[NSRCA-discussion] Outrunner Structural Failure - foam

Sun Aug 13 08:15:12 AKDT 2006

Hi Earl,
Ref your foam in nose technique-
Might building a form in nose, then filling voids between form and
fuse sides with an expandable foam be an equivalent technique?

Later, Ron Lockhart

----- Original Message ----- 
From: "Earl Haury" <ehaury at houston.rr.com>
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Sent: Sunday, August 13, 2006 11:13 AM
Subject: Re: [NSRCA-discussion] Outrunners and structural failures

> 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
> >>>
> >>>_______________________________________________
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> >>>
> >>>
> >>>
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