Was flying my Max-T around today on the slope, and went into a slight powered dive (slight being knee shakingly fast, as all good dives should be), and managed to shear off one of my folding blades (of a Graupner 6x3" folder).

It didn't shear at the bolt, but instead at the knee bend in the blade.

Um, is this a normal occurance. Did this happen because I exceeded the blade tip speed for this prop due to the (slight) powered dive? or is it just general wear and tear (there are microfractures on the other blade as well)

hmmmm... also found out planes don't fly with an unballanced rotating mess at the front of the plane.

Andrew

Closeup Image.

Looks like a standard fatigue break. Lots of flexing at that point. How old was the prop?
Any hard landings on it?

I think the CAM 6x3 folder (black blades) is probably a bit tougher (that's the one I use), but either should be fine on a standard S400. I'd suspect a manufacturing defect, or (as suggested) you stressed it on landing.

This particular prop is about 6 months old, and has only had about..20 flights. No hard landings at all (every landing has been a perfect ground slide on grass). Has had about 10 flights with my corflute pylon racer, and abother 10 on the Max-T.

Hasn't been left in the sun, purposely mishandled, nothing like that (that I know of).

I am also generally carefull with not slamming on full throttle, but I suspect that most of the stress comes at the lower half of throttle where the prop is slammed out from the folding position to the spnning position. After that, the extra stress of full throttle would work on any fractures started by the startup torque.

hmm...

A

Did the info with the prop specify any max rpm?

I believe some of the folders in the Hobby Lobby catalog do specify limits, but don't know about the Graupner. If they didn't say anything, one of the other causes the guys suggested is probably the cause.

If the other blade shows tiny fractures in the same area, you are probably exceeding the safe operating rpm IMHO. Sure sounds and looks like stress failure to me.

That bend in the prop blade has GOT to generate some severe stress points there.

Sure you wouldn't be better off with a non folder that will take any rpm? Yeah, you break one occaisionally landing but they are a lot cheaper.

There is no doubt that it is a stress fracture. And yes the elbows will concentrate an incredible amount of stress. (I should model this)

I am still unsure if it's caused by exceeding tip speed on this one occasion OR cumulative from rapid prop startup. Maybe people should check their props for this?

No max tip speed parameters were given with the instructions (Graupner do provide tip speed info for other props but unfortunately not for this one. Also the instructions have errors in them... they say in the safety info the prop should only be used for a Speed 500 motor in the english section, but every other language sais 400.. oh well)

Can't go to a standard prop on this as it needs to fold for high speed dives (this is a glider rather than a s400 pylon racer, however I have a gunther prop on it at the moment as a replacement). I have had so many problems with this graupner prop (and ESC combo.. schultze 24BE) in that it wouldn't fold properly (brake and flying style problem, my fault) or fold at all (until I modified the prop it'self at the anchor point, graupners fault).

So lessons learnt, on a mini hotliner don't do a powered dive (I think I must have been doing about 140-150KPh, but it is hard to tell), and be carefull with the startup throttle not to slam it on too fast.

Solution, keep the spinner hub, get some black CAM blades.

A