[NSRCA-discussion] Outrunner Structural Failure - foam

Sun Aug 13 08:22:12 AKDT 2006

Ron

Should be OK (maybe better) - but there's a potential for making a real mess 
and I didn't want to risk it on a finished airplane.

Earl

----- Original Message ----- 
From: "Ron Lockhart" <ronlock at comcast.net>
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Sent: Sunday, August 13, 2006 11:18 AM
Subject: Re: [NSRCA-discussion] Outrunner Structural Failure - foam

> 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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>> >>>NSRCA-discussion at lists.nsrca.org
>> >>>http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>> >>>
>> >>>
>> >>>
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