Under extreme load the spar pin is being twisted downwards (at the rear end). So
it is no longer purely in shear, thus the tendency for the rear spar to slide
off and the need for a pip pin and strap.
If the spar arrangement was a fork type (in many gliders) instead of the
scissors
that we currently have this twisting load would not exist.
Cheers,
Pete
> On Oct 27, 2013, at 7:27 AM, "David Joyce" <davidjoyce@doctors.org.uk> wrote:
>
>
> We know that the wings & spars have been proof tested to 10g and the force on
the spar pin is principally one of shear with the spar tip trying to depart
downwards,
and at 10g MAUW, the actual shear force is likely to be something in
excess of 3000kg.
> In comparison to this, the tendency for the spar tip to move backwards and
> disengage
itself from the spar pin must be very small and I have no doubt that the
clever fellows at Europa did their stress calculations and concluded that the
tension strength of the pip pin system was sufficient. I suspect that
reinventing
that particiular wheel will achieve nothing but risking not being able
to get your wings off somewhere down the line.
> Regards, David Joyce, G- XSDJ
>
> "craig" <craigb@onthenet.com.au> wrote:
>> However to improve on the pip pin being used for this purpose I think would
>> need a pin threaded for a nut to be tightened and if you can develop a way
>> of access to a fully assembled aircraft for applying a bolt and nut I would
>> be pleased to hear of it ! In the meantime the pip ball rules!
>> Bob, and all others of course
>> I like you and no doubt quite a few others have given the threaded bolt
>> option
>> You mention quite some thought, spending a lot of time traveling for work
>> Gives you the luxury of time to ponder such small things, the main issue
>> I have over switching to some sort of threaded arrangement is the alignment
>> issues
>> With the pin now can be troublesome, now say it has the be PERFECTLY aligned
>> to
>> Get the thread to start without cross threading it, and it just got a whole
>> lot
>> Harder, of course you could use some form of stepped alignment with say a
>> 3/8
>> Taper stepping up to the 1/2 inch thread and a caged nut that can move to
>> align
>> But not rotate, that was my idea anyway, but I don't think there is enough
>> space
>> Say 1/2 an inch, for the caged nut, plus then the tapered end of the bolt
>> sticking through
>> The nut. One option that did come to mind was something similar to a masonry
>> anchor
>> Where you slide the whole thing into a hole, and tighten the bolt(or nut)
>> depending
>> On the style and the unit in the hole expands at the end to lock itself into
>> the
>> Concrete. If you used an internal thread arrangement to pull a set of jaws
>> outwards
>> You could "grab" the back end of the socket where the balls are now, and
>> pull both spars
>> Then hard into the seat back, by simply tightening one bolt
>> HOWEVER
>> The device mentioned would likely be a custom made part, with springs etc to
>> ensure the jaws
>> Retract, I know of no such part at present.
>> Secondly, no doubt someone will weigh in with and it is a concern of mine
>> too
>> It is generally accepted the seat back is NOT contributing to transfer lift
>> loads etc
>> To the airframe, and that like most aircraft the wings do flex and move in
>> flight
>> If you have the spars firmly bolted to seat back then you remove that
>> movement potential
>> Completely and probably make the seat back a major load bearing component,
>> and who knows
>> What the result of that might be
>> Just my thoughts
>> Make of them what you will, the pin works ok, sure its not perfect, but
>> would
>> Something else be any better
>> Regards
>> craig Un/Subscription,
>> Forums!
>> Admin.
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