Elev. Pull pull Question

[brett terry]

I need to add my experience to this conversation.  I had a 6-32 bolt
aileron horn fail on a plane.  It was mounted in the center of the
aileron span, and I was using JR 8411 servos.  The airplane has a gas
engine (ZDZ 40), but doesn't really shake any more than a 1.4
4-stroke.

The rod sheared right at the junction with the aileron.  I think a
pull-pull rudder or elevator system will have more damping flexibility
than a direct mount aileron control setup with a titanium pushrod, but
the effect is similar.  In addition to the motion of the control
surface, remember the rod will undergo flexion about the surface
junction and at many additional frequencies.  Eventually these
frequencies will introduce cracking at the junction and eventual
failure.

After replacing the bolt in the wing it was encircled with a carved
cone of balsa to attenuate the vibrational flexion.  This added
stiffness to the system, will eliminate failure in the future, was a
simple retrofit, and looks more "finished" than the bare rod did.

I believe the ailerons are much more likely to experience this type of
failure than a tail surface.  I will use this method on all surfaces
just for peace of mind.  The cone was made from a 1/2" square balsa
stick with a hole drilled through the center.

The all-thread is a good idea as it is more ductile than a bolt and
less susceptible to stress cracking.

This is a great thread, I enjoy learning from all of you!

Brett




On Tue, 30 Nov 2004 10:54:26 -0600, Wayne Galligan
<wgalligan at goodsonacura.com> wrote:
> 
> And if you can find the threaded aluminum spacers(about 1/4 round) they make
> great stand offs and lock the bolt to the hard point with out having to glue
> them in. Also makes the control rod stiffer with nary any weight gain.  They
> make them in several lengths, 1/2, 3/4 and 1"  long.
> 
> 
>  
> Wayne G.
>  
> 
> ----- Original Message ----- 
> From: ronlock at comcast.net 
> To: discussion at nsrca.org ; discussion at nsrca.org 
> Cc: Karl G. Mueller 
> Sent: Tuesday, November 30, 2004 10:35 AM
> Subject: Re: Elev. Pull pull Question
> 
> 
> Another option- buy 6-32 all thread at local hardware store.
> Cut stud lengths as needed and thread into hard points with a bit of epoxy.
> Ive been using them for years for elevator, aileron, & rudder.
>  
> Ron Lockhart
> -------------- Original message -------------- 
> 
> Matt,
>  
> I have used # 6-32 socket cap screws for years now and never had a failure
> with these. Buy a box of 100 about 2" long at your local industrial supply
> for about $20-25, cut of the heads, put a couple of grooves in the non
> threaded
> part for the epoxy to hold and glue these into the hard point (1/2" dia
> dowel) in 
> your movable surface. Do no drill all the way through. Just up to the top
> sheeting.
> You can offset the hole in the hard point toward the hinge
> line to adjust for proper distance for the hole in the plastic end, that
> receives the
> clevis, to line up over the hinge line. Socket cap screw are heat treated,
> but not 
> all the way. They still have a soft core and will bend under enough force,
> but
> not with the amount of force used to move the surfaces.
>  
> Karl G. Mueller
> kgamueller at rogers.com
> 
>  
> 
> ----- Original Message ----- 
> From: Rcmaster199 at aol.com 
> To: discussion at nsrca.org 
> Sent: Tuesday, November 30, 2004 9:28 AM
> Subject: Re: Elev. Pull pull Question
> 
> 
> 
> Keith, please note that 4-40's are known to snap off a control surface. It
> has happened to me. The break is a straight forward fatigue fracture. If
> 6-32's are not useful to you, consider 5-40's. Personally, I do find 6-32s
> do just fine in this application
>  
> MattK
>  
> In a message dated 11/30/2004 3:17:03 AM Eastern Standard Time,
> tkeithb at comcast.net writes:
> 
> Cat's Meow? Maybe not, but darn close. The only thing that keeps them from
> being the Cat's Meow in my book is the 6/32 threads which don't offer as
> precision of adjustments as 4/40 or the MK horns. 
>  
> Lance's suggestion of offsetting the control arm at the servo is perfectly
> valid also.
>  
> Keith Black
>  
>
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