Duncan,
Thanks for your input and questions...answers indented below.
> <ami@mcfadyean.freeserve.co.uk>
>
> Fred,
> Nice execution of a good idea.
> CSA-VLA provides little guidance:
>
> http://www.easa.eu.int/doc/Agency_Mesures/Certification_Spec/
> decision_ED_2003_18_RM.pdf
> See pdf page 100 (or pages 2-8 and 2-9 at the back.
>
> It merely advises for "..adequate separation of ...straps to
> minimise... chafing of wearer's neck...." but recommends a maximum
> separation of 8" and maximum included angle of 60 degrees.
I really didn't want to re-invent the wheel here, so one of the
starting points was to not alter the attach point of the shoulder
straps...and to secure the "riser" to the same hard point used for the
straps. If our shoulder straps had had individual bolts to the hard
point(s), I would have maintained them; since they go back to a single
point, that's what this design works from. The key to
"minimise...chafing of wearer's neck" is the location of where the
shoulder straps connect to the seat belt; in my opinion, there is no
cause for concern here.
> Was your load testing applied to the straps as fitted to the device in
> the aircraft or merely to the device sat on a strong surface?
I've done no testing with this "riser" bolted to the CM. The photo
below shows the test rig I used. The riser is mounted on a piece of 4 x
10 wood with an angled face, set to duplicate the angle between the
back and top of the head rest. Unseen beneath the strapping are two AN3
bolts which correspond to those bolts which penetrate the hard point on
the back of the headrest and flank the AN4 bolt which secures the
typical shoulder straps; see other photos. Note that I've substituted
2" wide cargo straps for the standard issue shoulder harness straps;
since I'm testing this little "riser" and not the shoulder harness, I
thought this would have no effect. Note also that the straps are
clamped beyond the edge of the "flap"; the riser is held in place
solely by the two AN3 bolts (under the straps).
Also visable in the photo is a gauge resting on a loop in the straps
with a pressure plate beneath a 2 x 2 steel tube which receives the
force from a 20 ton hydraulic press.
The test rig presumes that tension forces on the shoulder straps will
be transmitted to the "riser" at its apex which has a radius of
3/8"...so the big question was...Would the "riser" be crushed? Well, I
really didn't want to destroy it, so I stopped at 1500#; this load was
reached incrementally...it was not an instantaneous load.
Full Disclosure: My buddy with the hydraulic press uses this gauge to
measure the tongue weight on trailers he uses w/ his business...he
swears that it measures weight in pounds. However, as you can see in
the next photo, the gauge is labeled in psi. You can note the size of
the pressure plate in relation to the 2 x 2 steel tube...it appears to
be about 2.5 inches in diameter...which means its area is just under
4.9 sq. in. This then implies (to this non-engineer) that the force on
the straps would be more like 7359# rather than just 1500#. Could this
be so? Help!
> And would it be better to wrap the forward edge of the device around
> the front edge of the headrest, to reduce the eccentricity of loading
> on that corner? thereby maximising the resistant of the front face of
> the headrest
Though it appears greater in the photos, the front face of the riser
is only about 1/4" back from the face of the head rest...the
intersection between the top and the front faces of the head rest has a
radius...I sized the "riser" so as to not extend beyond the start of
the radius. Personally, I'm unclear as to what portions of the loads
are transmitted to the front face and what portions are distributed
over the top head rest surface. The head rest construction appears to
me to be the same on all faces, so I didn't think aligning the front
faces would be significant.
> Would prefer to see the guides at the top made as part of (or recessed
> in to) the moulding, with broad rounded edges.
The beauty of homebuilt aircraft is the latitude (at least here in the
US) it gives to allow for personal preferences. The design I developed
was intended to both satisfy the functional requirement and be
sympathetic to the tapered forms of the head rests...guess I could live
with the "ears"!
Ron Pagoris has suggested I do some load tests of side loads such as
those which might be encountered in a ground loop...so notwithstanding
the pixs of these puppies on my CM, I guess it's back to the lab for a
bit.
Fred
A194
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