Microlighters

Scary stuff!

I have seen the same from a landing on the tail wheel of a sycamore but this happened in flight in turbulence.

Thoughts?

Eben sent me the same photo some time ago. He mentioned how the mast flexes in turbulence, and can result in the prop touching the keel in severe turbulence! Not good.

Has anyone else had such an experience?

I would be more concerned over the mast moving and causing fatigue than losing a prop.

Has anyone else had such an experience?

Glenn I recall reading about one or two similar reports on some of the overseas rotary wing forum - I think most were attributed to hard landings / turbulence etc.

Have heard of, (in this type of configuration) fatigue in the Diagonal support beam or its welds which is attached to the mast and the keel beam. This potentially allows flex outside of tolorence which could result in what you are experiencing. Not the easiest of checks but well worth it...

Mast flexing that much? That would be of grave concern.
I suggest you also check your engine mountings carefully, especially the lower two. If they are getting worn or they have play, then the entire engine can move resulting in exactly the same situation and allowing the prop to touch. Remember the thrust line and that all the weight (and drag) is being 'pushed' so those mountings take an incredible strain.

A student of mine pulled the nose up so sharply on takeoff in an MT0S that the tail dragged along the ground hard. (of course he made me sit bolt upright and slam the nose forward) No prop marks on the boom after that so it must take a lot for it to happen!

If your engine mounts aren't perfectly round with the bolts situated exactly in the middle then my call would be thats the problem! Replace them. Cheaper than a new prop.

Len

Same problem on Magnis, if your engine mounts are worn or weak, the prop will be much closer to the keel, and it can be seen without the engine running.
The best would be for each manufacturer to come up with a tolerance gap that can be checked at regular intervals. Statically should be good enough.

Len

Do you know what the prop clearance on the MT03 is?

I'll measure next time I'm out there. I would have thought the prop clearance on my MT-03 is between 2-3".

I'm not near an MT at the moment (Addis is far from CT) and can't say conclusively so if Glenn measures, we'll all know.
Thanks Glenn.

I think its at least two fingers (vertically) so at least 1.5 to 3 inches yes. Because of the 'arm' a millimeter or two sag in the mountings will result in a large difference/change in the gap.

Len

Found this pic. Looks like quite a distance to the keel.

I doubt that the keel is bending as there is a serious brace there, so it must be the mast.

The engine mountings would have to be very loose in order to get that amount of travel.

So Len what beer are they serving up there?

Here is a pic with the prop in the correct position. It looks like about three inches.

Flying-i

The local beer in Addis (if that is what you are referring to) is St Georges and it is actually very good!
Now swaer.... The picture you posted clearly doesn't have the lower prop blade in the center (the top two are offset) and therefore is misleading. (What were you drinking? ) PeterB's picture on the other hand has the blade centered and yes, I agree, it is a long way down to the keel which would mean the engine mounts would have had to have just-about collapsed. I concede.

I certainly hope its not the mast bending that caused this prop-strike! That would be a serious concern.
Perhaps someone in the know with regard to the initial photo can enlighten us?

Len

Another beer on you Len! Apparently it is mast bending. By the way, I would say the prop clearance is about 2-3 inches at least.

I'm no metallurgist, and this surprises me as well!

Anybody out there tell me if I am talking kak, but this is the way I understand it (Eben was the one that told me this):
Apparently in heavy turbulence, the increased rotor drag can pull the whole disc back. This causes an increases tension in the control rods of a gyro. This can cause the quite un-nerving experience of feeling a very heavy fore and aft, violent stick shake on the control column. I have experienced this on the older VPM models, and owners of Magni models tell me they have experienced it as well. It is actually a very scary feeling! It feels like the gyro wants to shake itself to pieces! Thank goodness, I haven't experienced this so far in my MT-03.

Because the MT-03 experiences this mast flexing, it prevents the horrific experience of the violent stick shake, but the disadvantage is that the prop and keel can momentarily touch. I've flown in some pretty heavy turbulence thus far, and can say that I have the smallest of small nicks on the prop and keel.

I also agree this is not great, but thus far it hasn't been a problem. Will try and make contact with the agent, and the manufacturers in Germany for further information on this matter.

Ok, but what you are describing is mast-flexing, as is built into the RAF2000, not bending.
If it's designed to do that fine, if not, it would worry me.

As a matter of interest, I have experienced the violent stick shake in a Magni M16 you describe. I was back seat with a student and he had control. As it happened I grabbed the stick, shouted that I had control, it stopped immediately and and I asked him what he did... he calmly said that he thought I had control and had done nothing. The stick hit him quite hard against the palm of his hand - it seems his hand was just hovering over the stick, not holding it. We were only doing about 80 in calm conditions. Tried to both let go again and re-create the situation but couldn't. Eric Torr (I think) told me it happened to him also and he was convinced that his passenger had reached back, grabbed the control rods and shook them to scare him. Seems not. Eric's suggestion was to tighten the lateral axis swivel bolt (right under the head) a bit, according to him this can happen if there is too much play. [Ek weetie, ek beskou myself nie as tegnies bevoeg nie]

I'd be happy to buy you guys a few beers in any event!

Len