev on a really soft mast.

Years ago, the first set on Hughes 269 blades that I acquired came from a gentleman that had tapered the root ends to resemble a Bensen blade, weakening them in an inplane direction to the extent that I didn’t think they were strong enough to be flown on a rigid hub.

I made up a non underslung hub with a coning/teeter hinge on the center of rotation, interleaved like a door hinge. Drag hinges were 16” outboard.

They flew reasonably well on a gyro with a round, floppy mast.

It’s not a scheme that will work on a stiff mast. The mast and rotor together are part of the same resonant structure.

[Master Roda]

I don't believe in magic Chuck, so it must be the latter.

Thanks for your response.

[Doug Riley]

I am going to expose my utter ignorance of resonant behavior here (despite playing stringed instruments once in awhile). My old man, a ham radio buff, used to blather about heterodynes, phase modulation, sidebands and suppressed carriers, but the apple fell far from the tree.

I get it that resonance is bad; it accumulates and gives back energy in a destructive way. What I do not get is why such problems as stick shake and/or frame shake in a gyro are necessarily problems of resonance.

If someone hits me on the head with a hammer every 5 seconds, he is unlikely to match a resonant frequency of my skull. It still hurts.

When a car navigates a potholed Vermont dirt road, the exciting frequencies are varying and apt not to be resonant. Without suitable shocks and springs, the ride will still be awful -- not as bad as if the frequency happened to match the suspension's resonant freq., but still awful.

Bottom line: from the depths of my ignorance, it seems that avoiding resonance is only the start. Yes, it's important, but suitably isolating the non-resonant vibes also should be a goal.

The vibes may be mostly 2/rev or higher (if the rotor has been balanced, tracked and put in pattern/string). They are not all in the same axis, though. Some are in the nature of "hop" (from varying rotor thrust through the rotor's rotation cycle) and others are mechanical (the reaction to cyclic feathering and the distribution of rotor mass above and below the teeter hinge).

It therefore seems to me that any version of a soft suspension that allows movement only in one axis (such as lag hinges, sliders and the RAF pivoting mast) are bound to do an incomplete job of keeping vibes out of the controls and the frame. The Bell suspension, IMHO, works better because, in addition to being very soft, it works in multiple axes.

[RotoPlane]

I certainly do not know as much about this stuff as you Doug and I agree with most of what you've said. I've wrapped my pea brain around this and have come to the conclusion that a rotorhead, if correctly designed, should be able to handle all bad vibes, even if the mast is stiff. I'm quite sure this will work with four blades but with two blades….I just know I don't know enough about vibes to be absolutely sure….and I won't really know until I try it.

[C. Beaty]

Isolation and damping are the other side of the coin.

Unfortunately, some attempts at damping also lead to altering resonant frequencies in a direction that exacerbates vibration problems.

Most any rotor that suffers mild 2/rev shake can be improved by stiffening it inplane.

Here was Mike Schallmann’s experience:

http://www.rotaryforum.com/forum/showthread.php?t=19613&highlight=rotor+shake

[helipaddy]

Maybe the MT03 blade cracking is nothing to do with in-flight loads, could it be taxying around with blades stopped on rough ground?

[Papa Smurf]

I looked at both sets of pictures in the documents but can't be sure which side of the blade is cracked.....Or both? I think the outboard end of the hub bar could be shaped differently.

Helipaddy might just be on to something......

There is some jagged-ness in the cracks that suggest more of an overload than fatigue...

[Redbaron]

good point paddy, when I taxi on grass I atleast put a little hand spin on my blades! In my opinion taxiing on rough ground without atleast a little rpm is harder on it than the roughest of treatment in the air!

Maybe the MT03 blade cracking is nothing to do with in-flight loads, could it be taxying around with blades stopped on rough ground?

[EI-GYRO]

I reckon those blades were flapped severely, or repeatedly, or both.
Assuming those cracks are on the bottom, and given that they are orientated fore-and-aft,
I cant see any other mechanism to cause cracks to form that way.

If they are fatigue cracks, we're in deep doo-doo, but I've never seen or heard of
previous incidences of this type.

It's not beyond the bounds of possibility that a pilot flapped the blades thoroughly,
but neglected to report it.

A metallurgist should be able to determine whether the failure is due to fatigue or overload.

[RotoPlane]

I think Fergus has a very good point....severe flapping could cause blade damage. I wonder what the teeter-stop area looks like....

[EI-GYRO]

Just re-read the CAA AD. No indication of machine hours, or whether it is a training
machine or not.
Given that they found some cracks on non-uk machines, one presumes they went looking for them.
Where better to look than training schools.?

Are students particularly prone to flapping these blades on takeoff? i.e. over-running
the blades by applying too much power too early.

Only an instructor can answer that one.

For such a potentially serious problem, there seems to be remarkably little detailed
information as yet.

I cant read German, but their report does not seem to contain much in the way
of explanatory text either.

[EI-GYRO]

A laborious babelfish translation of the German AD, makes reference to a permanent upward
deformation/bend and describes a procedure to check for it.

This further suggests to me that excessive upward bending/flapping of the blade is
involved.

The cracking is mainly on the outermost strap bolthole.

Ed makes a good point. Examination of the teeter stops and the bottom of the hub bar
might give an indication of how hard they've been flapped.
Again, a metallurgist might be able to determine whether excessive contact is involved.
Comparison with similar marks on non-crack machines might also be helpful.

[Papa Smurf]

We just can't see enough in the information have, yet there is a story or message in those blades...it just needs someone to help it out.

[Chopper Reid]

Is 1300 hours enough? Our trainer has seen the worst and our rotors and hub configuration seems to work very well. It has a rigid mast and shakes like crazy so all of the components are under alot of stress. No failure.

Here's a story. Two gentlemen take our trainer out for a checkride. Neither of them have the stick while in taxi. The blades start to flap and get cut in half at the root section from the prop. They fly it around the pattern. When they get back, they realise what had happened. They were thankful that our blades saved them from their stupidity. They didn't even notice a shake. I think the combination of the hub bar,torsional rigidity, and our spar strenghth is why they didn't really notice anything was wrong.

I'll post pics when I get back to the shop.

I also just noticed there aren't any good pics of our hubs either...I'll post those too.

Jon

1300 hours on a trainer without a problem is great going, are those 1300 hours equivalent to 1300 of mustering ? I'm not sure but I'm guessing that its an exremely good start.
Looking at your sportcopter hub bar /blade design, I'm fairly sure they would take the worst possible punishment that could be dished out eg, very rough airstrips and pilots who insist on pulling max G's every time they turn and really make the rotors work.

I see our Australian $ is very close to your US $ and I'm looking very closely at buying a set of 27 ft'ers to try out. I only do about 400 hours a year nowdays so it would take some time to get up over the 2000 hours.

[Master Roda]

Brian,

Our adjustable hub has been working very well since it was designed in 2000. We went to great lengths to design it. In minor accidents, such as ground strikes,flapping,etc.. the adjustable hub absorbs the impact and sometimes it's hard to tell that it had been wrecked. We've never had a failure, even in the event of accidents.

We have a dealer in Australia. We can put you in contact with him if you wish. Just call or email.

Jon

[OzyRuss]

Query..............do these blades drop straight into existing heads, .....ie...RAF heads, or is there is a bit more to it.

$ for $ exchange rate, makes this purchase more desireable......maybe.

( bean counters here in oz, are predicting our dollar overtaking yank dollar by 20%, sooner, rather than later.)

Must be all that dirt we is shipping all over the world, and making a squillion dollars from it...........( could i get going on this subject, we is givin it away .......chinese folks are queing up trying to buy our bleedin country........coal,iron ore,cattle stns, just to name a couple........i'm having nightmares thinking what our pollies will agree to.

[Master Roda]

I would be the first to move to Australia. I love your country. Too bad about the guns.

We have a fix for RAF's and others to accompany our blades and heads.

Too bad about the pollies. It affects all of us. Good luck.

Jon

[OzyRuss]

jon.........was about to log off, got your reply............Ok, so what changes etc are needed to a RAF

thanks............

[Chopper Reid]

I would be the first to move to Australia. I love your country. Too bad about the guns.

We have a fix for RAF's and others to accompany our blades and heads.

Too bad about the pollies. It affects all of us. Good luck.

Jon



I'm in a silmiliar boat to ozy russ, I dont want to change my Rosco head unless its absolutely neessary.

I know the Aussie dealer so I will contact him and have a yarn. Thanks

[Master Roda]

To convert to the rotors only, all you need is new cheek plates. We have those in stock I believe. This is probably what you would want if you like your existing head.

To do the complete conversion, we have a kit that replaces the mast plates to accept our head.

Jon

[PTKay]

Latest comment from Celier Aviation:

Rotor update info
4 grudnia 2010 15:05

Due to a number of emails asking us about rotor issues / problems from our competitor Autogyro in Germany, if we where touched, we want here to provide some complementary info.

They had (Autogyro) to publish a bulletin service through DULV-2010-003 and 04-2010, due to some rotor failures, which is a dramatic fact. The bulletin also mention the possibility on an Aircopter model.

We would like to remind that the ORIGINAL rotor made by Aircopter in Toulouse France, was modified against strong recommendations from Aircopter. Modification made is to drill 4 extra holes per blade for the fixation points. The original Aircopter rotor have 5, Autogyro made 9 per blade. The Aircopter rotor was physically tested by Eurocopter lab, as well re-validated by us, and we do confirm that ONLY 5 holes can guaranty the integrity and safety. It is AN ABSOLUTE NON-SENSE to have 9 close distance holes, as they are so close to each other, they produce the well known poststamp effect, weakening dramatically the blade fixation and as well the hub bar.

Next, they are using (and copied) the old version of our blade pitch profile holder, less resistant than our actual one for such a non necessary modification.

They then made a bad & poor copy of our now full owned rotor concept Aircopter – CELIER Aviation, just modifying few parameters without the consideration of the deeply studied and testing initially done by Aircopter and its collaborating partners like Aerospatiale and Eurocopter. The material and processing they use is far to match the highly confidential, certified and proven ones from our affiliated partners.

Extruding flying rotorcraft blades is a serious matter & business.

We do recommend the highest attention on this company not respecting others concepts and making copies, Aircopter – CELIER Aviation rotor is one of other examples, and aircraft quality issues.

For your own safety, fly originals not copies.

[EI-GYRO]

The cracks are at right angles to the line of holes.
If it was the 'postage stamp effect' would the cracks not be in line with the line of holes?.
The overload in these blades appears to be in the vertical direction, i.e. flapped.

[steveb]

The cracks on the Aircopter blades didn't originate at the bolt holes, they were in line with the end of the internal stiffener.

[PTKay]

Whatever it was, the sad thing in the gyrocopter constructions is,
that they are copies of the copies of the copies....

Except for Xenon, (maybe Sprocopter and Cruza) all the rest is copy of a copy of a copy.

Magni copied Trevamakki concept, then Ela copied Magni, MTO copied Ela...
The same with the rotor blades, tried this or that, then copied without understanding...

I do believe in machines, where the original construction is done
by the man, who then has the brain, common sense and eventually gonads
to test fly the prototype, and then every production unit himself...

[PTKay]

The cracks on the Aircopter blades didn't originate at the bolt holes, they were in line with the end of the internal stiffener.

Once again: THEY WERE NO AIRCOPTER BLADES,
THEY WERE COPIES...

[PTKay]

The cracks are at right angles to the line of holes.
If it was the 'postage stamp effect' would the cracks not be in line with the line of holes?.
The overload in these blades appears to be in the vertical direction, i.e. flapped.

The flapping moment on the blades crates longitudinal tension on the lower
and upper skins of the blade, especially at root, where the moments are
the highest.

It should be obvious to you, that if you perforate this skin 9 times instead of 5
you are interrupting the continuity of the said skin 9 times instead of 5,
so almost twice as much, hence weakening it almost twofold.

Also the distance of the holes plays a role, where at smaller distances
the tension and stress on one hole can interact with the stress on the other...

[rotormaster]

The blades may be o.k. But the Mast ist the Problem

[PTKay]

The blades may be o.k. But the Mast ist the Problem

I don't know, what do you mean???

AFAIK, this is an image of a Norwegian Xenon which hit a mole hole on the runway.
He tipped over.

What does it say about the mast strength???

The mast broke off where it was supposed to brake,
leaving the monocoque cockpit and pilot unharmed.

[Gyro_Kai]

Hello,

I don't want to get in what seems to evolve into a fight, here, let's please stay fact-based.
All cracked blades I heard of (6 cases) had the crack across the chord at the last bolt on the lower surface.

The other 4-8 holes apparently did not matter, Paul, as Fergus correctly pointed out. If the inner bolts had anything to do with it, there would have been damages there, as well.

MTs up to year 2005 had original aircopter blades.

MTs went from Aircopter blades to own, more beefy blades and from 5 to 9 bolts, because they wanted to improve the strength, as Aircopter blades started to fly off above 500 rrpm. One reason alledgedly was, that the British Section-T requires an excess of 3.5g as maximum load (hearsay).

Both, Aircopter as well as their own blades showed the cracks.

My theory, after talking to a few people, how this may have come about:
Normally the hub bar ends and the blades are always aligned in the close to constant coning angle.
If, however, you do wild banking and yanking, you may put g's on more quickly, than the rrpm can come up. In those cases the blades would be bent upwards. If the bracket in the hub-bar are not matching the bending shape, all load culminates in the last bolt, where the blade is apruptly bent up, thus creating a peak load at that point on the underside.

Magni has the bolts from front to back. thereby leaving top and bottom surface of the blade closed.

Open for discussion.

Kai.

[PTKay]

Kai,

very valid points.

It remains to clear, if the additional stiffness if the hub bar,
as mentioned by Raphael, caused the effect you suggest.

Hub bar elasticity properly adjusted to the blade elasticity
would prevent "knick coning" of the blades, but rather
result in rotor coning, including proper flexing of the hub bar.

Maybe the stiff bar, additionally stiffened by 9 bolts fixing it to
the blades was the reason?

[Vance]

Kai,

very valid points.

It remains to clear, if the additional stiffness if the hub bar,
as mentioned by Raphael, caused the effect you suggest.

Hub bar elasticity properly adjusted to the blade elasticity
would prevent "knick coning" of the blades, but rather
result in rotor coning, including proper flexing of the hub bar.

Maybe the stiff bar, additionally stiffened by 9 bolts fixing it to
the blades was the reason?

Hello Paul,

What is “knick coning”?

How is the stiff bar additionally stiffened by 9 bolts fixing it to the blades?

Thank you, Vance

[Gyro_Kai]

Clamping the blade for that length between too stiff hub-bar ends, with multiple bolts adding extra stiffness, may cause a peak of stress at the end of the hub bar, similar to a nick of the surface.

Vance, the two forking ends of the hub-bar being held by many bolts in honed holes (yes, they are reamed to fit exactly) will result in a stiffer piece than just the fork, or the fork with "loose" bolts.

Kai.

[Vance]

I have always assumed everything is flexing and that a sudden change in stiffness was poor design.

Fatigue in aluminum always fascinates me. I feel like I can manage 4130 but aluminum alloys and extrusions continue to confuse me.

I was not familiar with the term “knick coning”, is this what you are describing?.

Thank you, Vance

[steveb]

Just to stick to facts ....

The first cracks were found on Aircopter blades (my MT-03 has Aircopter blades) - my understanding is that the hub bars are an Auto Gyro design and that the blades have had the additional bolt holes installed - the Auto Gyro hub bar and the additional holes were required in order to meet BCAR Section T strength requirements (I couldn't comment on whether they unwittingly caused additional problems).

The cracks on the Aircopter blades did not originate at the bolt holes - they were further out along the blade. My understanding is that the Aircopter blades have some sort of internal stiffener in the blade root and the cracks formed at the end of this.

Following a fleet-wide check there were also come cracks found in the later Auto Gyro blades - those cracks started at the outboard bolt hole.

[PTKay]

I was not familiar with the term “knick coning”, is this what you are describing?.



Vance, this is a term I just created ad hoc.

What I meant is, that there is a "knick" in the stiffness of the hub-blades
continuous flexing beam in the rotor.

The coning angle, or the coning itself takes place at this "knick",
not on the total length of this "flexing beam".

This is a point of extreme stress and hence fatigue and failure.

If you look at the hub bar and fixing inserts of the Xenon rotor,
you will see, that the blades are held in place more at the leading
edge, so at the strongest part, rather than by the bolts in the middle.

[PTKay]

The cracks on the Aircopter blades did not originate at the bolt holes - they were further out along the blade. My understanding is that the Aircopter blades have some sort of internal stiffener in the blade root and the cracks formed at the end of this.

Following a fleet-wide check there were also come cracks found in the later Auto Gyro blades - those cracks started at the outboard bolt hole.

I don't know what cracks in the Aircopter blades along the blade you are talking about???

The image in the CAA Bulletin is clear.
The problem is the last bolt in the hub bar!

BTW, IMHO the number and spacing of bolts in this hub
look really ridiculous.

[PTKay]

All of you have obviously overseen the fact, that the problem
with the AutoGyro blades is not only cracking, but
also permanent bending, as mentioned in the SB:

http://www.auto-gyro.com/chameleon//outbox//public/28/2010_04_ServiceBulletin_Rotorsystem.pdf

Even if not cracked, the blades can be permanently bent at the
end of the stiff hub bar.

[Walter]

It seems clear that there is a jump in the stiffness at the end of the hub. This can be avoided by a tapered hub. However, I see no reason for 9 bolts. Disregarding elasticity, only one bolt will be holding all the load, namely whichever bolt comes under stress first. Elasticity will make the other bolts to take over a partial load. But too many bolts just weaken the rotorblade structure ("stamp effect") in the tangential direction and increase the stiffness of the hub assembly in the vertical direction, in my view.

[Vance]

Vance, this is a term I just created ad hoc.

What I meant is, that there is a "knick" in the stiffness of the hub-blades
continuous flexing beam in the rotor.

The coning angle, or the coning itself takes place at this "knick",
not on the total length of this "flexing beam".

This is a point of extreme stress and hence fatigue and failure.

If you look at the hub bar and fixing inserts of the Xenon rotor,
you will see, that the blades are held in place more at the leading
edge, so at the strongest part, rather than by the bolts in the middle.

Thank you Paul,

I try to stay up on the terminology related to structures. There are so many ways to do it wrong that precise descriptions have great value for me.

The picture with the squared off doublers you posted looks like an illustration of what they term “wrong” in many of the books about structural joints I have. They have pictures of “best” “correct” followed by “wrong”.

I only count 5 bolts on that one.

I was always taught to fish mouth or taper that sort of joint.

It is a wonder they don’t have more problems.

This seems to be the way it is done on many gyroplane rotor blades and as far as I know there have been no actual failures.

Putting the holes all in line doesn’t seem like best practice to me.

I was also taught never to weld across a tube, particularly doublers, and yet I see in that picture that the square control tube has a weld straight across the tube near the end. This would seem to me to be a flight critical part and worthy of special care in the design of the assembly.

There are so many things about gyroplane design that confuse me.

When I was building my shed the government regulations called for so many nails in my shear wall that it actually made it less than half as strong because it split the wood as the shear wall flexed in the wind.

If the extra bolts are because of section T this could be a similar circumstance of unintended consequences from the fantasy that if some fasteners are strong, more will be stronger.

I feel like the cracks in the rotor blades may be just a symptom of an underlying design challenge that is brought to the surface by a requirement for more fasteners.

Thank you, Vance

[EI-GYRO]

Steve, both the German and British documents are very specific, in that the cracks
originate at the outermost bolthole. There seems to be no reference to cracks further
out on the blades.

Can you provide a more detailed description of these other cracks, i.e. distance from
centre,(or from blade tip), crack orientation and length, top and/or bottom, depth of
penetration, etc.?

Why does the CAA MPD make no reference to these?

How common is the incidence of significant blade flap in training machines.?
Unless other convincing evidence emerges, I believe unreported flapping events are
probably to blame for the blade damage.
Unless machines are being routinely flown above MTOW, it is difficult to explain
how these blades are being overloaded in an upward direction.

On many videos, of MT-03s in particular, the takeoff rotation seems quite aggressive.
Is there a reason for this.? Or is it guys flying-by-numbers?
Could this cause a momentary blade overload, without actually hitting the teeter stops?

Are there enough of the five-bolt sets flying, uncracked, to indicate whether a change of stiffness due to more bolts, has had an undesirable side-effect.?

Finally, is there any follow-on research, after the MPD, to analyse the cracks for signs
of fatigue versus overload?

[C. Beaty]

Here is how the load shifts along the span of a rotor blade with rotation.

This view is for a helicopter; a gyro has the opposite load distribution.

As the load shifts along the span, it produces continuous flexing, the reason Bensen as well as helicopter manufacturers used segmented skins on metal blades.

Illustration from: “Aerodynamics of the Helicopter” by Gessow and Meyers.

[hillberg]

Bell Helicopters (Uh 1, 206. and 47s) has doublers (Fingers)on the blades to transfer bending loads to the grips. Would the approch to this problem be a doubler to take and spred out the bending (Flaping) forces ?

[StanFoster]

I had to fabricate 16 doubler plates for my Helicycle blades. 4 on the top and bottom of each blade. Out of these 16 doublers, there were 4 sizes , and I made 4 of each size. These all had to be shaped to the airfoil and each one was bonded to the one below it. They were progressively smaller as they were bonded. As mentioned, they help spread the load at the root of the blades. When rotor blades are new, you can go up and shake one tip and excite the natural frequency of the blades. They should vibrate in unison at the tips, both going up and down in time, and take awhile to subside. Then you can double excite the blades and these vibrations should end quickly. These two tests are supposed to show if you are getting some fatigued blades after lots of hours on them. Stan

[PTKay]

Are there enough of the five-bolt sets flying, uncracked, to indicate whether a change of stiffness due to more bolts, has had an undesirable side-effect.?



All Xenon rotorheads have 5 bolts, using originally Aircopter,
and now Aircopter licence blades.

Never heard about any cracks.

[steveb]

Guys

From the Rotorsport UK Service Bulletin:

"Using the x5 magnifier, carefully visually inspect the area of the outboard bolt hole on both the underside and
top-side of the rotor-blade. On Aircopter blades (these have a flat outer end cap) any crack is likely to be
around 10mm outboard of the hole, or at the hole. On AutoGyro blades (rounded outer end cap) any crack is
likely to be across the outboard bolt hole itself. Obviously, no cracks whatsoever are permitted in any area of
the blade."

The first cracks were found on Aircopter blades and were approx 10mm outboard of the final bolt hole. The cracks on Auto Gyro blades originated at the bolt hole.

[PTKay]

The first cracks were found on Aircopter blades and were approx 10mm outboard of the final bolt hole. The cracks on Auto Gyro blades originated at the bolt hole.

Steve,

when you mentioned "further down the blade" I thought about some
significant distance.
10 mm is nothing, it is still in the influence radius of the last hole
and (probably) under the hub bar plate.

So it is probably exactly at the edge of hub bar, so at the
"knick point" of the hubbar-blade beam.

[C. Beaty]

The problem seems to be too much flapwise stiffness at the root fittings. Scrambling bolt holes probably is not the answer.

A softer hub, perhaps filament wound fiberglass or better still, Robinson R-22 style coning hinges would be the better approach when using extruded rotor blades.

I’ve used coning hinges on a collective pitch hub and know from observation that motion at coning hinges is constant. Plain metal-on-metal coning bearings will gall and seize in short order. I had to resort to needle bearings.

[ckurz7000]

I'm hunting down some more information on this and maybe able to shed more light on the question in a couple of days. The notions of PTKay about the Xenon seem a bit too simple and quick to me, that's why I am going to do a little bit of research in that direction, too.

-- Chris.

[ckurz7000]

As promised, I’m reporting back with some more information about the cracks in rotor blades used on AutoGyro gyros. I got my information from a long and candid personal telephone conversation with Otmar Birkner, the owner, designer and chief test pilot of the company. After that conversation, Otmar emailed me a report hott off the press (dated 12/21/10), which details a scientific and thorough investiagtion in cooperation with Cranfield University and the British CAA.

Otmar reports that they, of course, took the problem very seriously and immediately launched an extensive investigation into possible causes and fixes. A metallurgical analysis of the cracks was commissioned, stress calculations of the blades and hub were performed, and extensive test flights werde done. In those test flights strain on several points on the blades top and bottom surfaces, rotor rpm and g-loading as the main parameters were recorded in a fine-grain temporal data stream. On the basis of these observations, let me try to separate fact from fiction:

Fact:

The result in a nutshell is, that these cracks are fatigue cracks due to high flapwise bending moments just beyond the end of the rotor hub. This doesn’t occur if the gyro is flown within the allowed limits set forth in the POH (max. 60° bank angle, no aerobatic maneuvers). But the MTOSport as well as the Calidus are very agile gyros that invite more extreme flight attitudes. Local stress limits were exceeded in a particular maneuver (without going into too much detail for obvious reasons) in which the rotor was first partially unloaded and then, very abruptly, loaded to about 2.5 G. At that point the rotor rpm takes a short time to spool up to the appropriate speed. During this time, blade coning significantly exceeds the pre-cone angle of the rotor hub and therefore result in high localized bending moments. Many such flights are necessary to result in an observable crack, so the blade doesn’t simply tear off when you do this. The metallurgical report even was able to state an approximate number of how many, which seems to point to heavily chartered gyros as, e.g., in a flight-school and charter setting.


Fiction:

The blade cracks are not in any way linked to the number of bolts used to fasten the blades to the rotor hub. In fact, the number of bolts were required to meet the Section T as well as German design specs for ultralight gyroplanes regarding tensile load limits of the blades. The fasteners used in the AutoGyro rotorsystem are not normal bolts but precision bolts in holes reamed to a very high tolerance.

It is apparently also fiction, that this has anything to do with a modified Aircopter blade or hub. It seems that the same flight maneuver flown repeatedly in any gyro with a similar rotor/hub system (and I think there are many!) would result in similar bending loads and ultimately cracks. But large side-by-side gyros such as the Xenon are not particularly amenable to such maneuvers.

Conclusion:

A reduction of bending stresses can be achieved by increasing the pre-cone angle of the rotor hub as well as by tapering the hub stiffness so that it becomes more flexible toward the end. AutoGyro has already done extensive flight testing on a modified rotor hub (larger pre-cone as well as a tapered and, toward the end, more flexible hub bar) in which even such radical maneuvers will not result in excessive loads.

Furthermore, according to Otmar, the MT03/MTOSport currently constitute the by far largest fleet of gyros from any single manufacturer with the largest number of hours worldwide. He is concerned that this problem is lurking in other commercial gyro rotor systems and will rear its head if the flight envelope is extended and sufficient hours are accumulated in the fleet.

In summary, the report -- to me -- seems well written und thorough, with original data presented to make any conclusions transparent. Negating the the possibility that such cracks may eventually also appear in other rotors is, in my opionion, just putting the head in the sand. No amount of proclamation, and rhetorics can change the physical facts. Only rigorous flight testing and in-flight measurements of local strain, rotor rpm and g-loading will tell the true story. I, for my part, would only accept such measurements as sufficient proof that any particular design (especially with 0° pre-cone angle) is immune to this problem.

Greetings, -- Chris.

[Doug Riley]

Sport Copter reported similar cracking in the McCutchen Skywheels carbon-fiber hub of the blades that Jim Vanek used for loops. The loop entry likely involves a loading pattern resembling the one described in the report -- an abrupt increase in rotor disk angle of attack, which, in turn increases the individual blades' angles of attack and lift. There is no such thing as an instantaneous increase in RRPM, so the blades cone more in response to increased lift.

To some extent, this temporary increase in coning angle and bending load happens with every landing flare.

[ckurz7000]

To some extent, this temporary increase in coning angle and bending load happens with every landing flare.

Yes. Theoretically and in pricipal it happens whenever you change the loading of the rotor, but to a much, much smaller degree. In the flight tests performed for the report it was also tested for spikes in g-loading when doing landings on surfaces of different nature (grass vs. asphalt). These apparently were insufficient to cause the observed cracks. Hard landing flares as well as flight in heavy turbulence was also simulated and found to be not satisfactory in explaining the cracks.

What precipitates these excessive moments is a violent maneuver involving an unloading of the rotor and bank angles near 90°. The unloading of the rotor tends to slow it down before you hit it hard with 2.5g, which exasperates the situation even more. It certainly looks really impressive. And, I admit, I like to do it, too, But I have no cracks in my rotor. It takes a lot of repetitions to accumulate. And now I'll wait for the new hub before doing it again :)

On another note, the presented data may also be used to settle an old question: "How many g can you pull in your gyro?" You don't get much over 2.5g.

-- Chris.

[jeff b]

Thanks Chris. Some excellent information to consider.

Regards,
Jeff.

[EI-GYRO]

Thanks for the feedback, Chris.

Any idea will the report be published in full?

I would be interested to read how they investigated this.

[DennisFetters]

Something I posted about extruded rotorblades back on 12-24-2007, 12:27 PM, and you can read the complete discussion at: <http://www.rotaryforum.com/forum/showthread.php?t=15292&highlight=extruded>:

I know this is going to start a big argument, so I will say now that after I give this information, I will opt out of any flaming war that starts.
Hmmmmm.
Why would you think that Dennis?
Coz sumone dosnt agree with wot you said?
Well, i dont agree, but that dont mean its gota start a " flaming war " dose it?

Very well David,
I'll answer your question;
Because of the sarcastic argumentative nature you have had in the past with answering my posts in an insulting way. But, let's assume that will not happen this time and we can carry on a civil conversation, which I would like very much.

Secondly, because I have already expressed my views on the subject, and it turned into a argument with the person I was talking to. I only expect it to again, but I apologize for my preemptive negativism. I would like to be wrong.

When someone disagrees with me is no problem. I will respect someones opinion, if they have something to base it on. I may not agree with them if I also have a good bases for my belief, but it rarely effects what I'm doing if someone disagrees with me, so no skin off my teeth. On the other hand, I learn something new everyday and I'm open to new ways of doing something.

I respect that you don't agree with me about extruded blades. But, I do have more experience with them, because not only do I fly them, but I manufacture them, and I see the imperfections that are regular in the extrusion process. I can't count how many feet must be discarded to find usable blades, and that is for a UAE helicopter where I limited the blade life to 250 hours. Imagine the scrutiny and discarded material necessary to provide a man carrying blade. And do the producers of the extruded blades provide this amount of scrunchy? I bet not.

and they are too heavy.
How heavy is too heavy? and why?.

I am not trying to be argumentative with you, but let me explain my ideology in the matter;

Every pound an aircraft weighs needs to be lifted into the air, which effects performance. If the rotor system is 30 or even 60 pounds heaver than they could be, then that is performance lost. It is mine, and most aircraft designers primary goal to make the aircraft as light as possible. The extruded blades are the heaviest way to go, and perform less than the composite blades they are copied after.

And as for the longevity, i know of a few sets with thousands of hours without a hint of crackn, and im not talkn bout your garden variety weekend warrier type flyer either.

Again, I am not trying to be argumentative with you, but let me explain my ideology in this matter too;

It is a simple fact that aluminum has only so many cycles of bending moments until it fails. Rotorblades are subjected to a harsh environment of bending, twisting and stretching, and are prime candidates for stress cracks. Lucky, gyroplanes are less severe on blades than helicopters, but they still strain the aluminum.

Rolled and formed sheet aluminum has higher qualities over extruded materials in many ways. It is pound for pound stiffer and stronger, and is able to perform more bending cycles, making it better for a blade skin material. The extruded aluminum is the worse for surviving bending cycles, unless it's thick like a spar.

As I said before, a rotor blade needs to be a composited structure, no matter the material, so to better survive the possibilities of overall failure from one member failing. An extruded blade will not offer that possibility, and if a stress crack forms, it can propagate and fail even during a single short flight, giving the pilot no opportunity to find it during preflight.

I understand that a few extruded blades may have 1000 hours on them. I have never seen an extruded blade with that many hours, but I have no reason to disbelieve you, so let's say a few do. If you add those few blades in with all the other extruded blades as a fleet time, it is a small fraction of the time the gyro industry has of composited metal blades over the last 50 years, and there are many instances where stress cracking occurred on composited metal blades.

It is just not an excepted policy in the rotorcraft field to use uncomposited blades for the reasons I mentioned above. Just because it has not yet failed does not mean it will not, and just because it has not failed does not mean that standard engineering principles of metal fatigue should be ignored.

[Papa Smurf]

Thanks Chris. I think they got it right....

[Arco]

Chris,
Could you tell us which the critical maneuver is? The one that creates bending effort at the root of the blade.
To know the maneuver is beneficial for all of us, because no matter which rotor blade type we have, at the end we are stressing them flying in that way. If we can avoid that maneuver we are flying safely.

[ckurz7000]

I don't want to post the maneuver in any kind of explicit detail here because there will always be some people who take this as an invitation to flirt with desaster.

In general terms, it is a maneuver in which the rotor is first partially unloaded followed by a very quick and extreme control input in more than one axis which results in near 90° bank angles for some amount of time.

The stressy part as far as rotor blades are concerned is the sudden change in g loading going from about 0.8 to 2.0 in too short a time for the rotor rpm to keep pace. Rotor rpm was found to vary between 390 (steady state before the maneuver) to 450 (rotor steady state in the steeply banked part). The rotor lags 1.5 seconds behind the g loading trace. That's what causes the higher than normal conus angle.

This was also confirmed by direct measurements with strain gauges placed on several locations on either side of the blade, which really takes all the guess work out.

-- Chris.

[C. Beaty]

Aluminum, curve 7 on the chart, isn’t good stuff for cyclical loading. The cycles on a rotor spinning 350-400 rpm accumulate quickly.

The only solution is to keep flapwise root stress levels low by using either a flexible hub or coning hinges.

Chart from Marks’ Handbook for Mechanical Engineers.

[Xavier AVERSO]

porte pales trop rigide.

hub too rigid.

[PTKay]

porte pales trop rigide.

hub too rigid.

This is the best comment in the whole thread.

Three words, hit the nail on the head...

:)

[ckurz7000]

This is the best comment in the whole thread.

Three words, hit the nail on the head...

:)

Sure, the hub's too stiff. But it it seems to me that at least 11 posts ago I said exactly this:

A reduction of bending stresses can be achieved by increasing the pre-cone angle of the rotor hub as well as by tapering the hub stiffness so that it becomes more flexible toward the end.

But don't forget the correct pre-cone of the hub bar either.

And now there's some hard data to back it up and quantify "how much" is "too stiff".

In addition I am concerned that other hub bars with zero pre-cone angle might have a problem lurking here. Are they sufficiently flexible so that they don't need ANY pre-cone? Given enough accumulated flight hours across the entire fleet would not a similar problem start to show up?

As I said: I would really like to see strain measurements and hard data before accepting a simple "no problem" as satisfactory answer.

-- Chris.

[EI-GYRO]

Two positive things about this problem;

It has been identified, quantified, investigated, and diagnosed without a single fatality.

It has put paid to the notion that gyros are immune to over-stress.

With more and more powerful and agile machines coming on the market, a bit of caution
is probably appropriate.

[Mark Humbke]

Chris..

"Thanks" ever so much for all your time in researching & explaining the problem that was initially started with this thread. Your approach is one to be admired how you systematically go thru it based on fact.

I'm the proud owner of an MTO Sport in Canada & I'm sure as with other owners of this machine it has set us at ease to know that we can all fly safe within the limits of the machines ability.

There are a lot of us out there as owner's that do not have the resources & are so willing to share them on this forum to set us at ease of what the root of the problem is..

I admire you for this.

Keep up the good work..

[willisbr]

Quick ignoramus question. How does one fly a MT03 in the US these days?

[ckurz7000]

Quick ignoramus question. How does one fly a MT03 in the US these days?

I did it by having a German license and pointing to the MT03's obvious German registration. The fact that nobody even asked me helped a bit.

Seriously, though, you'd have to register it Experimental for exhibition flights. There are some people who have done this before with Xenons, I think.

Could you import it by flying it in yourself across the US/Canadian border?

How about building one yourself as Experimental, using a real (uncertified one) as 1:1 model (and potentially parts supply)?

-- Chris.

[johne]

C Beaty has over many years pointed to the need for either hinges at the hub bar/rotor blade junction, or a flexible hub bar. I seem to recall him commenting years ago that once the rotor was up to speed a piece of rope could be substituted. Some have suggested a composite replacement, even recommending carbon fibre despite its being very stiff:).
When buying our MT03 I asked Otmar what the material was in the hub bar, pointing out to him that while the upper and lower elements gave it the advantage of redundancy they did increase the stiffness. I suggested he would do well to copy my spring steel hub bar. Perhaps he now understands what I was talking about:)

With a spring steel hub bar the rotor self cones for whatever load is applied, instead of having a rigidly enforced coning angle (which is only a calculated compromise anyway). I have watched with alarm hub bar failure problems attacked by thicker more rigid hub bars, some even being silly enough to use a straight hub bar with the coning angle provided by wedges or chamfering the ends of the hub bar instead of confining the safer bend to the centre of the hub bar.

As C.Beaty points out the favoured hub bar material is really quite unsuitable due to its fatigue characteristics. Things probably weren't so bad with single seat machines where the only cyclic loads resulted from flight maneuvers, whereas 2 seaters can be flown 1 or 2 up with similar flight maneuvers (unless forbidden by the Flight Manual).

While I only have limited hours on my 2 spring steel hub bars they do seem to solve serious problems. I am working on a third one which will be depitchable.

johne

[C. Beaty]

Automobile leaf springs do have remarkable durability, considering their environment,

Generally plain high carbon steel, never aluminum.

At least one model of the Chevy Corvette used a fiberglass leaf spring.

[Walter]

Johne, are there no electrolytic erosion problems in a steel-aluminum junction to be expected?

[PTKay]

Two positive things about this problem;

It has been identified, quantified, investigated, and diagnosed without a single fatality.

It has put paid to the notion that gyros are immune to over-stress.

With more and more powerful and agile machines coming on the market, a bit of caution
is probably appropriate.

I fully agree with it.

Well done.

[Irishpilot]

Brian/Chris:

You can also fly a factory built MTO Sport under the public use exemption. Our program has a MTO Sport in use and Friday we unloaded a factory built Calidus and MTO Sport to add to the program. We are hoping to get a factory built ELA, Xenon and Magni in our program as well. www.justnet.org/aviation

Fly safe,

Mike

[johne]

No Walter, no corrosion seen - the spring steel was painted with a two part paint and the bolts are plated - zinc I think.

If you want to see a fine example of corrosion between bare steel and aluminium (aluminum) you need look no further than the substandard unplated 9mm ground shoulder bolts attaching the rotor blades that came with our MT03. I removed them with great difficulty to comply with the Service Bulletin resulting from the recently found cracks. They have now been replaced with plated bolts - at considerable expense to me of course. I would not accept a machine with those bolts, and would condemn any machine that came to me for inspection until they were replaced.

There appear to be some worrying lapses in quality control in what I regard as a very good gyro.

Getting back to the spring steel hub bars, they were criticized for having the upper and lower leaves putting the blade attach bolts in shear (like a guillotine). This is not the case, when the hub bar cones upward above what is set into it the upper leaf is free to buckle slightly, while the lower leaf is now taking all of the centrifugal load its superior tensile strength is well above what is required.

[PTKay]

If you want to see a fine example of corrosion between bare steel and aluminium (aluminum) you need look no further than the substandard unplated 9mm ground shoulder bolts attaching the rotor blades that came with our MT03.

It is obvious, that Fe (iron) to Al (aluminium) galvanic
corrosion is dangerous.

The corrosion cavities on the inside of the bolt hole could
be the stress focus points and start the crack propagation.

[scandtours]

Service Bulletin AGAUS 02/2010 issued by Auto Gyro Australia.

The same did the Swedish LFV (FAA) with bulletins to all concerned.
See link below.

http://www.gyrokopter.se/files/101008/adaustralien.pdf




.

[johne]

Yes PT Kay, that rust on the ground shanks of those bolts was jamming those bolts in the hub bar holes, I was worried I'd twist the heads of as I loosened them. Crap engineering if you ask me. If I did something like that I'd be anxious to rectify it and apologise to the customer!

[PTKay]

I remember delivering to a customer a stainless steel (816) cabinet,
where one of my suppliers decided to fit a corrosion resistant socket
to connect a corrosion resistant waterproof plug.

Unfortunately he saved the money and socket and plug were some
soft metal chrome plated.

In the harsh environment of the food industry plant with all the
washing and disinfectants the socket and plug just disintegrated
within few months...

Galvanic corrosion at it's best. :)

Each of this products, the cabinet and the plug, were excellent
when used alone, together, a disaster.

[PTKay]

By another discussion coincidently I saw the original Averso blade cut
probably similar to the AutoGyro copy:

http://www.rotaryforum.com/forum/attachment.php?attachmentid=84896&d=1316286809

and the original recent Xenon (Celier Aviation) blades:

http://www.rotaryforum.com/forum/attachment.php?attachmentid=84889&d=1316265375

Mind the difference in the upper surface wall thickness exactly
at the position, where the hub bar fixing holes are drilled
and where the cracks in the AutoGyro rotor blades occurred.

http://www.gyroplanepassion.com/images/RotorBlade.JPG

Food for thought.

[PTKay]

porte pales trop rigide.

hub too rigid.

Or, maybe, looking on the images above,
the blades too flexible for the given hub?

[rotormaster]

Hi Kay,

than this profile my belongs to the Averso Blades, Not aware of any Cracks on Averso Blades at the Past, but a lot on MTO,s The MTO Blade is differed to the Pictures shown.

Steven

[PTKay]

Hi Kay,

than this profile my belongs to the Averso Blades, Not aware of any Cracks on Averso Blades at the Past, but a lot on MTO,s The MTO Blade is differed to the Pictures shown.

Steven

Do you have any picture of the AutoGyro GmbH blades section?

Would be interesting to compare.

[rotormaster]

Yes I have, not the best Picture, hope it helps

Steven

[Xavier AVERSO]

PTkay,mensonge les profils montrés ne sont pas de la pale Averso mais Aircopter monté chez MTO et Celier .
Jamais de fissure sur pale Averso.

PTkay, lie the shown profiles are not blade pitch Averso but Aircopter assembled to MTO and Celier.
Never of crack on Averso blade.

[PTKay]

PTkay,mensonge les profils montrés ne sont pas de la pale Averso mais Aircopter monté chez MTO et Celier .
Jamais de fissure sur pale Averso.

PTkay, lie the shown profiles are not blade pitch Averso but Aircopter assembled to MTO and Celier.
Never of crack on Averso blade.

Dear Xavier,

sorry for the mistake.
They are certainly AirCopter.

I found only this image on your web page:

http://www.averso.info/wpimages/wp4f93cb29_05.jpg

It looks very similar to AirCopter.

Could you publish a better picture
of the section of your blade.


Regards

Paul

[PTKay]

Yes I have, not the best Picture, hope it helps


Steven, thanks for the picture.

My comment comparing AirCopter and Celier/Xenon are valid
comparing MTO and Celier/Xenon.

The wall thickness at the critical bolt holes position is
significantly (almost 3x) thicker by Xenon.

http://www.rotaryforum.com/forum/attachment.php?attachmentid=84848&d=1316094136

[Xavier AVERSO]

Profil Blades AVERSO Stella


http://img101.imageshack.us/img101/2286/profilstella.jpg (http://imageshack.us/photo/my-images/101/profilstella.jpg/) Uploaded with ImageShack.us (http://imageshack.us)

[PTKay]

Profil Blades AVERSO Stella

Thanks a lot.

[Not Yet]

For compairson, the vortech blades now sold by Phoenix Rotocraft, LLC, look like this.

[Xavier AVERSO]

Rotors AVERSO

http://img683.imageshack.us/img683/5290/img1673dr.jpg (http://imageshack.us/photo/my-images/683/img1673dr.jpg/)