Rotorblade-cracks, MT03, MTOsport, Calidus

[OzGyro]

Hi All
Just found this on the US forum, thought it may be of interest to a few of you.

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


Cheers
Darren

[MPL Pilot]

Interesting

[FO Gyro]

I made contact regarding this issue with the UK agent, and his reply helped to shed some more light on the topic:

"We have, in the UK, one set of cracked and bent blades. We have checked 47~50 of the remaining UK fleet without any cracks or bend being found. We are now certain that the cause of the crack was a one-time instance where the blades were overloaded in flight, by reducing rotor rpm, and then imposing a large 'G' load on the rotor. This then distorts the ratio of centrifical load to bending load, and may result in a bend - and a resulting crack. The crack is a very slow propagation crack, possibly taking many hundreds of flights to make any great length (assessed using a scanning electron microscope), and the flight case that creates the load is well outside the operating envelope of most pilots - and of the handbook. As far as I am aware, two others have been found on AutoGyro rotors sets in Germany, from pilots who are well known to fly the aircraft beyond normal limits.
So there appears to be no cause for any concern for aircraft operated in the normal manner. We will finish the incredibly in-depth analysis with our CAA over the next few weeks, which has also involved monitoring blade loads in flight under a range of manoeuvres"

So, hopefully no cause for concern just yet. He also mentioned that the gyro in question was a 2006 model MT-03 with 850 hrs on it, and also that the gyro was being operated beyond it's normal flight envelope.

[MPL Pilot]

I certainly am not an expert, but have seen through the years in the engineering industry what stress does to many materials, especialy if it was a poor design for a particular application.
Having a look at coning angle's on the following aircraft during flight, all the coning angles look simular.
The photos might be deceiving, but the reality is, the advancing blade "bend" upwards and retreating blade has to give way, because of the rotor head design.

At wagtail, Johan has a gyro with well over 3000 hours, my conclusion is, have the rotors checked for safety reasons. What happened to the MT03 could be an isolated case. The aluminum manufactures could be to blame.

Just a thought, but time will teach us.
Ons sal seker nog hoor van die manne met die dik brille!

[Gyronaut]

Technically, this is not an MT03 unique potential problem. The MT's use Aircopter Rotor Blades and heads as do many other machines - like the Xenon for example. I am aware that Xenon has manufacturing rights on the Aircopter Rotor and heads so it appears theirs are not manufactured by Aircopter per se.

I would suggest a thorough inspection of all Aircopter blades, and not merely grounding one type, should be required by CAA in the interest of safety.

Len

[mak]

Len

The Xenons are grounded at the moment until their rotors have been inspected and signed off. I hope to be back in the air on Thursday.

[Gyronaut]

Aah, I didn't know that Mak, glad to hear it actually. Not for any other reason than the promotion of safety around these awesome machines. Our safety record is improving in leaps and bounds. Lets keep it that way.

[Vertical Tango]

Mine and Alan's have just been inspected and there is no problem.
However I have an issue with the way this problem is handled. I am going to be shot in flames by many for saying this but I'll face the music.
It is known and accepted by CAA that the aircraft that endured cracks was flown outside its enveloppe. I feel that CAA should not have grounded any gyro BUT recommended at next service to have a more detailed inspection in that area, for simple peace of mind. If gyro owners are uncomfortable with the wait, then it is their prerogative to inspect it sooner. If your aircraft is flown within the normal parameters, then it should not be any problem.
Following CAA's logic, are they going to ground all gyros when I'll have a hard landing and crack my undercarriage ? After all, it was flown also outside its enveloppe !

[Gyronaut]

You have a good point VT; however, also consider this perhaps?
If the crack problem, which was known to CAA to exist in a machine that was flown outside its envelope, developed in another prior to the 'next' inspection and resulted in an accident - they would be roasted for doing nothing. Perhaps they erred on the safe side - can't fault them for that.

I confess I am in the dark about the concept of "outside its envelope". Would someone please enlighten me what this means? Was it flown faster than its VNE? Was it overloaded? As far as I am aware those are the only two limits imposed by the manufacturers. Nowhere in any documentation have I seen G limits specified. I may be totally wrong here and would be the first to admit it but I have always been of the opinion that New Age Gyro's are engineered to stand more G's than they are physically able to execute. Am I wrong? For all I know I MAY be unknowingly flying "outside the envelope" ??

[Learjet]

The MT-03's and Xenons are great gyros which continue to contribute to the renaissance being brought about by a new era of modern, safe and stable gyros and I'm sure that most will agree that despite the inconvenience of being temporarily grounded, the pro-active safety and peace of mind in having the rotors checked far outweighs the consequences (however unlikely or remote) of a potential rotorblade crack .

[FO Gyro]

Gyronaut,

When I emailed the UK MT agent, he said that they had spent a fortune on trying to find out the exact cause of the cracked rotor. He said it was definitely caused by excessive bending of the rotor, and not through fatigue. Here's a copy from his email to me, " ...We've spent a fortune strain gauging rotors, stress assessments using Nastrans, datalogging G loads versus rotor rpm, engineering evaluations, etc, and the final result is that the blades were bent in flight - which then probably generated the fretting location for a crack to start. The bend is caused by an overload from a manoeuvre beyond the limits. And the crack has taken around 700 ground-air-ground cycle to get to the point of being found, so very, very slow propagation."

The guy that flew that particular gyro in the UK, apparently is known to fly rather radical manoeuvres. Gerry described the manoeuvre as follows, "... nose up, to the top of the 'hill', slow down everything, rpm decays by 10%, drop it one side or the other, rpm continues to decay as it is suddenly in freefall. Then pull nose up to level flight, hard. Watch the rotor accelerate back past normal rpm, approx +10% depending on the fall, which then slows back down to normal. Then think of the bending loads exerted.."

This guy was trying to pull hard "g" when the rotors were partially unloaded. Not sure why anyone would want to do such a strange manoeuvre.

[Gyronaut]

"... nose up, to the top of the 'hill', slow down everything, rpm decays by 10%, drop it one side or the other, rpm continues to decay as it is suddenly in freefall. Then pull nose up to level flight, hard. Watch the rotor accelerate back past normal rpm, approx +10% depending on the fall, which then slows back down to normal. Then think of the bending loads exerted.."

Thanks Glenn

Ok, I think I get it. In helicopter terms it would be a Torque Turn? In a gyro they are advanced maneuvers and can be executed safely, but beware! The speed can run away very quickly if you are not very careful. Correcting this excessive speed with stick hard back will cause a momentary high G loading.

As a matter of interest, the MGL Enigma I have in my machine has a G-load indicator. During a recent flight over the Karoo I hit such a sudden strong updraft that it read 1.8G Jonathan was flying just behind me and said I rocketed up into the sky. I didn't notice the rotor RPM but I guess this would be about the same effect on the machine as the maneuver described by Gerry? Severe turbulence will do the same, especially a strong downdraft followed by a strong updraft. I have always assumed the rotors and heads are over engineered to withstand many times more than the 3G max a gyro is able to pull.

I would be very interested to hear what an expert like Johan von Ludwig has to say on this topic.

[FO Gyro]

Len,

I think the main difference between what Gerry is describing, and your examples of very strong updraughts, and that in Gerry's case, the rotor RPM would be very low to begin with. The rotors obviously have less rigidity and are able to bend more. When hitting a very strong updraught when in straight and level cruise, at least the rotor RPM is normal as the gyro hits the updraught. The rigidity of the rotor should be sufficient to prevent any excessive bending of the rotor.

I haven't really paid too much attention to rotor RPM when hitting a downdraught however, but my gut feeling is that it is more easy to get oneself into a unloaded rotor situation through your own inputs in smoooth air, than by simply a strong downdraught. What I'm trying to say is that I think this problem is more to do with incorrect inputs from the pilot, than factors from the environment eg. up or downdraughts that could cause the excessive bending.

To put some numbers to it, when pulling up like in Gerry's example, and then easing the gyro "over the top", I've maybe seen my rotor RPM go from 360 RPM to maybe 320RPM. If one further rolled the machine, and allowed it to freefall some more, the rotor RPM might drop to 300RPM. In heavy turbulence however, I've maybe seen rotor drop to 340RPM briefly, but it never goes much lower. I would think that once rotor RPM is allowed to drop to less than 300RPM (just a guess) through going weightless or -tive "g", that would be taking the machine to the edge of the envelope.

I think us gyro pilots don't really take much notice of the rotor RPM, unlike the helicopter bunch, because we don't really have to. It might be worth taking more note of it, particularly when in heavy turbulence.

[MAGNIficent]

I think the fact that this crack was picked up, is a blessing in disguise.
Imagine what the consequences could have been, if the crack was ignored and the Rotor blade came off.

I do not think the Pilot mentioned by FO Gyro is the only pilot in the world that flew out of the envelope, there will always be pilots pushing the envelope.
Lets us see this as a wake up call, fly safe, do proper pre-flight inspections and work on further development.

[Vertical Tango]

For those flying with the Enigma, you have the "black box" a your disposal. I have used it extensively in my first 25 hours to understand better what's cooking and learn about my gyro. The best is to dowload it in a spreadsheet form and you can see everything that is displayed by your instruments one second at a time in your flight. There are few bugs that Rennier does not want to deal with but it is a good help, particularly your rotor RPM.