[NSRCA-discussion] Outrunners and structural failures

[Fred Huber]

I may be a bit wierd in how I did it... but it works. (Its also a somewhat 
smaller AXi than most of you will be using for Pattern)

Stuck my AXi 4120/18 on using 2 sets of glow engine mounts.  The bolt holes 
of the AXi fit the firewall holes on one mount set perfectly, and that had 
the shaft parallel to the bans, centered between em (pointed the wrong way) 
Put the other mount in as if installing a glow engine...  bolted the beams 
top to top...  Very secure mounting. (and very cheap using the ARF 
fiberglass mounts and a Du Bro fiberglass mount)

What I didn't do though was lighten the fuselage because of going electric 
instead of glow... and I avoided cutting the fuselage to install the 
batteries.
Lightening the structure and cutting things need to be VERY carefuly 
considered.  The original structure holds up under the glow engine you are 
replacing.  The prop is making as much (or more) power.....  Can you REALLY 
afford to take out structure?

Heres something I have also seen... the standoffs sometimes used in 
outrunner mounting (because the outrunner is short and the firewall tends to 
be back far enough for the glow engine...) just LOOK like they'd allow some 
flex.  The aluminum "X" that came with my AXi is meant to be bolted flat to 
a firewall... not to resist flex when bolted to a set of four 2 inch studs. 
Wood can give, and with the small standoff "footprint" you have a formula 
for the motor to move...  Those aluminum tube over bolt standoff mounts are 
EASY.,.. but are they right?

The flat square outline back of the mount setup I used cant rock on the 
firewall.  The bent and bolted together aluminum  or steel sheet (not sure 
which they are made of... I've just seen pictures of them) has a much more 
stable appearance.

Mounting the motor to the nose ring in the cowl?  Is that cowl meant to take 
the torque and other stresses of applying the motor power there?  Can it 
resist flexing under the loads?  Most are not designed for the loads. 
Torsional bracing frames need to be added.

Sometimes appearance is worth paying attention to...  If a structure looks 
unstable, it might be unstable.  Take the motor in hand and give it a bit of 
a twist (estimate the prop torque... apply it to where the moltor bolts 
on.)... if it flexes stuff in front of the firewall it needs more structure.

Food for thought.

****************
Re: my Quest with RF issues
I've been thinking of trying: stretch the firewall forward and get the 
batteries in the "extended fuel tank box" that would result.  Adding wood 
weight, but removing the weight of the doubled nylon mounts, fewer bolts. 
Might come out even. (and can be done to this model relatively easilly) 
Would let me get rid of a loop of wire that may be causing RF issues too... 
(most important reason to do it)  My battery access would still require cowl 
removal (bad) but the cowl should slide on and off more easilly.

Nothing tried so far has worked to get it to pass another range check... 
removed the steel pull-pulls last night.


----- Original Message ----- 
From: "Chris Moon" <cjm767driver at hotmail.com>
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Sent: Saturday, August 12, 2006 10:37 PM
Subject: [NSRCA-discussion] Outrunners and structural failures


> 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
>
> _______________________________________________
> NSRCA-discussion mailing list
> NSRCA-discussion at lists.nsrca.org
> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>
>
>
> -- 
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.405 / Virus Database: 268.10.9/417 - Release Date: 8/11/2006
>