[NSRCA-discussion] Outrunners and structural failures

[Chris Moon]

Several of us have been collecting info on the many (upwards of a dozen) 
fuse structural failures with people running the larger outrunners.  The 
common theme is that they are all front mounted to the nose ring without 
any support at the rear of the motor.  There have been some who have 
mounted their motors this way that have not had failures, but all that 
have failed were nose ring mounted without support for the back. Except 
for one, and that looks like it was a problem with the fuse 
manufacturer.  In that case (today) the seam split due to a poorly glued 
seam without good adhesion by the fiberglass seam tape.  The problem 
seems to be explained by a phenomenon called "whirl flutter" and 
basically is caused by an outside force causing the prop and motor to 
oscillate to the point where the structure will fail.  Here is a video 
of the phenomenon:

http://www.airspacemag.com/ASM/Web/Site/QT/PWFlutter.html

(Thanks to Jerry Budd for the research and video link)

So, if you are planning to use the big outrunners like the Axi or Hacker 
A60, the evidence is showing that a rear support of some kind is 
necessary to prevent failure.  I had 2 failures with my A60 set up until 
I added a rear support.  My first failure was on the 2nd flight and the 
2nd failure was on the 15th flight (at the NATS).I now have almost 20 
flight with the rear support and everything seems fine now.  Of course 
you can also mount the Axi to a firewall and we have not heard of any 
failures with that set up.  I spoke with Jerry Budd and he is planning 
to be making a rear support available for the outrunners similar to the 
one he now makes for the Hacker C50. Please if you are planning on using 
a front mount outrunner consider the rear support before you get too far 
along.  I was going to write a KFactor article on this, but it seems 
like a better idea to get this out more quickly via the mail list.



Chris Moon
D5 VP