[NSRCA-discussion] Outrunners and structural failures

[Jay Marshall]

I had a similar problem with a much smaller motor. The motor and gearbox
were mounted on a pylon which was in turn mounted to the firewall. There was
a hole in the firewall through which the rear of the motor could enter into
the fuse.

At certain motor/prop speeds there was a severe vibration which could have
destroyed something if allow to continue. The solution was to place 3 globs
of silicone sealant between the edges of the hole in the firewall and the
motor. This didn't impede much airflow into the battery compartment but
provided enough damping to quell the vibration.

People may also want to play some with Sorbothane, a great
damping/shock/vibration absorbing material. You can drop an egg from the
ceiling on a thin piece and the egg won't break.

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Jerry
Stebbins
Sent: Sunday, August 13, 2006 10:17 AM
To: chad at f3acanada.org; NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Outrunners and structural failures

Chad we are getting ready to try the EVO in the spinner mount. My guess is 
the location of the prop relative to the rear mount-plus all the prop 
related influences is the primary generation source. With the prop right at 
the mount (in the spinner) there would be a very moment short arm. I guess I

need to look at Jerry B's whirl flutter info trail and see what I find. Hope

he finds his lost parts and then has time to educate us on what he has 
found.
Jerry S
----- Original Message ----- 
From: "Chad Northeast" <chad at f3acanada.org>
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Sent: Saturday, August 12, 2006 10:46 PM
Subject: Re: [NSRCA-discussion] Outrunners and structural failures


> Very interesting...and here I was going to get Pletty to make me a front
> mounted 30-10 Evo....I guess there goes that idea :)
>
> What is being used for the front mount?
>
> I have always rear mounted (without a front support), and you can always
> hear some vibration when you get large AOA changes...like in snaps, or
> hard corners with slight rudder application.  Always figured it was the
> prop deflecting and vibrating the motor since is cantilivered so far
> away from the rotational axis.  I had figured a front mount would solve
> this.
>
> I did grab a couple of front mount setups while at the Nats and they all
> felt like they had more movement than my rear mount setup has, which
> surprised me somewhat.
>
> Chad
>
> Chris Moon wrote:
>
>>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
>>
>>_______________________________________________
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>>NSRCA-discussion at lists.nsrca.org
>>http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>>
>>
>>
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