[NSRCA-discussion] Outrunners and structural failures

Sun Aug 13 07:14:13 AKDT 2006

An example of the forces involved. I had (automotive) engine dynamometer 
stands fitted with inertial flywheels that were cantilevered behind the rear 
support. Shaft dia was 2 1/2", flywheel (350 # ft sq) mounted about 6" 
behind rear bearing, the shaft continued forward about 3' to another support 
with a dyno rotor (60 # ft sq) between the supports. A very small imbalance 
(<1/2 oz) would impart a wobble at 3500 rpm of several inches at the 
flywheel O.D. Watched with a strobe - the steel shaft looked like a rubber 
hose flexing! A much better system placed a support immediately before and 
after the flywheel.

So - scale that down and it's apparent the forces we're dealing with are 
easily underestimated. While cantilevered mounting of props is the norm (and 
only practical) - it's not the best situation and leads to all sorts of 
twisting forces when gyroscopic precession is included (changing the plane 
of rotation in pattern maneuvers). Mounting of an outrunner offers all sorts 
of complications compounded by the rotational forces of the motor housing / 
magnets (mass) - firewall mount it and the lever arm is long allowing any 
flex in the firewall to translate into a good deal of freedom of movement 
(oscillation) at the prop. Center mount it and - while the moments are 
shorter - the twisting forces fore / aft of the mounting also exert twisting 
to the mount, now in a weaker location (nose-ring). The long geared motors 
are a bit better in that they obviously demand a rear support and have the 
(motor) rotating mass closer to the centerline (shorter moment). But, add to 
all this a fuselage designed for power loading in the normal region of a 
firewall (light forward) and there's little structural rigidity to withstand 
the twisting forces on the more forward E mount.

Certainly mounting an outrunner with mid and rear support is a start. Adding 
structural strength to the fuse nose also seems prudent. Some have seen the 
(wing) foam I've installed in the nose of my E airplanes. I primarily did 
this for air ducting - however I quickly noticed a noise reduction. 
Obviously from reducing resonance of the composite of the fuse nose - 
certainly stiffening the composite structure lessens onset of  fatigue 
failure of the composite. Likely an overall strength addition to the fuse 
nose also. An easy / light retrofit with a some foam and poly glue.

Best fix is appears to be a combination of strengthening the fuse nose and 
limiting movement / twisting by two point (fore / aft) support of the motor.

Earl

----- Original Message ----- 
From: "Jerry Stebbins" <JAStebbins at worldnet.att.net>
To: <chad at f3acanada.org>; "NSRCA Mailing List" 
<nsrca-discussion at lists.nsrca.org>
Sent: Sunday, August 13, 2006 9:17 AM
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
>>>
>>>_______________________________________________
>>>NSRCA-discussion mailing list
>>>NSRCA-discussion at lists.nsrca.org
>>>http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>>>
>>>
>>>
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