Topic: warranty Stinks

3 hours.. thats 18 flights... i wish.

Yet another post disappears The gist of the last one was, model engines are not that high revving nowadays, so why the high bearing failures?

3 hours what does that tell you?

Manufacturing fault or possibly.....  User error

I don't see how you can easily quote running hours as there is too many variables and people run/treat there engines differently  

Prop size

Fuel Type

Needle value setting

How well was it run in - if at all

Could you quote reliably how many hours you expect a PC PSU to last?


Alan Ward

That's a bit of an assumption from a stats point of view.  50% of bearings will fail quicker than average.  Does that mean that were all faulty?

Actually 50% of bearings will fail before the *median*, not the average (mean).

PDR

Yet another post disappears The gist of the last one was, model engines are not that high revving nowadays, so why the high bearing failures?

What makes you think bearings wear quicker at high revs? When F3A motors first went to higher-pitched, lower-reving props in the late 80s the bearing life dropped dramatically (often down to half a dozen flights between bearing changes) even though the revs were 30-40% less...

PDR

What makes you think bearings wear quicker at high revs? When F3A motors first went to higher-pitched, lower-reving props in the late 80s the bearing life dropped dramatically (often down to half a dozen flights between bearing changes) even though the revs were 30-40% less...

PDR

Why did that happen? greater load?

Regards - J

lower revs, higher maximum torque and greater cyclic variation and shock loading at bdc?  C'mon PDR spill it. was it better dynamic balance and/or less crank flex at high revs? 

Of course we all balance our props and spinners, and make sure the props  run true don't we?  I can remember running KK and Tornado nylon props where you could see two clear paths being followed by the two blades with the engine running, the mouldings were that bad.  What effect did that have on crank bearings?

The cause was never really nailed down because model engines weren't (and still aren't) "designed" in the engineering sense - material specs and dimensions are done by experiment and rule-of-thumb (the only exception that I know of is the Metkymeyer team race and F3D pylon engines). It's therefore impossible to calculate which parameters were exceeded. Most likely candidate is a resonance of some kind, but it's only a guess.

PDR

"median", "mode" and "mean" are all averages.  It's a common error to refer to the mean as the only average.

You are correct that I should have been more specific.

Stroud disagrees with you - he states that "average" only ever refers to the arithmetic mean, and "mode" (the most frequently occuring value or the peak of the distribution) could never be cofused with the average. But it's a good, rich area for "general ignorance" (in the QI sense) with such statements as "The over-riding objective of the US education system must be to give all our children the right to be above average!"...



PDR

PDR

I'm sure they'll manage it, Pete, and wouldn't it warm your heart as well as mine to know that all the children of America are indeed above average, and that the entire concept of average could be abolished for once and for all.

Dave